3-Phase induction motor

 

 Introduction

This article will deal with those concepts of 3 phase induction motor which are essential prerequisite for proper selection, procurement, installation and maintenance of the same. Before any actual discussion on motor is started It will better to have a comparison of starting behavior of induction motor and transformer because as per the equivalent circuit representation a 3 phase induction motor is generalized transformer.

It is assumed that readers are already familiar with the elementary concept of principle of operation and construction of three phase induction motor.

Induction motor is a generalized transformer. Difference is that transformer is an alternating flux machine while induction motor is rotating flux machine. Rotating flux is only possible when 3 phase voltage (or poly phase) which is 120 degree apart in time is applied to a three phase winding (or poly phase winding) 120 degree apart in space then a three phase rotating magnetic flux is produced whose magnitude is constant but direction keeps changing. In transformer the flux produced is time alternating and not rotating.

There is no air gap between primary and secondary of transformer where as there is a distinct air gap between stator and rotor of motor which gives mechanical movability to motor. Because of higher reluctance ( or low permeability) of air gap the magnetizing current required in motor is 25-40% of rated current of motor where as in transformer it is only 2 -5 % of rated primary current.

In an alternating flux machine frequency of induced EMF in primary and secondary side is same where as frequency of rotor EMF depends on slip. During starting when S = 1 the frequency of induced EMF in rotor and stator is same but after loading it is not.

Other difference is that the secondary winding and core is mounted on a shaft set in bearings free to rotate and hence the name rotor.

 If at all secondary of a transformer is mounted on shaft set at bearings the rate of cutting of mutual magnetic flux with secondary circuit would be different from primary and their frequency would be different. The induced EMF would not be in proportion to number of turns ratio but product of turn ratio and frequency. The ratio of primary frequency to the secondary frequency is called slip.

Any current carrying conductor if placed in magnetic field experience a force so rotor conductor experience a torque and as per Lenz’s Law the direction of motion is such that it tries to oppose the change which has caused so it starts chasing the field.

Power flow diagram of induction motor

 

Stator input electrical power = A
Stator losses = B
Rotor losses = C
Mechanical output = P
A – ( B + C ) = P
Roughly B= 0.03A, C= 0.04A
A – 0.07A = P
0.93A = P, Hence efficiency = (P/A) x 100 = 93%

 

 

 

 

 Why LT motors are delta connected and HT motors are star connected?

Reason is techno commercial.

1. In star, phase current is same as line current. But phase voltage is 1/1.732 times line voltage. So insulation required in case of HT motor is less.
   
2. The starting current for motors is 6 to 7 times full load current. So start-up power will be large if HT motors are delta connected. It may cause instability (voltage dip) in case small Power system. In starred HT motor starting current will be less compared to delta connected motor. So starting power is reduced. Starting torque will also be reduced. (It will not be a problem as motors are of high capacity.)
   
3. Also as current is less copper (Cu) required for winding will be less.
   
4. LT motors are delta connected.
1. Insulation will not be problem as voltage level is less.
2. Starting current will not be problem as starting power in all will be less. So no problem of voltage dips.
3. Starting torque should be large, as motors are of small capacity.

 

 LT motors have winding delta connected.

1. In case it is having star delta starter than they are started as Star connected motor.
2. After it attains 80% of synch speed the changeover takes place from star to original configuration delta.
3. In star the voltages across the windings are lesser that is 1/1.732 times that available in delta so current is limited.
4. When it goes to delta again voltage is full line voltage so current increase even though it is lesser than the line current it remains higher than the line current drawn in star connection at reduced voltage. So cables for motor are sized for this current that is what it draws in delta connection.

Working of Three Phase Induction Motor

 Production Of Rotating Magnetic Field

 

 The stator of the motor consists of overlapping windings offset by an electrical angle of 120°. When the primary winding or the stator is connected to a 3 phase AC source, it establishes a rotating magnetic field which rotates at the synchronous speed.

Secrets behind the rotation:
According to Faraday’s law an emf induced in any circuit is due to the rate of change of magnetic flux linkage through the circuit. As the rotor windings in an induction motor are either closed through an external resistance or directly shorted by end ring, and cut the stator rotating magnetic field, an emf is induced in the rotor copper bar and due to this emf a current flows through the rotor conductor.

Thus from the working principle of three phase induction motor it may observed that the rotor speed should not reach the synchronous speed produced by the stator. If the speeds equals, there would be no such relative velocity, so no emf induction in the rotor, & no current would be flowing, and therefore no torque would be generated. Consequently the rotor can not reach at the synchronous speed. The difference between the stator (synchronous speed) and rotor speeds is called the slip. The rotation of the magnetic field in an induction motor has the advantage that no electrical connections need to be made to the rotor.

Thus the Three Phase Induction Motor is:
• Self-starting.
• Less armature reaction and brush sparking because of the absence of commutators and brushes that may cause sparks.
• Robust in construction.
• Economical.
• Easier to maintain.


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