Importance

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Abstract

Methods and control apparatus for operating a three-phase electric motor are described, in which the motor windings are switched between Star and Delta connections depending on torque requirements, and in which the motor windings are switched to a braking mode when braking torque is required. The electromagnetic torque of the motor is monitored, and a command to switch from Star to Delta is given when the electromagnetic torque rises to reach or exceed a threshold. A command to switch from Delta to Star is given when the electromagnetic torque falls to reach a second threshold. The invention discloses a method of determining instantaneous values for the electromagnetic torque of the motor based on measurements of phase current and phase voltage at particular instants in the voltage cycle. The application further discloses a method of timing the switching operation between Star and Delta configurations, or switching from Star or Delta configuration to a dynamic braking mode, to minimise inrush currents by monitoring the vector relationship between the EMF and the supply voltage and performing switching operations in each motor phase when EMF vector is aligned with the supply voltage vector. The application further discloses switching from Star to Delta or Delta to Star when the torque produced by the motor is substantially equal to the torque requirement of the load to maximise efficiency.

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First Claim