[Q:-01]
What is a single phase a.c. motor ?
Ans
:
The motor which works on a.c. single phase
supply is known as single phase a.c. motor. It has only one set of winding on
the stator.
[Q:-02]
Why are single phase motors not self-starting ?
Ans
:
Self-starting rotational torque in a
stationary rotor is produced by the rotating magnetic field of the stator.
The stator field produced in single phase
motor is only alternating but is not rotating as in the case of two or three
phase stator winding.
The pulsating field flux is incapable of
producing a rotational torque in a stationary rotor. That is why the single
phase motors are not self-starting.
[Q:-03]
How are a.c. single phase motors made self-started ?
Ans
:
To make it self-started an auxiliary
(starting) winding is provided in the stator frame connected parallel to the
main motor winding to form a second phase with some phase difference which
produces a field with its poles between the main poles. The effect of these two
fields is the production of a rotating magnetic field which makes the motor
self-started.
[Q:-04]
What are the main difference between a single phase induction motor and a three
phase induction motor ?
Ans
:
(i) Three
phase induction motor is a self-starting motor whereas single phase induction
motor has no starting torque.
(ii) For the
same output and speed a single phase induction motor has lower efficiency and
power factor than a three phase induction motor.
(iii) For the
same output and speed a single phase induction motor is heavier than a three
phase induction motor.
(iv) A single
phase induction motor requires always a starting device whereas a three phase
induction motor can start without any starting device.
[Q:-05]
What are the different types of a.c. single phase motors ?
Ans
:
(A) Induction
type—
(1) Split
phase.
(i) Resistance start type.
(ii) Capacitor type—
(a) Capacitor start.
(b) Capacitor start-Capacitor run.
(c) Capacitor run.
(iii) Shaded pole motor.
(2) Commutator
type—
(i) A.C. series Motor or Universal motor.
(ii) Repulsion motor—
(a) Plain Repulsion motor.
(b) Repulsion-Induction motor.
(c) Repulsion start Induction run
motor.
(3)
Synchronous type-
(i) Reluctance motor.
(ii) Hysteresis motor
[Q:-06]
What is a split phase motor ?
Ans
:
It is a single phase induction motor. It
has two windings on the stator, one is starting winding and the second is main
or running winding. The two windings are placed 900 electrical apart
and are connected in parallel across the supply. The rotor itself is of the
squirrel cage type. Due to phase difference in the currents of the two windings
a rotating magnetic field is set up thereby creating a rotational torque on the
rotor.
[Q:-07]
Why are two windings provided in the stator of split phase motor ?
Ans
:
To split up one phase into two phases in
order to produce a rotating magnetic field.
[Q:-08]
What is the high resistance winding in a single phase motor ?
Ans
:
Starting
winding.
[Q:-09]
Why does the starting winding have less turns than the main winding ?
Ans
:
To reduce the inductive reactance and to
allow more current.
[Q:-10]
How can the starting winding and the running winding be identified ?
Ans
:
The starting and running windings can be
identified with the help of a test lamp connected in series with the windings.
When a low voltage supply is applied to the winding-lamp series combination the
high resistance starting winding will give dim light and the running winding
will give comparatively bright light.
[Q:-11]
Does a split phase motor have a high or low starting torque ?
Ans
:
It has low
starting torque.
[Q:-12]
Where and why is a centrifugal switch used in split phase motor ?
Ans
:
Centrifugal switch is fitted on the shaft
of the rotor and electrically connected in series with the starting winding in
order to disconnect this winding automatically when the motor attains 90% of
its full load speed.
[Q:-13]
What will happen if the starting winding of single phase motor does not cut off
during running at full load speed ?
Ans
:
The motor will draw excessive current
giving a humming noise and the starting winding may be burnt out due to
excessive heating.
[Q:-14]
What types of windings are used in split phase motor ?
Ans
:
Double layer
chain or concentric type windings are used.
[Q:-15]
How can you reverse the direction of rotation of split phase motor ?
Ans
:
By interchanging the connections extremely
to either the main or the starting winding on the stator.
[Q:-16]
Why are split phase motors called resistance start motor ?
Ans
:
Because it has
a high resistance starting winding.
[Q:-17]
Why is a split phase motor called dual voltage motor ?
Ans
:
The two stator windings are so arranged
that the motor may work on either 110 volts or 220 volts. That is why it is
called dual voltage motor.
[Q:-18]
What are the uses of split phase motors ?
Ans
:
Split phase motors are used for constant
speed Operation even under varying load condition where moderate torque is
desirable e.g. Fans, Blowers, centrifugal pumps, Refrigerators etc.
[Q:-19]
What is a capacitor start motor ?
Ans
:
It is an a.c. single phase motor having
two windings as in split phase motor. One heavy duty electrolyte capacitor is
connected in series with the starting winding in order to produce the phase
difference between the currents windings and thereby producing starting torque.
As soon as the in the two motor reaches 90% of its full load speed the
capacitor and starting winding are out of the circuit and the pulsating field
is enough to maintain the speed of the motor.
[Q:-20]
Why and where is a capacitor used in the single phase a.c. motor ?
Ans
:
A capacitor is used in series with the
auxiliary starting winding to give a phase displacement in between starting and
running winding thereby improving starting torque.
[Q:-21]
Why a single phase capacitor motor will not start if the capacitor gets punctured
?
Ans
:
The phase difference between the magnetic
field of starting and main motor winding will not exist. As a result rotating
magnetic field will not be produced to make the motor self-started.
[Q:-22]
Does the capacitor start motor have a high or low starting torque ?
Ans
:
The capacitor start motor has a high
starting torque.
[Q:-23]
What will be the effect on starting torque if the capacitance of a capacitor
start motor is reduced ?
Ans
:
Starting
torque will be reduced.
[Q:-24]
What type of rotor is used in capacitor start motor ?
Ans
:
Squirrel cage
rotor.
[Q:-25]
How can the direction of rotation of capacitor start motor be reversed ?
Ans
:
By reversing either the running or starting
winding leads to the supply lines.
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