In field control method, the field exciting current is varied to control the speed of dc motors. In the case of dc shunt motors and dc compound motors external resistance is connected in series with the shunt field winding. But in case of dc series motors external resistance called diverter is connected across the series field as shown in the following figure.
In both arrangements, the current flowing through the field windings is decreased. Decrease in field exciting current reduces the flux. As the speed of the motor is inversely proportional to flux the speed of motor increases. Thus by field control method the motors can be operated at speeds above normal rated speed. Since torque developed is directly proportional to flux, the decrease in flux decrease the field excitation current, which results reduction in the torque. But at the same time the increase in speed due to decrease in flux keeps the power developed constant.
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