Solid state motor starting control

• US 3,916,274 A
• Filed: 07/29/1974
• Issued: 10/28/1975
• Est. Priority Date: 07/29/1974
• Status: Expired due to Term

• Current Assignee: Alexander J. Lewus
• Original Assignee: Alexander J. Lewus
• Inventors: Lewus, Alexander J.
• Primary Examiner(s): Rubinson, G. Z.
• Application Number:
  US05/492,587
• Time in Patent Office: 456 Days
• Field of Search: 318/221R,221E,227,229
• US Class Current: 318/787
• CPC Class Codes: H02P 1/44 by phase-splitting with a capacitor

Abstract

An improved solid state motor starting switch is disclosed, which switch is adaptable to single-phase induction motors of the split-phase, capacitor-start, or capacitor-start capacitor-run type, the switch circuit utilizing bidirectional semiconductor devices and a running winding current sensitive control device such as a bias resistor, inductor or transformer, for operating the semiconductor device, typically one or more triacs, to ON and OFF states, respectively, during motor starting and motor running conditions. Two rectifier or zener diodes, parallel connected back-to-back, are disposed in series connection in the gate circuit of the semiconductor switch such that the control device develops a control voltage in excess of the diode forward voltage during motor starting. During normal running, the control voltage from the control device falls below the diode forward voltage drop in the PN junction region such that leakage current is very small in order to reduce the triac gate voltage to a correspondingly small value during motor running conditions.

Claims

1. A control circuit for a single-phase electric motor of the split-phase or capacitor-start type including a main winding and a starting winding comprising:

power circuit means for connecting said main winding to a power supply;

starting circuit means for selectively connecting said starting winding to the power supply;

an a-c bidirectional semiconductor switch having first and second electrodes operatively connected in said starting circuit, and having a gate electrode for triggering the switch between a conduction state and a non-conduction state;

a current-sensitive control device connected in series with said main winding and said power circuit means; and

bias-variable impedance means including two parallel-connected, oppositely-poled diodes connected between the gate electrode of said switch and said control device, said current-sensitive control device responsive to starting current flowing through said main winding to produce a forward voltage drop across said diodes above the forward conductive threshold voltage of the diode PN junction thereof sufficient to render said diodes conductive to trigger said switch into conduction and establishing a low-impedance for said bias-variable impedance means during the conduction state of said switch, said currentsensitive control device responsive to running current through said main winding to produce a voltage across said diodes below the forward conductive threshold voltage of the diode PN junction thereof insufficient to forward bias such diodes providing sharp cut-off thereof and thereby establishing a high impedance for said bias-variable impedance means and causing said switch to be triggered to its non-conduction state.

2. A control circuit according to claim 1 wherein said current-sensitive control device comprises an inductor coil connected in series with said main winding and said power circuit means, and said bias-variable impedance means is coupled between the gate electrode of said switch and one end of said inductor coil and the first electrode of said switch is coupled to the other end of said inductor coil.

3. A control circuit according to claim 1 wherein said current-sensitive control device comprises a transformer having primary and secondary windings, the primary winding of which is connected in series with said main winding and said power circuit means and the secondary winding of which is coupled between said bias-variable impedance means and the first electrode of said switch.

4. A control circuit according to claim 1 wherein said current sensitive control device comprises a resistor connected in series with said main winding and said gate electrode of said bidirectional semiconductor switch is coupled to one side of said resistor and the first electrode of said switch is coupled to the other side of said resistor.

5. A control circuit according to claim 1 wherein said diodes comprise two rectifier diodes whereby upon motor starting conditions said rectifier diodes conduct substantially at their forward voltage drop to gate said bidirectional conductive semiconductor switch into condition, and whereby at motor running conditions said rectifier diodes become non conducting to offer high impedance in said gate electrode circuit of said bidirectional switch to cause said switch to be biased to the off state.

6. A control circuit according to claim 1 wherein said diodes comprise two zener diodes whereby upon motor starting conditions, said zener diodes conduct substantially at their forward voltage drop region to gate said bidirectional semiconductor switch to the on state and at motor running conditions, said zener diodes become non-conducting to offer high impedance in said gate electrode circuit of said bidirectional switch to cause said switch to be biAsed to the off state.

7. A control circuit according to claim 1 and further including an impedance means connected across the bidirectional semiconductor switch first and second electrodes for suppressing the transient feedback voltage peaks and the dv/dt network upon motor starting and running conditions.

8. A control circuit according to claim 7, wherein said impedance means includes a bias resistor connection in series with two zener diodes coupled back-to-back across the first and second electrodes of said bidirectional semiconductor switch to suppress the transient voltage feeback surge current and the dv/dt network upon motor starting and reversing condition.

9. A control circuit for a single-phase induction motor of the capacitor-start and capacitor-run type including a starting capacitor and running capacitor connected in parallel relation to each other and in series with starting winding circuit and running winding circuit angularly displaced in the stator core and inductively coupled to a rotor, comprising:

power circuit means for connecting said running winding circuit to a single-phase power supply;

sensing means coupled to said power circuit means for developing a control signal proportional to the load current to the running winding of the motor;

a starting circuit including a starting capacitor connected in series with said starting winding circuit and to the power supply;

a signal controlled bidirectional conductive semiconductor gate device having first and second electrodes connected in series in said starting circuit and having a gate electrode; and

variable impedance means including two parallel-connected, oppositely-poled diodes connected in circuit between said gate electrode of said bidirectional gate device and said sensing means for closing said starting circuit in response to said control signal whenever the load current to the main winding exceeds a first threshold amplitude and adapted to open said starting circuit means whenever the motor load current falls below a second threshold amplitude.

10. A control circuit according to claim 9, further including a suppression circuit connected across the input and output electrodes of said bidirectional conductive semiconductor gate device and adapted to suppress transient voltage feedback and the dv/dt network upon motor starting and running conditions.