Apparatus for controlling an input power to an induction motor

An apparatus for use with a three-phase AC induction motor having a switching device to apply an input power to the motor for controlling the input power, said apparatus comprising:

• means for detecting a phase voltage for each phase of the motor;
• means for deriving a phase signal from each of the phase voltages to determine a firing angle;
• means for detecting an input current for each phase of the motor;
• means for detecting a switch voltage across the switching device;
• means for sampling the detected switch voltage at a predetermined sampling rate;
• means for calculating an input power value drawn to the motor by averaging the summation of the products of the phase voltages and the currents whenever the detected switch voltage is sampled;
• means, having a plurality of memory locations, for storing the calculated input power values, wherein each of the calculated input power value is stored in a memory location indicated by the level of the sampled switch voltage corresponding thereto;
• means for comparing a first power sum with a second power sum, wherein the first power sum is the summation of the calculated input power values stored in a plurality of memory locations located at a lower level with respect to the level of the sampled switch voltage and the second power sum is the summation of the calculated input power values stored in the plurality of memory locations located at a higher level with respect to the level of the sampled switch voltage;
• and
• means for increasing the firing angle by a first predetermined amount at a time if the first power sum is greater than the second power sum and for decreasing the firing angle by the first predetermined amount at a time if the first power sum is less than the second power sum.