SPEED CONTROL OF UNIVERSAL MOTOR

There are various methods of controlling the speed of a universal motor. A wide range of speed control is possible by inserting a rheostat in the line circuit which causes variable voltage to appear across the motor terminals resulting in reduced motor speed. Another method of speed control, not very commonly used is by brush shifting mechanism. The speed of the motor increases when the brushes are moved backward relative to the direction of rotation. However, only a limited range of speed control is possible by this method. This is because when the brushes are moved further from the magnetic neutral, commutation worsens.

Another speed control method makes use of a tapped field winding. Universal motors are always bipolar. The number of turns on the two poles need not always be the same as the air gap flux is created by series combination of mmfs of the two pole windings.
As shown in Figure 1 the field winding having larger number of turns is tapped at three points thus making possible a total of four operating speeds. For a given value of load, minimum speed will be obtained when the entire winding is used (this gives maximum mmf and flux).

Figure: 1 Tapped field winding speed adjustment method for a universal motor

Maximum speed will result when the selector switch is on point H at which minimum flux is obtained. This method offers possibility of tapping the field winding at appropriate points to permit the motor to run at the same speed on direct current and also on alternating current for a particular input. This, however, does not mean that speeds will be the same on ac and dc supplies for some other values of input currents.

Another popular method of speed control is the governor controlled speed adjusting method. In this method of speed adjustment of a series motor, a governor consisting of an assembly of a spring loaded contacts is mounted on the shaft of the motor. The current enters the governor contacts through carbon brush and slip-ring arrangement. When the motor is running, governor contacts open and close very rapidly depending upon the natural resonant frequency of the moving contacts. For a given spring tension setting, the contacts vibrate at a certain rate. Figure 2 shows how a governor is connected to a series motor for non-reversing as well as for two field reversing service.

Figure: 2 Governor controlled speed adjustment method for a series motor

For a given spring tension setting, contacts vibrate at a certain rate. When the speed rises above the particular value set by the spring tension, the contacts remain open for a relatively longer period of time, than the time the contacts take for closing. This keeps a line resistance R in the circuit a little longer and acts to reduce the speed. The reverse is true when speed falls below the adjusted value. Now the closing time of contact is longer than its opening time and hence the resistance remains in circuit for smaller duration of time and as a consequence the motor speed tends to rise.

In this type of speed adjustment the motor does not run at a constant speed but runs in a narrow range above and below the set speed. A wide range of speed adjustment is thus possible by adjusting the spring tension of the contacts. A small capacitor is used across the contacts to prevent excessive arcing.