Electric motor control apparatus and method

An apparatus for controlling an induction motor, which is supplied by a source of three-phase electricity having a voltage alternating in polarity at a first frequency, said apparatus comprising:

• three bidirectional thyristor switch means, each coupling a phase of the source to one of three windings of the motor;
• means for generating a control signal;
• means for producing, during each of a plurality of consecutive half-cycles of the voltage from one phase of the source, indicia of the polarity of back emf voltage induced in each winding of the motor;
• means, coupled to said means for producing, for storing a plurality of polarity indicia for each winding of the motor;
• a sensing means for determining polarity of the voltage from the source;
• means, responsive to said means for producing and said sensing means, for detecting when the voltage from the source and the back emf voltage have opposite polarities;
• means for comparing the stored indicia to detect a predetermined relationship among a plurality of polarity indicia for each winding;
• a means for operating said thyristor switch means in a first mode in which current is sent through the motor during every half-cycle of current from each phase of the source, in a second mode commencing in response to the control signal in which said thyristor switch means are operated to send current through the motor in response to detection of the voltage from the source and the back emf voltage having opposite polarities, and in a third mode commencing upon detection of the predetermined relationship in which said thyrsitor switch means are controlled to sent current through the motor at an effective second frequency that is less than the first frequency.

An apparatus for controlling an induction motor, which is supplied by a source of three-phase electricity having a voltage that alternates in polarity, said apparatus comprising:

• three bidirectional thyristor switch means, each coupling a different phase of the source to one of three windings in the motor;
• means for generating a control signal;
• means for producing, during each of a plurality of consecutive half-cycles of the voltage from one phase of the source, an indicia of the polarity of back emf voltage induced in each winding of the motor;
• means, coupled to said means for producing, for storing a plurality of polarity indicia for each winding of the motor;
• a sensing means for determining polarity of the voltage from the source;
• means, responsive to said means for producing and said sensing means, for detecting when the voltage from the source and the back emf voltage have opposite polarities;
• means for comparing the stored indicia to detect a predetermined relationship among a plurality of polarity indicia for each winding;
• a means for operating said thyristor switch means in a first mode in which current is sent through the motor during every half-cycle of current from each phase of the source, in a second mode commencing in response to the control signal in which said thyristor switch means are operated to send current through the motor in response to detection of the voltage from the source and the back emf voltage having opposite polarities, and in a third mode commencing upon detection of the predefined relationship in which said thyristor switch means are controlled to send a current pulse through a winding of the motor during every other half-cycle of current in one phase of the source.

A method for controlling an induction motor, that is connected by a switch means to a source of three-phase electricity having a voltage which alternates in polarity, the method comprising the steps of:

• sensing polarity of back emf voltage induced across each winding in the motor during consecutive half-cycles of the voltage from one phase of the source to produce a set of polarity samples for each winding;
• comparing the polarity samples in each set to detect a predetermined relationship among the samples, which occurs when the motor has a speed within a given range;
• and
• altering the operation of the switch means to apply different sequences of current pulses to the motor in response to whether the predefined relationship has been detected.