SOLUTION

A universal motor is usually categorized as an AC motor. It gets its name from its ability to operate on either AC or DC voltage. Its construction is very similar to a series-wound DC motor.

Characteristics of Universal Motor

The speed/load characteristics of a universal motor is the same as that of D.C series motor. The speed of a universal motor is inversely proportional to the load, i.e., the speed is low at full load and it is high on no load. The speed may become dangerously high at no load due to low field flux. Therefore, to avoid its operation at no load, normally these motors are connected with permanent loads. The figure shows the typical torque-speed characteristics of this motor for both ac and dc operations. In actual practice, it will likely be found that the universal motor is not as effective on DC as a series motor designed for DC operation.

Since the universal motor is a series-wound, it has performance characteristics similar to a DC motor.

• It has high starting torque.
• The no-load speed is very high, but not high enough to break apart.
• It has poor speed regulation when the load to which it is connected changes.
• To minimize the effect of the load, most universal motors are designed to overcome this by operating at a very high speed of 3500 RPM or greater.
• In a router application, for example, the universal motor runs at 18,000 RPM.

There are three significant differences between the DC series motor and the universal motor:

1. In a DC motor, the iron cores are made of solid iron. The universal motor uses laminated iron to reduce energy loss from excessive heating due to eddy currents that are created from magnetic fields constantly reversing direction.
2. In a universal motor, the magnitude of the fields will fluctuate at twice the line frequency (120 times a second). This condition creates a reduction in output torque compared to a DC motor, which has constant field strength. This reduction of output power is partially compensated for by using more armature loops.
3. There is an excessive voltage drop across the series field windings of a DC series motor due to the high inductive reactance that develops when an AC voltage is applied. To minimize the voltage drop across the series coil of a universal motor, it is wound with a small number of turns on a low reluctance core.