Patent ID: 8030870

Claim:
A method for reducing the influence of a DC component in a load current of an asynchronous three-phase motor, in which the voltages of two of the three phases are controlled by adjusting firing angles of semiconductor devices of the type turning off at zero-crossing of the current therethrough, two connected in anti-parallel to each other in each phase, characterized in that the method comprises the following steps carried out for each said controlled phase: a) detecting the turn-off time of a first of said semiconductor devices and the subsequent turn-off time of the other, second semiconductor device, b) determining the time period, called second time period, of the current half-cycle of said second semiconductor device as the difference between said two turn-off times, c) calculating, based upon information about said second time period, a value of a change of firing angle of said first and/or second semiconductor device needed for changing the length of said second time period for compensating for the influence of any DC component of the current through this semiconductor device thereupon, and d) determining the firing angle of said first and/or second semiconductor device in dependence of the result of said calculation for reducing the influence of a possible DC component in the current.