Braking control

Abstract

A regenerative braking system for a linear induction motor includes a control device to increase the frequency slip in a braking mode and reduce the regenerative capacity of the motor. The control device is regulated by sensing means which monitor the voltage of the external power conductor and increase the slip if the voltage exceeds a predetermined value. This reduces the power regenerated by the motor and prevents excessive voltages in the conductor. A latch prevents cycling between the full and reduced regenerative braking to prevent oscillation of the braking force and is reset upon selection of a motoring condition for the motor.

I claim: 
     
       1. A regenerative braking system comprising an electric induction motor, .Iadd.said motor being operable in a motoring mode wherein said motor is conditioned to receive power from a source and a braking mode wherein said motor is conditioned to supply power to said source, .Iaddend.an external conductor connected to said motor to transfer power between said motor and a source, sensing means to sense the voltage in said external conductor, .[.and.]. control means to control the regenerative capacity of said motor, said control means being operable to increase the slip frequency between the synchronous frequency of said motor and the control frequency supplied to said motor to reduce said regenerative capacity upon said sensing means sensing a predetermined voltage in said conductor and thereby reduce the power delivered by said motor to the conductor .Iadd.and inhibiting means to inhibit operation of said control means whilst said motor is in said motoring mode.Iaddend.. 
     
     
       2. A regenerative braking system according to claim 1 wherein said control means includes latch means to maintain said motor at said reduced regenerative capacity upon reduction of said voltage in said conductor below said predetermined value. 
     
     
       3. A regenerative braking system according to claim 2 including command means to condition said motor from .[.a.]. .Iadd.said .Iaddend.braking mode to .[.a motor.]. .Iadd.said motoring .Iaddend.mode and wherein said command means includes reset means operable to release said latch means upon said motor being conditioned to said motoring mode. 
     
     
       4. A regenerative braking system according to claim 3 including signalling means to produce a signal dependant on vehicle speed, said signalling means being operable upon said control means to vary the reduction in regenerative capacity upon a reduction in vehicle speed. 
     
     
       5. A regenerative braking system according to claim 1 wherein said sensing means includes first detector means operable to produce a first reduction in regenerative capacity upon attainment of a first voltage in said conductor and second detector means operable to produce a further reduction in regenerative capacity upon attainment of a second voltage in said conductor. 
     
     
       6. A drive and braking system for a vehicle comprising an electric motor, power supply means to supply power from an external source to said motor, command means to condition said motor in either a .[.first condition.]. .Iadd.motoring mode .Iaddend.in which said motor receives power from said power supply to drive said vehicle or a .[.second condition.]. .Iadd.braking mode .Iaddend.in which said motor supplies power to said power supply to retard said vehicle, control means to control the efficiency of said motor and sensing means to sense the voltage of said power supply means, said control means .Iadd.being inoperative when said motor is in said motoring mode and .Iaddend.being operable when said motor is in said .[.second condition.]. .Iadd.braking mode .Iaddend.to reduce the efficiency of said motor and thereby reduce the power generating capacity thereof upon said sensing means sensing a predetermine voltage at said power supply means. 
     
     
       7. A control system for an induction motor to permit regenerative braking thereby comprising current supply means to connect said motor and an external source, frequency control means to control the frequency at which said current is supplied to said motor, said frequency control means including a synchronous frequency generator to provide a first signal indicative of the synchronous frequency of said motor, a slip frequency generator to generate a second signal proportional to the slip frequency, mode control means to condition said frequency control means to either a motoring mode in which said first and second signals are added increase the supply frequency above said synchronous frequency to provide power to said motor from said source or a braking mode in which said first and second signals are subtracted to reduce the supply frequency of said motor to less than said synchronous frequency, to provide power from said motor to said source, and slip modifying means operable in said braking mode to reduce further said supply frequency, said slip modifying means .Iadd.being inoperative in said motoring mode .Iaddend.being controlled by sensing means to sense the voltage of said external source whereby upon said voltage attaining a predetermined value, said slip modifying means reduces the supply frequency to reduce the regenerative capacity of said motor. 
     
     
       8. A control system according to claim 7 wherein said slip modifying means includes a signal generator whose output is directly proportional to the synchronous frequency of said motor. 
     
     
       9. A control system according to claim 8 including latch means to maintain said further reduction until said mode control means conditions said frequency control means to said motoring condition.