Speed/Load characteristics and Speed Control
Very similar to that of DC series motor, the universal motor also has varying speed characteristics. The speed is low at full loads. The speed is Very high and dangerous at no-loads. During no-loads the speed is limited only by its own frictional and windage load.
Speed control of universal motor is very important and the following methods are employed for the speed control of universal motors.
In this method of speed control a variable resistance is connected in series with the motor. The amount of resistance in the circuit can be changed. A foot pedal is used for this purpose. Usually this method is employed for motors used in sewing machines.
This method is involved whenever the application involves a number of speeds. Best example is home food and fruit mixers. Here a centrifugal device is attached to the motor. If the motor rises above the specified speed set by the lever, the centrifugal device opens the contact and R comes in contact with the circuit. This causes the motor speed to decrease below the set speed. When the motor runs slower than the speed set by the lever, the contact is established and resistance is short circuited. This causes the speed to increase. The variations in speed are noticeable as the process is repeated in a rapid manner.
As the name suggests the field is tapped at various points in this method. This is done by wounding the field pole.
Now how this tapping arrangement is established?
The tapping arrangement is established by two ways
· The various sections in the field pole are wounded and different sizes of wire and taps are brought out from those sections.
· In the second method, resistance made of Nichrome is wounded over the field pole. Various tappings are brought out from this wire.