236225174 psd-course-file-2011-12-1

----------------------------------------------------------------------- Power Semiconductor Drives
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POWER SEMICONDUCTOR DRIVES
UNIT – I
Control of DC motors by Single phase Converters: Introduction to Thyristor
controlled Drives, Single Phase semi and Fully controlled converters connected to d.c
separately excited and d.c series motors – continuous current operation – output voltage
and current waveforms – Speed and Torque expressions – Speed – Torque
Characteristics- Problems on Converter fed d.c motors.
UNIT - II
Control of DC motors by Three phase Converters: Three phase semi and fully
controlled converters connected to d.c separately excited and d.c series motors – output
voltage and current waveforms – Speed and Torque expressions – Speed – Torque
characteristics – Problems.
UNIT – III
Four Quadrant operation of DC Drives: Introduction to Four quadrant operation –
Motoring operations, Electric Braking – Plugging, Dynamic and Regenerative Braking
operations. Four quadrant operation of D.C motors by dual converters – Closed loop
operation of DC motor (Block Diagram Only).
UNIT-IV
Control of DC motors by Choppers: Single quadrant, Two –quadrant and four quadrant
chopper fed dc separately excited and series excited motors – Continuous current
operation – Output voltage and current wave forms – Speed torque expressions – speed
torque characteristics – Problems on Chopper fed d.c Motors – Closed Loop operation
( Block Diagram Only).
UNIT – V
B.V.R.I.T. Course File --------------------------------------------------------------------------- 1

Control of Induction Motor through Stator voltage: Variable voltage characteristics-
Control of Induction Motor by Ac Voltage Controllers – Waveforms – speed torque
characteristics.
UNIT – VI
Control of Induction Motor through Stator Frequency: Variable frequency
characteristics-Variable frequency control of induction motor by Voltage source and
current source inverter and cyclo converters- PWM control – Comparison of VSI and CSI
operations – Speed torque characteristics – numerical problems on induction motor drives
– Closed loop operation of induction motor drives (Block Diagram Only).
UNIT –VII
Control of Induction motor of Rotor side: Static rotor resistance control – Slip power
recovery – Static Scherbius drive – Static Kramer Drive – their performance and speed
torque characteristics – advantages applications – problems.
UNIT – VIII
Control of Synchronous Motors: Separate control & self control of synchronous motors
– Operation of self controlled synchronous motors by VSI and CSI cycloconverters. Load
commutated CSI fed Synchronous Motor – Operation – Waveforms – speed torque
characteristics – Applications – Advantages and Numerical Problems – Closed Loop
control operation of synchronous motor drives (Block Diagram Only), variable frequency
control, Cyclo converter, PWM, VFI, CSI.
TEXT BOOKS:
1. Fundamentals of Electric Drives – by G K Dubey Narosa Publications
2. Power Electronics – MD Singh and K B Khanchandani, Tata – McGraw-Hill
Publishing
company,1998
REFERENCES:
1. Power Semiconductor Controlled Drives – Gopal K Dubey PH International
Publications.
2. Power Semiconductor Drives - S B Dewan, G R Selmon, A Straughen
3. Power Electronic Control of AC drives – B K Bose
4. Thyristor Control of Electric drives – Vedam Subramanyam Tata McGraw Hill
Publilcations.
5. Electric Drives – By N K de and P K Sen , Prentice Hall of India Pvt. Ltd.
6. A First course on Electrical Drives – S K Pillai New Age International(P) Ltd. 2nd
Editon
7. Analysis of Thyristor Power – Conditioned Motors – By S K Pillai, Unversity Press
(India) Ltd. Orient Longman Ltd. 1995.
8. Fundamental of Electric Drives – by Mohd. AEL – Sharkawi by VIKAS Publishing
House.
UNIT – I
UNIT OBJECTIVE:
 To introduce the thyristor controlled drives.

 To study the operation of single phase semi and fully controlled converters
connected to DC separately excited motor with neat circuit diagram and
waveforms and also to derive the torque speed characteristics.
 To study the operation of single-phase semi and fully controlled converters
connected to DC series motor with neat circuit diagram and waveforms and also
to derive the torque speed characteristics.
IMPORTANT POINTS AND DEFINITIOS:
 A converter is a static device, which converts fixed AC voltage with fixed
frequency into variable DC voltage.
 A firing angle may be defined, as “the angle between the instant thyristor would
conduct if it were a diode and the instant it is triggered.”
 A semi (or half) controlled converter is one quadrant converter where as a full
converter is two-quadrant converter in which, voltage polarity can be reversible
but current polarity cannot be reversible because of the unidirectional properties
of the SCR.

The average value of DC output voltage of 1 - φ half controlled bridge converter
can be calculated by,
(i) For R – load:
VO TH = (Vm/ Π) (1 + Cos α)
Where VO TH is the theoretical average value of DC output voltage
Vm is maximum value of AC input voltage and
α is the firing angle.
(ii) For RL – load:
VOTH = (Vm / Π) (Cos α - Cos β)
The value of Extinction angle β can be calculated by,
β = (Π + φ); Where φ = tan-1
(ωL / R)
 The average value of DC output voltage of 1 - φ full controlled bridge converter
(i) For R – load:
VO TH = (2 Vm/ Π) (Cos α)
VOTH = (2 Vm / Π ) (Cos α - Cos β)
(ωL / R)
 In converters if the source inductance is considered the load current will not
transfer immediately from outgoing SCR’s to incoming SCR’s.

 “The period during which both outgoing SCR’s and incoming SCR’s are
conducting” is known as overlap period.
 µ (Commutation angle) is the angular period of overlap period.
by considering the source inductance for R – load can be calculated by,
VO TH = (2 Vm/ Π) Cos (α+µ) + (ωLs / Π) I0
µ is the commutation angle.
 In DC series motors the torque is directly proportional to the square of armature
current where as in DC separately excited motors torque is directly proportional to
the armature current only.
 In converter fed DC series motors the during the current zero period the output
voltage is equal to the back emf due to residual magnetism in the field, where as it
is equal to zero in DC separately excited drives.
OBJECTIVE QUESTIONS:
1. A single-phase half-wave controlled rectifier has 400 sin 314 t as the
input voltage and R as the load. For a firing angle of 60° for the SCR, the average output
voltage is
(a) 400/П (b) 300/ П
(c) 240/ П (d) 200/ П
2. A single-phase one-pulse controlled- circuit has resistance and counter
emf load and 400 sin 314 t as the source voltage. For a load counter emf of 200 V, the
range of firing angle control is
(a) 30° to 150° (b) 30° to 180°
(c) 60° to 120° (d) 60° to 180°
3. In a single-phase half-wave circuit with RL load, and a freewheeling
diode across the load, extinction angle 13 is more than 1t. For a firing angle a, the BCR
and freewheeling diode would conduct, respectively, for
(a) П -α, β (b) β-α,П-α
(c) П - α, β - П (d) П - α, П - β
4. In a single-phase one-pulse circuit with RL load and a freewheeling
diode, extinction angle 13 is less than 1t. For a firing angle a, the BCR and freewheeling
diode would, respectively, conduct for
(a) β - α, 0° (b) П - α, П-β
(c) α, β - a (d) β - α, α
5. A single-phase full-wave mid point thyristor converter uses a 230/200
V transformer with center tap on the secondary side. The P.I.V. per thyristor is
(a) 100 V (b) 141.4 V
(c) 200 V (d) 282.8 V
6. A single-phase two-pulse bridge converter has an average output
voltage and power output of 500 V and 10 kW respectively. The SCRs used in the two-
pulse bridge converter are now re-employed to form a single-phase two-pulse mid-point

converter. This new controlled converter would give, respectively, an average output
voltage and power output of
(a) 500 V, 10 kW (b) 250 V, 5 kW
(c) 250 V, 10 kW (d) 500 V, 5 kW
7. In a single-phase full converter, for continuous conduction, each pair
of SCRs conducts for
(a) П -α (b) П
(c) α (d) П + α
8. In a single-phase full converter, for discontinuous load current and
extinction angle β > П, each SCR conducts for
(a) α (b) β - α
(c) β (d) α +β
9. In a single-phase semi-converter, for continuous conduction, each SCR
conducts for
(a) α (b) П
(c) α + П (d) П -α
10. In a single-phase semi converter, for discontinuous conduction and extinction angle
β > П, each SCR conducts for
(a) П -α (b) β - α
(c) α (d) β
11. In a single-phase semi converter, for discontinuous conduction and extinction angle β
< П, each SCR conducts for
(a) П - α (b) П -α
© α (d) β
12. In a single-phase semi converter, for continuous conduction, freewheeling diode con-
ducts for
(a) α (b) П - α
© П (d) П + α
13. In a single-phase semi converter, with discontinuous conduction and extinction angle
β > П, freewheeling diode conducts for
(a) α (b) П - α (c) П + α (d) β
14. In a single-phase semi converter, with discontinuous conduction and extinction angle
β < β, freewheeling diode conducts for
(a) α (b) П - β (c) β - П (d) Zero degree
15. In a single-phase full converter, if α and β are firing and extinction angles respective-
ly, then the load current is
(a) discontinuous if (β-α) < П (b) discontinuous if (β - α) > П
(c) discontinuous if (β - α) = П (d) continuous if (β-α) < П
16. In a single-phase full converter with resistive load and for a firing angle α, the load
current is zero and non-zero, respectively, for
(a) α, П - α (b) П - α, α
(c) α, П+ α (d) α, П
17. In a single-phase semi converter with resistive load and for a firing angle a, each SCR

and free wheeling diode conduct, respectively, for
(a) α, 0° (b) П - α, α
(c) П+α,α (d) П-α, 0°
18. In controlled rectifiers, the nature of load current, i.e. whether load current is
continuous or discontinuous.
a) does not depend on type of load and firing angle delay
b) depends both on the type of load and firing angle delay
c) depends only on the type of load
d) depends only on the firing angle delay
19. In a single-phase full converter, if output voltage has peak and average values of 325
V and 133 V respectively, then the firing angle is
(a) 40° (b) 140°
(c) 50° (d) 130°
20. In a single-phase semi converter, if output voltage has peak and average values of 325
and 133 V respectively, the firing angle is
(a) 40° (b) 140°
(c) 73.40° (d) 80°
21. For a single-phase phase-controlled rectifier, with a freewheeling diode across the
load,
a) the instantaneous output voltage V0 is always positive
b) V0 may be positive or zero
c) V0 may be positive, zero or negative
d) V0 is always zero or negative
22. In a single-phase full converter, if load current is 1 and ripple free, than average
thyristor current is
(a) 1 / 2 I (b) 1 / 3 I
(c) 1 / 4 I (d) I
23. In a single-phase full converter, the number of SCRs conducting during overlap is
(a) 1 (b) 2
(c) 3 (d) 4
24. In a single-phase full converter, the output voltage during overlap is equal to
a) zero
b) source voltage
c) source voltage minus the inductance drop
d) inductance drop
SUBJECTIVE QUESTIONS:
1. Explain the operation of a single-phase semi converter fed DC separately excited
motor in continuous current mode and derive the equation relating speed and torque and
also draw their speed torque characteristics.
2. Explain the operation of a single phase semi converter fed DC series motor in
continuous current mode and derive the equation relating speed and torque and also draw
their speed torque characteristics.

3. Explain the operation of a single-phase full converter fed DC separately excited motor
in continuous current mode and derive the equation relating speed and torque and also
draw their speed torque characteristics.
4. Explain the operation of a single phase full converter fed DC series motor in
5. A 220 V, 960rpm, 80Aseparately excited DC motor has an armature resistance of
0.06ohms. Under rated conditions, the motor is driving a load whose torque is constant
and independent of speed. The speeds below rated speed are obtained with armature
voltage control (with full field), and the speeds above rated speed are obtained by field
control (with rated armature voltage). Determine (i) The motor terminal voltage when the
speed is 620rpm. (ii) The value of flux as a percentage of rated flux if the motor speed is
1200 rpm. Neglect the motor rotational losses.
6. A separately excited motor of 220V, 960rpm, 80A with an armature resistance of
0.06ohms is coupled to an over hauling load with a toque of 100 Nm. Compute the speed
at which the motor can hold the load by regenerative braking. Source voltage is 220V.
Neglect the motor rotational losses.
7. A separately excited DC motor is fed from a 230V, 50Hz supply via a single-phase
half controlled bridge rectifier. Armature parameters are: inductance 0.06H, Resistance
0.3ohms. Motor voltage constant is Ka = 0.9 V/A rad/s and the field resistance is Rf
104ohms. The field current is controlled by a semi converter and is set to max. possible
value. The load torque is 50Nm at 800 rpm. The inductances of armature and field circuit
are sufficient enough to make the armature and field currents continuous and ripple free.
Compute (i) The field current If (ii) The firing angle of converter in armature circuit. (iii)
The input power factor of the armature circuit converter. Neglect the system losses.
8. The speed of 10HP, 210V, 1000 rpm, separately excited DC motor is controlled by a
single-phase full converter as shown in fig. The rated motor armature current is 30Amps
and the armature resistance is Ra is 0.25 ohms. The AC supply volt is 230V. The motor
voltage constant is K 0.172 V/rpm. Assume that sufficing inductance is present in the
armature circuit to make the motor continuous and ripple free. (a) Rectifier operation
(Motoring action) for firing angle alpha = 450
and rated motor armature current,
Determine (i) Motor torque (ii) Speed of the motor (iii) the supply power factor (b)
Inverter operation. Reversing the field excitation reverses the motor back emf polarity.
Determine (i) The firing angle to keep the motor current at its rated value. (ii) The power
feed back to the supply
9. A 210V, 1200rpm, 10A separately exited motor is controlled by a single phase fully
controlled converter with an AC source voltage of 230V, 50Hz. Assume that sufficient
inductance is present in the armature circuit to make the motor current continuous and
ripple free for any torque greater than 25% of rated torque. Ra = 1.5ohm.
10. What should be the value of the firing angle to get the rated torque at 800rpm?
Compute the firing angle for the rated braking torque at –1200rpm.
Calculate the motor speed at the rated torque and alpha =1650
for the regenerative
braking in the second quadrant?
11. A small separately excited DC motor is supplied via a half controlled, single phase
bridge rectifier .The supply is 240Volts, the thyristors are triggered at 1100
, and the
armature current continues for 500
beyond the voltage zero. Determine the motor speed at

a torque of 1.8 Nm, Given the motor torque characteristics 1.0 Nm/amps and its armature
resistance is 6ohms. Neglect the all converter losses.
12. The speed of 20HP, 210V, and 1000-rpm series motor is controlled by a single-phase
(a) semi converter (b) full converter. The combined field and armature circuit resistance
is 0.25ohms. Motor constants are Kaf = 0.03N-m.amp2
, Kres = 0.075V-sec/rad, the supply
voltage is 230V. Assume the continuous ripple free motor current, Determine the
following for firing angle alpha = 300
and speed N = 1000rpm; (i) motor torque (ii) motor
current (iii) supply power factor
PREVIOUS SUBJECTIVE QUESTIONS:
1) A dc series motor has Ra = 3 Ω, Rf = 3 Ω and Maf = 0.15 H. A phase-controlled bridge
varies the motor speed. The firing angle is π/4 and the average speed of the motor is 1450
rpm. The applied ac voltage to the bridge is 330 Sin wt. Assuming continuous motor
current find the steady state average motor current and torque. Sketch the waveforms for
output voltage, current and gating signals.
2) (a) Explain how the speed of a dc series motor is controlled using converters.
(b) A series motor is supplied from a full converter whose α= 650
, 1 φ supply of 230V
rms, 50HZ frequency. The armature and field resistance together equal 2 Ω. The torque
constant Maf is 0.23H and the load torque is 20Nm. Neglect damping and find the
average armature current and speed.
3) Two independent single-phase semi-converters are supplying the armature and field
circuits of the separately excited dc motor for controlling its speed. The firing angle of
the converter, supplying the field, adjusted such that maximum field current flows. The
machine parameters are: armature resistance of 0.25 Ω, field circuit resistance of 147 Ω,
motor voltage constant Kv=0.7032 V/A-rad/s. The load torque is T=45 N-m at 1000 rpm.
The converters are fed from a 208 V, 50 Hz ac supply. The friction and windage losses
are neglected. The inductance of the filed and armature circuits are sufficient enough to
make the armature and field currents continuous and ripple free. Determine
(a) the field current,
(b) the delay angle of the armature converter,
(c) input power factor of the armature circuit converter..
4) A single-phase fully controlled thyristor converter is supplying a dc separately excited
dc motor. Draw the neat waveform diagrams and explain various operating modes of the
drive both in motoring and regenerative braking for
(a) γ < α,
(b) γ > α,
Where α: is the firing angle, γ: is the angle at which the source voltage equal to the motor
back emf. Assume the armature of the separately excited dc motor can be replaced by
simple R-L and back emf load.
5) (a) A DC shunt motor operating from a 1 φ half controlled bridge at a speed of 1450
rpm has an input voltage 330 Sin 314t and a back emf 75V. The SCRs are fired
symmetrically at α = π/4 in every half cycle and the armature has a resistance of 5Ω.
Neglecting armature inductance, find the average armature current and the torque.
(b) Sketch the speed-torque characteristics for the above problem.

UNIT – II
UNIT OBJECTIVE:
 To study the operation of three phase semi and fully controlled converters
connected to DC separately excited motor with neat circuit diagram and
waveforms and also to derive the Torque - Speed characteristics.
 To study the operation of three-phase semi and fully controlled converters
connected to DC series motor with neat circuit diagram and waveforms and also
to derive the Torque – Speed characteristics.
 To derive the Torque – Speed expressions for the entire converter fed DC motor
drives.
IMPORTANT POINTS AND DEFINITIONS:
 The average value of DC output voltage of 3 - φ half controlled bridge converter
(i) For R – load:
VO TH = (3 Vml / 2 Π) (Cos α)
Vml is maximum value of AC input line voltage and
VOTH = (3 Vml / 2 Π) (Cos α - Cos β)
(ωL / R)
 The average value of DC output voltage of 3 - φ half controlled bridge converter
(i) For R – load:
VO TH = (3 Vml / Π) (Cos α)
VOTH = (3 Vml / Π) (Cos α - Cos β)
(ωL / R)
by considering the source inductance for R – load can be calculated by,
V0 TH = (3 Vml / Π) Cos (α+µ) + (3 ωLs / Π) I0
Where V0 TH is the theoretical average value of DC output voltage
µ is the commutation angle.

 The three phase converter SCR’s are triggered at a faster rate when compared
with single phase converter SCR’s, causing the output current to be more
continuous in the three phase converters.
1. Each diode of a 3-phase half-wave diode rectifier conducts for
(a) 60° (b) 120°
(c) 180° (d) 90°.
2. Each diode of a 3-phase, 6-pulse bridge diode rectifier conducts for
(a) 60° (b) 120°
(c) 180° (d) 90°.
3. In a 3-phase half-wave diode rectifier, if per phase input voltage is 200 V, then the
average output voltage is
(a) 233.91 V (b) 116.95 V
(c) 202.56 V (d) 101.28 V
4. In a 3-phase half-wave diode rectifier, the ratio of average output voltage to per-phase
maximum ac voltage is
(a) 0.955 (b) 0.827
(c) 1.654 (d) 1.169.
5. In a 3-phase half-wave rectifier, dc output voltage is 230 V. The peak inverse voltage
across each diode is
(a) 481.7 V (b) 460 V
(c) 345 V (d) 230 V.
6. In a 3-phase full-wave diode rectifier, the peak inverse voltage in terms of average
output voltage is
(a) 1.571 (b) 0.955
(c) 1.047 (d) 2.094
7. In a 3-phase half-wave diode rectifier, if Vm is the maximum value of per phase
voltage, then each diode is subjected to a peak inverse voltage of
(a) Vm (b) (√3) Vm
(c) 2Vm (d) 3Vm,
8. In a 3-phase full-wave diode rectifier, if Vm is the maximum value of line voltage, then
each diode is subjected to. a peak inverse voltage of
(a) Vm (b) (√3) Vm
(c) 2Vm (d) 3Vm
9. In a 3-phase full-wave diode rectifier, if V is the per phase input voltage, then average
output voltage is given by
(a) 0.955 V (b) 1.35 V
(c) 2.34 V (d) 3 V
10. A converter which can operate in both 3-pulse and 6-pulse modes is a
(a) 1-phasefull converter (b) 3-phase half-wave converter
(c) 3-phase semi converter (d) 3-phase full converter.
11. In a 3-phase semi-converter, for firing angle less than or equal to 60°, each thyristor
an diode conduct, respectively, for
(a) 60°, 60° (b) 90°, 30°

(c) 120°, 120° (d) 180°, 180°
12. In a 3-phase semiconverter, for firing angle less than or equal to 60°, freewheeling
diode conducts for
(a) 30° (b) 60°
(c) 90° (d) zero degree
13. In a 3-phase semiconverter, for a firing angle equal to 90° and for continuous conduc-
tion, each SCR and diode conduct, respectively, for
(a) 30°, 60° (b) 60°, 30°
(c) 60°, 60° (d) 30°, 30°
14. In a 3-phase semi converter, for a firing angle equal to 90° and for continuous
conduction, free wheeling diode conducts for
(a) 30° (b) 60°
(c) 90° (d) 0°
15. In a 3-phase semiconverter, for firing angle equal to 120° and extinction angle equal
to 110°, each SCR and diode conduct, respectively, for
(a) 30°, 60° (b) 60°, 60°
(c) 90°, 30° (d) 110°, 30°
to 110°, freewheeling diode conducts for
(a) 100
(b) 200
(c) 50° (d) 110°
to 100°, none of the bridge elements conduct for
(a) 100
(b) 200
(c) 30° (d) 60°
18. A 3-phase semi converter can work as
converter for α = 0° to 180° (b) converter for α = 0° to 90°
(c) inverter for α =90° to 180° (d) inverter for α = 0° to 90°.
19. In a 3-phase semiconverter, the three SCRs are triggered at an interval of
(a) 600
(b) 900
(c) 120° (d) 180°
20. In a 3-phase full converter, the six SCRs are fired at an interval of
(a) 30° (b) 60°
(c) 90° (d) 120°
21. In a 3-phase full converter, three SCRs pertaining to one group are fired at an interval
(a) 30° (b) 60°
(c) 90° (d) 120°
22. The frequency of the ripple in the output voltage of a 3-phase semiconverter depends
upon
a) firing angle and load resistance
b) firing angle and load inductance
c) the load circuit parameters
d) firing angle and the supply frequency
23. In a 3-phase full converter, if load current is 1 and ripple free, then average thyristor
current is
(a) 1/ 2 I (b) 1 / 3 I

(c) 1 / 4 I (d) I
24. The effect of source inductance on the performance of single-phase and three-phase
full converters is to
a) reduce the ripples in the load current
b) make discontinuous current as continuous
c) reduce the output voltage
d) increase the load voltage
25. In a 3-phase full converter, the output voltage during overlap is equal to
a) zero
b) source voltage
c) source voltage minus the inductance drop
d) average value of the conducting-phase voltages.
26. The total number of SCRs conducting simultaneously in 3-phaseftill converter with
overlap considered has the sequence of
(a) 3, 3, 2, 2 (b) 3, 3, 3, 2
(c) 3,2,3,2 (d) 2,2,2,3.
27. A 3-phase full converter has an average output voltage of 200 V for 0° firing angle
and for resistive load. For a firing angle of 900, the output voltage would be
(a) zero (b) 50 V
(c) 100 V (d) 26.8 V
28. In a 3-phase full converter, the output voltage pulsates at a frequency equal to
(a) supply frequency, f (b) 2 f
(c) 3 (d) 6f
29. The three-phase ac to dc converter, which requires neutral point connection, is
(a) 3-phase semi converter (b) 3-phase full converter
(c) 3-phase half-wave converter (d) 3-phase full converter with diodes
30. A 3-phase full converter can function as
a) converter for α = 00
to 1800
b) converter for α == 00
to 900
c) inverter for α = 900
to 1800
d) inverter for α = 00
to 900
31. In a 3-phase full converter, the number of SCRs working during overlap is
a) 1 from positive group, 1 from negative group
b) 2 from positive group, 1 from negative group
c) 1 from positive group, 2 from negative group
d) 2 from positive group, 2 from negative group
32. In a 3-phase semiconverter, frequency of the ripple in the output voltage may be
a) 3 times the supply frequency/for firing angle α < 600
b) 3 f for α > 600
c) 6 f for α < 600
d) 6 f for α > 600
33. The peak inverse voltage in ac to de converter systems is highest in
a) single-phase full wave mid-point converter
b) single-phase full converter
c) 3-phase bridge converter

d) 3-phase half-wave converter
1. Explain the operation of a three-phase semi converter fed DC separately excited motor
2. Explain the operation of a three phase semi converter fed DC series motor in
3. Explain the operation of a three phase full converter fed DC separately excited motor
4. Explain the operation of a three phase full converter fed DC series motor in continuous
current mode and derive the equation relating speed and torque and also draw their speed
torque characteristics.
5. A 80KW, 440V, 800rpm, DC motor is operating at 600rpm and developing 75% rated
torque is controlled by 3-phase, 6-pulse thyristor converter. If the back emf at the rated
speed is 410V, determine the triggering angle of converter. The input converter is 3-
phase, 415V, 50Hz,AC supply?
6. A 3-phase full converter controls the speed of 150HP, 650V, 1750rpm, separately
excited DC motor. The converter is operating from a 3-phase 460V, 50Hz supply. The
rated armature current of the motor is 170Amps. The motor parameters are Ra= 0.099
ohms, La = 0.73mH and Ka = 0.33 v/rpm. Neglect the losses in the converter system.
Determine (a) No load speed of firing angle alpha = 00
, 300
. Assume that at no load the
armature current is 10% of rated current and it is continuous. (b) The firing angle to
obtain rated speed of 1750 rpm at rated motor current and also computes the supply
power factor. (c) The speed regulation for the firing angle obtained in part.
UNIT – III
UNIT OBJECTIVE:
 To study the different types of braking methods used for DC motors.
 To study the operation of dual converter fed DC series motor in both circulating
non circulating current modes.
 To study the operation of dual converter fed DC separately excited motor in both
circulating non circulating current modes.
 To introduce the four quadrants operation of DC motors, i.e., motoring operations.
 To study the block diagram of closed loop operation of converter fed DC motors.
 The main basic procedure for braking of DC motors is to make the current flow in
reverse direction during the braking period i.e., from armature to source.

 The current flow direction can be reversible by making the back emf of the motor
greater than terminal voltage.
 In all the three methods of braking DC motors, regenerative braking is advantages
since the power developed during the braking period is usefully employed to the
other loads, which are connected to the source.
 Dual converter is a four-quadrant converter, in which two full converters will be
connected in anti parallel and are controlled in such a way that the sum of two
firing angles should be 1800
.
 The condition (α1 + α2) = 1800
, implies that if one converter is operating in
conversion mode the other one in inversion mode.
 In practical dual converter with out circulating current, only one convert is in
operation at a time where as in, dual converter with circulating current two
converters are in operation at a time.
1. A four-quadrant operation requires
a) two full converters in series
b) two full converters connected back to back
c) two full converters connected in parallel
d) two semi converters connected back to back
2. In circulating-current type of dual converter, the nature of voltage across reactor is
(a) alternating (b) pulsating
(c) direct (d) triangular
3. In a dual converter, converters 1 and 2 work as under:
a) 1 as rectifier, 2 as inverter
b) l as inverter,2 as rectifier
c) both as rectifiers
d) both as inverters
1. a) Describe the operation of ideal dual converter.
b) Describe the operation of dual converter in non-circulating current mode with neat
circuit diagram and waveforms.
2. Describe the operation of dual converter in circulating current mode with neat circuit
diagram and waveforms and derive the expression for peak value circulating current.
(a). Explain the need of commutation in thyristor circuits. What are the different
commutation schemes? Explain class A commutation with neat diagrams.
(b). A circuit employing parallel resonance turn off (class B commutation) circuit has C =
50 µF, L = 20 µH, V= 200 V and initial voltage across the capacitor is 200 V. Determine
the circuit turn off time for main thyristor for load R = 1.5 Ω.
3. (a) Distinguish clearly between voltage commutation and current commutation in
thyristor circuits.

(b) Discuss how the voltage across a commutating capacitance is reversed in a
commutating circuit.
(c) A circuit employing resonance pulse commutation has C= 20µF and L= 3µH the
initial capacitor voltage = source voltage, Vs = 230 V DC. Determine conduction time for
auxiliary thyristor and circuit turn off time for main thyristor in case constant load current
is 300 A.
4. Explain the operation of dual converter fed DC separately excited motor in both
circulating current and non-circulating modes and also draw their speed torque
characteristics.
5. Explain the operation of dual converter fed DC series motor in both circulating current
and non circulating modes and also draw their speed torque characteristics.
6. Explain the operation of single quadrant chopper fed DC series and separately excited
motor in both circulating current and non-circulating modes and also draw their speed
7. Explain the operation of two-quadrant chopper fed DC series and separately excited
8. Explain the operation of four-quadrant chopper fed DC series and separately excited
9. Explain the regenerative braking method used for DC series and separately excited
motors with neat circuit diagrams.
10. Explain the dynamic braking method used for DC series and separately excited
motors with neat circuit diagrams.
11. Explain the plugging method used for DC series and separately excited motors with
neat circuit diagrams.
12. A DC chopper is used to control the speed of DC shunt motor. The supply voltage to
the chopper is 220V. The on time and the off time of the chopper are 10 ms and 12ms,
respectively. Assuming continuous conduction of the motor current, and neglecting the
armature inductance, determine the average load current when the motor runs at a speed
of 146.60 rad/ sec and has a voltage constant Ka of 0.495 V/ A rad/sec.
1) (a) Explain how four-quadrant operation is achieved by dual converters each of 3 φ full
wave configuration for d.c. separately excited motor.
(b) Distinguish between circulating current and non-circulating current mode of operation
2) a) What is a dual converter? Explain the principle of operation of a dual converter in
circulating current mode. How the same is used for speed control of dc drive.
b) A 230v separately excited dc motor takes 50A at a speed of 800rpm. It has armature
resistance of 0.4 Ω. A chopper with an input voltage of 230v controls this motor and
frequency of 500Hz. Assuming continuous condition throughout, calculate and plot
speed-torque characteristics for:
Motoring operation at duty ratios of 0.3 and 0.6.
Regenerative braking operation at duty ratios of 0.7 and 0.4.

3) a) Deduce the mathematical expression for minimum and maximum currents for a
class A chopper operated dc motor with back emf.
b) A 220v, 24A, 1000rpm separately excited dc motor having an armature resistance of
2Ω is controlled by a chopper. The chopping frequency is 500Hz and the input voltage is
230v. Calculate the duty ratio for a motor torque of 1.2 times rated torque at 500rpm.
4) (a) Draw the circuit diagram and explain the operation of closed-loop speed control
with inner-current loop and field weakening.
(b) A single-phase fully controlled double bridge converter is operated from 120v, 60Hz
supply and the load resistance is 10 ohms. The circulating inductance is 40mH. Firing
delay angle for converter I and II are 600
and 1200
respectively. Calculate the peak
circulating current and the current through converters.
5) Describe the relative merits and demerits of the following types of braking for dc
motors: mechanical braking, dynamic braking and regenerative braking with neat
diagram.
UNIT – IV
UNIT OBJECTIVE:
 To study the operation of single quadrant chopper fed DC separately excited
motor and DC series motor.
 To study the operation of two-quadrant chopper fed DC separately excited motor
and DC series motor.
 To study the operation of four-quadrant chopper fed DC separately excited motor
and DC series motor.
 To study the speed – torque characteristics of chopper fed DC motors.
 To study the block diagram of closed loop operation of chopper fed DC motors.

 The function of a chopper is to convert Fixed DC voltage in to Variable DC
voltage.
 In a chopper output voltage can be controlled by two strategies:
(i) TRC (Time Ratio Control)
(ii) CLC (Current Limit Control)
 In TRC schemes PWM (Pulse Width Modulation) scheme is advantageous than
frequency modulation scheme.
 The output voltage of a step down chopper is given by
VO DC = δ (VI DC)
Where VO DC is the average value of the DC output voltage,
δ is the duty cycle and
VI DC is the average value of the DC input voltage
 Duty cycle, δ is the ratio of ON time of the chopper to the TOTAL time of the
chopper.
 The output voltage of a step up chopper is given by
VO DC = (1/(1 - δ)) (VI DC)
Where VO DC is the average value of the DC output voltage,
δ is the duty cycle and
 VI DC is the average value of the DC input voltage
1. In dc choppers, if Ton is the on period and f is the chopping frequency, then output
voltage in terms of input voltage Vs is given by
(a) Vs. Ton l f (b) Vs .f / Ton
(c) V / f. Ton (d) Vs. f. Ton
2. In dc choppers, the waveforms for input and output voltages are respectively
(a) discontinuous, continuous (b) both continuous
(c) both discontinuous (d) continuous, discontinuous
3. In PWM method of controlling the average output voltage in a chopper, the on time
is (varied / kept constant) but the chopping frequency is(varied / kept constant).
4. In FM method of controlling the average output voltage in a chopper, chopping
period is (varied / kept constant) but on time is (varied / kept constant) or off time is
(varied / kept constant).
5. For type-A chopper, Vs is the source voltage, R is the load resistance and α is the
duty cycle. The average output voltage and current for this chopper are respectively
(a) αVs, α. (Vs / R) (b) (1-α) Vs, (1- α) Vs / R
(c) Vs / Vα, V s / αR (d) Vs / (1 -α), Vs /(1 - α) R
6. A chopper has Vs as the source voltage, R as the load resistance and α as the duty
cycle. For this chopper, RMS value of output voltage is
(a) αVs (b) (α)1/2
.Vs
(c) Vs/ (α)½
(d) (1-α)1/2
Vs
7. For a chopper, Vs is the source voltage, R is the load resistance and a is the duty'
cycle. RMS and average values of thyristor currents for this chopper are

(a) α.Vs / R, (√α)Vs / R (b) (√α).Vs / R , (√α) .Vs / R
(c) (√α) Vs / R, αVs / R (d) (1-α)1/2
.Vs / R, (1-α)1/2
Vs / R.
8. In dc choppers, per unit ripple is maximum when duty cycle a is
(a) 0.2 (b) 0.5
(c) 0.7 (d) 0.9.
9. A voltage-commutated chopper has the following parameters:
Vs = 200 V, Load circuit parameter: 1Ω, 2 mH, 50 V
Commutation circuit parameters, L = 25 µH, C = 50µF
Ton = 500 µs, T = 2000 µs
For a constant load current of 100 A, the effective on period and peak current
through the main thyristor are respectively
(a) 1000 µs, 200 A (b) 700 µs, 382.8 A
(c) 700 µs, 282.8 A (d) 1000 µs, 382.8 A.
10. For the voltage-commutated chopper of Prob. 10, the turn-off times for main and
auxiliary thyristors are, respectively,
(a) 120 µs, 60 µs (b) 100 µs, 0.5 µs
(c) 120 µs, 55 µs (d) 100 µs, 55.54µs.
11. A load commutated chopper, fed from 200 V dc source, has a constant load current
of 50 A. For a duty cycle of 0.4 and a chopping frequency of 2 kHz, the value of
commutating capacitor and the turn-off time for one thyristor pair are respectively
(a) 25 µF, 50 µs (b) 50 µF, 50 µs
(c) 25 µF, 25 µs (d) 50 µF, 25 µs
12. A dc battery is charged from a constant dc source of 200 V through a chopper. The
dc battery is to be charged from its internal emf of 90 to 120 V. The battery has
internal resistance of 1 Ω. For a constant charging current of 10 A, the range of duty
cycle is……….. to……......
13. For type-A chopper; Vs, R, Io and a are respectively the dc source voltage, load
resistance, constant load current and duty cycle. For this chopper, average and RMS
values of freewheeling diode currents are
(a) α Io, (√α) Io (b) (1 - α) Io, (1-α)1/2
Io
(c) α Vs / R, (√α) Vs / R (d) (1 - α) Io, (√α) Io
14. A step-up chopper has Vs as the source voltage and a as the duty cycle. The output
voltage for this chopper is given by
(a)Vs (1+ α) (b) Vs / (l - α)
(c) Vs (1 - α) (d) Vs / (l + α).
15. A dc chopper is fed from 100 V dc. Its load voltage consists of rectangular pulses
of duration 1 m sec in an overall cycle time of 3 m sec. The average output voltage
and ripple factor for this chopper are respectively
(a) 25 V, 1 (b) 50 V, 1 (c) 33.33 V, (√2) (d) 33.33 V, 1
16. When a series LC circuit is connected to a dc supply of V volts through a thyristor,
then the peak current through thyristor is
(a) V. √ LC (b) V / √CL
(c) V. √(C / L) (d) V. √ (L / C)
17. In dc choppers, if T is the chopping period, then output voltage can be controlled by
PWM by varying
(a) T keeping Ton constant (b) Ton keeping T constant

(c) Toff keeping T constant (d) T keeping Toff constant.
18. In dc choppers, for chopping period T, the output voltage can be controlled by FM
by varying
(a) T keeping Ton constant (b) T keeping Toff constant
(c) Ton keeping T constant (d) Toff keeping T constant
1. (a) What is time ratio control of chopper? Explain the operation.
(b) A battery is charged from a constant DC source of 220 V through a chopper.
The DC battery is to be charged from its internal emf of 90 V to 122 V. The battery
has internal resistance of 1Ω. For a constant current charging of 10 A, compute the
range of duty cycle.
2. (a) Explain class C type of commutation used for thyristors with appropriate current
and voltage waveforms.
(b)Explain the merits and demerits of self-commutation of SCR and its other
methods of commutation.
3. (a) Describe the principle of operation of a step down chopper. Derive an
expression for the average output voltage in terms of input voltage and duty cycle.
(b) A chopper circuit is operating on TRC principle at a frequency 1 KHz on a 220
V DC supply. If the load voltage is 180 V, calculate the conducting and blocking
period of thyristor in each cycle
1) (a) Explain the principle of speed control of a dc motor and show how it can be
achieved by a chopper.
(b) A 230v, 1200rpm, 15A separately excited motor has an armature resistance of 1.2Ω.
Motor is operated under dynamic braking with chopper control. Braking resistance has a
value of 20Ω.
i. Calculate duty ratio of chopper for motor speed of 1000rpm and braking torque equal to
1.5 times rated motor torque.
ii. What will be the motor speed for duty ratio of 0.5 and motor torque equal to its rated
torque.
2) (a) Explain with neat circuit diagram the basic principle of operation of a class A type
of chopper. The chopper is connected to R-L-E load. Analyze the same for continuous
current mode of operation.
(b) A dc supply of 200v supplied power to separately excited dc motor via a class A
thyristors chopper. The motor has an armature circuit resistance of 0.33Ω and inductance
of 11mH. The chopper is fully on at the rated motor speed 1200rpm when the armature
current is 20A. If the speed is to be reduced to 800rpm with the load torque constant,
calculate the necessary duty cycle. If the chopper frequency is 500Hz, is the current
continuous?
3) (a) Discuss with the suitable diagrams I quadrant and II quadrant choppers.
(b) A constant frequency TRC system is used for the speed control of dc series traction
motor from 220v dc supply. The motor is having armature and series field resistance of
0.025Ω and 0.015Ω respectively. The average current in the circuit is 125A and the

chopper frequency is 200Hz. Calculate the pulse width if the average value of back emf is
60 volts.
4) (a) List the advantages offered by dc chopper drives over line-commutated converter
controlled dc drives.
(b) A dc chopper controls the speed of dc series motor. The armature resistance Ra =
0.04Ω, field circuit resistance Rf = 0.06Ω, and back emf constant Kv = 35 mV/rad/s. The
dc input voltage of the chopper Vs=600v. If it is required to maintain a constant
developed torque of Td = 547N-m, plot the motor speed against the duty cycle K of the
chopper.
5) A class-A chopper, operating in time-ratio control, is supplying the armature of the
separately excited dc motor. Show that the motor speed-torque relationship is ωm =
(δ.V/K)-(Ra.Ta/K2
) , Where V - chopper input voltage, Ra - Armature resistance, Ta -
motor torque, K- torque constant.
UNIT – V
UNIT OBJECTIVE:
CONTROL OF INDUCTION MOTOR FROM STATOR SIDE
 To study the operation of stator voltage control method of induction motor by AC
voltage controllers both for star and delta connected stator windings. And their
speed –torque characteristics.
AC VOLTAGE CONTROLLERS:
 The function of AC voltage controller is to convert the fixed AC voltage into
variable AC voltage with out changing the frequency.
 Varying the firing angle can control the output voltage.

 The harmonics in the output voltage are minimized by using more and more
number of AC voltage controller units in Synchronous tap changer.
 The voltage is calculated by using the formula
(i) For R – load:
V0 rms = Vph {[(Π - µ) + (1/2) Sin 2µ] / Π}1/2
Where V0 rms is the theoretical RMS value of the output voltage,
Vph is the phase voltage of the input voltage and
µ is the firing angle
Vo rms = Vph {[(β - α) + (1/2)(Sin 2α - (1/2) Sin 2β)] / 2Π}1/2
Theoretically the value of Extinction angle can be calculated by,
(ωL / R)
 In AC voltage controllers Self-commutation is used.
 The function of inverters is to convert Fixed DC voltage into Variable voltage AC
with variable frequency.
 In a Voltage source inverters, the DC source has negligible impedance where as
inn Current source inverters, the DC source is having high impedance.
 In all the inverters self-commutation is employed except for Mc Murray and Mc
Murray – Bedford inverters. For these two inverters forced commutation is used.
 In Mc Murray inverter, Auxiliary commutation is used and in Mc Murray –
Bedford inverter Auxiliary impulse commutation is used.
 In all of the voltage control techniques used for inverters, Pulse Width modulation
(PWM) technique is more advantageous one and also it reduces the harmonic
content in the output waveform.
 In single-phase parallel inverter, the capacitor is used for commutation.
OBJECTIVE QUESTIONS :
1. A single-phase voltage controller feeds an induction motor (A) and a heater (B)

(a) In both the loads, fundamental and harmonics are useful
(b) In A only fundamental and in B only harmonics are useful
(c) In A only fundamental and in B harmonics as, well as fundamental are useful (d)
In A only harmonics and in B only fundamental are useful.
2. A load resistance of 10 Ω is fed through a I-phase voltage controller from a voltage
source of 200 sin 314 t. For a firing angle delay of 90°, the power delivered to load
in kW, is
(a) 0.5 (b) 0.75
(c) 1 (d) 2
3. A single-phase voltage controller is employed for controlling the power flow from
260 V, 50 Hz source into a load consisting of R = 5 Ω and ωL = 12 Ω. The value of
maximum RMS load current and the firing angle are respectively
(a) 20 A, 0° (b) 260/10.91A, 0°
(c) 20 A, 90° (d) 260/10.91 A, 90°
4. A "load, consisting of R = 10 Ω and ωL = 10 Ω, is being fed from 230 V, 50 Hz
source through a I-phase voltage controller. For a firing angle delay of 30°, the
RMS value of load current would be
(a) 23 A (b) 23/√2 A
(c) > 23/√2 A (d) < 23/√2 A
5. In a single-phase voltage controller with RL load, ac output power can be controlled
if
(a) firing angle α > (load phase angle) and conduction angle Ø = Π
(b) α > Ø and Ø < Π
(c) α << Ø and Ø = Π
(d) α < Ø and Ø > Π
6. A single-phase voltage controller feeds power to a resistance of 10 .0. The source
voltage is 200 V rms. For a firing angle of 90°, the RMS value of thyristor current
in amperes is
(a) 20 (b) 15
(c) 10 (d) 5
7. A single-phase voltage controller is connected to a load of resistance 10.0 and a
supply of 200 sin 314t volts. For a firing angle of 90°, the average thyristor
current in amperes is
(a) 10 (b) 10 / Π
(c) 5√2 / Π (d) 5√2
8. A single-phase voltage controller, using two SCRs in anti parallel, is found to be
operating as a controlled rectifier. This is because
(a) load is R and pulse gating is used
(b) load is R and high-frequency carrier gating is used
(c) load is RL and pulse gating is used
(d) load is RL and continuous gating is used
9. A single-phase ac voltage controller (or regulator) fed from 50 Hz system supplies a
load having resistance and inductance of2.0 .0 and 6.36 mH respectively. The
control range of firing angle for this regulator is

(a) 0° < α< 180° (b) 45° < α < 180°
(c) 90° < α < 180° (d) 0° < α< 45°
10. If, for a single-phase half-bridge inverter, the amplitude of output voltage is Vs and
the output power is P, then their corresponding values for a single-phase full-bridge
inverter are
(a) Vs, P (c) 2 Vs, 2P
(b) V/2, 2P (d) 2 Vs, P
11. In voltage source inverters
(a) load voltage waveform Vo depends on load impedance Z, whereas load current
waveform io does not depend on Z
(b) Both Vo and io depend on Z
(c) Vo does not depend on Z whereas io depends on Z
(d) both Vo and io do not depend upon Z
12. A single-phase full bridge inverter can operate in load-commutation mode in case
load consists of
(a) RL (b) RLC over damped
(c) RLC under damped (d) RLC critically damped
13. A single-phase bridge inverter delivers power to a series connected RLC load with
R = 2 Ω, ωL = 8 Q. For this inverter-load combination, load commutation is
possible in case the magnitude of l/ωC in ohms is .
(a) 10 (b) 8
(c) 6 (d) zero
14. For a 3-phase bridge inverter in 180° conduction mode, Fig. A-33, the sequence of
SCR conduction in the first two steps, beginning with the initiation of thyristor 1, is
(a) 6, 1,2 and 2,3, 1 (b) 2,3, 1 and 3, 4, 5
(c) 3, 4, 5, and 5, 6, 1 (d) 5, 6, 1 and 6, 1, 2
(a) 6, 1 and 1, 2 (b) 1,2 and 2,3
(c) 1, 6 and 5, 6 (d) 1, 3 and 3, 4
16. In single-pulse modulation of PWM inverters, third harmonic can be eliminated if
pulse width is equal to
(a) 300
(b) 600
(c) 1200
(d) 150°
17. In single-pulse modulation of PWM inverters, fifth harmonic can be eliminated if
(a) 300
(b) 720
(c) 36° (d) 108°
18. In single-pulse modulation of PWM inverters, the pulse width is 120°. For an input
voltage of 220 V dc, the RMS value of output voltage is
(a) 179.63 V (b) 254.04 V
(c) 127.02 V (d) 185.04 V
19. In single-pulse modulation used in PWM inverters, Vs is the input de voltage. For
eliminating third harmonic, the magnitude of RMS value of fundamental
component of output voltage and pulse width are respectively

(a) 2√2 Vs, 120° (b) 4 Vs, 60°
Π Π
(c) 2√2 Vs, 60° (d) 4 Vs, 120°
Π Π
20. In multiple-pulse modulation used in PWM inverters, the amplitudes of reference
square wave and triangular carrier wave are respectively 1 V and 2 V. For
generating 5 pulses per half cycle, the pulse width should be
(a) 36° (b) 24° (c) 18° (d) 12°
21. In multiple-pulse modulation used in PWM inverters, the amplitude and frequency
for triangular carrier and square reference signals are respectively 4 V, 6 kHz and 1
V, 1 kHz. The numbers of pulses per half cycle and pulse width are respectively
(a) 6, 90° (b) 3, 45°
(c) 4, 60° (d) 3, 40°
22. In sinusoidal-pulse modulation used in PWM inverters, amplitude and frequency for
triangular carrier and sinusoidal reference signals are respectively 5 V, 1 kHz and 1
V, 50 Hz. If zeros of the triangular carrier and reference sinusoid coincide, then the
modulation index and order of significant harmonics are respectively
(a) 0.2,9 and 11 (b) 0.4,9 and 11
(c) 0.2, 17 and 19 (d) 0.2, 19 and 21
23. Which of the following statement/statements is/are correct in connection with in-
verters:
(a) VSI and CSI both require feedback diodes
(b) Only CSI requires feedback diodes
(c) GTOs can be used in CSI
(d) Only VSI requires feedback diodes
24. In a CSI, if frequency of output voltage is f Hz, then frequency of voltage input to
CSI is
(a) f (b) 2f
(c) f/2 (d) 3f
25. In sinusoidal-pulse modulation used in PWM inverters, amplitude and frequency of
V, 50 Hz. If peak of the triangular carrier coincides with the zero of the reference
sinusoid, then the modulation index and order of significant harmonics are
(a) 0.2,9 and 11 (b) 0.4,9 and 11
(c) 0.2, 17 and 19 (d) 0.2, 19 and 21
26. In sinusoidal PWM, there are 'm' cycles of the triangular carrier wave in the half
cycle of reference sinusoidal signal. If zero of the reference sinusoid coincides with
zero/peak of the triangular carrier wave, then number of pulses generated in each
half cycle is respectively
(a) (m - 1)/m (b) (m - 1)/(m - 1)
(c) m/m (d) m/(m - 1)
27. In an inverter with fundamental output frequency of 50 Hz, if third harmonic is
eliminated, then frequencies of other components in the output voltage wave, in Hz,
would be
(a) 250, 350, 450, high frequencies (b) 50, 250, 350, 450
(c) 50,250,350,550 (d) 50, 100, 200, 250

28. A single-phase CSI has capacitor C as the load. For a constant source current, the
voltage across the capacitor is
(a) square wave (b) triangular wave
(c) step function - (d) pulsed wave
29. A single-phase full bridge VSI has inductor L as the load. For a constant source
voltage, the current through the inductor is
(c) sine wave (d) pulsed wave
SUBJECTIVE QUESTIONS
1) A pump has a torque-speed curve given by TL = (1.4/103
)N2
Nm. It is proposed to use
a 240V, 50 Hz, 4 pole, star connected Induction motor with the equivalent circuit
parameters (referred to stator turns) R1 = 0.25Ω, R2 = 0.6Ω, X1 = 0.36Ω, X2 = 0.36Ω,
Xm = 17.3Ω. The pump speed N is to vary from full speed 1250 RPM to 750 RPM by
voltage control using pairs of inverse-parallel connected thyristors in the lines. Calculate
the range of firing angles required.
2) A 3 phase, 4 pole, 50 Hz squirrel cage Induction motor has the following circuit
parameter.
r1 = 0.05ohm, r2 = 0.09ohm,X1 + X2 = 0.55ohm.
The motor is star connected and rated voltage is 400V. It drives a load whose torque is
proportional to the speed and is given as T1 = 0.05ω Nw-m. Determine the speed and
torque of the motor for a firing angle of 450 of the AC Voltage Controller on a 400V, 50
Hz supply.
3) What is an AC Voltage Controller?
Explain with suitable diagrams the various types of solid state 3 phase AC Voltage
Controllers that can be used for speed control of 3-phase Induction motors from stator
side. Mention the advantages of the AC Voltages Controllers over the other methods of
solid-state speed control techniques of 3-phase Induction motor.
4) (a) For stator voltage control scheme of a 3-phase Induction motor discuss about speed
range, regeneration, harmonics, torque pulsating, power factor, cost, efficiency and
applications.
(b) Draw a block schematic diagram for automatic speed control of 3 phase cage
Induction motor using solid state AC Voltage Controller on stator side.
5) (a) Using 3 phase solid state AC Voltage Controllers explain clearly how it is possible
to achieve 4 quadrant operation of 3 phase Induction motors.
(b) Draw a closed loop block schematic diagram for the above speed control technique.
Mention the merits of the above method of speed control.

UNIT VI
UNIT OBJECTIVE:
CONTROL OF INDUCTION MOTOR THROUGH STATOR FREQUENCY
 To study the operation of stator supply frequency control method of induction
motor by VSI (voltage source inverter), CSI (current source inverter) and Cyclo
converters. And their speed –torque characteristics.
 To know about the PWM (pulse width modulation) control of induction motors
 To study the comparisons between VSI and CSI.
To study the block diagram of closed loop operation of induction motor drives.

IMPORTANT POINTTS:
 In Voltage source inverters, the DC source has negligible impedance where as inn
Current source inverters; the DC source is having high impedance.
CYCLO CONVERTERS:
 Cyclo converters are single stage frequency conversion devices.
 Their function is to convert Fixed AC voltage with fixed frequency into Variable
AC voltage with variable frequency.
 Cyclo converters are classified into mid point type and Bridge type cyclo
converters.
1. If, for a single-phase half-bridge inverter, the amplitude of output voltage is Vs and
the output power is P, then their corresponding values for a single-phase full-bridge
inverter are
(a) Vs, P (c) 2 Vs, 2P
(b) V/2, 2P (d) 2 Vs, P
2. In voltage source inverters
(a) load voltage waveform Vo depends on load impedance Z, whereas load current

waveform io does not depend on Z
(b) Both Vo and io depend on Z
(c) Vo does not depend on Z whereas io depends on Z
(d) both Vo and io do not depend upon Z
3. A single-phase full bridge inverter can operate in load-commutation mode in case
load consists of
(a) RL (b) RLC over damped
(c) RLC under damped (d) RLC critically damped
4. A single-phase bridge inverter delivers power to a series connected RLC load with
R = 2 Ω, ωL = 8 Q. For this inverter-load combination, load commutation is
possible in case the magnitude of l/ωC in ohms is
(a) 10 (b) 8
(c) 6 (d) zero
(a) 6, 1,2 and 2,3, 1 (b) 2,3, 1 and 3, 4, 5
(c) 3, 4, 5, and 5, 6, 1 (d) 5, 6, 1 and 6, 1, 2
(a) 6, 1 and 1, 2 (b) 1,2 and 2,3
(c) 1, 6 and 5, 6 (d) 1, 3 and 3, 4
7. In single-pulse modulation of PWM inverters, third harmonic can be eliminated if
(a) 300
(b) 600
(c) 1200
(d) 150°
8. In single-pulse modulation of PWM inverters, fifth harmonic can be eliminated if
(a) 300
(b) 720
(c) 36° (d) 108°
9. In single-pulse modulation of PWM inverters, the pulse width is 120°. For an input
voltage of 220 V dc, the RMS value of output voltage is
(a) 179.63 V (b) 254.04 V
(c) 127.02 V (d) 185.04 V
10. In single-pulse modulation used in PWM inverters, Vs is the input de voltage. For
eliminating third harmonic, the magnitude of RMS value of fundamental
component of output voltage and pulse width are respectively
(a) 2√2 Vs, 120° (b) 4 Vs, 60°
Π Π
(c) 2√2 Vs, 60° (d) 4 Vs, 120°
Π Π
11. In multiple-pulse modulation used in PWM inverters, the amplitudes of reference
square wave and triangular carrier wave are respectively 1 V and 2 V. For
generating 5 pulses per half cycle, the pulse width should be

(a) 36° (b) 24° (c) 18° (d) 12°
12. In multiple-pulse modulation used in PWM inverters, the amplitude and frequency
for triangular carrier and square reference signals are respectively 4 V, 6 kHz and 1
V, 1 kHz. The numbers of pulses per half cycle and pulse width are respectively
(a) 6, 90° (b) 3, 45°
(c) 4, 60° (d) 3, 40°
13. In sinusoidal-pulse modulation used in PWM inverters, amplitude and frequency for
V, 50 Hz. If zeros of the triangular carrier and reference sinusoid coincide, then the
modulation index and order of significant harmonics are respectively
(a) 0.2,9 and 11 (b) 0.4,9 and 11
(c) 0.2, 17 and 19 (d) 0.2, 19 and 21
14. Which of the following statement/statements is/are correct in connection with in-
verters:
(a) VSI and CSI both require feedback diodes
(b) Only CSI requires feedback diodes
(c) GTOs can be used in CSI
(d) Only VSI requires feedback diodes
15. In a CSI, if frequency of output voltage is f Hz, then frequency of voltage input to
CSI is
(a) f (b) 2f
(c) f/2 (d) 3f
16. In sinusoidal-pulse modulation used in PWM inverters, amplitude and frequency of
V, 50 Hz. If peak of the triangular carrier coincides with the zero of the reference
sinusoid, then the modulation index and order of significant harmonics are
(a) 0.2,9 and 11 (b) 0.4,9 and 11
(c) 0.2, 17 and 19 (d) 0.2, 19 and 21
17. In sinusoidal PWM, there are 'm' cycles of the triangular carrier wave in the half
cycle of reference sinusoidal signal. If zero of the reference sinusoid coincides with
zero/peak of the triangular carrier wave, then numbers of pulses generated in each
half cycle are respectively
(a) (m - 1)/m (b) (m - 1)/(m - 1)
(c) m/m (d) m/(m - 1)
18. In an inverter with fundamental output frequency of 50 Hz, if third harmonic is
eliminated, then frequencies of other components in the output voltage wave, in Hz,
would be
(a) 250, 350, 450, high frequencies (b) 50, 250, 350, 450
(c) 50,250,350,550 (d) 50, 100, 200, 250
19. A single-phase CSI has capacitor C as the load. For a constant source current, the
voltage across the capacitor is
(c) step function - (d) pulsed wave
20. A single-phase full bridge VSI has inductor L as the load. For a constant source
voltage, the current through the inductor is

(c) sine wave (d) pulsed wave
21. A cyclo converter is a
(a) frequency changer (fc) from higher to lower frequency with one-state
conversion
(b) fc from higher to lower frequency with two-stage conversion
(c) fc from lower to high frequency with one-state conversion
(d) either (a) or (c).
22. The cyclo converters (CCs) require natural or forced commutation as under:
(a) natural commutation in both step-up and step-down CCs
(b) forced commutation in both step-up and step-down CCs
(c) forced commutation in step-up CCs
(d) forced commutation in step-down CCs.
23. For converting 3-phase supply at one frequency to single-phase supply at a lower
frequency, the basic principle is to_______(vary/keep) the firing angle
__________ (constant/gradually).
24. Three-phase to three-phase cyclo converters employing 18 SCRs and 36 SCRs have
the same voltage and current ratings for their component thyristors. The ratio of VA
rating of 36-SCR devices to that of 18-SCR devices is
(a) 2 (b) 1 (c) 2 (d) 4.
25. Three-phase to 3-phase cyclo converters employing 18 SCRs and 36 SCRs have the
same voltage and current ratings for their component thyristors. The ratio of power
handled by 36-SCR devices to that handled by 18-SCR devices is
(a) 2 (b) 1 (c) 2 (d) 4.
26. The number of thyristors required for single-phase to single-phase cyclo converter
of the mid-point type and for three phase-to three-phase 3-pulse type cyclo
converters are respectively -.
(a) 4, 6 (b) 8, 18 (c) 4, 18 (d) 4, 36.
27. A 3-phase to single-phase conversion device employs a 6-pulse bridge cyclo
converter. For an input voltage of 200 V per phase, the fundamental rms value of
output voltage is
(a) 600/Π V (b) 300√3/Π V (c) 300/Π V (d) 600√3/Π V.
1. A 3 – Ф, squirrel cage induction motor is developing a torque of 1500sync. Watts at
50GHz and 1440rpm (Ns = 1500rpm). If the motor frequency is now increased to
75Hz using constant power mode, determine the new value of torque developed by
motor at constant slip.
2. A 3 – Ф star connected, 50Hz, 4-pole induction motor has the following parameters
in per phase referred to the stator:
R1= R2= 0.034Ω and X1=X2=0.18Ω
The motor is controlled by the variable frequency control with a constant (V/ f).
Determine the following for an operating frequency of 15Hz.

(a) The brake down torque as a ratio of its value at the rated frequency for motoring and
braking.
(b) The starting torque and rotor current in terms of their values at the rated frequency.
(c) The frequency, stator current and the voltage at rated braking torque and 1200rpm.
3. An inverter supplies a 4 – pole, 3 – Ф cage induction motor rated at 220V,
50Hz.Determine the approximate output required of the inverter for motor speeds of
(i) 900rpm (ii) 1200rpm (iii) 1500rpm (iv) 1800rpm
4. A 3 – Ф 400V delta connected induction motor has the following parameters at
50 Hz.
R1= 0.5Ω, R2= 1.5Ω and X1= X2 =2.5Ω, Xm= 130Ω
This motor is fed from a square waved inverter. The voltage waveform is such that
it’s fundamental is equal to the rated voltage of the motor. Determine input current
wave from corresponding to a rotor frequency of 2Hz. When the supply frequency
is 50Hz and 10Hz and the voltage applied is proportional to frequency. What
waveform do you expect at 10Hz if the voltage is varied to keep air gap flux
constant.
5. A 3 – Ф 400V, 50 Hz, 4 – pole; 1450rpm star connected squirrel cage
induction motor has the following parameters per phase referred to the stator.
R1= 0.11Ω, R2= 0.09Ω and X1=0.4Ω, X2 =0.6Ω, Xm= 12Ω, 6-stepinverter
controls the motor. The DC input to the inverter is provided by a 6 pulse fully
controlled rectifier.
6. What should be the rectifier firing angle for getting the rated fundamental voltage
across the motor if the rectifier is fed by an AC source of 400V, 50Hz.
If the machine is operated at a constant flux then determine
a. The inverter frequency at 560rpm and rated torque.
b. The inverter frequency at460 rpm and half the rated torque.
Also determine the motor current.
7. A 3 – Ф, 400V, 50Hz 980rpm, 6pole star connected squirrel cage induction motor
has the following parameter. per phase referred to the stator
R1= 0.20Ω, R2= 0.12Ω and X1= 0.18Ω, X2 =0.4Ω, Xm= 10.3Ω
The current source inverter controls the motor at the rated value, flux maintain constant.
Compute the following
(a) The stator current and DC link current, when the machine operates at rated torque and
50Hz.
(b) The inverter frequency and DC link current for a speed of 500rpm and rated torque .
The motor speed stator current and DC link current for half of the rated torque and
inverter frequency of 25Hz.
1) With the help of circuit diagram and waveforms explain the induction motor with
current source inverter. Draw the circuit diagram of the Auto-sequentially commutated
inverter.
2) A 460V, 100-HP (74.6 KW), 1775 RPM, three-phase, squirrel cage Induction motor
has the following equivalent circuit parameters.
Rs = 0.060 ohm R’r = 0.0302 ohm

L1s = 0.638mH, Lms = 23.3 mH, L1r’ = 0.957 mH
The motor is to be driven from a current source inverter with the rotor frequency
controlled at the rated value. Maximum output power is to be limited to 80% of the rated
value. Motor friction, windage, and core losses may be neglected. The load is to consist
of a pump presenting a load characteristic described by the equation. T = ωm
2
/110 N.m.
Determine the maximum values of motor speed, inverter frequency, rms motor line
current, and fundamental line-to-line motor terminal pd at maximum power output.
3) Explain the operation of voltage source inverter (180-degree conduction mode), used
for induction motor speed control. Draw neat waveforms of line voltages (Vab, Vbc,
Vca) and hence show that the phase voltage, Van, is six-step voltage waveform.
4) Discuss in detail the role of Cyclo converters for speed control of Induction motor.
Draw neat circuit diagram for speed control of 3-phase Induction motor using
Cycloconverters. Mention the merits and limitations of the above scheme.
5) A three phase star connected 50 Hz, 4-pole induction motor has the following
approximate per-phase equivalent circuit parameters referred to stator side:
Rs=Rr’=0.024, Xs=Xr’=0.12. The motor is controlled by the variable frequency control
with constant (V/f) ratio. For an operating frequency of 12 Hz, calculate
(a) The breakdown torque as a ratio of its value at the rated frequency for the motoring
operation,
(b) The starting torque and rotor current in terms of their values at the rated frequency.
6) While explaining the principle of varying the speed of 3 phase Induction motor by v/f
method discuss if for the following two different modes.
(a) Operation below rated frequency
(b) Operation above rated frequency.
UNIT VII
UNIT OBJECTIVE:
CONTROL OF INDUCTION MOTOR FROM ROTOR SIDE
 To study the performance Static rotor resistance control of the induction motor
drives.
 To study the performance Slip power recovery scheme of the induction motor
drives.
 To study the performance Static Scherbius drive.
 To study the performance Static Krammer drive.

 To derive the Torque – Speed characteristics of all the induction motor drives.
 To know the advantages, disadvantages and applications of all the induction
motor drives.
1. Explain the Static rotor resistance control scheme of induction motor drive with neat
circuit and waveforms and draw their speed – torque characteristics.
2. Explain the Slip energy recovery scheme of induction motor drive with neat circuit
and waveforms and draw their speed – torque characteristics.
3. Explain the operation of Static Scherbius Drive with neat circuit and waveforms and
draw their speed – torque characteristics.
4. Explain the operation of Static Krammer drive with neat circuit and waveforms and
draw their speed – torque characteristics.
List out the advantages and applications of any three rotor side controlled induction
motor drives.
1) A 3-phase, 420V, 50Hz, star connected induction motor has the following parameters:
R1 = 2.95Ω, R2’ = 2.08Ω, X1 = 6.82Ω, X2’ = 4.11Ω per phase. Neglect core loss. The
motor draws a current 6.7A at no load and controlled by rotor resistance controller. A
resistance Re Ωhas been controlled by chopper. Determine the value of Re to get a speed
range of 1500 to 500 rpm, assuming a turns ratio of two between stator and rotor. The
torque and speed of the load are related by T α N. Determine the characteristics giving
the speed Vs time ratio of the chopper.
2) A 3-phase, 400V, 50Hz, 4 pole, 1400rpm, star connected wound rotor induction motor
has the following parameters referred to the stator R1 = 2Ω, R2’ = 3Ω, X1 = X2’ = 3.5Ω.
The stator to rotor turns ratio is 2. The motor speed is controlled by static Scherbius
drive. The inverter is directly connected to the source. Determine.
(a) The speed range of the drive when αmax = 1650
(b) The firing angle for 0.4 times the rated motor torque and speed of 1200 rpm.
(c) Torque for a speed of 1050rpm and firing angle of 950
.
3) The speed of a 3-phase slip ring induction motor is controlled by variation of rotor
resistance. The full load torque of the motor is 50Nm at a slip of 0.3. The motor drives
load having a characteristics T α N2
. The motor has 4 poles and operates on 50Hz, 400V
supply. Determine the speed of the motor for 0.8 times the rated torque. The operating
condition is obtained with additional resistance in the circuit. Chopper in the rotor circuit
controls the resistance. Determine the average torque developed for a time ratio of 0.4.
4) A 3-phase 400V, 4 pole, 50Hz, Star connected induction motor has the following
parameters referred to the stator: R2’ = 0.2Ω, X2’ = 0.35Ω. Stator impedance and the
magnetizing branch can be ignored. When driving a load with its torque proportional to
speed, the motor runs at 1450rpm. Calculate the magnitude and phase of the voltage
(referred to the stator) to be impressed on the slip rings in order that the motor may
operate at 1200 rpm and unity power factor.

5) A 3-phase, 50Hz Star connected, 970rpm, 6-pole induction motor has the following
parameters referred to the stator. R1 = 0.2Ω, R2’ = 0.15Ω, X1 = X2’ = 0.4Ω. Stator to
rotor turns ratio = 3.5. The motor is controlled by the static Kramar drive. The drive is
designed for a speed range of 30% below the synchronous speed. The maximum value of
firing angle is 1700
. Calculate
a) Turns ratio of the transformer
b) Torque for a speed of 750rpm and α = 1400
.
c) Firing angle for half the rated motor torque and a speed of 850rpm.
6) A 600V, 50Hz, 30kW, 3-phase induction motor is used as the drive motor in an SER
system. It is required to deliver constant (rated) motor torque over the full range from
100rpm to the rated speed of 1000rpm. The motor equivalent circuit parameters are:
R1 = 0.05Ω,R′ = 0.07Ω,R0 = 53Ω,X0 = 23Ω, X1+ X2’ = 0.153Ω.
Stator to rotor turns ratio is 1.3. Calculate the motor currents, efficiency and power factor
at 300 rpm.
7) (a) Draw and explain a closed loop operation for a static Kramer controlled drive.
(b) In which way static Kramer Control is different from static Scherbius drive?
UNIT VIII
UNIT OBJECTIVE:
CONTROL OF SYNCHRONOUS MOTORS
 To know about the separate control of synchronous motors.
 To know about the self-control of synchronous motors.
 To study the operation of self controlled synchronous motor drives by VSI.
 To study the operation of self controlled synchronous motor drives by CSI.

 To study the operation of self controlled synchronous motor drives by Cyclo
converters.
 To study the operation of load commutated CSI fed synchronous motor drives and
to draw the Speed – Torque characteristics.
1. Explain the difference between separate and self-control of synchronous motor.
2. Explain the operation of VSI fed self-controlled synchronous motor drive with
neat circuit and waveforms and also draw their speed torque characteristics.
3. Explain the operation of CSI fed self-controlled synchronous motor drive with
neat circuit and waveforms and also draw their speed torque characteristics.
4. Explain the operation of Cyclo converter fed self - controlled synchronous motor
drive with neat circuit and waveforms and also draw their speed torque
characteristics.
5. Explain the closed loop control of synchronous motor drives with neat block
diagrams.
List out the advantages, disadvantages and applications of self controlled synchronous
motor fed by VSI, CSI and Cyclo converter.
1) A 6 MW, three phase, 11 kV, 50 Hz, unity power factor, 6-pole, star-connected
synchronous motor has the following parameters: armature resistance = 0, synchronous
reactance = 9 ohms, rated field current = 60 A. The machine is controlled by variable
frequency at constant V/f ratio up to base speed and at constant V above base speed.
Calculate the torque and field current for rated armature current, 750 rpm and 0.8 leading
power factor. Draw motor characteristics and waveforms under the above method of
control.
2) Describe self-controlled and load-commutated inverter controlled synchronous motor
drives in detail and compare them.
3) Describe the open loop and closed loop methods of speed control of a synchronous
motor using VSI.
4) Discuss the VSI method of speed control of synchronous motor describe the operation
of the converter with waveforms.
5) A 500 kW, 3-phase, 6.6 kV, 60 Hz, 6-pole, Y-connected wound-field synchronous
motor has the following parameters: Xm = 78, Xsf = 3, rated pf = 1, n = 5, Rs =
negligible. The motor speed is controlled by variable frequency control with a constant
V/f ratio up to base speed and rated terminal voltage above base speed. Calculate and plot
T, Pm, V, Im, and IF versus speed for the motor operation at rated armature current and
unity pf. What is the range of constant power operation? Neglect friction, windage and
core loss. Draw motor characteristics and waveforms under the above method of control.
6) Describe the converter and control systems used for
(a) constant air gap flux density and
(b) constant V/f operation of a synchronous motor. Draw the characteristics of the drive
for the two cases.
7) Draw the block diagram of a closed loop synchronous motor drive fed from VSI and
explain.

Old Question Papers
Co de No: 07A70201 R07 Set No. 2
IV B.Tech I Semester Examinations,MAY 2011
Electrical And Electronics Engineering
Time: 3 hours Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
? ? ? ? ?
1. (a) Generally the stator voltage control is suitable for speed control of Induction
motor in fan and pump drives. Discuss in detail why the above method is
useful.
(b) Explain why the stator voltage control is not an efficient method of control.
[8+8]
2. Draw the circuit diagram and explain the operation of rotor- resistance control
using chopper. Mention the advantages and disadvantages of the above method of
control. [16]
3. Derive the Speed, Torque Equations of a fully controlled converter connected to
separately excited D.C motor with continuous current operation with necessary
waveforms. [16]
4. Explain the basic operational aspects of three phase fully controlled converters
with
neat sketches of the waveforms and the circuit diagram. What is the e
ect of free
wheeling diode. [16]
5. What is continuous current operation of chopper fed DC motors? Explain with
relevant waveforms. [16]
6. With suitable circuit diagrams discuss in detail the principle of operation of Self
controlled Synchronous motor drive employing a Cyclo converter. [16]
7. A 50 hp 440 V, 50 Hz six pole Star connected Induction motor has the following
equivalent circuit parameters:
R1 = 0.1 Ohm, R2 ' = 0.12 Ohm,
X1 = 0.3 Ohm, X2 ' = 0.3Ohm . Xm = 15 Ohm. For a slip of 0.03 at rated frequency
determine the torque developed. [16]
8. (a) With neat circuit diagram and waveforms, explain dynamic braking of sepa-
rately excited motor by single phase converter.
(b) A dc shunt motor has the armature resistance of 0.04 oms and the field winding
resistance of 10
. Motor is coupled to an over hauling load with a torque of
400N-m. Following magnetization curve was measured at 600 rpm:
Field Current, A 2.5 5 7.5 10 12.5 1.5 17.5 20 22.5 25
Back emf, V 25 50 73.5 90 102.5 110 116 121 125 129
Calculate the value of RB when the motor is required to hold overhauling load
at 1200rpm. [8+8]

?????
1
? ? ? ? ?
1. Explain static motor resistance control for speed control of I.M. Draw speed &
torque charecterstics.
[16]
2. Explain in detail the braking operation of a controlled separately excited d.c
motor.
[16]
3. Explain in detail the operation of a full- converter feeding a D.C series motor with
reference to voltage and current waveforms, Assuming that the motor current is a
continuous one. [16]
4. Draw and explain the speed torque curves with variable frequency control for the
two di
erent modes.
(a) Operation at constant
ux
(b) Operation at constant (V/f ) ratio. [16]
5. A 3 half wave bridge comprising three thyristors is fed from a 277Vrms, line to
neural, 60Hz supply and provides an adjustable dc voltage at the terminals of a
separately excited dc motor. The motor has Ra=0.02, La =.001H, Ka=1.2 and full
load Ia = 500A. Find the ring angle so that the motor operates at full load current
and at rated speed of 200rps. Assume continuous conduction and neglect thyristor
forward voltage drop. [16]
6. Explain the principle of operation of self control of synchronous motor fed from
VSI source. [16]
7. A 3-, 8 pole, 50Hz IM has the following circuit parameters r2 =0.15
x2 = 0.7
.
The motor speed is controlled by varying the applied voltage by an AC voltage
controller, which operates from a 380v, 50Hz supply. Detemine the applied voltage
perphase of the motor to have a slip of 0.15. The motor drives a load with a
charectersitc of T1 = 0.014W2 NW-m. Determine the ring angle of the converter.
[16]
8. A class-A chopper, operating in time-ratio control, is supplying the armature of
the
separately excited dc motor. Show that the motor speed-torque relationship is ,
!m= :V
K Ra

K2 Ta
Where V - chopper input voltage, Ra - Armature resistance, Ta - motor torque, K-
torque constant. [16]
? ? ? ? ?
2
? ? ? ? ?
1. In variable frequency control of asynchronous motor why (V/f) ratio is
maintained
constant up to base speed and V constant above base speed. Draw the relevant
characteristics. [16]
2. Write down the basic performance equations for a D.C Series motor Sketch char-
acteristics of constant torque drive and constant power drive regions. [16]
3. Explain how braking mode of operation is obtained in D.C drives. [16]
4. (a) State the major features of Rotor resistance control of Wound Rotor Induction
motor.
(b) In the rotor resistance control, what type of motor speed - torque character-istics
will be obtained if one phase has a loose contact? [8+8]
5. Discuss in detail the principle of operation of a 3 Phase Induction motor when it
is operated below rated frequency and above rated frequency. Draw relevant speed
torque characteristics. [16]
6. What is a Chopper? Explain the Chopper control of a separately excited D.C
motors
(a) Motoring Mode
(b) regenerative braking mode and also draw the Speed-Torque Curves in each
mode. [8+8]
7. Describe how the speed of a separately excited dc motor is controlled through the
use of two 3- phase full converters. Discuss how two quadrant drive can be obtained
from the scheme. Derive expressions for rms values of source and thyristor currents.
State assumptions made. [16]
8. The rotor resistance and stand still reactance referred to stator of a 3 phase, 4
pole,
50 Hz Squirrel cage Induction motor is 0.2 ohm and 0.8 ohm per phase respectively.
The full load slip of the motor is 4 percent. Neglect stator resistance and leakage
reactance. Determine how much stator voltage should be reduced in order to get a
speed of 1200rpm if the load torque remains constant. [16]
? ? ? ? ?
3

? ? ? ? ?
1. Derive the Speed, Torque Equations of a fully controlled converter connected to
D.C series motor with continuous current operation with necessary waveforms.
[16]
2. Mention the reasons
(a) Why V/f ratio is maintained constant when the motor is operated below the
base speed.
(b) Why the terminal voltage is maintained constant, when Induction motor is
operated above base speed. Draw relevant speed torque characteristics. [8+8]
3. Explain the operation of four quadrant chopper fed to the D.C separately excited
motor and also draw the current and voltage wave forms for continuous current
operation. [16]
4. What are the various possible combinations of voltage source DC link converters
to
obtain a variable voltage variable frequency supply to feed a Synchronous motor?
Draw the circuit diagrams and discuss in detail. [16]
5. (a) Starting from fundamentals prove that torque developed by the Induction
motor is proportional to square of the supply voltage.
(b) Draw the speed torque curves for di
erent voltages fed from stator voltage
controller. [8+8]
6.The field circuit and armature circuit of a separately excited dc motor are
controlled
by two identical three phase fully controlled converter and are fed from a 400V 50
Hz supply .The rated armature current of the motor is 170A.motor parameters are
Ra=0.2
Rf=320
motor constant is 0.5. Field converter has zero degree

ring
angle delay. For rated load torque of 60Nm at 2000rpm. Calculate
(a) Rated armature current
(b) Firing angle delay of the armature converter
(c) Speed regulation at rated load. [16]
7. Explain with block diagrams closed loop torque control and closed loop speed
con-trol of drives. [16]
4
8. A 3 phase,400 V,50 Hz 4 pole,1400 rpm, Star connected Induction motor has the
following parameters referred to the stator.
Rs = 2 ohm, Rr' = 3ohm, Xs = 3.5 ohm, Xr'= 3.5. The stator to rotor turns ratio
is 2 ohm. The motor speed is controlled by Static Scherbius drive. The Inverter is
directly connected to the source. Determine the ring angle for 0.4 times the rated
motor torque and a speed of 1200 rpm. [16]
? ? ? ? ?
5

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