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 range of the torque/speed plane with low losses and matching the behaviour of the motor to the needs of the mechanical load. 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 nominal voltage — would result in an excessive current of up to 10 times nominal value. Under normal operating conditions this is prevented by the induced armature voltage e, which cancels most of the applied voltage ua so that only the difference is driving the armature current ia; these two quantities are performing the actual electromechanical energy conversion. In transient operation, for example while accelerating or braking, there is always the danger of excessive current due to the rapidly changing armature voltage or speed; the same is true with a steady state overload on the motor. It is therefore important to provide a fast current or torque limit to protect the motor, the power supply and the load; this is best realised by feedback control establishing an effective safeguard against electrical or mechanical stresses. At the same time an unequivocal criterion is gained for distinguishing tolerable temporary overcurrents from currents caused by a functional fault which call for immediate clearance by a breaker or by fuses.
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