Series AC Motors

A series AC motor is electrically similar to a DC series motor. Refer to following figure and use the left-hand 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 affects both the armature and the field. The AC input causes these reversals to take place continuously.

FIGURE

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 only in small sizes and do not operate on polyphase AC power.