POLY PHASE INDUCTION MOTOR

• Before to start of polyphase induction motor, we will learn introduction about three-phase induction motor.

• Three-phase induction motor is the most widely used ac motor in an industrial and commercial application. The three-phase induction motor can be considered among the most reliable electrical machines. These motors find their application in the most differential sector such as food, chemical textiles, metallurgical and paper industries. 
  
  
  
• Thee motor is widely used for most of the industrial application such as centrifugal pump, conveyers, compressor, crusher,s, punch press etc. In fact, three-phase induction motors find their application everywhere from a small workshop to large industry.

• Following are the main advantages of three-phase induction motors:

1. Simple and rugged construction

2. Required little maintenance

3. Good operating or performance characteristics such as power factor and high efficiency

4. Don’t have commutator so, low maintenance required.

5. Low cost

6. Very efficient, robot and reliable

7. Self-started motor

8. It’s a single excited machine

• This three-phase induction motor is called an asynchronous motor. As it does not run at asynchronous speed. It always runs at a speed slightly less than the synchronous speed. 

• Varies types of AC induction motors are available in the market. In particular, the squirrel cage design is the most widely used electric motor in industrial applications. 
• Although AC induction motors are easier to design than DC motors.

• We can classification of AC motors by base on speed, operation and nature of current.

• Construction of three-phase induction motors:

A three-phase induction motor consists of two main parts,

1. Stationary Part,(Stator)

2. Rotating Part (Rotor)

1. Stationary Part (Stator)

The stator is the stationary part of the three-phase induction motor. It consists of stator core & 3-phase AC distributed winding.

The stator core is made of laminated stampings with slots on its circumference to receive the windings. These stampings are made up of high-grade silicon steel lamination.

This stator core is laminated and the thickness of lamination is kept minimum possible to reduce the eddy current losses. The thickness of lamination maybe 0.35mm - 0.5mm.

The stator has 3-phase winding and its supplied from 3-phase, 50 Hz, AC supply. This stator winding may e either on star or delta connected. It is wound for a definite number of poles, greater the number of poles, lesser the speed.

2. Rotating Part (Rotor)

      The rotor is rotating part of three-phase induction motor. There are 2 types of rotor construction is used for 3-phase induction motor.

1. Squirrel cage rotor

2. Wound rotor

(a) Squirrel Cage Rotor

Motors using this type of rotor are called cage type or squirrel cage

induction motors. Almost 90% of induction motors are provided

with squirrel-cage rotor because of its very simple, robust and most

rugged construction.

The squirrel cage rotor has copper bars placed parallel in rotor stampings. The

copper bars are short-circuited at both ends by solid end rings made

of copper or brass. The induction motor with squirrel cage rotor has

good running performance, however, its starting torque is low.

The copper bars are short-circuited at two ends and hence it is not

possible to add or insert any external resistance in series with the

rotor circuit for starting purpose.

• The rotor slots are generally skewed. The skewing of rotor bars offer

the following advantages

1. reduce the noise and vibrations during operation.

2. minimize the tendency of cogging (magnetic locking between

stator and rotor)

3. reduce the torque defects or to obtain a uniform torque.

The resistance of the squirrel cage rotor is fixed and small and hence

the starting current is very large and its starting torque is low.

(ii) Wound rotor

Motors using this type of rotor are called Phase wound' or Wound
motors' or slip ring induction motors. Fig.1.3 shows the wound rotor
along with slip rings mounted on the shaft.
The rotor is wounded with insulated winding. He wound rotor consists of the rotor core and 3-phase stator connected winding. The number of rotor slots is normally lesser than the stator slots and fewer turn per phase are used for the rotor winding.

The wound rotor has three-phase stator connected winding whose terminal is connected to three-phase respective slip ring mounted on the shaft. The starting torque of slip ring induction motor a be increase by improving power factor with a reduction in starting current by connecting external resistance in the rotor circuit. This external resistance is cutout and bypass during the normal running condition.

Other parts of three-phase induction motors:-
1, Frame:- Function_ to support the stator core and winding & protect the inner part of the winding.

2. Shaft:- The rotor of the induction motor is mounted on a shaft which is made of mild steel.

3. Slip Ring:- slip ring are used in wound rotor induction motor. Slip rings are made of phosphor bronze.

4. Bearing:- the function of earing is to support the rotor of the induction motor and reduced the friction.

5. Fan:- Light aluminium fan use for circulation of cooling air and its fix at the end of the shaft.

6. Terminal box:- there is a total of 6 number of terminals in the terminal box.

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