- Fractional-horsepower series motors that are adapted for use on either D.C. or A.C. circuits of a given voltage are called universal motors.
- The universal motor is designed for commercial frequencies from 60 cycles down to D.C. (zero frequency), and for voltage from 250 V to 1.5 V. A commercial universal motor may have a somewhat weaker series field and more armature conductors than a D.C. series motor of equivalent horsepower. It is manufactured in ratings up to 3/4 H.P., particularly for vacuum cleaners and industrial sewing machines. In smaller sizes of ± H.P. or less; it
is used in electric hand drills.
Like all series motors, the no-load speed of the universal motor is universally high. Quite frequently, gears trains are built into the motor housing of some universal motors to provide exceedingly high torque at low speeds.
When these motors are used in commercial appliances such as electric shavers, sewing machines, office machines, and small hand hair dryers or vacuum cleaners, they are always directly loaded with little danger of motor runaway.
Advantages of a universal motor:
- High speed from above 3600 r.p.m. to around 25000 r.p.m.
- High power output in small physical sizes for use in portable tools.
- High torque at low and intermediate speeds to carry a particularly severe load.
- Variable speed by adjustable governor, by line voltage or especially by modern pulse techniques.
- Increased service requirement due to use of brushes and commutators. The life of these parts is limited in severe service.
- Relatively high noise level at high speeds.
- Moderate to severe radio and television interference due to brush sparking.
- Requirement for careful balancing to avoid vibration.
- Requirement for reduction gearing in most portable tools.
Fig. 21. Field core of a two pole universal motor.
Universal motors are manufactured in two types:
- Concentrated-pole, non-compensated type (low H.P. rating).
- Distributed field compensated type (high H.P. rating).
Fig. 21 shows the laminated field structure of a typical concentrated field universal motor.
Operation of a Universal Motor. As explained in Art. 5.4, such motors develop unidirectional torque regardless of whether they operate on D.C. or A.C. supply. The production of unidirectional torque when the motor runs on A.C. supply can be easily understood from Fig. 22.
The motor works on the same principle as a D.C. motor i.e., the force between the main pole flux and the current carrying armature conductors. This is true regardless of whether current is alternating or direct.
- Fig. 23 shows the typical torque characteristics of a universal motor both for D.C. and A.C. supply.
- The speed of a universal motor may be controlled by the following methods:
(i) Reactance method.
(ii) Tapped-field method.
(iii) Centrifugal mechanism.