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Rating of 3-phase Induction Motors & Standard Class types of 3- phase Squirrel cage Induction Motors

Rating of 3-phase Induction Motors 


Three - phase Induction Motors are rated of power output in kw, speed, Voltage, frequency, phase ( single or three ), line Current and Temperature rise in a specified time.

If motor is designed to be operated on more than one voltage or at more than one speed by reconnecting the winding, a Connection diagram is also often given on the name plate. 

Class of motors ( A, B, C, D, E or F ) is also to be mentioned in the name plate.


Standard Class types of 3- phase Squirrel cage Induction Motors

These motors are available in a range of standard Rating up to 150 kw at various standard frequencies, Voltages and speeds in order to meet the usual Requirements of industry. According to their Electrical characteristics, such Induction Motors types are divided into six types as given below ; 

Standard types 1) Class A Motors
         
                             2) Class B Motors

                             3) Class C Motors
                        
                             4) Class D Motors

                             5) Class E Motors 

                             6) Class F Motors


Induction Motor Rating
Standard Induction Motors



1) Class A Motors

These motors have 

            a) low resistance and  

            b) low Reactance and 

            c) low slip  
           
             But high efficiency full load.

The high starting Current (5 to 8 times full -load Current at rated Voltage) is the main drawback. Such as Motors are used in small Rating for machine tools, centrifugal pumps, fans, blowers etc.


2) Class B Motors

These motors have

                              a) high Reactance. 

This Design is common in the 5- 150 kw range. 

Such Motors have largely replaced class A class motors for new installations as its running characteristics are quite similar to those of Class A Motor but have smaller starting Current ( around 5 times the full - load Current at rated Voltage ).

Note :- Class A Motor & Class B Motors similar small Starting Current both are same. 


3) Class C Motors 

These motors are

               a) double cage Motors and 
               
               b) Providing high starting torque
                    high starting torque with low 
                    Starting Current.

Typical applications are 

  •  Driving air compressor. 


  •  Conveyors, mixers


  •  Reciprocating pumps, crushers


  •   Large refrigerating Machine etc.

Standard type Induction Motors
Class Types Induction Motors


4) Class D Motors

Such Motors are provided with a 

         a) high resistance Squirrel cage Rotors
              And therefore give

         b) low operating Efficiency and

         c) their uses is limited to driving                          intermittent loads involving high 
             Accelerating duty and 

         d) Drive high - impact loads such as Punch presses, shears, bulldozers, small hoists etc.

Note :- Class C Motors similar high Starting torque with low starting Current both are same.


5). Class E Motors

These motors have a alternatively low starting torque, Normal starting Current and have a relatively low slip at rated load.


6). Class F Motors

These motors have a relatively low starting torque, low starting Current and normal slip.


Starting Methods of Squirrel cage Induction Motors

At Start the squirrel cage Induction Motor is just like a transformer with short- Circuited secondary and, therefore, it will draw heavy Current ( 5 to 7 times the rated full - load Current ) from the supply lines if rated Voltage were applied at Start.

Of course, the induced emf in the rotor Circuit decrease with the increase in speed of Rotor and consequently both the rotor and Stator current drop to values determined only by Mechanical load. 

It is essential to reduce the starting Current or an extent that the line Voltage drop does not affect the operation of other equipment connected to the same Distribution work. 

One obvious way to reduce the starting Current is to impress the lower Voltage across Stator winding at Start. 

The objection to it is that it reduces the starting torque as the starting torque varies as the square of impressed Voltage. The various Methods of starting Squirrel cage Induction Motor are below.

1) Direct- on - line starting Method or full -
Voltage method.

2) Reduced Voltage or line resistance starter method.

3) Auto - Transformer starting Method.

4) Star Delta switching method.

Note:-  Next Topic is Methods of Starting Squirrel cage Induction Motor fully understood structure.

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