CONTROL SYSTEMS FOR AN ELECTRIC DRIVE

摘要

CASE 2975 CONTROL SYSTEM FOR AN ELECTRIC DRIVE A control system for an icebreaker drive has a ship's telegraph for inputting a speed reference signal representing a desired speed at which the electric motors propelling the ship are to run. The motors receive power from a controllable converter which, in turn, receives power from an AC generator or generators driven by respective prime movers. The prime movers run at a governed rated speed but slow down with overload and may stall if the load is not decreased quickly. A sensor determines the frequency of the generator and provides a frequency feedback signal. This is compared to a set frequency signal related to the speed of the prime mover and the generator frequency. The frequency feedback signal and the set frequency signal are compared and a speed reduction signal is derived related to the amount by which the frequency feedback signal is below the set frequency signal. The speed reference signal is reduced in value in accordance with the speed reduction signal. A second speed reduction signal may be derived by comparing a signal representing the actual kilowatts of power being provided by the generators with a signal representing rated kilowatts of power for the number of generators operating at the time. Thus the speed reference signal may be reduced by either or both of the first and second reduction signals. The speed reference signal is compared to a sensed actual speed signal and the result integrated and limited to provide a torque reference signal. The torque reference signal controls the operation of the converter providing controlled current to the motor, and thus controls speed. The speed is kept constant until an overload occurs with respect to the prime mover or the generator and then the speed is reduced. This decreases the load and also indicates a change to the Captain.