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In the previous article we discussed about the basic principles of repulsion motors and how the repulsion principle aids in rotating a motor. Now let us discuss the various disadvantages of a conventional repulsion motor and how the various types are used to minimize those disadvantages, followed by repulsion induction motor wiring study.
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Disadvantages of conventional repulsion motor
· No load speed is very high and dangerous.
· The power factor is poor at low speeds.
· Occurrence of sparks at brushes
· Commutator and brushes wear out quickly. This is primarily due to arcing and heat generated at brush assembly.
To minimize the above specified shortcomings, various other types of repulsion motors are introduced.
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Types of repulsion motors
The various types of motors which works under the repulsion principle are:
· Compensated repulsion motor
· Repulsion-start Induction-run motor
· Repulsion Induction motor
Now let us discuss the various types of repulsion motors in detail.
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Compensated repulsion motor
In this type of motor an additional stator winding called compensation winding is provided. This is the modified form of the basic repulsion motor. The compensation winding serves for two purposes:
· To Improve the Power factor
· For better speed regulation
This type of motor is used whenever there is a need for motor to run at constant speed and at higher power factor
So an additional stator winding called compensating winding is used. The additional winding which is connected in series with the armature, is smaller than stator winding and wounded to the inner slots of main pole. It also consists of additional set of brushes which are placed mid way between the short circuited brushes.
Such a type of modification reduces the quadrature drop and improves the power factor. And speed regulation also improves due to this compensation.
Quadrature drop occurs in salient pole types due to non-uniform air gap length. Due to quadrature drop cross-magnetizing effect occurs which opposes the mmf waves. By providing such a type of compensation, this effect can be reduced which increases power factor. Further the leakage between armature and field is reduced.
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Repulsion start induction run motors
As the name suggests this motor starts as a repulsion motor and runs as an Induction motor. How this is achieved? Is it possible to start a motor as repulsion type and run as induction type? The answer is YES, it is possible.
This type of motor starts as a normal Repulsion motor and after achieving three-fourths of its full speed, it runs as an Induction motor. For this purpose a centrifugal force-operated device is used. This centrifugal device short circuits the commutator segments and this aids in running the motor as a squirrel cage motor. As soon as the commutator is short circuited, the brushes present do not carry any current. So the brushes can be removed to avoid the wear and tear.
What is the advantage in running the motor as a squirrel cage one?
The main advantage is that it provides high starting torque without excessive current. Also constant speed is ensured for wide range of torque.
Further there are two different designs in repulsion start motors:
BRUSH LIFTING TYPE- In this type the brush is lifted as soon as the commutator is short circuited to avoid unnecessary wear and tear and losses due to friction. So in this type the brush is present only when the motor is started as a repulsion one.
BRUSH RIDING TYPE- In this type of motors the brushes ride along with the commutator at all times. So the brushes are present even after the commutator is short circuited.
Applications of this motor include:
· Machine tools
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Repulsion induction motor
This type of motor is a combination of repulsion motor and induction motor. It is also referred as squirrel cage repulsion motor.
This motor possesses the characteristic of both induction motor and repulsion motor. It combines the desirable starting characteristics of repulsion motor and constant speed characteristics of an induction motor.
Here the stator winding is same as every other repulsion motor but there are two separate rotor windings
· A squirrel cage winding
· A Commutator winding
The commutator winding lies on the outer slots while the squirrel cage winding is located in the inner slots. Both the windings operate during the entire period of operation of motors. The brushes are in contact with the commutator all the time.
The biggest advantage in such type of motors is that they don’t need a separate centrifugal short-circuit system as in Repulsion-start Induction-run motors.
As soon as the motor is started, the squirrel cage winding is practically inactive for a small period of time due to high reluctance. Only the commutator winding supplies most of the torque. But during normal running condition, the squirrel cage winding supplies most of the torque and commutator winding supplies relatively lower torque when compared to Squirrel cage winding. So the squirrel cage winding takes up most of the load as the rotor accelerates.
· Petrol pumps
· Mixing machines
· Lifts and Hoists
But now a days brushless repulsion motors are preferred to reduce the amount of wear and tear and frictional losses that occur at the brush terminals.
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Image and Content Credits
"Alternating Current Machines" - R.K.Rajput