SYNCHRONOUS MOTOR DRIVES

1. SYLLABUS
WOUND FIELD CYLINDRICAL ROTOR MOTOR
EQUIVALENT CIRCUITS
PERFORMANCE EQUATION OF OPERATION FROM A VOLTAGE SOURCE
POWER FACTOR CONTROL AND V CURVES
STARTING AND BRAKING
SELF CONTROL-LOAD COMMUTATED SYNCHRONOUS MOTOR DRIVES,
BRUSH AND BRUSH LESS EXCITATION
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2. Basic parts of a synchronous motor:
 Rotor - dc excited winding
 Stator - 3-phase winding
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3. Cylindrical or round-rotor synchronous machine
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4. Salient-pole synchronous machine
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5. Cylindrical or round-rotor synchronous machine
 They are smaller in diameter but having longer axial
length.
 Cylindrical rotors are used in high speed electrical
machines, usually 1500 RPM to 3000 RPM.
 Windage loss as well as noise is less as compared to
salient pole rotors.
 Their construction is robust as compared to salient
pole rotors.
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6. Cylindrical or round-rotor synchronous machine
 Number of poles is usually 2 or 4.
 Damper windings are not needed in non-salient pole
rotors.
 Flux distribution is sinusoidal and hence gives better
emf waveform.
 Non-salient pole rotors are used in nuclear, gas and
thermal power plants.
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7. Comparison of Salient pole rotor
and Cylindrical rotor
Sl.No Salient pole rotor Cylindrical rotor
1 Large diameter Smaller diameter
2 Shorter axial length Longer axial length
3 Poles are projected
No projection .
Smooth cylindrical
one
4 Need damper winding No need
5
It is suitable for low speed hydro
Generator
Suitable for high
speed turbo generator
6 Windage loss is higher Lesser
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8. Advantages of rotating field and stationary
armature type
 High voltage generation
 Better insulation
 Rotor weight – less
 Current collection - easy
 Lesser number of slip rings
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9. Power factor control and V curves
The V-curve
The rotor current can be
adjusted to vary the power factor
of the stator
Unity power factor is achieved
when stator current is at its
minimum
This machine can be used to
correct power factor of induction
motors when connected in
parallel
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10. Power factor control and V curves
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11. Power factor control and V curves
 Curves of armature current vs. field current (or
excitation voltage to a different scale) are called V
curves,
 The curves are illustrate the effect of the variation
of field excitation on armature current and power
factor for typical shaft loads.
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12. Power factor control and V curves
 It can be easily noted from these curves that an
increase in shaft loads require an increase in field
excitation in order to maintain the power factor at
unity.
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13.  For motor with increased (decreased) excitation
power factor becomes leading (lagging)
 For generator with increased (decreased)
excitation power factor becomes lagging
(leading)
 Unloaded overexcited synchronous motors are
sometimes used to improve power factor. They
are known as synchronous condensers
Power factor control and V curves
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14. Equivalent circuit and performance equation of
operation from a voltage source
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15. Equivalent circuit and performance equation of
operation from a voltage source(cont’d)
 The equivalent-circuit model for one armature phase
of a cylindrical rotor three phase synchronous motor is
shown in Fig,
 All values are given per phase.
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16. Equivalent circuit and performance equation of
operation from a voltage source(cont’d)
 Applying Kirchhoff’s voltage law to Fig.
VT = IaRa + jIaX l + jIaXas + E f -------- 1
 Combining reactances, we have
Xs = Xl + Xas ---------------- 2
 Substituting Eqn. 2 in Eqn.1
VT = E f + Ia(Ra + jXs)------ 3
or
VT = E f + IaZs
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17. Equivalent circuit and performance equation of
operation from a voltage source(cont’d)
where:
Ra = armature resistance (/phase)
Xl = armature leakage reactance (/phase)
Xs = synchronous reactance (/phase)
Zs = synchronous impedance (/phase)
VT = applied voltage/phase (V)
Ia = armature current/phase(A)
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•The phase angle δ between the terminal voltage VT
and the excitation voltage E f in Fig , is usually
termed the torque angle.
•The torque angle is also called the load angle or
power angle.

18. Equivalent circuit and performance equation of
operation from a voltage source(cont’d)
Phasor diagram corresponding to the equivalent-circuit model
The counter EMF which is obtained from the phasor equation;
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E f = VT − IaZs ---------- 4

19.  Except for very small machines, the armature
resistance of a synchronous motor is relatively
insignificant compared to its synchronous reactance,
so that Eqn. 3 to be approximated to
 VT = E f + jIaXs -------------- (5)
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Equivalent circuit and performance equation of
operation from a voltage source(cont’d),
Synchronous-motor power equation

20. Equivalent circuit and performance equation of operation from a
voltage source(cont’d),
Synchronous-motor power equation
 From this phasor diagram, we have,
IaXs cosθi = −E f sinδ ------- 6
 Multiplying through by VT and rearranging terms we
have,
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------------- 7

21. Equivalent circuit and performance equation of operation from a
voltage source(cont’d),
Synchronous-motor power equation
 The left side of Eqn. 7 is an expression for active power
input
 Power input will also represent the electromagnetic
power developed, per phase, by the synchronous
motor. Thus,
P in ,ph = V T I a cosφi ------ 8
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22. Equivalent circuit and performance equation of
operation from a voltage source(cont’d),
Synchronous-motor power equation
 Thus, for a three-phase synchronous motor,
Pin = 3 ∗ V T I a cosφ i ------------ 9
or
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------------ 10
Equ 10 is the Synchronous motor power equation the electro magnetic power
developed per phase by a cylindrical-rotor motor, in terms of its excitation voltage and
power angle.
P ∝ Ia cosθ
P ∝ Ef sinδ

23. Equivalent-circuit of a synchronous-motor,
assuming armature resistance is negligible
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24. Phasor diagram model for a synchronous-motor, assuming
armature resistance is negligible
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25. Methods of starting synchronous motor
 Motor Starting by Reducing the supply Frequency
 Motor Starting with an External Motor
 Motor Starting by Using damper (Amortisseur)
Winding
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26. Motor Starting by Reducing the supply Frequency
 If the rotating magnetic field of the stator in a
synchronous motor rotates at a low enough speed,
there will be no problem for the rotor to accelerate and
to lock in with the stator’s magnetic field.
 The speed of the stator magnetic field can then be
increased to its rated operating speed by gradually
increasing the supply frequency f up to its normal 50-
or 60-Hz value.
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27. Motor Starting with an External Motor
 Method of starting a synchronous motor is to attach
an external starting motor (pony motor) to it and
bring the synchronous machine to near about its rated
speed
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28. Motor Starting by Using damper (Amortisseur) Winding
 Most of the large synchronous motors are provided
with damper windings,
 In order to nullify the oscillations of the rotor
whenever the synchronous machine is subjected to a
periodically varying load.
 Damper windings are special bars laid into slots cut in
the pole face of a synchronous machine and then
shorted out on each end by a large shorting ring,
similar to the squirrel cage rotor bars.
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29. Braking of Synchronous Motors
 There are three types of braking i.e, regenerative,
dynamic and plugging type braking.
 But for synchronous motor drives only dynamic
braking can be applied though plugging can be
applied theoretically.
 Regenerative braking cannot be applied to them as
they need higher speed than synchronous speed.
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30. Braking of Synchronous Motors
 Dynamic braking is done by disconnecting the motor
from supply and connecting it across a three phase
resistor
 At that time the motor works as a synchronous
generator and energy is dissipated at the resistors.
 Plugging is not used for synchronous motors as high
plugging current can cause severe disturbance and
damage in line.
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31. SELF CONTROL
 Advantages
 The operation of synchronous motor in the
self controlled mode eliminates the hunting
and stability problems
 Self control permits the realization of
versatile control characteristics of a dc motor
without the limitations associated with
commutator and brushes
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32. SELF CONTROL
 Advantages(cont’d)
 The self controlled synchronous motor drives have
been build of power ratings of tens of megawatts and
speed approaching 6000 rpm which are beyond the
capability of dc and induction motor drives
 They have been good dynamic response
 Smooth starting and braking operation with a high
torque to current ratio
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33. Disadvantage of synchronous motor in the self
controlled mode
 Complex control
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34. SELF CONTROL
Applications
 Compressors
 Extruders
 Induced and forced draft fans
 Blowers
 Conveyors
 Aircraft test facilities
 Mainline traction
 Steel rolling mills
 Large ship propulsion
 Flywheel energy storage ,and so on.
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35. selfcontrol
In self control as the rotor speed
changes ,the armature supply
frequency is also changed
proportionately
 The armature field always moves at the
same speed as the rotor
The armature and rotor fields move in
synchronism for all operating points
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36. Self controlled synchronous motor drives
 Self controlled synchronous motor drives are popularly
known as commutatorless dc and ac drives depending
on whether the synchronous motor is fed from a dc
supply through an inverter or from an ac supply
through a cycloconverter.
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37. brushless dc and ac drives
 When Self controlled synchronous motor drives
employ a wound field motor with a brushless
excitation system or a permanent magnet motor then
they are called brushless dc and ac drives
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38. LOAD COMMUTATED SYNCHRONOUS MOTOR DRIVES
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Load commutated brushless dc motor drive with terminal voltage sensor and
permanent magnet motor

39. LOAD COMMUTATED SYNCHRONOUS MOTOR DRIVES(cont’d)
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Load commutated brushless dc motor
drive with terminal voltage sensor and
permanent magnet motor

40. LOAD COMMUTATED SYNCHRONOUS MOTOR DRIVES(cont’d)
 The firing pulses derived either from the rotor position
encoder or machine terminal voltage sensor
 The machine may be fed from a load commutated
current source inverter or a load commutator
cycloconverter
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41. BRUSH AND BRUSH LESS EXCITATION
 What is brushless excitation system of alternator
and it's working?
 The main function of excitation system is to keep
machine output voltage constant irrespective of the
load.
 There are mostly two types of A.C
excitation system used in large synchronous
machine as static excitation and brush-less excitation.
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42. Brush-less excitation-
 Now a days most of large capacity generators are used
Brush less excitation.
 In this system there is no brushes and slip ring are
used for excitation purpose,
 That’s why it’s name is Brush-less excitation.
 Here only a pair of brushes are used for generator
protection and earthing purpose.
 As there is no brushes it’s maintenance cost is less as
compared to static excitation.
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43. Parts of Brush-less excitation
system
a) Three phase pilot exciter.
b) Three phase main exciter.
c) Rotating rectifier wheels.
d) Cooler system.
E) Metering and supervisory system.
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44. 1/9/2020 44

45. Three phase pilot exciter.
 The three phase pilot exciter has a revolving field with
permanent magnet poles.
 The controlled rectified d.c. is fed to the main exciter
field.
 The induced three phase a.c. voltage is rectified in the
rotating rectifier bridge and fed to the generator
rotor winding through the d.c. leads in the shaft.
 The pilot exciter magnets, the main exciter rotor and
the rotating diodes are all mounted on a single shaft.
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46. Three phase main exciter.
 The three phase main exciter is a six pole rotating armature
unit.
 The field poles with the damper windings are arranged in
the stator frame.
 Laminated magnetic poles are arranged to form the field
winding.
 Bars are provided on the pole shoes, and their ends are
shorted to form a damper winding.
 To measure the exciter current a quadrature axis coil is
fitted between two poles.
 The rotor is formed by stacking laminations together and
these are compressed by through-bolts over compression
rings.
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47. Three phase main exciter.
 The three phase winding is inserted in the slots of the
rotor.
 The winding conductors are transposed within the core
length, and the ends turns of the rotor winding are secured
with steel bands.
 The connections are made at rectifier wheel end. A ring bus
is formed at the winding ends and the leads to rotating
rectifier wheels are also connected to the same.
 The complete rotor is shrunk fit on the shaft.
 The rotor is supported on a journal bearing positioned
between the main and pilot exciters.
 Lubrication of the bearing is from the turbine oil system.
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48. Rotating rectifier wheels.
 The silicon diodes are arranged on the rectifier wheels in a
three phase configuration.
 The diodes are so made that the contact pressure increases
during rotation due to the centrifugal force.
 There are two diodes in parallel on each heat sink and.
these are protected by one fuse.
 The RC suppression network consists of one capacitor and
one damping resistor each, and there are six RC circuits per
wheel.
 The two diode wheels are identical but differ only in the
forward direction of the diodes.
 The d.c. leads run through the bore of the shaft and the
connection to the diodes is made via radial bolts.
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49. Cooling arrangement.
 The exciter is air cooled.
 This is a closed loop system and the hot air is cooled in two
coolers arranged alongside the exciter.
 The entire exciter is housed in an enclosure through which
the cooling air circulates.
 The main exciter receives the cool air from the fan which
draws the cold air through the pilot exciter.
 Air enters the main exciter from both ends and is passed on
to the ducts below through radial holes.
 The warm air passes over the coolers and return to the
main enclosure.
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50. Metering and supervisory system.
 The supervision of exciter consists of the stroboscope
for fuse monitoring and rotor ground fault detections
circuit.
 The generator field current is measured through
a quadrature axis coil mounted on the exciter stator.
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51. Advantage of Brushless excitation:-
 No headache for replacement of brushes in this excitation .
 No external power supply required during starting.
 The shaft driven permanent magnetic generator pilot
exciter provides a reliable source of exciter field power that
eliminates bulky power transformers and dependence on
station battery for field flashing.
 Short circuit sustaining capability provides fault current
support.
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52. Advantage of Brushless excitation
 No heavy bus work or cable connections are required
between excitation cubicles and the generator,
 thus simplifying installations.Large expensive field
field circuit breaker and field discharge resistors are
not required.
 Compact voltage regulator hardware for installation
and control panel or switchgear eliminates large
excitation cubicles.
 Suitable for large size generator.
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