Q. Show Process of Speed Control of DC Motors?

Equation showed that the speed of a dc motor can be varied by control of the field flux, the armature resistance, and the armature applied voltage. The three most common speed controlmethods are shunt-field rheostat control, armature circuit-resistance control, and armature terminal-voltage control.The base speed of themachine is defined as the speedwith rated armature voltage and normal armature resistance and field flux. Speed control above the base value can be obtained by varying the field flux. By inserting a series resistance in the shunt-field circuit of a dc shunt motor (or a compound motor), we can achieve speed control over a wide range above the base speed. It is important to note, however, that a reduction in the field flux causes a corresponding increase in speed, so that the generated emf does not change appreciably while the speed is increased, but the machine torque is reduced as the field flux is reduced. The dc motor with shunt-field rheostat speed control is accordingly referred to as a constant-horsepower drive.

This method of speed control is suited to applications in which the load torque falls as the speed increases. For a machine with a series field, speed control above the base value can be achieved by placing a diverter resistance in parallel with the series winding, so that the field current is less than the armature current.

When speed control below the base speed is required, the effective armature resistance can be increased by inserting external resistance in series with the armature. This method can be applied to shunt, series, or compound motors. It has the disadvantage, however, that the series resistance, carrying full armature current, will cause significant power loss with an associated reduction in overall efficiency. The speed of the machine is governed by the value of the voltage drop in the series resistor and is therefore a function of the load on themachine. The application of this method of control is thus limited. Because of its low initial cost, however, the series-resistance method, or a variation of it, is often attractive economically for short-time or intermittent slowdowns.