Classic AC motors: Some practical aspects of AC series motors

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. FIG. 1 illustrates some of the practical aspects of series motors. The schematic diagram shown in FIG. 1A is the same as that for a DC series machine, except for the connection to an AC source. Notice that a double-pole, double-throw switch would be needed to accomplish reversal of rotation (either the field terminals or the armature terminals would have to be transposed). The connection of compensating and inter- pole windings is shown in FIG. 1B. Here, again, the technique duplicates that used in DC series motors.

The AC series motor depicted in FIG. also has compensating and interpole windings. However, these auxiliary windings, rather than being conductively connected to the armature circuit, are inductively coupled. In AC applications, such a technique produces beneficial results. First, the armature can develop more torque because its applied terminal voltage is not diminished by the reactive voltage drops that would occur across these auxiliary windings if they were conductively connected to the armature circuit. Second, it’s found that the power factor of the AC motor is improved.

The general concept of transformer action inside motors is relevant to control techniques. In the DC motor, the ac-carrying armature does not induce currents in the main field windings. This is because the magnetic flux from the main field poles is in space quadrature with the armature flux. These two magnetic fields interact to be sure, but the magnetic lines of force from the armature don’t link the turns of wire in the main field poles. The situation is analogous to that prevailing when an AC carrying solenoid is oriented at right angles to the longitudinal axis of a second sole noid. Despite the physical proximity of the two solenoids, the second one won’t become a “secondary” current source by transformer action.

To facilitate reversal of rotation, some series motors are equipped with two main field windings. These are polarized, or phased, to produce opposite rotation—only one such winding is active when reversal is accomplished with a single-pole, double- throw switch as shown in FIG. iD. The characteristic curves of a typical small universal motor are shown in FIG. 2. At 60 Hz, AC series motors generally exhibit the highest horsepower-to-weight ratio of any AC motor. This is largely because of the high speed at which these series motors can operate.


FIG. 1 The AC series motor.

  • A. General circuit connection for AC series motor, or universal motor. AC source; Series field.
  • B. AC series motor, or universal motor, with conductively connected compensating and interpole windings.
  • C. AC series motor, or universal motor, with inductively coupled compensating and interpole windings. Interpole windings; Compensating windings
  • D. Reversible AC series motor, or universal motor, with split field windings. Reversal switch


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