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Self protecting and automatic resetting capacitor synchronous switch apparatus for control of AC power to inductive loads

  • US 4,933,798 A
  • Filed: 04/13/1989
  • Issued: 06/12/1990
  • Est. Priority Date: 10/22/1987
  • Status: Expired due to Fees
First Claim
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1. An electrical control system for controlling the average electrical power applied to at least one electrical load device, said system comprising a DC power supply, electronic switching means connected between an alternating current source and said load device and comprising a first electronic switch comprising thyristor and second electronic switch connected in shunt with said first electronic switch and comprising a transistor, a capacitor connected to said alternating current voltage source and to said load device, and a control means, connected to the DC power supply and to said switching means, for reducing the level of average electrical power applied to the electrical load device via plurality of electrical current pathways provided by the first and second switching means, and dependent upon the switch states of the first and second electronic switches, for sequentially controlling the states of said electronic switches as follows during each AC half cycle of steady state operation:

  • (a) at the time of initial energization both switches are in the first, open state thereof, a predetermined level of electrical current flows through the electrical load in the closed circuit formed by a series circuit comprising said capacitor, said AC voltage source, and said load device;

    (b) when said system is energized and the DC power supply reaches a system operating level, and when the electric energy stored in said capacitor has declined to a level which does not cause any harmful effect when the first switch changes from the first, open state to the second, closed state thereof, to provide a further current path including the first switch, the AC voltage source, and the load device;

    (c) at a point in time within an AC half wave of system operation, the second switch changes from the first, open state thereof to the second, closed state thereof at which time the electrical current flowing in the voltage source and electrical load will momentarily divide between the two current paths formed by the first and second switches both being in the second, closed states thereof;

    (d) while the second switch is in the second, closed state for the predetermined period of time within a half wave cycle of controlled operation, the first switch changes from the second, closed state thereof back to the first, open state thereof at which time the load current flows in the current path formed by the second switch in the second, closed state thereof, the AC supply, and the load device, the second switch then returning to the first open, state thereof at the end of the predetermined time period in which said second switch is in the second, closed state thereof so that with the first and second switch both in the first, open states thereof within the AC half cycle of operation, the electrical current continues to flow by reverting to an alternate current path formed by the capacitor, the voltage source, and the electrical load device, and the load current continues to flow through the alternate current path until the electrical energy stored in the capacitor declines to a level which will not cause any harmful effect in the first switch changes from the first, open state to the second, closed state thereof, at which time the foregoing switching sequence repeats itself again in the next half cycle of operation.

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