Monday, 24 November 2014

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Speed Control of Series Motors

1. Flux Control Method: 

: Variations in the flux of a series motor can be brought about in any one of the following ways:

 (a) Field Diverters:

 The series winding are shunted by a variable resistance known as field diverter. Any desired amount of current can be passed through the diverter by adjusting its resistance. Hence the flux can be decreased and consequently, the speed of the motor increased.

 (b) Armature Diverter:

A diverter across the armature can be used for giving speeds lower than the normal speed. For a given constant load torque, if Ia is reduced due to armature diverter, the  must increase (TaIa ) This results in an increase in current taken from the supply(which increases the flux and a fall in speed (N  I/ )). The variation in speed can be controlled by varying the diverter resistance.

(c) Trapped Field Control Field: This method is often used in electric traction. The number of series filed turns in the circuit can be changed. With full field, the motor runs at its minimum speed which can be raised in steps by cutting out some of the series turns.
 (d) Paralleling Field coils: this method used for fan motors, several speeds can be obtained by regrouping the field coils. It is seen that for a4-pole motor, three speeds can be obtained easily.

 Variable Resistance in Series with Motor

By increasing the resistance in series with the armature the voltage applied across the armature terminals can be decreased. With reduced voltage across the armature, the speed is reduced. However, it will be noted that since full motor current passes through this resistance, there is a considerable loss of power in it.

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