SERIES AC MOTOR
A series ac motor is the
same electrically as a dc series motor. Refer to figure(1) and use the
lefthand rule for the polarity of coils. You can see that the instantaneous
magnetic polarities of the armature and field oppose each other, and motor
action results. Now, reverse the current by reversing the polarity
of the input. Note that the
field magnetic polarity still opposes the armature magnetic polarity. This is because
the reversal effects both the armature and the field. The ac input causes these
reversals to take place continuously.
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| fig. 1 |
The construction of the ac
series motor differs slightly from the dc series motor. Special metals, laminations,
and windings are used. They reduce losses caused by eddy currents, hysteresis,
and high reactance. Dc power can be used to drive an ac series motor
efficiently, but the opposite is not true.
The characteristics of a
series ac motor are similar to those of a series dc motor. It is a
varying-speed machine. It has low speeds for large loads and high speeds for
light loads. The starting torque is very high. Series motors are used for
driving fans, electric drills, and other small appliances. Since the series ac motor
has the same general characteristics as the series dc motor, a series motor has
been designed that
can operate both on ac and
dc. This ac/dc motor is called a universal motor. It finds wide use in small electric
appliances. Universal motors operate at lower efficiency than either the ac or
dc series motor. They are built in small sizes only. Universal motors do not
operate on polyphase ac power.
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