Series motor for high torque; compound motor characteristics

Why a series motor is used for high torque

In a series motor the field carries the full armature current, so (below saturation) $\phi \propto I_a$ and

$$T \propto \phi I_a \propto I_a^2$$

• Doubling the current gives about four times the torque, so a large starting torque is obtained with moderate current. A shunt motor ($T \propto I_a$) would need far more current for the same torque.
• At heavy load the speed falls automatically, so power $T\omega$ stays nearly constant and the supply is not overloaded.
• Hence series motors suit traction (electric trains, trams), cranes, hoists, elevators and conveyors, where high starting torque is needed. They must never run without load, because speed becomes dangerously high.

Compound motor characteristics

A compound motor has both shunt and series fields. In a cumulative compound motor the series flux aids the shunt flux; in a differential one it opposes it.

1. Electrical characteristic ($T$ vs $I_a$)

• Cumulative: $\phi = \phi_{sh} + \phi_{se}$ rises with load, so torque rises faster than in a shunt motor (between shunt and series curves). Good starting torque.
• Differential: $\phi$ falls with load, so torque rises slowly; starting torque is poor.

2. Speed characteristic ($N$ vs $I_a$)

• Cumulative: since flux rises with load, speed falls more than in a shunt motor, but there is a definite no-load speed (set by the shunt field), so there is no runaway.
• Differential: flux falls with load, so speed stays nearly constant or rises with load; operation can be unstable.

3. Mechanical characteristic ($N$ vs $T$): obtained from the two above; cumulative gives a drooping curve, differential a flat/rising one.

 T
 |      series
 |     /  cumul.
 |    /  /  shunt
 |   /  / /
 |  / / / differential
 | ///_-----
 |/____________ Ia

 N
 |                         _.-' differential
 |____________________.--''
 |'-.______________________   shunt
 |  '-._
 |      '-.______________     cumulative
 | \
 |  '.
 |    '-.___
 |          '------.______    series
 |_____________________________ Ia