Voltage control

Claim 1

Voltage control apparatus in conjunction with a machine having a work member and a DC motor for driving it, an electrical circuit including controls for controlling the machine, and including the motor and means for braking the motor, the circuit also including a rectifier representing an instrumentability subject to damage by the counter emf developed in braking the motor, said voltage control apparatus comprising the controls of the machine including a first relay, and the apparatus including normally open contacts of that relay, in series with the motor and the rectifier, a second subcircuit including a second relay controlling the first relay, and when energized retaining the first relay energized, said second subcircuit also including an electrical valve, a plurality of resistances operative for imposing a negative bias on the valve to render it non-conducting, the motor being operative in response to development of a predetermined value of counter emf, for driving sufficient current through the remaining resistances and render the valve non-conducting, and thereby open the circuit to the second relay and de-energize the latter and open the said contacts of that relay and thereby put all of the resistances in circuit with the relay until that current lowers to a predetermined value, the second relay being operative for de-energizing the first relay, and thereby opening the first contacts between the motor and the rectifier, while the counter emf is above a predetermined value.