Synchronous motor torque control device

• US 4,931,715 A
• Filed: 02/16/1988
• Issued: 06/05/1990
• Est. Priority Date: 11/03/1986
• Status: Expired due to Term

• Current Assignee: Power Paragon, Inc. (L3Harris Technologies, Inc.)
• Original Assignee: Teledyne Inet (Teledyne Technologies Incorporated)
• Inventors: Mak, Chi, Lee, Robert H., Smith, Gerald L., Levran, Alexander, Marcinkowski, Jacek J.
• Primary Examiner(s): Smith, Jr., David
• Application Number:
  US07/156,289
• Time in Patent Office: 840 Days
• Field of Search: 318/778, 318/806, 318/798, 318/705, 318/706, 318/707, 318/709, 318/711, 318/721-723
• US Class Current: 318/709
• CPC Class Codes: H02P 25/024 controlled by supply frequency

Abstract

A method is described for controlling synchronous motor torque, utilizing a three-phase switch connected in a circuit to the input of a polyphase synchronous motor, and a switch control circuit. The switch control circuit senses the AC voltages input to the synchronous motor and produces signals that open and close the three-phase switch in phase with the input motor voltage, producing positive motor torque during normal operation. If input AC power is lost, the switch control circuit acts to prevent a reverse flow of high current from the motor from entering the power source bus. When input power, after a short time, returns, the switch control circuit acts to close the three-phase switch in such a manner as to permit only real power applied to the synchronous motor and thus positive torque that accelerates the motor speed and quickly restores the motor to synchronism with the input power source.

Claims

1. A method for controlling the torque developed by an electrical polyphase synchronous motor that receives energy from an electrical three-phase sinusoidal power source, said motor coupled to a generator and/or to an inertial load;

comprising the steps of;

(a) sensing motor line-to-neutral voltage for each phase at the motor terminals and transmitting the sensed voltages to switch control circuits;

(b) converting the sensed motor voltage sine waves to square waves that are exactly in phase with the sensed motor voltages;

(c) processing and advancing the input square waves from step b to produce thyristor gate firing pulses that lead the motor voltage by 90 degrees;

(d) applying the thyristor gate firing pulses to the thyristors in a three-phase static switch which is in series with the motor input lines, and turning on the thyristors in sequence;

(e) prohibiting the application of thyristor gate firing pulses if an input power outage is detected;

(f) detecting a first coincidence of input source voltage and motor voltage zero crossing after the input power returns, and resuming the application of thyristor gate firing pulses to the thyristors in the three phase static switch; and

(g) controlling the switching of the three phase static switch to permit net positive power input to the motor, producing positive motor torque and therefore an acceleration of the motor to synchronism with the source power as quickly as possible.

2. The method of claim 1 wherein step (e) includes:

prohibiting the application of thyristor gate firing pulses if an input overvoltage or overcurrent is detected.

3. The method of claim 1 wherein step (e) includes:

prohibiting the application of thyristor gate firing pulses if an out-of-synchronism frequency condition, greater than a pre-determined limit, is detected between the source bus voltage and the motor voltage.

4. In combination with a circuit for controlling operation of a polyphase synchronous motor to maintain synchronous speeds;

said circuit utilizing a three-phase contactor that is connected in series with said polyphase synchronous motor;

said circuit having sensed inputs connected to the terminals of said synchronous motor and to the input power lines of a three-phase power source, and having a `contactor close` circuit and a `contactor trip` circuit which produce signals that are connected to relays, that in turn, energize or deenergize the coil causing said three-phase contactor to close or to trip open;

the improvement claimed being the modification of said `contactor close` circuit;

said contactor close circuit comprising a motor voltage detector subcircuit and an identical source voltage detector subcircuit, an AND gate, a third `one-shot` circuit, the auxiliary contacts of said three-phase contactor and a startup circuit means;

each identical voltage detector subcircuit comprising a stepdown transformer, a current limiting resistor, a comparator and a `one-shot` circuit;

in said source voltage detector subcircuit, the primary coil of a second stepdown transformer being coupled line-to-neutral Phase A with said power source, receiving a sinusoidal input voltage signal;

the secondary coil of said second stepdown transformer being connected to the input of a third comparator via a third current limiting resistor;

said third comparator producing a DC square wave which is in phase with the positive half cycle of the sensed source voltage;

the output signal of said third comparator being communicated to the input of a first `one-shot` circuit which processes said DC square wave input and outputs a 10 microsecond or suitable width voltage pulse coinciding with the leading edge of said DC input square wave;

said motor voltage detector subcircuit being identical in every way to said source voltage detector circuit described above, except that its input is coupled line-to-neutral, Phase A with the terminals of said synchronous motor;

the 10 microsecond voltage pulses from both the said motor voltage and source voltage detector subcircuits being communicated to the input of said AND gate;

said AND gate also having connected a status input line originating at the auxiliary contacts of said three-phase contactor and indicating contactor status by a `low` for contactor closed and a `high` for contactor open states;

said AND gate looking at the phase tolerance windows represented by said voltage pulses;

if there is an overlap in the `windows` and if the contactor status line is high, said AND gate will output a spike voltage pulse to the input of said third `one-shot` circuit;

said third `one-shot` circuit upon receiving said spike voltage, outputting a high `close contactor` signal which is communicated to said control relay means causing closure of said three-phase contactor;

the input of said third `one-shot` circuit also being connected to the output of said startup circuit means, and responding to receipt of such `startup` signal by outputting a high `close contactor` signal.

5. A method for providing quick recovery to synchronous speeds for a polyphase motor upon return of input power following a power outage, said polyphase motor receiving energy from an electrical polyphase sinusoidal power source, said polyphase motor coupled to a generator and/or to an inertial load;

comprising in combination the steps of;

(a) sensing power source line to neutral voltages and transmitting the sensed voltages to switch control circuits;

(b) sensing motor line to neutral voltages and transmitting the sensed voltages to switch control circuits;

(c) producing pulsed drive signals in a sequence that is related to the sensed condition of said power source and motor voltages;

(d) applying the pulsed drive signals to the gates of switches in a polyphase static switch means which is in series with the motor input lines and turning the switches on in sequence; and

(e) controlling the switching of the polyphase static switch means to permit positive power input to the motor, producing positive motor torque and therefore an acceleration of the motor to synchronism with the source power as quickly as possible.