THE UNIVERSAL MOTOR

THE UNIVERSAL MOTOR

Perhaps the simplest approach to the design of a motor that will operate on a

single-phase ac power source is to take a dc machine and run it from an ac supply.

If the polarity of the voltage applied to a shunt or series dc motor is reversed, both

the direction of the field flux and the direction of the armature current reverse, and

the resulting induced torque continues in the same direction as before. Therefore,

it should be possible to achieve a pulsating but unidirectional torque from a dc

motor connected to an ac power supply.

Such a design is practical only for the series dc motor (see Figure 1(,

since the armature current and the field current in the machine must reverse at exactly the same time. For shunt dc motors, the very high field inductance tends to

delay the reversal of the field current and thus to unacceptably reduce the average

induced torque of the motor.

In order for a series dc motor to function effectively on ac, its field poles

and stator frame must be completely laminated. If they were not completely laminated, their core losses would be enormous. When the poles and stator are laminated, this motor is often called a universal motor, since it can run from either an ac or a dc source.

When the motor is running from an ac source, the commutation will be

much poorer than it would be with a dc source. The extra sparking at the brushes

is caused by transformer action inducing voltages in the coils undergoing commutation. These sparks significantly shorten brush life and can be a source of

radio-frequency interference in certain environments.

A typical torque-speed characteristic of a universal motor is shown in Figure

2. It differs from the torque-speed characteristic of the same machine operating

from a dc voltage source for two reasons:

1. The armature and field windings have quite a large reactance at 50 or 60 Hz.

A significant part of the input voltage is dropped across these reactances, and

therefore E_A is smaller for a given input voltage during ac operation than it is

during dc operation. Since E_A = Kfw, the motor is slower for a given armature current and induced torque on alternating current than it would be

on direct current.

2. In addition, the peak voltage of an ac system is V2 times its rms value, so

magnetic saturation could occur near the peak current in the machine. This

saturation could significantly lower the rms flux of the motor for a given current

level, tending to reduce the machine 's induced torque. Recall that a decrease

in flux increases the speed of a dc machine, so this effect may partially

offset the speed decrease caused by the first effect.

Applications of Universal Motors The universal motor has the sharply drooping torque- speed characteristic of a dc series motor, so it is not suitable for constant-speed applications. However, it is compact and gives more torque per ampere than any other single-phase motor. It is therefore used where light weight and high torque are important.

Typical applications for this motor are vacuum cleaners, drills, similar

portable tools, and kitchen appliances.

Speed Control of Universal Motors As with dc series motors, the best way to control the speed of a universal motor is to vary its rms input voltage. The higher the rms input voltage, the greater the resulting speed of the motor. Typical torque-speed characteristics of a universal motor as a function of voltage are shown in Figure 3.

In practice, the average voltage applied to such a motor is varied with one

of the SCR or TRIAC circuits. Two such speed control

circuits are shown in Figure 4. The variable resistors shown in these figures

are the speed adjustment knobs of the motors (e.g., such a resistor would be the

trigger of a variable-speed drill).