Control of a Separately Excited DC Machine

In Chap. 5 the steady state and dynamic behaviour of a separately excited DC machine with adjustable armature and field voltage has been explained; this discussion is now extended by considering the machine as part of a feedback control system. The reason for this is that in practice the choice of a DC drive is normally motivated by the possibility of operating over a wide speed range with low losses and matching the behaviour of the motor to the needs of the load. In order to achieve the desired operating characteristics in the presence of supply- and load-disturbances, feedback control is usually necessary. Another reason why DC drives are normally contained in feedback loops is that the armature of a larger motor represents a very small impedance which — when supplied with rated voltage — would result in an excessive current of up to 10 times rated value. Under normal operating conditions this is prevented by the induced armature voltage e, which cancels most of the applied voltage u a so that only the difference determines the armature current i a . It is these two quantities which are performing the actual electromechanical energy conversion.