COMPOUND DC MOTOR
· In dc compound motors both shunt field and series field will be acting simultaneously.
· In cumulative compound motors the series field assists the shunt field.
· In such motors when armature current increases the field flux increases.
· So for given armature current the torque developed will be greater and speed lower when compared to a shunt motor.
· In differentially compounded motors the series field opposes the shunt field.
· Therefore when armature current increases the field flux decreases.
· So for given armature current, the torque developed will be lower and the speed greater when compared to shunt motor.
Torque Vs armature current and speed Vs armature current characteristics of dc compound motors are shown in the following figure.
The speed Vs torque characteristics are compared with that of shunt motor as shown in the following figure.
The Dc compound motor has the series as well as the shunt field.
· Regenerative braking
· Dynamic braking
· Counter braking
· In the regenerative braking operation of the compound motor, the direction of the armature and the series field are reversed.
· This may be demagnetized the motor to avoid the demagnetization, the series field winding of the motor is shunt as soon as the speed raises to Wo .
· Therefore the speed torque characteristics of regenerative braking is the straight line.
· The dynamic braking of the compound motor is similar to the dynamic braking of the shunt motor.
· During dynamic braking the armature of the motor is disconnected from the supply and is connected across the braking resister and only the shunt field winding is excited.
· Therefore the field flux is constant.
· Counter current braking of the compound motor is similar to the series motor .This is because of the influence of series field winding.
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