Three phase induction motor

              Three phase induction motor

Introduction

Three phase induction motor is the most popular type of AC motor it is very commonly used for industrial drive since it is cheap, robust, efficiency and reliable. It has good speed regulation and high starting torque. It required little maintenance. It has a reasonable overload capacity.

Construction

Three phase induction motor essentially consist of two part the stat or and rotor. The stat-or is the stationary part and the rotor is the rotating part. The stat-or is build up of high grade alloy Steel lamination to reduce Eddy current losses. The lamination are slaughtered on the inner periphery and are insulator from each other. This lamination's are supported in a state of frame of cast iron are fabricated steel plate the insulated stat-or conductor placed in this loss. The stat-or conductors are connected to form a three phase winding. The phase winding may be either star or delta connected.

The rotor is also build up of thin lamination of the same material as status. The laminated core is mounted directly on the side of a spider carried by the Smurfs. This lamination are slaughtered on their outer periphery to receive the road per conductor. There are two types of induction motor rotor

1.   1. Squirrel cage rotor of simpler cage rotor

2.   2. Face bond or ground road police Stop motor using a type of water also called slip ring motor.

Comparison of cage and bound rotors

The advantage of the cage rotor are as follow:

1.     1. Robust construction and cheaper

2.     2. The absence of brushes reduce the risk of sparking

3.    3.  Lasser maintenance

4.     Higher efficiency and higher power factor.

1. The found rotor have the following merits :

1.    2.  High starting torque and low starting current

2.     3. Additional resistance can be connected in the rotor circuit to control speed.

Production of rotating field

when three phase winding this place in space by 120 degree are supplied by three phase current display in time by 120 degree a magnetic flux is produce which rotate in space.

Parts of 3 phase induction motor

1.    1.  Sta-tor

2.     2. Rot-ar

3.     3. Shaft

4.    4.  Bearings

5.     5. Fan

6.     6. Terminal box

Advantage of 3 phase induction motor

1.    1.  Its construction is very simple

2.     2. Its cost is low and is very reliable

3.    3.  It can produce sufficient torque

4.     4. Minimum maintenance

5.    5.  In running normal no processes are required and hence friction losses are reduced.

6.    6.  It has higher efficiency.

7.    7.  It has good power factor

8.     8. Speed control by using thyristor.

Disadvantage of three phase induction motor

1.     1. It has low starting torque

2.     2. Speed control is difficult

Application

1 lathes

2 drilling machine

3 fans

4 blowers

Principle of operation of three phase induction motor

Latest consider of one conductor on the stationary rotor let this conductor be subjected to the rotating magnetic field produced when three phase supply is connected to the three phase-winding of the stat-or. Light the rotation of the magnetic field b clockwise. A magnetic field moving clock wise has the same effect as a conductor moving anticlockwise in a stationary field. By Faraday's law of electromagnetic induction, a voltage will be induced in the conductor. Since the rotor circuit is complete, either through the and rings or external resistance the induced voltage current to flow in the rotor conductor. By right hand rule we can determine the direction of induced current in the conductor. Since the magnetic field is rotating clockwise, and the conductor is stationary we can assume that the conductor is in motion in the anticlockwise direction with respect to magnetic field. By right hand rule the direction of the induced current is outward as given hand rule the direction of the induced current is outward show by Dot as given the current in the rotor conductor produces its on magnetic field.

We know that when a conductor carrying current is put in a magnetic field a force is produced on it.does, a force is produced on the router conductor. The direction of the force can be found by left hand rule.it is seen that the force acting on the conductor is in the same direction as the direction of the rotating magnetic field. Since the rotor conductor is in a slot on the circumference of the rotor, this force act in a tangent equal direction to the rotor and develop a torque on the rotor.similar talk are produced on all the rotor conductor since the router is free to move it is start rotating in the same direction as the rotating magnetic field. Does, three phase induction motor is self starting. Since the operation of this motor depend upon the induced voltage in its rota conductors, it is called an induction motor.

Slip and speed

induction motor cannot run at synchronous speed. Latest considered for a moment that is rotor is rotating at synchronous speed. Under the condition there would be no cutting of flux by the rotor conductors, and there would be no generate voltage, no current and not torque. The rotor speed is there for slightly less than the synchronous speed. And induction motor may also be called as a synchronous motor as it does not run at synchronous speed. The difference between the synchronous speed and the actual rotor speed is called the slip speed.

Condition for torque production

1.     1. Two magnetic field present in the machine

2.     2. Both magnetic field having same displacement

3.     3. Both are stationary with respect to each other

4.     4. Both having same 

magnitude

 

 

 

 

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