Speed Control of D.C. Motors

Speed Control of D.C. Motors

Shunt-wound motors

The speed of a shunt-wound d.c. motor, n, is proportional to VIaRa/8 (see equation (21.9)). The speed is varied either by varied the worth of flux, 8, or by varied the worth of Ra. the previous is achieved by employing a resistance nonparallel with the field winding, as shown in Figure twenty one.29(a) and such a electrical device is termed the shunt field regulator. because the price of resistance of the shunt field regulator is redoubled, the worth of the field current, If, is minimized.

This ends up in a decrease within the price of flux, 8, and thus a rise within the speed, since n/1/8. therefore solely speeds on top of that given while not a shunt field regulator is obtained by this technique. Speeds below those given by VIaRa/8 are obtained by increasing the resistance within the coil circuit, as shown in Figure twenty one.29(b), where

Since electrical device R is nonparallel with the coil, it carries the complete coil current and ends up in massive|an out sized|an over sized} power loss in large motors wherever a substantial speed reduction is needed for long periods. These ways of speed management square measure incontestible within the following worked drawback.


Series-wound motors

 The speed control of series-wound motors is achieved using either(a) field resistance, or (b) coil resistance techniques.

(a) The speed of a d.c. series-wound motor is given by:

where k may be a constant, V is that the terminal voltage, R is that the combined resistance of the coil and series field and eight is that the flux.

 Thus, a discount in flux ends up in a rise in speed. this is often achieved by putt a variable resistance in parallel with the field winding and reducing the field current, and therefore flux, for a given worth of offer current. A circuit diagram of this arrangement is shown in Figure twenty one.30(a).
A resistor connected in parallel with the series-wound field to regulate speed is termed a diverter. Speeds higher than those given with no diverter square measure obtained by this technique. downside twenty nine below demonstrates this technique.

 (b) Speeds below traditional square measure obtained by connecting a resistor nonparallel with the field winding and coil circuit, as shown in Figure twenty one.30(b). This effectively will increase the worth of R within the equation