Engineering

1. 1

2. A. Braking methods: Rheostatic, Plugging and Regenerative.
Closed loop control of drives: current limit control, torque
control and speed control.
B. Single phase and three phase fully controlled converter
drives and performance of converter fed separately excited
DC Motor for speed control operations. Chopper controlled
drives for separately excited and series DC Motor operations.
Numerical based on above. Closed loop speed control of DC
motor below and above base speed.
DC Motor Drives 2
CONTENTS

3. DC Motor Drives 3
INTRODUCTION
• The important of applications in DC motor drives:
1. Adjustable speed.
2. Good speed regulation.
3. Good frequent starting, braking and reversing.
• E.g.: Rolling mills, paper mills, mine winders,
hoists, machine tools, traction, printing, textile
mills, excavators and cranes.
• Low cost and simple control – DC drives AC drives

4.  Common used DC motors:
• Separately excited motor – field and armature voltage can
be controlled independently.
• Shunt motor – field and armature are connected to a
common source.
• Series motor – field flux is a function of armature current.
• Cumulatively compound motor – magneto-motive
force (mmf) of the series field is a function of
armature current and in the same as mmf of the
shunt field.
DC Motor Drives 4
DC MOTOR AND PERFORMANCES

5. DC Motor Drives 5
DC MOTOR AND PERFORMANCES

6. DC Motor Drives 6
DC MOTOR AND PERFORMANCES
Where  is the flux per pole, Webers; Ia is the armature current, A; V is the source
voltage, V; E is the armature voltage, V; Rais the resistance of the armature circuit,
𝜔𝑚 is the speed of armature , rad/sec; T is the torque developed by the motor, N-
m; and Ke is the motor constant.
T
e
a
e e e
m 2
K 
K  K  K 
V

Ra V Ra
  I  
• The basic important of expressions for DC motor:
E  Kem
V  E  Ra Ia
T  KeIa

7. DC MOTOR AND PERFORMANCES
Shunt and Separately excited motors:
• Constant field current, the flux can be assumed
constant.
Ke  K constant
T  KIa
E  Km
𝜔𝑚 =
𝑉
𝐾
−
𝑅𝑎
𝐾
𝐼𝑎 =
𝑉
𝐾
−
𝑅𝑎
𝐾2 𝑇

8. DC MOTOR AND PERFORMANCES
• Separately excited motor – applications that requiring
good speed regulation and adjustable speed.

9. DC MOTOR AND PERFORMANCES
Series motors:
• The flux is a function of current.
• In unsaturated region of magnetization characteristic,
flux is proportional to armature current.
  K f Ia
V  E  Ra Ia
T  K K I 2
e f a
𝜔𝑚 =
𝑉
𝐾𝑒𝐾𝑓𝐼𝑎
−
𝑅𝑎
𝐾𝑒𝐾𝑓
=
𝑉
𝐾𝑒𝐾𝑓
1
𝑇
−
𝑅𝑎
𝐾𝑒𝐾𝑓

10. DC MOTOR AND PERFORMANCES
Series motors:
• Series motors – for application requiring high starting
torque and heavy torque overloads.
• Since torque is proportional to the squared armature
current, the increasing of motor current is less for the
same increase of torque compared to the separately
excited motor (torque is proportional to the armature
current).
• As speed varies inversely as the squared root of
torque, machine runs at large speed at light load.

11. DC MOTOR AND PERFORMANCES
• Shunt motor.

12. DC MOTOR AND PERFORMANCES
Example 1:
A 200 V, 10.5 A, 2000 rpm shunt motor has the armature and
field resistances of 0.5 Ohm and 400 Ohm respectively. It drives
a load whose torque is constant at rated motor torque. Calculate
motor speed if the source voltage drops to 175 V.

13. DC MOTOR AND PERFORMANCES

14. Example 2:
A 220 V DC series motor runs at 1000 rpm (clockwise) and
takes an armature current of 100 A when driving a load with
constant torque. Resistances of the armature and field windings
are 0.05 Ohm each. Find the magnitude and direction of motor
speed and armature current if the motor terminal voltage is
reserved and the number of turns in field winding is reduced to
80%. Assume linear magnetic circuit.
DC MOTOR AND PERFORMANCES

15. DC MOTOR AND PERFORMANCES
When the number of turns is reduced to 80%, the value of flux
for same field (or armature) current will also reduced to 80%.

16. DC MOTOR AND PERFORMANCES
Armature Current has a negative sign because the supply
voltage has been reversed.

17. Compound motors:
• No-load speed depends on the strength of shunt
field and slope of the characteristic on the
strength of series field.
• Compound motors – drooping characteristic similar
to the series motor that required and no-load speed
must be limited. E.g. lifts and winches.
• Application varies from almost no-load to very
heavy load.
DC MOTOR AND PERFORMANCES

18. • Compound motor.
DC MOTOR AND PERFORMANCES

19. • Maximum current (twice of the current rating) can be
commutated without sparking for safety.
• If full supply voltage across its terminal, a very high
current will flow, which may damage the motor due to
heavy sparking at commutator and heating of the winding.
• Thus it is necessary to limit the current to a safe value
during starting.
STARTING

20. • When motor speed is controlled by armature voltage
control, the controller which controls the speed can
also be used for limiting the motor current during
starting to a safe value.
• In a absence of a controller, a variable resistance
controller is used for starting as in Fig. 5.5.
• As motor accelerates and back emf rises, one section
of the motor is cut out at a time, either manually or
automatically with the help of contactors by
maintaining the current within the specified
maximum and minimum values.
STARTING

21. STARTING

22. • Braking is required in the electric motor.
• In braking, motor works as a generator and
developing a negative torque which oppose
the motion.
• Three types:
• Regenerative braking.
• Dynamic or rheostatic braking.
• Plugging or reverse voltage braking.
BRAKING

23. • Generated energy is supplied to the source.
E > V and negative Ia (armature current)
• Field flux cannot be increased substantially
beyond rated value due to saturation.
• For a source of fixed voltage of rated value,
regenerative braking is possible only for
speeds higher than rated value.
• For a variable of voltage source, it is also
possible below rated speeds.
• The speed-torque characteristic is shown in Fig.
5.6 for a separately excited motor.
REGENERATIVE BRAKING

24. REGENERATIVE BRAKING

25. • Series motor, as speed increases, armature current
and flux decreases. Thus regenerative braking
cannot be achieved.
• Regenerative braking should only be used when
there are enough loads to absorbed the
regenerated power.
• Alternatively, an arrangement must be made to
divert the excess power to a resistor bank for heat
dissipation.
REGENERATIVE BRAKING

26. Example 3
A 220 V,200 A, 800 rpm DC separately excited motor has an
armature resistance of 0.06 Ohm. The motor armature is fed
from a variable voltage source with an internal resistance of
0.004 Ohm. Calculate internal voltage of the variable voltage
source when the motor is operating in regenerative braking at
80% of the rated motor torque and 600 rpm.
REGENERATIVE BRAKING

27. REGENERATIVE BRAKING

28. DYNAMIC BRAKING
Braking> Dynamic braking.
• Motor armature is disconnected from the source
and then connected across braking resistor Rb and
armature resistor Ra for generated
energy/heat dissipation.

29. Braking> Dynamic braking.
• Speed-torque curves and transition from motoring to braking
modes.
• For fast braking, Rb is consists of a few sections
• Separately excited motor can be converted as a self- excited
generator even when supply fails.
DYNAMIC BRAKING

30. Example 3:
A 220 V DC series motor runs at 1000 rpm (clockwise) and
takes an armature current of 100 A when driving a load with
constant torque is operated under dynamic braking at twice the
rated torque and 800 rpm. Resistances of the armature and field
windings are 0.05 Ohm each . Calculate the value of braking
current and resistor. Assume linear magnetic circuit.
DYNAMIC BRAKING

31. PLUGGING
Braking> Plugging .
• Separately excited motor – the supply voltage of is reversed
and it assists the back emf in forcing armature in reverse
direction.
• The Rb is also connected to limit the current.
• Series motor – armature alone is reversed.

32. Braking> Plugging .
• Fig. 10, motor rotation in
reverse direction arises when
a motor is connected for
forward motoring.
• Counter-torque braking
– torque direction
remains +ve.
PLUGGING

33.  In a control system, two types of systems
 open loop control system
 output has no effect on the input, i.e the controlling
phenomenon is independent of the output.
 closed loop control system
 much more advanced and scientific
 the output is fed back to the input terminal which
determines the amount of input to the system
for ex. if the output is more than predetermined value the
input is reduced and vice-versa.
CLOSED LOOP CONTROL OF DRIVES

34. In electrical drives feedback loops or closed loop control
satisfy the following requirements:
1. Protection
2. Enhancement of speed of response
3. To improve steady-state accuracy
CLOSED LOOP CONTROL OF DRIVES

35.  To limit and sense the current fed to the motor below safe limit during
starting.
 Contains current feedback loop with threshold logic circuit.
 The feedback loop does not effect the normal operation of the drive.
 If the current exceeds the predetermined safe limit, the feedback loop
activates and the current is brought down below the safe limit.
 Once the current is brought down below the safe limit the feedback
loop again deactivates.
CURRENT LIMIT CONTROL

36.  mainly in battery operated vehicles like cars, trains etc.
 the accelerator present in the vehicles is pressed by the driver to set
the reference torque T*.
 The actual torque T follows the T* which is controlled by the driver via
accelerator.
CLOSED LOOP TORQUE CONTROL

37.  two control loops, a) inner loop and b)outer loop.
 Inner current control loop
 limits the converter and motor current or motor torque below
the safe limit.
 Suppose the reference speed ωm* increases, there is a positive error
Δωm, which indicates that the speed is needed to be increased.
CLOSED LOOP SPEED CONTROL

38.  Inner loop increases the current keeping it under maximum allowable
current.
 Driver accelerates and when the desired speed achieved,
motor torque Te = load torque Tl
 decrease in reference speed 𝜔𝑚 indicates there must be deceleration
 braking done by the speed controller at maximum allowable current.
 during speed controlling the operation transfers from motoring to braking
and vice versa continuously for smooth operation and running of motor.
CLOSED LOOP SPEED CONTROL

39.  mainly in battery operated vehicles like cars, trains etc.
 the accelerator present in the vehicles is pressed by the driver to set
the reference torque T*.
 The actual torque T follows the T* which is controlled by the driver via
accelerator.
CLOSED LOOP TORQUE CONTROL

40. CONTROLLED RECTIFIER FED DC DRIVES
 Controlled rectifier fed DC drives are also known as static Ward-
Leonard drives
 Controlled rectifiers used to get variable dc voltage from an ac
source of fixed voltage
 Controlled rectifier fed DC drives are widely used in applications
requiring a wide range of speed control and/or frequent starting,
braking and reversing.
 Applications: rolling mills, paper mills, printing presses, mine
winders, machine tools.

41. CONTROLLED RECTIFIER FED DC DRIVES

42. SINGLE PHASE FULL WAVE FULLY CONTROLLED
RECTIFIER FED SEPARATELY EXCITED DC
MOTOR

43. SINGLE PHASE FULL WAVE FULLY CONTROLLED
RECTIFIER FED SEPARATELY EXCITED DC
MOTOR
 Controlled rectifiers used to get variable dc voltage from an ac
source of fixed voltage
 Controlled rectifier fed DC drives are widely used in applications
requiring a wide range of speed control and/or frequent starting,
braking and reversing.
 Applications: rolling mills, paper mills, printing presses, mine
winders, machine tools.

44. SINGLE PHASE FULL WAVE FULLY
CONTROLLED RECTIFIER FED SEPARATELY
EXCITED DC MOTOR
• Duty interval (  t  )
• Zero current interval
(  t   )
Discontinuous conduction
Modes of operation