Abstract: A three-phase motor is operated by a controller having sets of thyristors which trigger electricity from each phase of a supply to the motor. The controller measures the interval .phi. between the voltage and the current when one phase of the motor makes a zero crossing and measures the interval .gamma. when the thyristors are non-conductive during a half-cycle of the voltage for one phase. A control angle .theta. is calculated according to the equation .theta.=.phi.+K.gamma., where K is a positive number. The control angle .theta. is compared to a reference value to derive an error value. The times at which the thyristors are triggered is controlled in response to the error value. To start the motor, the reference value is gradually decreased to trigger the thyristors progressively earlier during each half cycle of the supply voltage. To stop the motor, the reference value is gradually increased to trigger the thyristors progressively later during each half cycle of the supply voltage.

Abstract: An AC electric motor can be braked by applying current pulses to the motor when the polarity of the supply voltage is opposed to the polarity of the back electromotive force (emf) voltage across a stator coil of the motor. This current application continues until the motor has stopped. The stoppage is detected by sensing when the current is being applied during every cycle of the supply voltage. Thereafter, the polarity pattern of back emf voltage induced across each of the motor stator coils is sensed. The occurrence of a back emf voltage polarity pattern that remains unchanged for a given number of cycles is detected. Thereafter, a change in this pattern is sensed and the braking current application either ceases or continues for an additional predetermined interval.

Abstract: A three-phase motor controller includes a bidirectional thyristor switch to couple each phase of an electrical supply to a motor. The motor is started by rendering the thyristors conductive during successively greater portions of each cycle of AC electricity from the supply. In order to reduce the cooling requirements of the controller, a contactor is connected in parallel with the thyristors to provide apply electricity to the motor once it reaches full speed. When the controller receives a signal to stop the motor, the contactor is deactivated and the controller goes through a sequence to synchronize the thyristor triggering to the AC electricity. The synchronizing process provides a smooth transition from contractor control back to thyristor control of the application of electricity to the motor. Once thyristor control resumes, the controller executes a triggering routine which decreases the voltage applied to the motor and produces a braking effect.

Abstract: The speed of an induction motor is reduced by initially using dynamic electrical braking to slow the motor to a speed at which it will not remain synchronized to the A.C. supply frequency. At this point, A.C. cycle skipping is employed to apply current to the motor at an effective frequency which is a fundamental frequency component of the A.C. supply line frequency. The motor becomes synchronized at this fundamental frequency component. If desired, the motor can be brought to a complete stop by discontinuing the cycle skipping and using the dynamic braking again for a given period of time. This technique provides a very controlled stopping of the motor enabling equipment driven by the motor to be accurately positioned.

Abstract: An electric motor has first, second and third stator windings supplied by a source of alternating voltage having three phases A, B and C, respectively. A separate thyristor switch assembly couples each stator winding to an alternating voltage phase when said switch means is rendered conductive by a trigger signal. When the speed of the motor is above a first predefined level, the thyristor switch assemblies are triggered, in response to the polarity of the voltage between phases A and B being opposed to the polarity of back emf voltage induced across the third winding, to apply current through said first and second stator windings. However, when the speed of the motor is below the first predefined level, the thyristor switch assemblies are triggered, in response to the polarity of the voltage betwen phases B and C being opposed to the polarity of back emf voltage induced across the third winding, to apply current through said second and third stator windings.

Abstract: An electric motor is braked by sensing the polarity of voltage of the alternating electricity for powering the motor and sensing the polarity of the voltage induced in the motor by the back electromotive force. When these polarities are opposite the electricity is applied to the motor to generate a braking torque. The application of the electricity is stopped a given interval after it begins to be applied during consecutive voltage cycles. This interval is derived from the time period between when braking commenced to when the electricity was applied during consecutive cycle.