Three phase Induction motor [best study]|electricaltechguru

Three phase induction motor

Contents in this chapter

• Types of three phase induction motor 

1. Structure basis
2. Speed basis 

Construction

Working

Advantages

Disadvantages

Applications

• Starting method of Induction motor
• Speed control method of Induction motor

Three phase induction motor is widely used motor in industrial and commercial for much more purposes, because of squirrel cage induction motor is having lower maintenance cost, is working is very easy, also it can be use in any kind of environment with lower maintenance.

Related post: single phase inducton motor details

Types of motor on its rotor structure basis.

1. Squirrel cage induction motor

2. Slip ring or rotor wound induction motor

1.squirrel cage induction motor

Basically, this is asynchronous type of induction motor.

In this type of motor stator having a winding and rotor is squirrel cage rotor.In rotor, bars are inserted which is work as rotor winding. This bars are shorted by both sides by end rings on rotor. This type of motor always running with a slip that slip is decide a motor torque with the variation of  load

Squerrel cage induction motor 

Squirrel cage rotor internal bars

Slip=Ns-N
Where,
Ns=synchronous speed (speed of rotating magnetic field)
N=actual rotor speed

Working

how induction motor starts?

When stator winding carry current from external 3 phase source the rotating magnetic field created in air gaps between stator and rotor this will induce EMF in rotor bar or rotor winding according to 'Faraday's law of electromagnetic induction' and these rotor winding will create is own magnetic field which is then interact with stator field.
According to lenz's law force exerted on rotor and rotor start rotate in one direction,
The direction of this type of motor can be reversed by interchanging any of the two phases of 3 phase system.
Speed variation of this motor can be achieved by given formula
Ns=120f/p

Where,
Ns=synchronous speed
F=frequency
P=poles of induction motor

From above formula we can say frequency is directly proportional to synchronous speed, if frequency is reduced then speed of motor will reduced proportionally.
Pole is inversely proportional to synchronous speed, so if pole reduced speed Will increase and  if pole will increase synchronous speed will decrease.

Advantages

1. Low maintenance cost
2. Suitable for any environment

Disadvantages

1. Speed decrease with increasing load.

Applications of squirrel cage induction motor

1. Pump
2. Conveyor
3. Drilling 
4. Belt drives
5. Fan

2. slip ring induction motor

This type of motor having a two windings that is stator winding and rotor winding.
Rotor having slip ring arrangement for energies to rotor, slip rings are used for this are made up of copper and its brushes are connected to its .3 phase rotor wound motor has three slip ring used.

Rotor wound induction motors rotor

This type of motor is rarely use because of maintenance cost is more than the squirrel cage induction motor.

Working

When stator winding connected to 3 phase supply it produces magnetic field,that is rotating magnetic field. due to Faraday's law of electromagnetic induction EMF induced in rotor winding that EMF force to current flow through this rotor winding and due to this rotoflux are created.
Now there are two fluxes, stator flux and rotor flux due to this two fluxes force exerted on rotor and rotor start rotate.

Advantages

1. Initial torque better than squirrel cage induction motor
2. Speed control is easy because of rotor resistance starter can be used.

Disadvantages

1. More maintenance cost

application of slip ring induction motor

1. Cranes
2. Hoist
3. Textile mills
4. Lift

Related post:Working principle of transformer

Types of induction motor basis of speed

1. Synchronous motor
2. Asynchronous motor

1.Synchronous motor

This type of motor is running with its synchronous speed without any slip that's why its called synchronous motor.
Synchronous motor having a 3 phase stator winding and the and winding is required 3phase AC supply and rotor having a winding which is energies with direct current.

Working

how synchronous motor starts?
When we energies stator winding the magnetic field produce, which is rotating in air gap with its synchronous speed between stator and rotor, at this stage rotor cannot self start so external force is required to rotate the rotor near to these synchronous speed. After achieving this speed DC supply of rotor winding switch ON. due to this magnetic locking is done between stator and rotor and motor runs with its synchronous speed. Without any slip.

Advantages

1. Speed is constant.
2. Rpm accuracy

Disadvantages

1. Its not self start. for doing its self start need to some additional arrangements, like damper winding on the same shaft of motor
2. AC and DC both type of power needed.
3. With increasing load humming sound increase.

Applications

1. Constant rpm and constant torque drive
2. Power factor correction as like synchronous condenser
3. Voltage regulation
4. In timing clock due to high accuracy in rpm.

2.Asynchronous motor

Above mentioned squirrel cage induction motor is also called as asynchronous type of motor.

Staring method of three phase inducton motor

1. Autotransformer starter
2. Direct online starter
3. Star delta starter

1. Autotransformer starter

Autotransformer starter circuit diagram

  To reduce starting current and voltage across each winding the this type method is used. Torque of motor also control by this method.
By the auto transformer method induction motor started by increasing voltage from zero to its higher value, here due to slightly increasing voltage motor torque increasing slightly and that's why motor started very slowly.

 2.direct online starter

DOL starter power circuit diagram

DOL starter control circuit diagram

  This method also called as D.O.L. starter. In this method induction motor is directly started in in delta connection.

3.star delta starter

Star delta starter power circuit diagram

 At starting in this method voltage across each winding applied by 1/1.73 time of line voltage .we can say its phase voltage is applied across winding. after archiving rated speed line voltage is applied. This is most widely used.
In star delta starter motor first run in star and then is connected in delta, in Star connection phase voltage applied across the winding of stator and then line voltage is applied across the winding. Because of the phase voltage motor starting at low voltage that’s why torque of the motor is low at starting,

Speed controlling methods

Rotor speed of slip ring induction motor is

Nr=Ns(1-s)
Where,
Nr=rotor speed
Ns=synchronous speed
S=slip

Rotor speed can be controlled by

1. Slip control method
2. Synchronous speed control method

1.slip control method

• Voltage control method: this method is used for Both slip ring and squirrel cage induction motor
• Rotor resistance method: This method used only for slip ring induction motor.
• Rotor emf injection method: This method used only for slip ring induction motor

2.synchronous speed control method

• Frequency control method: used for Both
• Pole changing method: only used for squirrel cage induction motor
• Cascading of two induction motor: in this method one must be slip ring induction motor, and another can both of them.