Power inverter

• US 4,638,418 A
• Filed: 08/26/1985
• Issued: 01/20/1987
• Est. Priority Date: 08/26/1985
• Status: Expired due to Fees

• Current Assignee: Tocco, Inc. (Park-Ohio Holdings Corporation)
• Original Assignee: Tocco, Inc. (Park-Ohio Holdings Corporation)
• Inventors: Alexander, Jonathan W.
• Primary Examiner(s): Wong, Peter S.
• Assistant Examiner(s): Jones, Judson H.
• Application Number:
  US06/769,246
• Time in Patent Office: 512 Days
• Field of Search: 363/37, 363/49, 363/57, 363/58, 363/136, 363/137, 363/138, 219/10.77
• US Class Current: 363/49
• CPC Class Codes: H02M 7/517 with special starting equipment

Abstract

A power inverter particularly suited for use in applications requiring serially inductively heating workpieces in a predetermined manner, having a main inverter and an auxiliary starting inverter, in an arrangement in which the starting inverter is selectively applied and removed from the load in predetermined timed relationship with the application of the main inverter across the load, and providing a decrease in the inherent instability of the main inverter operation through an increase in the apparent Q of the load, while adjusting the output of the starting inverter to a value greater than the instability level of the main inverter.

Claims

1. In a power inverter for converting direct current from a constant current supply into alternating current, for use in applications requiring frequent energizations of serial loads having varying Q characteristics, said power inverter including a starter inverter providing initial energization of each said load, said starting inverter having a direct current input from said supply and an alternating current output which may be selectively applied across each load, means responsive to an energization level of said load to remove said starting inverter from said load when a preselected energization level is reached, and means to energize said power inverter in timed relationship with the removal of said starting inverter with said load, said power inverter having an output voltage level below which there is starting instability, the improvement comprising:

means in said power inverter for increasing the apparent Q of the load to be energized, whereby the voltage level of output instability of the power inverter is substantially reduced to a lower output voltage; and

,

means providing an output voltage at said starting inverter at least as high as said lower output voltage, whereby commutation failure of said power inverter is precluded.

2. The improvement as defined in claim 1, wherein said means increasing said apparent Q includes a D.C. choke between said power inverter and said constant current power supply.

3. The improvement as defined in claim 2 wherein said D.C. choke has an induction value of approximately 5.3 millihenries.

4. The improvement as defined in claim 2 wherein said apparent Q of the load is increased by said D.C. choke to about 6.

5. The improvement as defined in claim 1 wherein said starting inverter energization level is approximately about 10 to 15 percent of the power inverter energization level.

6. A method of operating a power inverter to reduce commutation failure in operation, the inverter of the type used in induction heat treating of sequential loads, having a constant current power supply and a starting inverter removably connectable across each load to initiate energization thereof until a preselected energization value is achieved whereupon said power inverter is energized and said starting inverter is removed from said load, said power inverter having an output voltage level below which there is starting instability, said method comprising the steps of:

(a) reducing the output voltage of starting instability to a new output voltage by increasing the apparent Q of the load; and

,

(b) providing a voltage energization level of said starting inverter at least as great as the new output voltage of starting instability in said power inverter.

7. The method as defined in claim 6 wherein said output voltage instability is reduced by providing a D.C. choke between said power inverter and said constant current power supply.

8. A method as defined in claim 7 wherein said D.C. choke is provided with an induction value of approximately 5.3 millihenries.

9. The method as defined in claim 8 wherein said preselected energization value is approximately about 10 to 15 percent of the power inverter energization level.

10. A method of operating a current source power inverter to prevent starting instability when used to provide power for heating cycles of an induction heating load, said inverter having a constant current power supply input and a small starting inverter with an output voltage removably connectable across the output of said power inverter during each heating cycle whereby said power inverter has an output voltage level below which there is starting instability and whereby said output voltage of said starting inverter is less than said instability voltage level of said power inverter, said method comprising increasing the inductance at the constant current input by an amount sufficient to reduce said instability output voltage level to a voltage only slightly below output voltage of said small starting inverter whereby the size of said inductance is minimized.

11. A method as defined in claim 10 wherein said inductance amount is sufficient to reduce said instability output voltage level less than 10% below said output voltage of said small inverter.