Patent ID: 7176653

Claim:
An electric motor drive for driving an alternating-current motor comprising: a frequency converter for controlling the electric motor, wherein the frequency converter comprises a rectifier and an inverter; an intermediate circuit connecting an output of the rectifier and an input of the inverter for reducing the surge current from the power source; and a current measuring unit, in a three-phase network, measuring or monitoring a three-phase current to the electric motor, wherein the inverter includes, a plurality of inverter contactless switches for controlling the conduction of current for supply to the electric motor, and an auxiliary switch connected to selected ones of said inverter contactless switches, the auxiliary switch includes a switching transistor having an emitter, base and collector with the emitter connected to a safety device, a control signal input to said auxiliary switch is supplied to the respective inverter contactless switch when the safety device is not actuated, the safety device, when actuated, disabling said respective inverter contactless switch by controlling the conduction of said auxiliary switch so that supply of current to the electric motor by said inverter may be disabled by said safety device.