Importance

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Abstract

An electric motor has three windings which are supplied by a three-phase source of electricity that has one polarity during a first half-cycle and another polarity during a second half-cycle. The motor is controlled by an apparatus that has three thyristor switches coupling the phases of the source to the motor windings. A switch controller triggers the thyristor switches to apply electricity that produces movement of the motor. One thyristor switch is operated to be in a first non-conductive state during a portion of every first half-cycle and in a second non-conductive state during a portion of every second half-cycle. The first non-conductive states are longer in duration than the second non-conductive states. A current sensor provides a first value that indicates the magnitude of the DC component of electric current flowing through one motor winding that is connected to the one switch. A voltage sensor provides a second value that indicates the magnitude of the DC component of voltage across the one motor winding. The resistance of the one motor winding is determined from the first and second values and the resistance is used to calculate the temperature of the motor. Should the temperature reach a maximum operating temperature, the triggering of the thyristor switches ceases thereby bringing the motor to a stop.

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First Claim