Electric motor controller with temperature protection

A method of controlling an electric motor having three windings connected by three solid state switches to a three-phase source of electricity wherein electricity in each phase changes polarity between alternating first and second half-cycles, steps of said method comprising:

• (a) triggering the three solid state switches to apply electricity from the three-phase source to the electric motor to produce rotation of the motor shaft, wherein triggering of one solid state switch is delayed by a first phase angle delay after said first thyristor switch has become non-conductive in each one of a plurality of first half-cycles to create a first non-conductive period during a portion of said each one of a plurality of first half-cycles, wherein triggering of said first thyristor switch is delayed by a second phase angle delay after said first thyristor switch has become non-conductive in each one of a plurality of second half-cycles to create a second non-conductive period during a portion of said each one of a plurality of second half-cycles, in which said plurality of second half-cycles are interleaved with the plurality of first half-cycles, and wherein the first non-conductive period is longer in duration than the second non-conductive period;
• (b) sensing a DC component of electric current flowing through a motor winding that is connected to the one solid state switch and providing a first value indicative of a magnitude of the DC component of electric current;
• (c) sensing a DC component of voltage across the motor winding that is connected to the one solid state switch and providing a second value indicative of a magnitude of the DC component of voltage;
• (d) determining a resistance value R.sub.T from the first and second values;
• (e) determining a temperature of the motor from the resistance value;
• (f) repeating steps (a) through (e) to provide sufficient excitation of the motor windings to produce rotation of the motor shaft, while at the same time monitoring the temperature of the motor obtained in step (e), and if the temperature exceeds a thermal limit for the motor, then altering the triggering of the three solid state switches to apply a monitoring pulse of electric current through the motor winding that is connected to one solid state switch, in which said monitoring pulse is insufficient to produce rotation of the motor shaft thereby also allowing the motor to cool;
• and thereafter
• (g) repeating application of said monitoring pulse and steps (b) through (e) to continue determining the temperature of the motor during application of said monitoring pulse, and when the temperature drops below said thermal limit by a predetermined amount, then discontinuing application of the monitoring pulse and resuming to perform step (f) of providing sufficient excitation of the motor windings to produce rotation of the motor shaft and monitoring the temperature of the motor.

An apparatus for controlling an electric motor having three windings supplied by a three-phase source of electricity wherein electricity in each phase changes polarity between alternating first and second half-cycles, said apparatus comprising:

• first, second and third switches, each of which coupling a phase of the source to a winding of the electric motor;
• a switch controller connected to operate said first, second and third switches in order to apply electricity to produce movement of the motor, wherein said first switch is operated to be in a conductive state for a longer period of time during each one of a plurality of first half-cycles than during each one of a plurality of second half-cycles which are interleaved with the plurality of first half-cycles;
• a first sensor is connected to said first switch and provides a first value that indicates a DC component of electric current flowing through one motor winding that is connected to said first switch;
• a second sensor is connected across the motor and provides a second value that indicates a DC component of voltage across the one motor winding;
• a first mechanism connected to said sensors for compensating said first and second values;
• and
• a second mechanism connected to said sensors for determining the resistance.

In a motor control apparatus for determining the resistance of a winding within an alternating current electric motor by energizing the motor with asymmetrical voltage pulses to thereby introduce a DC component in the winding, by sensing the magnitude of the DC voltage and DC current resulting from the asymmetrical voltage pulses, and by computing the resistance of the winding based upon the sensed magnitude of the DC voltage and DC current, the improvement comprising:

• means for operating the electric motor with symmetrical voltage pulses, without introducing a substantial DC component in the winding;
• means for storing a DC voltage offset value and a DC current offset value, respectively obtained by reading the output of the sensing circuit while the motor is operated without a DC component;
• means for producing a corrected DC voltage measurement value and a corrected DC current measurement value by subtracting the respective DC voltage offset value and DC current offset value from the raw DC voltage measurement value and the raw DC current measurement value when the motor is operated with a DC component introduced by energizing the motor with asymmetrical voltage pulses;
• and
• means for computing the resistance of the winding utilizing the corrected DC voltage measurement value and the corrected DC current measurement value to thereby increase the accuracy of the resistance value which is computed.