Speed Control For Dc Motors

EE JULY/AUGUST 1987 Simple DC operated motors with a permanent magnetic stator behave as an indepen- dently energized motor. The speed of an ideal motor with an infinitely low internal resistance is in direct proportion to the voltage applied, irrespective of the torque. The motor thus runs at a speed at which its reverse electromotive force (e.m.f.) equals the supply voltage. The reverse e.m.f. is directly pro- portional to the force of the (constant) magnetic field, and the motor speed. In theory, therefore, the motor speed can be held constant with a constant supply voltage. The speed reduction observed in practice arises from the voltage drop across the internal resistance, cation. Capacitors Ci and C2 suppress noise on the reverse e.m.f. from the motor. Due care should be taken, however, in so extending the circuit, because this readily leads to instability. The motor itself already forms a fairly complex load, since the revolving rotor winding is mainly inductive, and the ro...