Motor-INDUCTION MOTORS

INDUCTION MOTORS AND SPECIAL MACHINES

 Describe the main parts of a single phase induction motor-

The single phase induction motor has two main parts.  They are the stator and the rotor.  The stator is made up of laminated sheets and has slots on the inner periphery to carry the stator winding or the main winding.  The main winding is excited by a single phase ac supply.  The rotor is of squirrel cage type which has number of conducting bars short circuited at the two ends using end rings.

What is synchronous speed?-

The speed at which the rotating magnetic field rotates is called the synchronous speed Ns, which is given by,

                              

      where f is the frequency of ac supply and P is the number of poles on stator.

 What is slip of an induction motor?  Give its importance-

The ratio of the difference between the synchronous speed and the normal running speed of the induction motor to the synchronous speed is called the slip of an induction motor.

                              

State double field revolving theory-

The double field revolving theory states that, any alternating quantity can be resolved into two rotating components, which rotate in opposite directions and each having a magnitude as half of the maximum magnitude of the alternating quantity.

What is the direction in which the rotor of single phase induction motor rotates?  Give reason-

The rotor of a single phase induction motor rotates in the direction of the rotating magnetic field. 

The cause producing the rotation in the rotor is the relative speed between the rotating magnetic field and the rotor.  According to Lenz’s law the rotor rotation will oppose the cause, hence it rotates in the same direction as the rotating magnetic field so that the relative speed can be reduced.  But the rotor can never rotate at synchronous speed because if the rotor rotates at synchronous speed the relative speed which is the cause producing the rotation will become zero and the motor will come to halt.

What is the necessary condition to obtain a rotating magnetic field?

To obtain the rotating magnetic field there should be at-least two alternating fluxes present.  The resultant of two or more alternating fluxes will be a rotating magnetic field.

How can we obtain rotating magnetic field in a single phase induction motor?

We can obtain rotating magnetic field in a single phase induction motor by introducing an auxiliary winding called the starting winding which produces additional alternating flux at some phase angle.

Why is the single phase induction motor not self starting?

The single phase induction motor is not self starting because of the absence of a rotating magnetic field.  Since it is operated on single phase, there exists only one flux which is alternating in nature.  But for the motor to start there should be a rotating magnetic field.

What are the types of single phase induction motors?

There are four main types of single phase induction motors.  They are,

a)    Split phase induction motor

b)    Capacitor start induction motor

c)    Capacitor start capacitor run induction motor

d)    Shaded pole induction motor

Why is the starting torque of a capacitor start induction motor high, when compared to that of a split phase induction motor?

The starting torque of the single phase induction motor is directly proportional to the phase angle between the currents carried by the two windings, namely the main winding and the starting winding.  Split phase induction motor uses a highly resistive starting coil, which gives the phase angle nearly 90°.  But in capacitor start induction motor, the phase angle obtained is greater than 90°.  Hence the starting torque is still higher in this type.

What is a stepper motor?

Stepper motor is an electromechanical device which actuates a train of step movements of shaft in response to train of input pulse.  The step movement may be angular or linear.

What is a step angle?  How is it calculated in a variable reluctance stepper motor?

The angle through which the motor turns or shaft moves for each pulse is known as the step angle, expressed in degrees.

The step can also be defined as the angular displacement of the rotor in response to each input pulse.  The step angle in variable reluctance stepper motor is given by,

                              

where, q is the number of phases and N_r is the number of rotor poles.

What are the types of stepper motors?

 The various types of stepper motors are,

a)    Variable reluctance stepper motors

b)    Permanent magnet stepper motors

c)    Hybrid stepper motors

How will you calculate the number of poles required in the rotor of a variable reluctance stepper motor?

The number of poles required in a rotor is,

                              

      where, N_s is the number of poles in the stator and q is the number of phases.

Give the expression for Torque developed in a variable reluctance stepper motor-

                  The torque developed in a variable reluctance stepper motor is given by,

                             

               

where L is the inductance of the relevant phase at an angle q. 

Define rotor tooth pitch in a variable reluctance stepper motor-

The distance through which the rotor moves when all three phases are excited once is called one rotor tooth pitch.(RTP)

                               

Give the advantages of a variable reluctance stepper motor-

 The advantages of a variable reluctance stepper motor are,

a)    High torque to inertia ratio

b)    High rates of acceleration

c)    Simple and low cost machine

d)    Fast dynamic response

e)    Rotor construction is robust

List out the applications of stepper motors-

The applications of stepper motor are,

a)    Compatible to digital circuit, hence widely used in computer peripherals such as printers, Floppy disc drives, etc.

b)    Numeric control machine tools

c)    Robotic control systems

d)    Watches

Define Holding torque, Critical torque and limiting torque-

The holding torque is defined as the maximum static torque that can be applied to the shaft of an excited motor without causing a continuous rotation.

The critical torque is defined as the maximum load torque at which rotor does not move when an exciting winding is energized.  This is also called pullout torque.

The limiting torque is defined as the maximum load torque at which motor follows the control pulse without missing any step.  This is also called pull in torque.

What is slewing rate?

The slewing rate is defined as the maximum rate at which the motor can step uni-directionally.