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Universal motor

A series motor built to operate on either alternating current (ac) or directcurrent (dc). It is normally designed for capacities less than 1 hp (0.75kW). It is usually operated at high speed, 3500 revolutions/min loadedand 8000 to 10,000 revolutions/min unloaded. For lower speeds,

reduction gears are often employed, as in the case of electric hand drillsor food mixers. As in all series motors, the rotor speed increases as theload decreases, and the no-load speed is limited only by friction and

windage. To obtain more constant speed with variations in load, acentrifugal governor may be used to switch in or out a small resistor in

series with the armature as in Fig. 1.

If an alternating current is applied to any dc series motor, the motor

would still rotate. Since the current is reversed simultaneously in thearmature and the field, the torque would pulsate but would not reversedirection. However, a universal motor designed to operate on ac shouldhave certain modifications: laminated cores to avoid excessive eddy

currents, fewer turns in the field coils than in a dc motor, and more polesand usually more commutator segments.

The series ac motor is an alternating-current commutator motor which

has great flexibility of performance. It can be operated over a wide rangeof speeds and is readily controllable. The series ac commutator motor is

in many respects similar to the dc series motor and the universal motor.

The ac series motor, like the dc series motor and the repulsion motor,consists fundamentally of these windings or their equivalent: (1) rotatingarmature winding, (2) stationary field winding, and (3) compensatingwinding (Fig. 2).

Fig. 1. Universal motor diagram.

Fig. 2. Series motor diagram. (a) Without compensatingwinding. (b) With compensating winding.

A major problem in larger (up to 1000 hp or 750 kW) ac series motors isin commutation. Because of the transformer action between the field andarmature coils, voltage is produced in the armature coils which are short-circuited by the brushes as the commutator bars pass under them.The

Fig. 3. Single-phase traction motor circuit diagram.

coils which are short-circuited act like a short-circuited secondary of a

static transformer. The resulting large currents are interrupted as the barspass the brushes, causing bad sparking. In addition, these inducedcurrents reduce the magnetic flux of the field and reduce the torque of themotor. Interpoles shunted with noninductive resistance are required on ac

series motors as in Fig. 3.

The single-phase commutator motor usually has a large number of turnsin the armature winding, more commutator segments, and a smal

number of turns in the field winding, as compared with the dc motorwhich is designed for relatively strong field and weaker armature. The acmotor usually has more poles and operates at a lower voltage than its dccounterpart.