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2.1 INTRODUCTION
Direct-current motors are extensively used in variable-speed drives and positioncontrol
systems where good dynamic response and steady-state performance are
required. Examples are in robotic drives, printers, machine tools, process rolling mills,
paper and textile industries, and many others. Control of a dc motor, especially of the
separately excited type, is very straightforward, mainly because of the incorporation of
the commutator within the motor. The commutator brush allows the motor-developed
torque to be proportional to the armature current if the field current is held constant.
Classical control theories are then easily applied to the design of the torque and other
control loops of a drive system.
2.2 DC MOTORS AND ITS CHARACTERISTICS
When a DC supply is applied to the armature of the dc motor with its field excited
by a dc supply, torque is developed in the armature due to interaction between the axial
current carrying conductors on the rotor and the radial magnetic flux produced by the
stator. If the voltage V is the voltage applied to the armature terminals, and E is the
internally developed motional e.m.f. The resistance and inductance of the complete
armature are represented by Ra and La in Figure 2.1(a). Under motoring conditions, the
motional e.m.f. E always opposes the applied voltage V, and for this reason it is referred
to as ‘back e.m.f.’ For current to be forced into the motor, V must be greater than E, the
armature circuit voltage equation being given