1
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Why
the induction motor is called as rotating transformer?
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Rotating Transformer
- The power transfers from stator to rotor in the induction motor is same
as that of in the transformer ( from primary side to the secondary side ) but
the rotor of the induction motor rotates.
- Therefore the induction motor is called as rotating transformer.
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2
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What do you mean by the conduction motor and
induction motor?
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Conduction motor
- The power transfer in the armature of the DC motor through brushes and
commutator therefore it is called as conduction motor.
Induction motor
- The power transfers in the induction motor stator to rotor via air gap
or inductively so it is called as induction motor.
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3
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Describe the advantages of induction motor over DC
motor.
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Advantages of induction motor over DC motor
- Simple and rugged construction
- Low cost
- No brushes are required so friction losses are reduced
- Low maintenance
- Spark less operation
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4
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Define : Synchronous speed, Actual speed
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Synchronous speed
- The speed of the stator magnetic field in the three phase squirrel
cage induction motor is called as synchronous speed.
Synchronous speed NS
= 120f / P
f = Supply frequency
P = Number of poles
Actual speed
- The speed of the rotor in the induction motor is called as actual
speed. It is given by N = NS ( 1 – s )
N = Rotor speed
NS =
Synchronous speed and
s = Slip
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5
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Describe the different types of induction motor.
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Types of induction motor
- Squirrel cage induction motor
- Slip ring induction motor
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6
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How squirrel cage induction motor rotor construction
is different from slip ring induction motor rotor?
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Squirrel cage induction motor
- The rotor of the squirrel cage motor consists of copper or aluminum
bars which are placed in the slot.
- The rotor bars are bolted or welded together to the short circuit end
rings.
- It is not possible to insert external resistance in the rotor circuit.
- The starting torque of the squirrel cage induction motor is very low.
- The rotor of the small induction motor is molded and casting in one
piece.
Slip - ring induction motor
- The rotor of the slip – ring induction motor consists of three phase
star connected winding.
- It is possible to insert external resistance to the rotor circuit via
slip rings.
- The starting torque of the slip – ring induction motor can be
increased by means of external resistor.
- However the external resistor does
not affect the maximum and running torque.
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7
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Why rotor slots are kept slight skew to the rotor
shaft in the squirrel cage rotor?
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The rotor slots are not quite parallel to the rotor shaft due to
following reasons
- Motor run quietly by reducing magnetic hum.
- To reduce locking of rotor teeth to the stator teeth.
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8
|
Which type of rotor construction is used in small
rating squirrel cage induction motor?
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Rotor construction
- The small rating squirrel cage induction motor rotor consists of
entire rotor core in the mould and casting all the bars and end – rings in
the one piece.
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9
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Whether rotor winding is wound for two phases or
three phases when the stator winding is wound for two phase supply?
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The rotor is always wound with three phase even when stator is wound
with two phase or three phase
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10
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Describe the position of slip – rings and brushes in
the wound rotor induction motor under running condition?
OR
Explain : The working of the phase wound induction
motor is same as that of Squirrel cage induction motor under running
condition?
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- The slip rings of the wound rotor induction motor in the running
condition are automatically short circuited by means of a metal collar which
is pushed along the shaft and connects all the slip –rings are together.
- The brushes are automatically lifted from the slip – rings to reduce
the wear tear and friction loss.
- Therefore the wound rotor induction motor works as a squirrel cage
induction motor under running conditions.
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11
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Which material is used for slip – ring in the wound
rotor induction motor?
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Phosphorous – bronze
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12
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When the three phase supply is given to stator
winding, resultant flux rotates around the stator at synchronous speed….True
or false. Explain the above statement.
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Revolving flux
- There is no actual revolving of the flux.
- The flux due to each phase
change periodically according to change in the phase current but the magnetic
flux itself does not move around the stator.
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13
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How the rotor does rotate in the three phase
induction motor?
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Rotation of rotor
- When a three phase supply is given to induction motor, a magnetic
field of constant magnitude is produced.
- The magnetic field rotates at synchronous speed.
- The rotor conductor cuts this magnetic field via air gap.
- The emf induced in the rotor according to Faraday’s law of
electromagnetic induction.
- The magnitude of induced emf is directly proportional to relative
velocity between two flux and the conductors.
- The direction of the induced emf is given by Fleming’s right hand
rule.
- As the rotor itself closed circuit, current passing through it.
- According to Fleming’s left hand force act on rotor conductors which
is same as that of direction of stator magnetic field.
- Therefore the rotor rotates is in the same direction as that of stator
magnetic field.
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14
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Give reason : The induction motor rotor speed is
always less than the stator magnetic field.
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Speed of rotor
- If the rotor magnetic field and stator magnetic field rotates same
speed then there is no relative speed between them resulting no rotor emf and
hence no rotor current and no torque.
- Therefore speed of the rotor is always
less than the stator magnetic field.
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15
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What is speed of stator and rotor magnetic speed?
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The stator and rotor field rotates synchronously to each other it means
that they are stationary with respect to each other.
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16
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What is frequency of rotor current in the induction
motor?
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Frequency of rotor current
- The frequency of the rotor current is slip times the supply frequency.
Therefore f ’ = sf
f ’ = Rotor frequency
s = Slip
f = Stator supply frequency
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17
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What is meaning of the starting and running torque
in the induction motor?
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Starting torque
- The torque developed is the induction motor at starting is called as starting
torque of the induction motor.
- It is roughly 1.5 to 2 times that of full load torque.
Running torque
- The torque developed in the induction motor during running condition
is called as running torque.
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18
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Describe the effect of rotor power factor on the
induction motor torque.
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Effect of rotor power factor on rotor torque
- The torque in the induction motor is given by T = kE2I2 Cos Φ2
Cos Φ2 = Rotor power
factor
- As the rotor power factor increases the torque increases and vice
versa
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19
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How the starting torque of the slip – ring induction
motor can be increased?
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The starting torque of the slip – ring induction motor can be
increased by adding external resistance in the rotor circuit by slip – rings.
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20
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Describe the effect of rotor torque in the induction
motor when rotor is ( a ) Purely resistive ( b ) Purely inductive
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Rotor – Purely resistive
- The torque is always positive.
Rotor – Purely inductive
- The backward and forward torque have equal and opposite value
resulting total torque becomes zero.
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