K.Rajkumar, Department of Electrical and Electronics Engineering
1. Explain the constructional details of Alternator in detail with
neat sketches. (16 Marks)
Construction of Alternators:
An alternator has 3,-phase winding on the stator and a d.c. field
winding on the rotor.
a. Stator
It is the stationary part of the machine and is built up of silicon
steel laminations having slots on its inner periphery. A 3-phase
winding is placed in these slots and serves as the armature
winding of the alternator. The armature winding is always
connected in star and the neutral is connected to ground.
b. Rotor
The rotor carries a field winding which is supplied with direct
current through two slip rings by a separate d.c. source. This
d.c. source (called exciter) is generally a small d.c. shunt or
compound generator mounted on the shaft of the alternator.
Rotor construction is of two types, namely;
1. Salient (or projecting) pole type
2. Non-salient (or cylindrical) pole type
Salient pole type:
In this type, salient or projecting poles are mounted on a large
circular steel frame which is fixed to the shaft of the alternator as
shown in Fig. (1). The individual field pole windings are connected
in series in such a way that when the field winding is energized by
the d.c. exciter, adjacent poles have opposite polarities.
Dhanalakshmi Srinivasan Institute of Technology, Samayapuram, Trichy.
Fig. 1. Salient Pole Rotor
Low and medium-speed alternators (120-400 r.p.m.) such as
those driven by diesel engines or water turbines have salient pole
type rotors due to the following reasons: (a) The salient field poles
would cause an excessive windage loss if driven at high speed and
would tend to produce noise. (b) Salient-pole construction cannot
be made strong enough to withstand the mechanical stresses to
which they may be subjected at higher speeds.
Since a frequency of 50 Hz is required, we must use a large
number of poles on the rotor of slow-speed alternators (Used in
hydro turbines and Diesel Engines). Low- speed rotors always
possess a large diameter to provide the necessary spate for the
poles. Consequently, salient-pole type rotors have large diameters
and short axial lengths.
Non-salient pole type:
In this type, the rotor is made of smooth solid forged-steel radial
cylinder having a number of slots along the outer periphery. The
field windings are embedded in these slots and are connected in
series to the slip rings through which they are energized by the d.c.
exciter. The regions forming the poles are usually left unslotted as
shown in Fig. (2). It is clear that the poles formed are non-salient
i.e., they do not project out from the rotor surface.
1
K.Rajkumar, Department of Electrical and Electronics Engineering
Fig. 2. Cylinderical Rotor
High-speed alternators (1500 or 3000 r.p.m.) are driven by steam
turbines and use non-salient type rotors due to the following
reasons:
a. This type of construction has mechanical robustness and gives
noiseless operation at high speeds.
b. The flux distribution around the periphery is nearly a sine wave
and hence a better e.m.f. waveform is obtained than in the case
of salient-pole type.
Since steam turbines run at high speed and a frequency of 50 Hz
is required, we need a small number of poles on the rotor of highspeed alternators (also called turboalternators) (Used with steam
tubines). We can use not less than 2 poles and this fixes the highest
possible speed. For a frequency of 50 Hz, it is 3000 r.p.m. The next
lower speed is 1500 r.p.m. for a 4-pole machine. Consequently,
turboalternators possess 2 or 4 poles and have small diameters and
very long axial lengths.
Dhanalakshmi Srinivasan Institute of Technology, Samayapuram, Trichy.
2. Discuss the powerfactor improvement using Synchronous
Condenser.
(Or)
Write
notes
on
Synchronous
Condensor.(8Marks)
SYNCHRONOUS CONDENSER
A synchronous motor takes a leading current when over-excited
and, therefore, behaves as a capacitor. An over-excited synchronous
motor running on no-load is known as synchronous condenser.
When such a machine is connected in parallel with induction
motors or other devices that operate at low lagging power factor,
the leading kVAR supplied by the synchronous condenser partly
neutralizes the lagging reactive kVAR of the loads. Consequently,
the power factor of the system is improved.
The following figure shows the power factor improvement by
synchronous condenser method. The 3 phase load takes current IL at
low lagging power factor cosϕL. The synchronous condenser takes a
current Im which leads the voltage by an angle ϕm. The resultant
current I is the vector sum of Im and IL and lags behind the voltage
by an angle f. It is clear that ‘ϕ’ is less than ‘ϕL’ so that cosϕ is
greater than cosϕL. Thus the power factor is increased from cosϕL to
cosϕ.
Synchronous condensers are generally used at major bulk supply
substations for power factor improvement.
Advantages
i. By varying the field excitation, the magnitude of current
drawn by the motor can be changed by any amount. This
helps in achieving stepless control of power factor.
ii. The motor windings have high thermal stability to short
circuit currents.
iii. The faults can be removed easily.
Disadvantages
i. There are considerable losses in the motor.
ii. The maintenance cost is high.
iii. It produces noise.
iv. Except in sizes above 500 RVA, the cost is greater than that
of static capacitors of the same rating.
2
K.Rajkumar, Department of Electrical and Electronics Engineering
Dhanalakshmi Srinivasan Institute of Technology, Samayapuram, Trichy.
The phasor diagram shown in Fig.3 neglects the armature
resistance. i.e, Ra=0.
 tan 
Xs
o
  Hence =90 .
Ra
Input power per phase =VIacosϕ
For the case Ra=0, stator copper loss, Ia2Ra=0.
Hence input power is equal to the mechanical power developed by
the motor (Pm).
i.e, Pm= VaIacosϕ
--- (1)
Referring to the phasor diagram in Fig 3,
AB  Er cos  I a X s cos
Also, AB  Eb sin   I a X s cos
E sin 
or I a cos  b
--- (2)
Xs
Fig 2. Powerfactor improvement using Synchronous Condenser
v. As a synchronous motor has no self-starting torque, thenfore, an auxiliary equipment has to be provided for this
purpose.
3. Derive the power developed by the synchronous motor.
(8Marks)
Fig.3. Phasor Diagram of Under excited Synchronous Motor
Substituting (2) in (1),
VE b sin 
Pm 
Xs
It is clear from the above relation that mechanical power increases
with torque angle δ and its maximum value reached when δ=90o.
Pm max  
VE b
per phase
Xs
Under this condition, the poles of the rotor will be mid-way
between N and S poles of the stator.
4. Compare the Synchronous Motor with Induction Motor. (6
Marks)
S.No Remarks
Synchronous Motor Induction Motor
1
Speed
Remains Constant Decreases with load
irrespective of load
2
Powerfactor Can be operated at Operates at lagging
any powerfactor
p.f only
3
Excitation
Requires
D.C No excitation is
Excitation at the required.
rotor
3
K.Rajkumar, Department of Electrical and Electronics Engineering
S.No
4
Remarks
Economy
Synchronous Motor Induction Motor
Economical for the Economical
for
speed below 300 speed above 600
r.p.m
r.p.m
5
Self-starting No self starting. It Self-starting
requires additional
arrangement.
6
Construction Complicated
Simple
7
Starting
More
Starting Less Starting torque
Torque
torque
5. Explain how the V and inverted V curves can be obtained in
synchronous motor. (8Marks)
A synchronous motor is a double-excited machine, its armature
winding is energised from an a.c source and its field winding from
d.c source.
When synchronous motor is working at constant applied
voltage, the resultant air gap flux demanded by applied voltage
remains constant. This resultant air gap flux is established by both
a.c in armature winding and d.c in the field winding.
If the field current is sufficient enough to set up the air-gap flux,
as demanded by constant applied voltage then magnetizing current
or lagging reactive VA requied from the a.c source is zero and
therefore motor operates at unity power factor. This field current,
which causes unity power factor operation of the synchronous
motor, is called normal excitation or normal field current.
If the current less than the normal excitation, i.e, the motor is
under excited, then the deficiency in flux must be made up by the
armature winding m.m.f. In order to do the needful, the armature
winding draws a magnetizing current or lagging reactive VA from
the a.c source and as a result of it, the motor operates at a lagging
power factor. In case the field current is made more than its normal
Dhanalakshmi Srinivasan Institute of Technology, Samayapuram, Trichy.
excitation, i.e the motor is over-excited, operates at leading power
factor.
Fig(1) shows the variation of armature current and power factor
with field current at no load, half load and full load conditions.
6. Discuss the procedure to obtain Xd (Direct axis reactance) and
Xq (Quadrature Axis Reactance) of a synchronous generator.
(8Marks)
The unsaturated values of Xd and Xq of a 3-Phase synchronous
machine can be easily determined experimentally by conducting the
4
K.Rajkumar, Department of Electrical and Electronics Engineering
following test known as slip test. The rotor of the synchronous machine is
driven by means of a prime mover (usually a DC motor in the laboratory)
at a speed close to the synchronous speed in the proper direction but not
equal to it. The armature is supplied with a low voltage 3-Phase balanced
supply through a variac, while the field circuit is kept open.
Dhanalakshmi Srinivasan Institute of Technology, Samayapuram, Trichy.
2 MARKS Questions and Answers
Unit 1 - Synchronous Generator
1. Why the stator core is laminated?
The stator core is laminated to reduce eddy current loss of the
machine.
2. Define voltage regulation.
The voltage regulation of an alternator is defined as the change
in terminal voltage from no-load to full-load (the speed and
field excitation being constant) divided by full-load voltage.
The armature current varies between two limits since it moves
through, since the synchronously rotating armature MMF acts through the
varying magnetic reluctance paths as it goes from inter-polar axis to pole
axis region. The values of Xsd and Xsq are determined based on the applied
voltage and the armature current values. The ratio of applied voltage to
the minimum value of the armature current gives the direct axis
synchronous reactance Xsd. The ratio of applied voltage to the maximum
value of the armature current gives the the quadrature-axis reactance Xsq.
For more accurate determination of these values the oscillogram of the
armature current and voltage can be recorded.
V MaximumLine Voltage
Xd  t
3  I a ,min
Xq 
Vt MinimumLine Value
3  I a ,max
3. Write the causes of harmonics in the voltage and current
waves of electrical machinery.
The voltage and current
waveform
generated
in
armature winding of electrical
machines are assumed to be
smooth sinusoidal. But in
practice, the armature is not
smooth
due
to
slotted
periphery. Due to this slotting,
certain harmonics are produced in e.m.f’s. The ripples due to
slotting are called slot harmonics.
4. What are the conditions for parallel operation of alternators?
 Terminal voltage of the incoming machine must be same as
that of busbar voltage.
 Frequency of the incoming machine must be same as that of
busbar voltage.
5
K.Rajkumar, Department of Electrical and Electronics Engineering
Dhanalakshmi Srinivasan Institute of Technology, Samayapuram, Trichy.

Phase sequence of the incoming machine must be same as
that of busbar voltage.
5. What is meant by armature reaction in alternator?
The effect of the armature flux on the main flux affecting its
value and distribution is called armature reaction.
6. What are the essential elements for generating EMF in
alternators?
The essential elements for generating EMF in alternators:
i. Stator – Armature Core, Armature Winding
ii. Rotor – Field Pole Core, Field Coil
iii. Yoke or Enclosure
iv. Slipring and Brushes
7. Two reaction theory is applied only to salient pole machines.
State the reason.
Since the airgap of salient pole machines is not uniform
throughout the stator periphery, the effect of armature reaction
cannot be same in any point at airgap. Hence, it is considered as
two components-one acting along the direct axis and other
acting along the quadrature axis. Blondel’s two-reaction theory
considers the effects of the quadrature
components of the armature reaction separately.
and
direct-axis
8. State the causes of voltage drop in an alternator when loaded.
(or) Write down the causes for reduction in terminal voltage
of alternator from it’s no load value E0 to V for a lagging
powerfactor.
The voltage drop occurs in alternator during loaded condition
due to,
i. Voltage drop IaRa where Ra is the armature resistance
per phase.
ii. Voltage drop IaXL where XL is the armature leakage
reactance per phase.
iii. Voltage drop because of armature reaction.
9. What do you mean by single layer and double layer winding?
If a slot consists of only one coil side, the winding is said to be
single layer. Whereas the double layer winding contains two
coil sides per slot, one at the bottom and one at the top.
Generally, double layer winding is preferred to avoid wastage
of stator space.
10. What are the various function of damper winding provided
with alternator?
 The damper windings are useful in preventing the hunting
(momentary speed fluctuations) in generators.
 The damper winding also tends to maintain balanced 3-phase
voltage under unbalanced load conditions.
11. How will you distinguish between the two types of large
synchronous generator from their appearance?
 Slow Speed Salient pole alternators are larger in diameter
and shorter in axial length.
 High speed Cylindrical alternators are shorter in diameter
and axially lengthier.
12. What is meant by alternator on infinite busbars?
Synchronous generators are rarely used to supply individual
loads. These generators are generally connected to a power
supply system known as an infinite bus in which a large number
6
K.Rajkumar, Department of Electrical and Electronics Engineering
Dhanalakshmi Srinivasan Institute of Technology, Samayapuram, Trichy.
of synchronous generators are connected together, the voltage
and frequency of the infinite bus hardly change.
13. Calculate the pitch factor for the under given winding: 36
stator slots, 4 poles, coil span 1 to 8.
No. of Slots, S=36
No. of Poles, p=4
Unit 2 - Synchronous Motor
Slots( S ) 36

9
Pole( p) 4
The coil span falls short by,
2 slots. i.e.,
2
9
   180o  40o
 
Pitch factor, k p  cos   cos 20o  0.94
2
14. Calculate the distribution factor for a 36 slots, 4 poles, single
layer three phase winding.
No. of Slots, S=36; No. of Poles, p=4
Slots( S ) 36
180o 180o


 20o

9
n
9
Pole( p) 4
Slots( S )
36
m

3
Pole( p)  No. of phaseP  4  3
n


 3  20o 
m





sin
sin 


2
2 


  0.96
kd 

o
 20 
m sin   

2 sin 
 2

 2 
1. What is meant by torque angle?
The angular displacement between stator and rotor poles is
called ‘torque angle’. This torque angle causes the phase of
back e.m.f Eb to change with respect to supply voltage V.
2. What is meant by hunting in synchronous motor?
The quick forward and backward motion of the rotor as it
revolves at the average constant speed is called ‘hunting’.
3. List the inherent disadvantages of synchronous motor.
 Synchronous motor is not self starting,
 It is not suitable for variable speed operation. Variable
speed operation needs separate arrangement to vary the
supply frequency.
4. When is a synchronous motor said to receive 100%
excitation?
The motor is said to be normally excited if the field excitation is
such that Eb = V.
5. What are the uses of damper winding in synchronous motor?
The damper windings are used for the following purposes in
synchronous motors:
i. To avoid hunting
ii. To enable self-starting
6. What is meant by synchronous condenser?
An over-excited synchronous motor running on no-load in
connection with busbar to improve powerfactor is known as
synchronous condenser.
7. What could be the reasons if a 3-phase synchronous motor
fails to start?
It is usually due to the following reasons:
1. Voltage may be too low
7
K.Rajkumar, Department of Electrical and Electronics Engineering
Dhanalakshmi Srinivasan Institute of Technology, Samayapuram, Trichy.
2. Some faulty connection in auxiliary apparatus
3. Too much starting load
4. Open-circuit in one phase or short-circuit
5. Field excitation may be excessive.
8. What are V-curves?
V-Curve is the plot of the stator current against the rotor
excitation current.
9. Name any two important characteristic of a 3 phase
synchronous motor not found in 3 phase induction motor.
i. Synchronous motor can be operated at any powerfactor.
ii. Speed remains constant from no-load to full-load.
10. Give any two methods of starting a synchronous motor.
i. Using DC Motor
ii. Using Damper winding
11. Define pull-out torque in synchronous motor.
As the load increases, the torque angle α also increases so that a
stage is reached when the rotor is pulled out of synchronism and
the motor comes to a standstill. This load torque at which the
motor pulls out of synchronism is called pull-out or breakdown
torque.
12. Write down the applications of synchronous motor.
 Used to improve the powerfactor of a plant.
 Used to improve the voltage regulation of transmission
lines.
 Used to drive constant speed loads like blowers, fans,
crushers and rotary kilns etc.
13. Write down the equation for power developed by a
synchronous motor.
Pem 
3E a V a
sin 
Xs
8