Some important questions:
A. Fill in the blanks
1.Single phase induction motor is _______ starting. Ans: Not
self starting.
2. Pull out torque is the ______torque devolved without losing
synchronism. Ans: Maximunm
3.Synchronous motor runs _____speed. Ans: Synchronous
4. In a synchronous motor, if field excitation is increased
then armature current _______ Ans: First decreased then increased.
5. The direction of rotation of three phase induction motor
can be reversed by_______ Ans: By changing phase sequence.
6 Table fan is the
application of _____type single phase in.
Ans:Shaded pole
7. Servo motor can operate in _______.. Ans: AC ,DC both
8. A synchronous motor is a ______excited machine. Ans: Doubly.
10.Running torque is determined by ________ . Ans: Power rating
and speed
11. The resultant flux setup by a three phase winding makes
____ revolutions per cycle. Ans: One.
12. The percentage full load sleep in case of a synchronous
motor is ________. Ans : 0. It is a constant speed motor.
13. The synchronous motor operating under excited will have
____ power factor. Ans : Lagging..
14. If the air gap of the induction motor is increased the
magnetising current of the motor will ____ . Ans : Increased.
B. Multiple choice
type question
1. Inverted V-curve is the relation between a) Armature current and
field excitation. b) Power factor and field excitation. C) Speed and
armature current d) Speed and power factor.
2. Pitch factor of a full pitch coil alternator is a) 1 b) 0.5 c) half d)2
3.Synchronous motor can operate wide range of ) leading b) lagging
c)both d)only unity power factor.
4. The speed of synchronous motor depends on ) load b) excitation
c) both d) independent of load.
5. Distribution factor is always a) <1 b)>1 c)=1 d) 2
6. In capacitor start motor capacitor is connected in
a)series b)parallel c) series -parallel d)inductor is connected
7.The two winding of a capacitor start motor are displaced
in a)90 b)0 c) 180 d) 360 degree in space.
C. Answer any two
1. What is hunting, V- curve and inverted V-curve of
synchronous motor.Draw the V-curve.
2.Explain pull in torque and pull out torque of synchronous
motor.
3. What are the reason of parallel operation of alternator?
What are the conditions of parallel operation?
4.Mention application of servo motor, shaded pole motor and capacitor
start single phase induction motor. Draw
the circuit diagram of single phase induction motors.
5.Explain cogging and crawling of induction motor.
6. Discuss effects of varying field current on synchronous
motor.
7. Write a short note on servo motor.
Ans: 1.
a. Hunting- The phenomenon of oscillation of the
rotor about its final equilibrium position is called hunting or phase swinging.
b.
V-curve: The curve drawn between armature
current and field current for different constant loads are known as V curves .
c.
Inverted V – curve: the curve drawn between
power factor and field current for different constant loads are known as
inverted V-curves.
Ans 2.
Pull in Torque: A synchronous motor is started as induction
motor till it runs 2 to 5 per cent below the synchronous speed. The dc.
excitation is then applied and the rotor pulls into step with the synchronously
rotating stator field.
The pull-in torque is the maximum constant torque at rated
voltage and frequency under which a motor
will pull a connected load into synchronism when the d c.
motor excitation is applied.
Pull out torque: It is the maximum value of torque which a
synchronous motor can develop at rated voltage and frequency without losing
synchronism .
Ans 3.
Reason of parallel operation:
I) To maintain the continuity of the supply to the load
ii) To serve the load increasing demand of the consumer
iii) To reduce the cost and cost of energy generation.
Conditions:
1. The phase sequence of the busbar voltages and the
incoming machine voltage must be the same.
2. The busbar voltages and the incoming machine terminal
voltage must be in phase.
3. The terminal voltage of the incoming machine should be
equal to that of the alternator with which it is to be run in parallel or with the
busbar voltage.
4. The frequency of the generated voltage of the incoming
machine must be equal to the frequency of the voltage of the live busbar.
Ans:4:
Applications of motor:
Servo moto- Radars,robots
Shaded pole Single phase induction motors: Table fans,record
player,tape recorder.
Capacitor Start Single phase induction motors:
Circuit diagram of capacitor start single phase induction
motors.
Ans.5:
Cogging:
Sometimes, even when full voltage is applied to the stator
winding, the rotor of a 3-phase cage induction motor fails to start. This
phenomenon of magnetic locking between stator and rotor teeth is called cogging
or teeth locking.
. In order to reduce or eliminate cogging the number of
stator slots are never made equal to or have an integral ratio. Cogging can
also be reduced by using skewed rotor.
Crowling:
The tendency of the motor to run at a stable low as one-seventh of the normal speed N, and being
unable to pick up its normal speed is
known as crawling of the induction motor.
Crawling can be reduced by reducing 5th and 7 . This can be done by using a chorded or short pitched winding.
DIFFERENT TORQUES IN A SYNCHRONOUS MOTOR The following
torques are considered in the selection of a synchronous motor for a particular
application:
1. Locked-rotor torque
2. Running torque
3. Pull-in torque
4. Pull-out torque
Locked Rotor Torque
It is the minimum torque at any angular rotor position that
is developed with the rotor locked (that is, stationary) and rated voltage at
rated frequency is applied to the terminals. This torque is provided by the
stator windings
Running Torque
It is the torque developed by the motor under running
conditions. It is determined by the power rating and speed of the driven
machine.
Pull-in torque
A synchronous motor is started as induction motor till it
runs 2 to 5 per cent below the synchronous speed. The dc. excitation is then
applied and the rotor pulls into step with the synchronously rotating stator
field. The pull-in torque is the maximum constant torque at rated voltage and
frequency under which a motor will pull a connected load into synchronism when
the d c. motor excitation is applied.
Pull-out torque
It is the maximum value of torque which a synchronous motor
can develop
at rated voltage and frequency without losing synchronism
Parallel operation of alternators:
REASONS OF PARALLEL OPERATION
Alternators are operated in parallel for the following
reasons:
1. Several alternators can supply a bigger load than a single
alternator.
2. During periods of light load, one or more alternators may
be shut down, and those remaining operate at or near full load, and thus
more efficiently. 3. When one machine is taken out of
service for its scheduled mainte nance and inspection, the remaining machines
maintain the continuity of supply.
4. If there is a breakdown of a generator, there is no
interruption of the power supply.
5. In order to meet the increasing future demand of load
more machines can be added without disturbing the original installation.
6. The operating cost and cost of energy generated are
reduced when
several generators operate in parallel Thus, parallel
operation of alternators ensures greater security of supply and enables overall
economic generation.
CONDITIONS NECESSARY FOR PARALLELING ALTERNATORS
Most synchronous machines will operate in parallel with
other synchronous machines and the process of connecting one machine in
parallel with another machine or with an infinite busbar system is known as
synchronizing. Those machines already carrying load are known as running
machines, while the alterna tor which is to be connected in parallel with the
system is known as the incoming machine. Before the incoming machine is to be
connected to the system, the follow ing conditions should be satisfied:
1. The phase sequence of the busbar voltages and the
incoming machine voltage must be the same.
2. The busbar voltages and the incoming machine terminal
voltage must be in phase.
3. The terminal voltage of the incoming machine should be
equal to that of the alternator with which it is to be run in parallel or with the
busbar voltage.
4. The frequency of the generated voltage of the incoming
machine must be equal to the frequency of the voltage of the live busbar.
Note about question 5:
COGGING OR MAGNETIC LOCKING
Sometimes, even when full voltage is applied to the stator
winding, the rotor of a 3-phase cage induction motor fails to start. This
happens when the number of stator and rotor slots are either equal or have an
integral ratio. With the number of stator slots equal to or an integral
multiple of rotor slots, strong alignment forces are produced between stator
and rotor at the instant of starting. These forces may create an alignment
torque greater than the accelerating torque with consequent failure of the
motor to start. This phenomenon of magnetic locking between stator and rotor
teeth is called cogging or teeth locking.
The reluctance of the magnetic path is minimum wher, the
stator and rotor teeth face each other. Under this condition there is a
magnetic locking between stator and rotor teeth. In order to reduce or
eliminate cogging the number of stator slots are never made equal to or have an
integral ratio. Cogging can also be reduced by using skewed rotor.
Cogging and crawling are much less prominent in wound rotor
motors because of their higher starting torques. RP.
Crawling:
At a stage the motor
will not accelerate upto its normal speed but running at a speed which is nearly (1/7) of
its normal speed . This tendency of the motor to run at a stable low as one-seventh of the normal speed N, and being
unable to pick up its normal speed is
known as crawling of the induction motor.
Reduction of crawling:
Crawling can be reduced by reducing 5th and 7 . This can be done by using a chorded or short pitched winding.
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