Abstract: Timing the firing of thyristors in a motor controller is done by using a microprocessor for timing relative to the event of current reaching zero, and by using hardware that generates firing signals in response to the voltage crossing zero. A motor controller has: thyristors for connecting the motor to a source of AC electrical energy; motor voltage zero crossing circuitry and motor current zero crossing circuitry for measuring a LAG TIME indicative of the time interval between the event of voltage applied; to the motor crossing zero and current flow to the motor reaching zero; a microprocessor generating a desired OFF TIME indicative of a desired time interval between the event of current flow to the motor reaching zero and firing of the thyristors; phase angle controller circuitry for firing the thyristors at a firing time after the event that the voltage applied to the motor crosses zero, the firing time being computed by adding the LAG TIME and the OFF TIME.

Abstract: An indication that a motor has reached full speed operation is produced by generating an up to speed signal based upon measurement of the phase angle between the voltage applied to the motor and electric current flowing to the motor. A motor up to speed detector has; means for connecting an electric motor to an electric power source; means for measuring the time interval between the event of voltage applied to the motor crossing zero and the event of current flow to the motor reaching zero to obtain a measured value of current lag; and, means for comparing successive measured values of the current lag at predetermined time intervals after electric power is applied to the motor as the motor is started, and for generating a signal indicative that the motor has reached an up to speed condition in the event that successive measured values of the current lag increase.

Abstract: A phase loss detection circuit for a three phase electrical system providing three phase power as to a motor, the three phase electrical being subject to a phase loss condition and a transient condition is disclosed. The transient condition includes starting transients, stopping transients or severe load transients. The phase loss detection circuit distinguishes between a phase loss condition and a transient condition and indicates when a loss of phase has occurred.

Abstract: A reference current lag for a motor controller is selected so that the reference current lag approximates the motor current lag when the motor is fully loaded. A motor controller has a plurality of thyristors for controlling electric current flow to the motor. A firing circuit fires the thyristors at a selected firing angle. A current lag, as a phase angle between a power line voltage applied to the motor and electric current flow to the motor, is measured. The selected firing angle is adjusted so that the measured current lag is substantially equal to a reference current lag. A temporary value of the reference current lag is selected. The motor current is monitored, and in the event that the motor current becomes between an upper predetermined motor current and a lower predetermined motor current, the reference current lag is given a final value.

Abstract: Gradual starting of an electric motor is provided by increasing the voltage applied to the motor. A reduced voltage starter controls the application of a voltage derived from a line voltage to a motor by using a voltage ramp acceleration control circuit comprising, means for generating a ramp voltage having a predetermined rate of rise, means for controlling the voltage applied to the motor in response to the ramp voltage, means for sensing that the motor is running at substantially full speed, and means responsive to the means for sensing that the motor is running at substantially full speed for increasing the predetermined rate of rise of the ramp voltage, whereby under conditions where the motor is running at substantially full speed before the voltage applied to the motor reaches the line voltage, then the voltage applied to the motor will rise at the increased predetermined rate of rise of the ramp voltage.

Abstract: An induction motor controller with a scaling convertor for both providing selective alteration of the overload sensitivity characteristic of the controller and converting the input current to a proportional control voltage. The feedback circuit includes a plurality of feedback resistors which are selectively connectable between the output of the current-to-voltage convertor and its input to selectively change its gain and thus the sensitivity of the controller.