Low stress soft charge circuit for diode front end variable frequency drive
A variable frequency drive comprises a diode rectifier receiving multiphase AC power from a source and converting the AC power to DC power. An inverter receives DC power and converts the DC power to AC power to drive a load. A link circuit is connected between the diode rectifier and the inverter and comprises a DC bus to provide a relatively fixed DC voltage for the inverter. A bus capacitor is across the bus. A soft charge circuit limits inrush current to the bus capacitor. The soft charge circuit comprises an inductor connected between the source and the link circuit and a switch circuit in the link circuit for selectively providing a semiconverter configuration or a full bridge converter configuration to provide two stage charging of the bus capacitor.
1. A voltage source inverter comprising:
a rectifier having an input for receiving multi phase AC power and converting the AC power to DC power at an output;
an inverter for receiving DC power and converting the DC power to AC power;
a link circuit connected between the rectifier circuit and the inverter circuit and comprising a DC bus having first and second rails to provide a relatively fixed DC voltage for the inverter, and a bus capacitor across the first and second rails; and
a soft charge circuit to limit inrush current to the bus capacitor, the soft charge circuit comprising an inductor connected between the rectifier input and the second rail and a switch circuit in the second rail between the rectifier output and the bus capacitor to provide two stage charging of the bus capacitor.
2. The voltage source inverter of claim 1 wherein the inductor comprises a wye connected inductor having a neutral point connected to the second rail.
3. The voltage source inverter of claim 1 wherein the soft charge circuit further comprises a blocking diode connected between the second rail and the inductor.
4. The voltage source inverter of claim 1 wherein the soft charge circuit further comprises a sensing circuit for sensing bus voltage and a control circuit for controlling the switch circuit responsive to sensed voltage.
5. The voltage source inverter of claim 4 wherein the sensing circuit comprises a voltage divider connected across the link circuit.
6. The voltage source inverter of claim 1 wherein the soft charge circuit further comprises a control circuit for controlling the switch circuit responsive to a select time period.
7. The voltage source inverter of claim 4 wherein the control circuit turns the switch off if sensed voltage is below a select brown out level voltage.
8. A variable frequency drive comprising:
a diode rectifier receiving multi phase AC power from a source and converting the AC power to DC power;
an inverter for receiving DC power and converting the DC power to AC power to drive a load;
a link circuit connected between the diode rectifier and the inverter and comprising a DC bus to provide a relatively fixed DC voltage for the inverter, a bus capacitor across the bus; and
a soft charge circuit to limit inrush current to the bus capacitor, the soft charge circuit comprising an inductor connected between the source and the link circuit and a switch circuit in the link circuit for selectively providing a semiconverter configuration or a full bridge converter configuration to provide two stage charging of the bus capacitor.
9. The variable frequency drive of claim 8 wherein the inductor comprises a wye connected inductor having a neutral point connected to the link circuit.
10. The variable frequency drive of claim 8 wherein the soft charge circuit further comprises a blocking diode connected between the link circuit and the inductor.
11. The variable frequency drive of claim 8 wherein the soft charge circuit further comprises a sensing circuit for sensing DC bus voltage and a control circuit for controlling the switch circuit responsive to sensed voltage.
12. The variable frequency drive of claim 11 wherein the sensing circuit comprises a voltage divider connected across the link circuit.
13. The variable frequency drive of claim 8 wherein the soft charge circuit further comprises a control circuit for controlling the switch circuit responsive to a select time period.
14. A soft charge circuit for a diode rectifier front end variable frequency drive comprising:
a DC bus having a positive rail and a negative rail to provide a relatively fixed DC voltage, a bus capacitor across the bus, an inductor connected between a diode rectifier input and the negative rail and a switch circuit in the negative rail to provide two stage charging of the bus capacitor.
15. The soft charge circuit of claim 14 wherein the inductor comprises a wye connected inductor having a neutral point connected to the second rail.
16. The soft charge circuit of claim 14 wherein the soft charge circuit further comprises a blocking diode connected between the second rail and the inductor.
17. The soft charge circuit of claim 14 wherein the soft charge circuit further comprises a sensing circuit for sensing DC bus voltage and a control circuit for controlling the switch circuit responsive to sensed voltage.
18. The soft charge circuit of claim 17 wherein the sensing circuit comprises a voltage divider connected across the link circuit.
19. The soft charge circuit of claim 14 wherein the soft charge circuit further comprises a control circuit for controlling the switch circuit responsive to a select time period.
20. The soft charge circuit of claim 17 wherein the control circuit turns the switch off if sensed voltage is below a select brown out level voltage.