Download 1. Explain the constructional details of Alternator in detail with neat

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
Dhanalakshmi Srinivasan Institute of Technology, Samayapuram, Trichy.
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 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.
Vt
Xd 
i min
2
Vt
Xq 
i max
2
5