Start winding switch protection circuit

• US 5,103,154 A
• Filed: 05/25/1990
• Issued: 04/07/1992
• Est. Priority Date: 05/25/1990
• Status: Expired due to Fees

• Current Assignee: Texas Instruments, Inc.
• Original Assignee: Texas Instruments, Inc.
• Inventors: Dropps, Kevin J., Thompson, Herbert A.
• Primary Examiner(s): Shoop, Jr., William M.
• Assistant Examiner(s): SALATA, ANTHONY J
• Application Number:
  US07/529,034
• Time in Patent Office: 683 Days
• Field of Search: 318/753, 318/778, 318/786, 318/782, 318/289, 318/788, 318/817, 307/632, 361/91, 361/23, 361/30, 361/33, 361/54, 361/56, 361/88, 361/86, 361/31
• US Class Current: 318/782
• CPC Class Codes: H02H 9/047 Free-wheeling circuits
• CPC Class Codes: H02P 1/44 by phase-splitting with a capacitor
• CPC Class Codes: H03K 17/08144 in thyristor switches

Abstract

A single phase, capacitor start electric motor is shown in which energization of the auxiliary winding is controlled by a triac. The triac is protected from excessive voltages caused by the vector sum of capacitor, line and induced voltages which can occur at turn-off of the triac. The triac is protected in several embodiments by a gated semiconductor switch serially connected to a current limiting or discharge resistor, both being coupled across the triac. The semiconductor switch has a voltage breakdown device connected in its gate circuit. When voltage across the triac exceeds a selected level the gated semiconductor is turned on thereby drawing sufficient current momentarily to limit the voltage across the triac to a safe level and allowing the capacitor to discharge its voltage through the auxiliary winding circuit. In another embodiment a stack of high voltage bilateral triggering devices is placed across the triac.

Claims

1. A single phase AC induction motor comprising a main winding and an auxiliary winding both connectable to an AC power source, a start capacitor for providing a phase shifted field, auxiliary winding switch means for connecting and disconnecting the start capacitor and auxiliary winding to and from the AC source, auxiliary winding switch protection means including a gated semiconductor switch serially connected to a discharge resistor coupled across the auxiliary winding switch and a voltage breakdown device connected to the semiconductor switch and connected to be directly responsive to the voltage across the auxiliary winding switch, the breakdown device selected to breakdown at a voltage level to prevent excessive voltage appearing across the auxiliary winding switch.

2. A single phase AC induction motor according to claim 1 in which the auxiliary winding switch is a triac.

3. A single phase AC induction motor according to claim 2 in which the gated semiconductor switch is a triac.

4. A single phase AC induction motor according to claim 3 in which the breakdown device comprises a plurality of serially connected sidacs selected to breakdown at approximately 700 volts.

5. A single phase AC induction motor according to claim 3 in which the breakdown device is a sidac connected to the junction between a resistor and a capacitor coupled across the triac.

6. A single phase AC induction motor according to claim 2 in which the gated semiconductor switch is an SCR.

7. A single phase AC induction motor according to claim 2 in which the breakdown device comprise a plurality of serially connected zener diodes selected to breakdown at approximately 700 volts.

8. A single phase AC induction motor according to claim 2 in which the breakdown device is a zener diode connected to the junction between a resistor and a capacitor coupled across the triac.

9. A single phase AC induction motor according to claim 1 in which the auxiliary winding switch comprises back to back SCR'"'"'s.