Control systems for cranes

Claim 1

A control system for a reversible electric crane or hoist motor having a brake associated therewith, comprising an electrical supply source; supply circuit means including motor reversing means, brake actuating means, and thyristor means connecting said supply source to said motor; controller circuit means including a tachogenerator driven by said motor, a static control circuit receiving a feedback signal from said tachogenerator and connected to and applying a control signal to said thyristor means for adjusting the firing angle of said thyristor means in response to said feedback signal, thereby accurately and smoothly controlling the speed of said motor; said controller circuit means also including hoisting and lowering contact means operable to actuate said motor reversing means to a hoisting or lowering position and to actuate said brake actuating means to a brake release condition, and means associated with said static control circuit for permitting a sufficiently heavy load to initially accelerate downwardly under gravity to a predetermined speed when said hoisting and lowering contact means has been set for lowering and for then causing said static control circuit to change the firing angle of said thyristor means to apply current to said motor in a hoisting direction, thereby applying sufficient motor braking power to maintain said predetermined speed.