Associated Electrical Industries Limited

John David Markham

A dynamic braking system for a D.C. traction motor which is driven from an asymmetric bridge rectifier having freewheel diodes to permit circulation of motor generated current. The system employs field reversal to ensure a generated (braking) current direction the same as for motoring and this facility permits injection current braking by phasing up the supply bridges when the motor is braking with full excitation. The brake resistance is in two parts one of which forms an uncontrolled arm of the bridge, this part of the resistance being shunted by a thyristor. Part way through an injection braking process the supply bridge is phased down, the resistance bridge arm shorted out and the supply bridge phased up again. Lower losses and greater braking efficiency result.

Dynamic braking of direct current motors

A braking system for, primarily, separately excited D.C. traction motors, in which rheostatic braking is effected by field reversal to generate armature current freewheeling through the low impedance supply and through a brake resistor in series with the armature. A contactor shorts out the brake resistor for normal motoring. Field excitation in conjunction with armature current or armature voltage polarity reversal is detected to indicate the motor generating function, lock open the motoring contactor and prevent short circuiting of the motor/generator. Plug braking by energisation of the armature supply is thus always permissible either by brake handle and power handle or reverse handle and power handle. The armature current then contains a rheostatic braking component and a tractor braking component. What would normally be mis-use of the reversing control to achieve rapid deceleration thus becomes acceptable.

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Manchester, United Kingdom

John David Markham