STARTINGANDSPEED CONTROL OF
THREEPHASEINDUCTION MOTOR
1. What are the types of starters?
Stator
rheostat, Autotransformer Star to Delta starter and rotor resistance starter.
2. List out the methods of speed control of cage type
3-phase induction motor? a) By changing supply frequency
b) By
changing the number of poles c) By operating two motors in cascade
3. Mention
different types of speed control of slip ring induction motor?
e) By
changing supply frequency
f) By
changing the number of stator poles
g) By
rotor rheostat control
h) By
operating two motors in cascade
4.State the advantages of capacitor start run motor
over capacitor start motor.
Running
torque is more; Power factor during running is more.
5. Explain why single-phase induction motor is not
self-starting one.
When the
motor is fed from a single phase supply its stator winding produces an
alternating or pulsating flux, which develops no torque which is explained in
Double revolving field theory..
6.What type of motor is used for ceiling fan?
Capacitor
start and capacitor run single-phase motor is used for ceiling fans.
7.What is the type of induction motor used in wet
grinders?
Capacitor
start capacitor run single-phase induction motor.
8.What kind of motor is used in mixie?
Single-phase
ac series motor is used in mixie.
9.What is the application of shaded pole induction
motor?
Because
of its small starting torque, it is generally used for small fans, toys,
instruments, hair driers, ventilators, electric clock etc.
10. In which direction does a shaded pole motor
run?
The rotor
starts rotation in the direction from unshaded part to the shaded part.
11.Why single-phase induction motor has low power
factor?
The
current through the running winding lags behind the supply voltage by a very
large angle. Therefore power factor is very low.
12.Diffrentiate between “capacitor start “and
“capacitor start capacitor run “induction motor?
In
capacitor start motor, capacitor is connected in series with the starting
winding. But it will be disconnected from the supply, when the motor picks up
its speed. But in capacitor start
capacitor
run motor the above starting winding and capacitor are not disconnected, but
always connected in the supply .so it has high starting and running torque.
13.
State the
application of an induction generator?
Used in
windmill for generating electric power. Used in regenerative breaking places
like traction.
14.
What do
you mean by residual EMF in a generator.
The EMF
induced in the armature conductor only due to the residual flux in the field
poles is known as residual EMF
15.
State the
effect of rotor resistance on starting torque?
Starting
torque increases with increase in value of rotor resistance.
16.
How can
varying supply frequency control speed?
We know
that Ns = 120f / P
From the
equation it is clear that by varying frequency speed can be varied it is vary
rarely.
17.How is speed control achieved by changing the
number of stator poles?
Here
change in stator poles is achieved by having two or more independent stator
windings in the same slot. Each winding gives different number of poles and
different speeds. At a time only one winding is used and other is closed.
18.What are the main disadvantages of rotor
rheostatic control?
Ø The speed
can be decreased by increasing the rotor resistance, but increases I2R loss and
hence decreases efficiency.
Ø Speed
depends on load also and so used for small periods only.
19.What are the methods of speed control preferred
for large motors?
Ø Kramer
system
Ø Scherbius
system
20.What is an induction regulator?
An
induction regulator is used to obtain the constant voltage at the feeder end.
Varying the range between the magnetic axes of the primary and secondary
windings controls the voltage; it may be a single phase. Rotor is moved usually
by a maximum of 180 degree.
21.Define-Slip frequency.
The
relation motion of the stator flux and the rotor conductors induces the voltage
of frequency Sf called slip frequency.
22.Define- Asynchronous torque.
When
stator and rotor fields are stationary with respect to each other, a steady
torque is produced and rotation is maintained. Such a torque existing at any
mechanical speed other than
synchronous
speed is called as an asynchronous torque.
23.What is the main use of squirrel cage winding in
synchronous motor starting?
When a
squirrel cage winding called the amortissuer or damper winding is inserted in
the rotor pole faces, the rotor comes up to the synchronous speed by induction
motor action with the field winding unexcited.
24.What is breakdown torque?
From the
torque verses slip charactertics, we can infer that as the torque increases,
slip increases upto a maximum torque developed is called a breakdown torque.
25.What is the function of rotary converter? Where
it is used?
Rotary
converter converts low slip ac power. It is used in Kramer system, which is for
the speed control of three-phase induction motor.
26.What are the advantages of Kramer system of
speed control?
Any speed
with in the working range can be obtained
When
rotary converter is overexcited, it will take leading current, compensates with
the lagging current drawn by the motor, thus improving power factor.
27.Write the expression for concatenated speed of
the set.
Pa – no
of poles of motor A
Pb – no
of poles of motor B
28.Canthe starting torque of of a slip ring
induction motor being increased?
Yes. It
can be increased by adding resistances to the rotor.
29.What would happen if a3 phase induction motor is
switched on with one phase disconnected?
The motor
is likely to burn .
30.What happens if the air gap flux density in an
induction motor increases?
The
increase in air gap flux increases iron loss and hence efficiency decreases.
31.State the advantages of skewing?
Itreduces
humming and hence quite running of motor is achieved. Itreduces magnetic
locking of the stator and rotor.
32.State the condition at which the starting torque
developed in a slip-ring induction motor is maximum.
When
R2=X2
33.What are the effects of increasing rotor resistance on starting
current and starting torque?
The
additional external resistance reduces the rotor current and hence the current
drawn from the supply. It improves the starting torque developed by improving
the power factor in high proportion to the decrease in rotor current.
34.What is slip of an induction motor?
The slip
speed expressed as the ratio of synchronous speed is defined as slip.
Percentage
slip S=Ns-N/Ns*100
35.Howthe magnitude of rotor emf is related to the
slip in an I M?
Rotor
circuit emf per phase E2r=SE2
36.Howthe frequency of rotor emf is related to the
slip in an I M?
Frequency
of rotor emf/current fr=Sfs
37.What is the normal value of slip of an I M
operating at full load?
3 - 5%
38.Why is not possible for the rotor speed of an IM
to be equal to the speed of its rotating magnetic field?
The
machine will not be able to develop any mechanical torque to run as a motor.
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