Speed Control of DC Motor

Speed Control of DC Motor Points : Speed Control of DC Motor Speed control means intentional change of the drive speed to a value required for performing the specific work process. This concept of speed control or adjustment should not be taken to include the natural change in speed which occurs due to change in the load on the drive shaft. The desired change in speed is accomplished by acting accordingly in the drive motor or on the transmission connecting it to the unit it serves to drive. This may be done manually by the operator or by means of some automatic control device.

Any given piece of industrial equipment may have its speed changed or adjusted mechanically by means of stepped pulleys, sets of change gears, variable speed friction clutch mechanism, and other mechanical devices. Historically, this proved to be the first step in transition from non-adjustable speed to adjustable speed drive. The electrical speed control has many economical as well as engineering advantages over mechanical speed control.

The nature of the speed control requirement for an industrial drive depends upon Its type. Some drives may required continuous variation of speed for the whole of the range from zero to Ml speed, or over a portion of this range; while the others may require two or more fixed speeds, Some machines may require cheeping speed for adjusting or setting up the work. Such a speed is of the order of few rpm. For most of the drives, however, a control of speed within the range of 20% may be suitable.

One of the attractive features the dc motor offers over all other types is the relative case with which speed control can be achieved and, therefore, dc motors are indispensable for many adjustable speed drives, The various schemes available for speed control can be deduced from the expression of speed for a dc motor which is repeated here with one modification
N = K V-I_a (R + R)/Ф

The modification involves the inclusion of an external resistance in the armature circuit. The above expression reveals that the speed can be controlled by adjusting any one of the three factors appearing on the right hand side of the expression; (i) applied voltage to the armature terminals, V (ii) external resistance in the armature circuit, R and (iii) flux per pole, (iv). The first two possibilities involve adjustment affecting the armature circuit, whereas the third involves change in the magnetic field. Therefore, speed control methods are broadly classified as armature control methods and field control methods.

Sometimes a combination of the two methods is employed. With armature control the speed decreases as the voltage applied to the armature terminals is reduced, whereas with field control the speed increases as the flux is reduced.