10.1 THE UNIVERSAL MOTOR
Perhaps the simplest approach to the design of a motor that will operate on a
single-phase ac power source is to take a dc machine and run it from an ac supply.
Recall from Chapter 8 that the induced torque of a dc motor is given by
(8-49)
If the polarity of the voltage applied to a shunt or series dc motor is reversed, both
the direction of the field flux and the direction of the armature current reverse, and
the resulting induced torque continues in the same direction as before. Therefore,
it should be possible to achieve a pulsating but unidirectional torque from a dc
motor connected to an ac power supply.
Such a design is practical only for the series dc motor (see Figure 10- 1),
since the annature current and the field current in the machine must reverse at exactly
the same time. For shunt dc motors, the very high field inductance tends to
delay the reversal of the field current and thus to unacceptably reduce the average
induced torque of the motor.