The Series A-C Motor (Universal Motor)

Author: E.E. Kimberly

This motor is called a universal motor because it can operate equally well on direct current or alternating current. Its construction is similar to that of the direct-current series motor with salient poles or to that of the repulsion motor with distributed stator windings.

Fig. 21-14. Universal-Motor Characteristics

However, the field and armature are connected in series. All of the magnetic circuit must be laminated to minimize eddy-current loss. The operating characteristics are similar to those of the series direct-current motor in that both the starting torque and the speed regulation are high. Curves for various conditions are shown in Fig. 21-14. With the exception of those built for railway-traction service, series a-c motors are built in fractional-horsepower sizes only. They are inherently best adapted to high-speed service (3000 to 10,000 rpm), and are commonly used in floor sanders, vacuum sweepers, food mixers, and other household and office appliances. To avoid the evils of poor commutation, it is customary to shift the brushes in these motors a few degrees against the direction of rotation, or to place the brushes on the neutral axis with respect to the physical poles but to rotate the commutator on the shaft a few degrees in the direction of rotation with respect to the armature winding. Such a motor is suitable for operation in only one direction of rotation. Motors provided with compensating windings like those described on page 137 will have identical characteristics in both directions of rotation. Universal motors of the cheaper construction are not provided with compensating windings.