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 high- speed 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 I
L
at low lagging power factor cosϕ
L
. The synchronous condenser takes a current I m
which leads the voltage by an angle ϕ m
. The resultant current I is the vector sum of I m
and I
L
and lags behind the voltage by an angle f. It is clear that ‘ ϕ’
is less than ‘ ϕ
L
’
so that cosϕ is greater than cosϕ
L
. Thus the power factor is increased from cosϕ
L
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
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
Dhanalakshmi Srinivasan Institute of Technology, Samayapuram, Trichy.
The phasor diagram shown in Fig.3 neglects the armature resistance. i.e, R a
=0.
 tan  
X
R a s   Hence

=90 o
.
Input power per phase = VI a cosϕ
For the case R a
=0, stator copper loss, I a
2
R a
=0.
Hence input power is equal to the mechanical power developed by the motor ( P m
). i.e, P m
= V a
I a cosϕ --- (1)
Referring to the phasor diagram in Fig 3,
AB  E r cos   I a
X s cos 
Also, AB or

I a
E b sin  cos  
 I a
E b
X
X sin s
 cos  s
--- (2)
Substituting (2) in (1),
P m

VE b
X sin s

It is clear from the above relation that mechanical power increases with torque angle δ and its maximum value reached when δ=90 o
.
P m
 max


VE
X s b per phase
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 Decreases with load
2
3
Speed Remains Constant irrespective of load
Powerfactor Can be operated at any powerfactor
Excitation Requires D.C
Excitation at the rotor
Operates at lagging p.f only
No excitation required. is
3

K.Rajkumar, Department of Electrical and Electronics Engineering
S.No Remarks
4
5
Economy
Self-starting
Synchronous Motor Induction Motor
Economical for the speed below 300 r.p.m
Economical r.p.m for speed above 600
No self starting. It requires additional
Self-starting
6
7 arrangement.
Construction Complicated
Starting
Torque
More torque
Starting
Simple
Less Starting 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 X d
(Direct axis reactance) and
X q
(Quadrature Axis Reactance) of a synchronous generator.
(8Marks)
The unsaturated values of X d
and X q
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. 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 X sd
and X sq
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
X sd
. The ratio of applied voltage to the maximum value of the armature current gives the the quadrature-axis reactance X sq
. For more accurate determination of these values the oscillogram of the armature current and voltage can be recorded.
X d
 i min
V t
2
X q
 i max
V t
2
Dhanalakshmi Srinivasan Institute of Technology, Samayapuram, Trichy.
5