Page 568 - The Mechatronics Handbook

0066_Frame_C20 Page 38 Wednesday, January 9, 2002 5:49 PM

Applied voltage, V
Speed
Torque
EMF
Torque
E

Speed
Armature current

FIGURE 20.62 The series-wound motor load characteristics.

Torque

Speed

FIGURE 20.63 The series-wound motor torque–speed characteristics.

then the speed may become dangerously high. A series-wound motor should never be used, therefore,
in situations where the load is likely to be suddenly relaxed.
The main advantage of the series-wound motor is that it provides a large torque at low speeds. Series-
wound motors are eminently suitable, therefore, for applications where a large starting torque is required.
This includes, for example, lifts, hoists, cranes, and electric trains.
The Compound-Wound Motor
Compound-wound motors are produced by including both series and shunt fields. The resulting char-
acteristics of the compound-wound motor fall somewhere in between those of the series-wound and the
shunt-wound machines.

Starting dc Motors

With the armature stationary, the induced emf is zero. If while at rest, the full voltage is applied across the
armature winding then the current drawn would be massive. A typical 40-kW motor might have an armature
resistance of about 0.06 Ω. If the applied voltage is 240 V, the current drawn is 4000 A. This current would
undoubtedly blow the fuses and thereby cut off the supply to the machine. To limit the starting current a
variable external resistance is connected in series with the armature. On start-up the full resistance is
connected in series. As the machine builds up speed and increases the back emf, the external resistance can
be reduced until at rated speed the series resistance is disconnected. Alternatively, a series resistance can be
momentarily activated in conjunction with the starter switch.

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