The Series Motor

Author: E.E. Kimberly

The general construction of a series motor is identical with that of a shunt or compound motor, but its field flux is derived solely from a series-field winding. The field flux is great at start, and the starting torque is very high. As the speed increases, the back emf Eg increases and therefore the armature current decreases. The decrease of armature current also flowing in the field causes the field strength to decrease and the motor speed to increase still further. This cumulative action continues until the load, the armature-iron loss, the windage and If the load of a series motor be too greatly reduced, the speed may become excessive and cause damage to the windings. Therefore, it is not safe to connect a series motor to a load by a belt or other insecure means which might fail and allow a dangerous speed.

Fig. 11-8. Series-Motor Characteristic

Series motors are used for tractive purposes on locomotives, street cars, ore bridges, and cranes, and for small household devices.

Fig. 11-9 shows superposed the characteristic speed-load curves of a shunt motor, a compound motor, and a series motor, all rated at the same horsepower and speed.

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The starting handle should be moved slowly, about 15 seconds being allowed for full operation. See Chapter 15. other friction are just great enough to require the torque available at some particular speed. When the load is varied, the speed varies greatly; and the series motor is therefore unsuited to service requiring relatively constant speed. Fig. 11-8 shows a typical series-motor speed-torque curve. The torque is almost inversely proportional to the speed.