Eaton Corporation

Dennis H Braun

Robert W Spink

A method is provided for measuring the torque of an AC motor (4) without a mechanical transducer by sensing input voltage, and input current regardless of phase, and determining the torque therefrom.

Method of measuring torque of an AC motor

The control system has application to polyphase AC wound rotor motor which employ opposed parallel connected phase controlled thyristors together with series connected fixed resistors in each leg of a delta connected secondary impedance network. The line-to-line secondary terminals of the motor provide a source of polyphase variable frequency and variable amplitude AC reference signals. The variable frequency, variable voltage reference signals are converted into complementary pairs of square wave synchronization voltages. These same reference voltages are also concurrently integrated during their respective positive and negative half cycles to provide complementary pairs of constant peak amplitude, variable frequency voltages which are combined with associated ones of the aforementioned synchronization voltages to provide synchronization locks. These synchronization locks will not allow the synchronization voltages to recognize multiple sinusoidal zero crossings which appear on the original AC reference voltages due to space harmonics generated by the motor or spurous electrical noise until a time dependent on the fundamental frequency, well beyond the desired control recognized zero crossing of the fundamental waveforms. The generated synchronization voltages are additionally used for speed detection, integrator reset and gate pulse lockout. The variable frequency, variable voltage reference signals are also integrated and compared with a DC command signal to provide a ramp and pedestal type 180.degree. gate current firing period for their associated pairs of the thyristors. The aforementioned control of the secondary thyristors is shown and described with a polyphase AC wound rotor motor as used in an industrial hoist system which additionally employs an auxiliary eddy current retarding brake. Establishment of proper directional power connection to the motor primary terminals is afforded through operation of a multiposition master controller. The master controller in certain positions in both hoist and lower direction effects shunting of the controlled thyristor and series resistors to permit the motor to run in an open loop mode. The master controller in accordance with its positioning concurrently effects changes in a speed reference voltage which is used in conjunction with the gate current firing control for the secondary thyristors. Operation of the controller also coordinates the energization of the eddy current brake in certain lower speed positions, and during plugging modes to insure smooth transition when going from lowering to hoisting modes.

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Brookfield, WI, United States of America

Dennis H Braun