Theory

DC motors are in general much more adaptable to adjustable speed drives than ac motors which are associated with a constant speed rotating fields. Indeed this susceptibility of dc motors to adjustment of their operating speed over wide ranges and by a variety of methods is one of the important reasons for strong competitive position of dc motors in modern industrial drives. It is thus necessary to gather an idea about speed control methods along with their associated characteristics.

The torque, T developed and speed, n of a dc motor are given as,

T=KΦIa
N= Vf-IaRa/KΦ

where k is constant decided by the design of the machine (total number of conductors, number of parallel paths and number of poles). The above equation explains the concepts for different methods of speed control.

Varying Field Excitation (Φ)

In shunt and compound motors speed control can be achieved by varying the shunt field circuit resistance. The lowest speed corresponds to zero resistance in field rheostat. Speed can be increased by increasing the field rheostat resistance. The highest speed is limited by armature reaction under weak field conditions, causing motor instability or poor commutation. The dc motors with the shunt field-rheostat speed control is generally referred to as a constant horsepower drive, since back emf remains practically constant. The torque on the other hand varies directly with flux and therefore has its highest allowable value at the lowest speed. Field rheostat control is thus best suited to drives requiring decreased torque at high speed.

Varying Armature Terminal Voltage (Vt)

A change of the armature terminal voltage results in a change in speed with constant excitation. Usually the power available is constant voltage ac, so the auxiliary equipment in the form of rectifier is required to provide the controlled armature voltage for the motor. In this mode the torque developed by motor is almost equal to rated torque at any speed from zero to rated speed. In this region motor operates in constant torque mode.