Q.31 What are the various method of measuring slip?
We can measure the slip of the induction motor by following methods.
- The actual measurement of rotor speed
- Measurement of rotor frequency
- Stroboscopic method
Q.32 What is crawling in an induction motor?
Crawling is the phenomenon of running the motor at less than its synchronous speed. The motor may run at 1/7 th its synchronous speed if the motor is lightly loaded. The crawling phenomenon may happen when the motor is operated through variable frequency drive.
Q.33 What is the rotor speed?
The rotor speed is the actual speed of the motor.
Nr = Ns(1-s)
Q.34 What is the stator?
The stator is the stationary part of the induction motor. The stator has three winding placed 120 degrees of electrical apart. The winding placed 120 degrees apart produces a rotating magnetic field in the air gap.
Q.35 What is the rotor?
The rotor is the rotating part of the induction motor. The rotor rotates in the air gap. The flux produced by the stator links to the rotor conductor and induced EMF in it. The rotor EMF sets up the rotor current which interacts with the magnetic field of the stator and produces torque.
Q.36 What are the data required to draw a circle diagram of an induction motor?
The following data are required to draw a circle diagram.
- Block rotor test data
- Stator resistance data
- no-load test data
Q.37. What is the material of slip ring in wound rotor motor?
Slip rings are made of Phosphorous Bronze and steel.
Q.38 What is the condition for maximum torque in the induction motor?
Starting torque will be maximum when
R2 = s X2
R2 = X2 ( slip=1 at start)
The rotor resistance/phase is equal to the standstill rotor reactance/phase.
Q. 39 What are the advantages & disadvantages of Slip ring induction motor compared to squirrel cage motors?
Advantages:
- Low starting current
- High Sr tarting torque
- Smooth acceleration under heavy loads
- No abnormal heating during starting
- Good running characteristics after external rotor resistances are cut out
- Adjustable speed
Disadvantages:
- The initial and maintenance costs are greater than those of squirrel cage motors because of additional part counts.
- The speed regulation is poor when run with resistance in the rotor circuit because of heating in the resistance
Q.40 Why V/f ratio is kept constant when the motor is run through VFD?
The flux in the motor is proportional to the ratio of V/f. The induction motor is a constant flux machine and therefore V/f ratio is kept constant in VFD.
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