Speed control system for conveying machine energized by three-phase current

Claim 1

A speed control system for a conveying machine energized by three-phase current, said conveying machine having a rotor and a stator, said speed control system comprising an ideal value transmitter for providing an ideal value voltage corresponding to a desired speed of said conveying machine, an actual value transducer coupled to the rotor of said conveying machine for providing an actual value voltage corresponding in magnitude to the actual speed of said conveying machine, three-point controller means connected to said ideal value transmitter and to said actual value transducer for providing a control signal equal to the difference in magnitude between said ideal and actual value voltages, reversing contactor means coupled to said controller means and connected to the stator of said conveying machine for supplying three-phase current to the stator of said conveying machine under the control of the control signal provided by said controller means, and direct current contactor means coupled to said controller means and connected to the stator of said conveying machine for supplying direct current to the stator of said conveying machine under the control of the control signal provided by said controller means, further comprising braking current controller means for providing maximum braking current for said conveying machine, input connecting means for selectively applying said ideal value voltage to actuate said braking current controller means, and output connecting means for supplying said maximum braking current to said conveying machine via said direct current contactor means, and wherein the input connecting means of said braking current controller means further comprises emergency switch means connected between said ideal value transformer and said time relay means for initiating emergency braking.