Variable Frequency Drive System Apparatus and Method for Reduced Ground Leakage Current and Transistor Protection

• US 20080303469A1
• Filed: 12/30/2005
• Published: 12/11/2008
• Est. Priority Date: 12/30/2005
• Status: Active Grant

• Current Assignee: SMC Electricial Products Incorporated (Accton Technology Corporation)
• Original Assignee: SMC Electricial Products Incorporated (Accton Technology Corporation)
• Inventors: Nojima, Geraldo
• Granted Patent: US 8,503,180 B2
• Time in Patent Office: Days
• US Class Current: 318/503
• CPC Class Codes: H01L 2924/00 Indexing scheme for arrangements or methods for connecting or disconnecting semiconductor or solid-state bodies as covered by H01L24/00
• CPC Class Codes: H01L 2924/0002 Not covered by any one of groups H01L24/00, H01L24/00 and H01L2224/00
• CPC Class Codes: H02M 7/003 Constructional details, e.g. physical layout, assembly, wiring or busbar connections
• CPC Class Codes: H02P 27/06 using dc to ac converters or inverters H02P27/05 takes precedence

Abstract

As applications of variable frequency drives (VFD) (50) continue to grow so do challenges to provide VFD (50) systems meeting application specific requirements. For multiple reasons to include safety standards and electromagnetic interference, reduced ground leakage current is desirable. Building high output voltage VFDs (50) using transistors rated at voltages lower than the VFD output voltage is desirable for economic reasons. The apparatus and method described herein meet these challenges and others, in part by placing an electrically insulating plate (cp176) having high thermal conductivity, a low dielectric constant, and high dielectric strength between the heat sink plate of a VFD power semiconductor module and a grounded cooling plate (80 TE). The positive effects of this plate installation include reducing ground leakage current induced by system capacitances to ground upon high frequency voltage steps and increasing the effective dielectric strength of the VFD'"'"'s (50) transistor modules engaging in high reliable VFD (50) voltage output for a given transistor rating.

Claims

1. A method of reducing ground leakage current in a VFD system comprising:

floating a neutral point on a DC bus of the VFD, and

disposing an electrically insulating plate which has high thermal conductivity, a low dielectric constant, and high dielectric strength between a VFD power semiconductor module and a cooling plate.

2. The method of reducing ground leakage current in a VFD system according to claim 1, further comprising:

connecting a first end of cables to respective output terminals of the VFD and connecting a second end of the cables to a device to be driven by the VFD.

3. The method of reducing ground leakage current in a VFD system according to claim 2, further comprising:

applying a common mode filter to the cables connected to the respective output terminals of the VFD.

4. The method of reducing ground leakage current in a VFD system according to claim 2, wherein the ground leakage current is reduced to less than 1.5 amps.

5. The method of reducing ground leakage current in a VFD system according to claim 3, wherein the ground leakage current is reduced to less than 1 amp.

6. The method of reducing ground leakage current in a VFD system according to claim 4, wherein the ground leakage current is reduced to less than 1.5 amp, and wherein reduction to less than 1.5 amp is achieved despite a capacitance to ground due to the cables connected to the output terminals of the VFD.

7. The method of reducing ground leakage current in a VFD system according to claim 4, further comprising

driving a motor using an output voltage of the VFD, wherein the ground leakage current is reduced to less than 1.5 amp, and wherein reduction to less than 1.5 amp is achieved despite a motor capacitance.

8. The method of reducing ground leakage current in a VFD system according to claim 5, wherein the ground leakage current is reduced to less than 1 amp, and wherein reduction to less than 1 amp is achieved despite a capacitance to ground due to the cables connected to the output terminals of the VFD.

9. The method of reducing ground leakage current in a VFD system according to claim 5, further comprising

driving a motor using an output voltage of the VFD and wherein the ground leakage current is reduced to less than 1 amp, and wherein reduction to less than 1 amp is achieved despite a motor capacitance.

10. A method of reducing ground leakage current in a medium voltage VFD system comprising:

floating a neutral point on a DC bus of the VFD; and

disposing an electrically insulating plate which has high thermal conductivity, a low dielectric constant, and high dielectric strength between a VFD power semiconductor module and a cooling plate, wherein the ground leakage current is reduced to less than 1.5 amps.

11. A medium voltage VFD system having low ground leakage current comprising:

a DC bus which has a floating neutral point;

an inverter bridge, of a VFD power semiconductor module, electrically connected to the DC bus;

an electrically insulating plate which has high thermal conductivity, a low dielectric constant, and high dielectric strength; and

a cooling plate, wherein the VFD power semiconductor module is mounted on the electrically insulating plate, having high thermal conductivity, a low dielectric constant, and high dielectric strength;

wherein the insulating plate is mounted on the cooling plate; and

wherein the cooling plate is grounded.

12. The medium voltage VFD system having low ground leakage current according to claim 11, further comprising:

a first end of cables connected to respective output terminals of the VFD, a common mode filter applied to the cables and filtering an output of the VFD, and a second end of the cables connected to a device to be driven by the VFD.

13. A medium voltage VFD system having low ground leakage current according to claim 12, wherein

the ground leakage current is reduced to less than 1 amp.

14. A medium voltage VFD system having low ground leakage current according to claim 11, wherein

the ground leakage current is reduced to less than 1.5 amps.

15. A medium voltage VFD system having low ground leakage current according to claim 13, wherein the cables connected to the output terminals of the VFD are in excess of 30 feet.

16. A medium voltage VFD system having low ground leakage current according to claim 12, wherein an output voltage on the VFD output terminals is greater than 690 volts.

17. The medium voltage VFD system having low ground leakage current according to claim 11, wherein the insulating plate is a ceramic material.

18. The medium voltage VFD system having low ground leakage current according to claim 11, wherein the insulating plate is made of boron nitride.

19. The medium voltage VFD system having low ground leakage current according to claim 11, wherein a capacitance is formed across the electrically insulating plate between the VFD power semiconductor module and the cooling plate, and wherein said capacitance between the VFD power semiconductor module and the cooling plate is less than an inverter bridge transistor internal capacitance to ground, and a total capacitance to ground of the VFD system is reduced.

20. The medium voltage VFD system having low ground leakage current according to claim 11, further comprising at least one mounting device which enables electrical insulation and thermal conduction between the VFD power semiconductor module and the grounded cooling plate and secures the VFD power semiconductor to the insulating plate and secures the insulating plate to the grounded cooling plate.

21. A medium voltage VFD system comprising:

a DC bus which has a grounded neutral point;

VFD power semiconductor modules, wherein

transistor modules of an inverter bridge are connected in series, the inverter bridges is electrically connected to the DC bus, and an insulation of transistor modules is rated at less than half of a full maximum DC bus voltage rating;

an electrically insulating plate which has high thermal conductivity, a low dielectric constant, and high dielectric strength; and

a grounded cooling plate, wherein the VFD power semiconductor module is mounted on the electrically insulating plate, having high thermal conductivity, a low dielectric constant, and high dielectric strength;

wherein the insulating plate is mounted on the grounded cooling plate; and

wherein an effective insulation capability to ground of the VFD system is increased.