Patent ID: 8319490

Claim:
A method of synchronizing an inverter to a three-phase motor after a supply network failure, comprising: determining a voltage induced on an intermediate circuit by the motor, wherein the intermediate circuit is connected to the inverter and includes upper and lower controllable switches for each phase of the motor, the upper and lower controllable switches are connected in series between upper and lower potentials of the intermediate voltage circuit, a free-wheeling diode is connected in parallel with each upper and lower controllable switch, and a resistive circuit is connected in parallel with each of the lower controllable switches; measuring voltages induced on each resistive circuit in the lower controllable switches of the intermediate circuit; selecting the voltage induced on a first resistive circuit corresponding to a first phase of the motor as a reference voltage, wherein the voltage induced on a second resistive circuit corresponding to a second phase of the motor and the voltage induced on a third resistive circuit corresponding to a third phase of the motor are at first voltage levels, respectively; establishing a first time instant when the induced voltage associated with one of the second or third phases of the motor changes to a second level, wherein a value of the phase associated with the induced voltage that does not change at the first time instant is a first peak value; establishing a second time instant when the induced voltage associated with the first peak value changes to a second level, wherein a value of the phase associated with the induced voltage that does not change at the second time instant is a second peak value; determining a polarity of the phases associated with the first and second peak values by comparing the associated induced voltages with the reference voltage; and determining an angle and direction of rotation of the motor at each time instant based on the polarities of the first and second peak values.