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Alternating current motor control system with emergency control responsive to failure of power supply

  • US 5,070,290 A
  • Filed: 03/13/1989
  • Issued: 12/03/1991
  • Est. Priority Date: 05/20/1987
  • Status: Expired due to Fees
First Claim
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1. A control system supplied by a fixed frequency alternating current (AC) source (10) for controlling an AC machine (40) in a motor mode in which variable frequency AC is supplied thereto and a generator mode in which nonfixed frequency AC is generated therein, comprising:

  • first means (20), connected to said fixed frequency AC source, for converting said fixed frequency AC to DC in said power mode and for converting DC to fixed frequency AC for supply to said source in said generator mode;

    second means (30), connected to said first means to receive DC therefrom, in said motor mode, and responsive to a variable speed control signal (50a), for converting said DC to AC of a controlled frequency and amplitude for supply to said AC machine;

    third means (58 or

         66) responsive to the level of current or the level of voltage exceeding a predetermined level in said first means in said generator mode for providing a dynamic braking signal for commencing dynamic braking;

    fourth means (35), associated with said AC machine or its load (34), for applying a mechanical brake (35a, 35b) thereto in response to a mechanical braking signal (36); and

    fifth means (50) for providing said variable speed control signal for controlling said second means to drive said motor at an adjustable speed in said motor mode and, in response to a power failure signal at a time T0, for preventing said first means from providing said fixed frequency AC to said source and for reducing the magnitude of said controlled frequency AC to a predetermined minimum limit low enough to maintain a synchronous relationship between said controlled frequency AC and said machine for a selected period (TD) and, after said period elapses, for causing said third means to provide said dynamic braking signal for slowing said machine (for a period from t2 to t5) and thereafter supplying said mechanical braking signal, and, on the other hand, during said generator mode, in response t one of current level and voltage level in said first means exceeding said predetermined level, for providing said braking signal for activating said fourth means at an initial braking torque level and then monitoring (104,

         108) said power source for detecting power failure during a timed period while said fourth means being so activated, said fifth means preventing said first means from providing said fixed frequency Ac and for reducing the magnitude of said controlled frequency AC to said predetermined minimum level for said selected period (TD) when power failure is detected while said fourth means being so activated, and, after said timed period expires, otherwise controlling said fourth means for adjusting braking torque of said mechanical brake at a torque level greater than said initial braking torque level for decelerating said motor to stop.

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