Electric Motor Speed Torque Characteristics:

A Electric motor speed torque characteristics is defined as the relationship between the speed at which it operates and the torque it develops, ω = f(T).

Practically all electric motors—shunt wound, series wound, compound dc motors, squirrel-cage and slip-ring induction motors, and ac commutator motors, have drooping speed-torque characteristics, i.e., their speed falls off as the load torque increases. However, the degree by which the speed changes with the change in torque differs for various types of motors, it being characterized by the so-called hardness of their speed-torque characteristics.

Electric motor speed torque characteristics may be classified into three main groups.

1. Absolutely hard (flat) speed-torque characteristic:

A characteristic exhibiting no change in speed with change in load torque. Synchronous motors operate with such a characteristic (horizontal straight line I in Fig. 1.8).

2. Hard speed-torque characteristic:

A characteristic showing a speed which drops only slightly with increase in torque. A hard characteristic is exhibited by a shunt wound dc motor, this also being true of induction motors over the operating region of the speed-torque characteristic (curve II in Fig. 1.8).

The speed-torque characteristic of an induction motor exhibits a “hardness” that differs according to what point along the characteristic is taken into consideration (Fig. 1.9). Between the points of the maximum torque in motor operation T_{max M} and the maximum torque in generator operation T_{max G} an induction machine will exhibit a rather hard characteristic.

3. Soft speed characteristic:

A characteristic showing considerable drop in speed with rise in torque. Series wound dc motor possesses such characteristic, especially along the low-torque portion of the characteristic (curve III in Fig. 1.8). For such motors, the degree of hardness of the characteristic varies all along the curve.

Compound wound dc motors, depending on the degree of hardness their speed-torque characteristics display, may be considered as hard- or soft-characteristic motors.