Universal Commutator Motors

Any dc motor can in principle be run off an ac line because the motor torque that is dependent on the product of the armature cur­rent and the air-gap field flux does not reverses direction when the directions of the armature current and field flux reverse simultaneously.

For a large torque to be develop­ed, it is necessary that the armature current and field flux should

reverse direction simultaneously, Fig. 8.18. Wiring diagram of a i.e. they should be in phase. In a universal motor shunt motor, such a phase coincidence is impossible to reach because the flux produced by the field winding lags the impressed volt­age by about 90°. In a series motor the armature current is at the same time the field current. Neglecting the phase shift between the field current and magnetic flux, the armature current and flux may be regarded to be in phase.

Small commutator motors are made universal, i.e. adapted for operation from a dc and an ac power line. This type of motor is usually built without a compensating winding. The motor has dc terminals labeled 0 and =, and ac terminals 0 and ~ for its connec­tion to the dc and the ac supply respectively (Fig. 8.18). Thus, when the motor runs off the ac line, the number of turns of the field winding is much smaller than when the motor operates on dc, with the result that the power factor proves rather high despite the ab­sence of the compensating winding.

The characteristics of a universal commutator motor operated from the ac supply are similar to those of a dc series-wound motor. Single-phase small-power commutator motors find use in automat­ic, communication, and domestic facilities. They differ substantially from dc motors in design. The magnetic circuit of the stator for this type of motor is built up of steel sheets to minimize the eddy-current loss. The flux due to armature reaction produces the emf of self-induction which heavily reduces the power factor. To avoid the effect of armature reaction, the stator is fitted with a compensating winding whose flux opposes the armature flux. This winding can be placed in series with the armature or coupled inductively with it, or wound on the stator so as to perform the functions of both the field winding and the compensating winding simultaneously.

Use is sometimes made of the motors with a transformer coupling between stator and roto_r. These motors, called induction, or repul­sion, commutator motors, are mainly designed for household faci­lities and operated directly from the ac line. In such a motor, the winding connected across the line acts as the field and the compen­sating winding, the armature winding being shorted out by the commutator brushes.