Variable frequency drive circuit with overvoltage protection

Patent metadata

Field	Value
Publication number	US-2016268797-A1
Application number	US-201514656049-A
Country	US
Kind code	A1
Filing date	Mar 12, 2015
Priority date	Mar 12, 2015
Publication date	Sep 15, 2016
Grant date	—

Abstract

A variable frequency drive (VFD) circuit includes an input connectable to an AC source, a rectifier to convert an AC power input to a DC power, a DC link to receive DC power from the rectifier and having a DC link voltage thereon, a DC link capacitor bank with one or more capacitors connected to the DC link, and a pre-charge circuit coupled to the DC link capacitor. The pre-charge circuit further includes one or more resistors, one or more pre-charge relays each operable in on and off states to selectively control a current flow through the resistor(s) so as to control an initial pre-charge of the DC link capacitor, and an overvoltage relay operable in on and off states to selectively cut-off a current flow to the DC link capacitor bank, so as to prevent an overvoltage condition in the DC link capacitor bank.

First claim

What is claimed is:

1 . A variable frequency drive (VFD) circuit comprising:
an input connectable to an AC source; a rectifier connected to the input to convert an AC power input to a DC power; a DC link coupled to the rectifier to receive the DC power therefrom, the DC link having a DC link voltage thereon; a DC link capacitor bank comprising one or more capacitors connected to the DC link to smooth the DC link voltage; and a pre-charge circuit coupled to the DC link capacitor bank, the pre-charge circuit comprising:
one or more resistors;
one or more pre-charge relays each operable in an on and off state to selectively control a current flow through the one or more resistors, so as to control an initial pre-charge of the DC link capacitor; and
an overvoltage relay operable in an on and off state to selectively cut-off a current flow to the DC link capacitor bank, so as to prevent an overvoltage condition in the DC link capacitor bank.

2 . The VFD circuit of claim 1 further comprising:
one or more voltage sensors to measure the DC link voltage; and
a controller that receives an input from the one or more voltage sensors regarding the measured DC link voltage, the controller being programmed to control the overvoltage relay and the one or more pre-charge relays based on the measured DC link voltage.

3 . The VFD circuit of claim 2 wherein the rectifier comprises a six-diode bridge and the one or more pre-charge relays in the pre-charge circuit comprises a pre-charge relay; and
wherein the overvoltage relay is coupled in series with the one or more resistors and the pre-charge relay is coupled in parallel with the one or more resistors and the overvoltage relay.

4 . The VFD circuit of claim 3 wherein the controller is programmed to:
compare the measured DC link voltage to a normal DC link voltage value;
operate each of the overvoltage relay and the pre-charge relay in the off state if the measured DC link voltage is greater than the normal DC link voltage value;
operate the overvoltage relay in the on state and the pre-charge relay in the off state if the measured DC link voltage is less than the normal DC link voltage value; and
operate the pre-charge relay in the on state if the measured DC link voltage is equivalent to the normal DC link voltage value.

5 . The VFD circuit of claim 2 wherein the rectifier comprises a silicon-controlled rectifier bridge and the one or more pre-charge relays in the pre-charge circuit comprises three pre-charge relays, with a pre-charge relay being provided for each of three phases of the input.

6 . The VFD circuit of claim 5 wherein the controller is programmed to:
compare the measured DC link voltage to a normal DC link voltage value;
operate the overvoltage relay in the off state and operate each of the three pre-charge relays in the on state if the DC link voltage is greater than the normal DC link voltage value;
operate the overvoltage relay and the three pre-charge relays in the on state if the measured DC link voltage is less than the normal DC link voltage value; and
operate the three pre-charge relays in the off state if the measured DC link voltage is equivalent to the normal DC link voltage value.

7 . The VFD circuit of claim 5 wherein the silicon-controlled rectifier bridge comprises:
three silicon-controlled rectifiers; and
a resistor coupled to a gate of each of the three silicon-controlled rectifiers;
wherein each resistor and its associated silicon-controlled rectifier is in series with a respective one of the three pre-charge relays.

8 . The VFD circuit of claim 5 wherein the pre-charge circuit further comprises a diode arranged in series with each respective pre-charge relay of the three pre-charge relays;
wherein the overvoltage relay is in series with all of the three pre-charge relays; and
wherein the one or more resistors is in series with the overvoltage relay.

9 . The VFD circuit of claim 1 wherein the overvoltage relay has a small signal level and does not carry a high current, such that no additional power source is required to switch the overvoltage relay between the on and off states.

10 . The VFD circuit of claim 1 further comprising an electromagnetic interference (EMI) filter positioned between the input and the rectifier; and
wherein the overvoltage relay, when in the off state, prevents failure of the DC link capacitor bank due to voltage notches present in AC power provided from the AC source and due to overvoltage ringing at input terminals of the rectifier.

11 . A method of operating a variable frequency drive (VFD) circuit that includes a rectifier, a DC link having a DC link capacitor bank with one or more capacitors thereon, a pre-charge circuit, and an inverter, the method comprising:
receiving an AC power at an input of the VFD circuit; providing the AC power to the rectifier to covert the AC power to a DC power on the DC link, the DC power comprising a DC voltage; measuring the DC voltage via one or more voltage sensors on the DC link; comparing the measured DC voltage to a list of defined voltage conditions in order to identify a voltage condition on the DC link, the voltage conditions comprising:
an undervoltage condition;
an overvoltage condition; and
a normal voltage condition; and
controlling the pre-charge circuit based on the identified voltage condition, wherein controlling the pre-charge circuit comprises:
selectively operating one or more pre-charge relays in the pre-charge circuit in an on state or an off state to control an initial pre-charge of the DC link capacitor bank; and
selectively operating an overvoltage relay in the pre-charge circuit in an on state or an off state to prevent an overvoltage condition in the DC link capacitor bank.

12 . The method of claim 11 further comprising providing the measured DC voltage to a controller in operable communication with the one or more voltage sensors and the pre-charge circuit, wherein the controller performs the comparing of the measured DC voltage to the list of defined voltage conditions and the controlling of the pre-charge circuit based on the identified voltage condition.

13 . The method of claim 11 wherein the rectifier comprises a six-diode bridge rectifier and the one or more pre-charge relays in the pre-charge circuit comprises a single pre-charge relay; and
wherein controlling the pre-charge circuit comprises:
operating each of the overvoltage relay and the single pre-charge relay in the off state if the measured DC link voltage is in the overvoltage condition;
operating the overvoltage relay in the on state and the single pre-charge relay in the off state if the measured DC link voltage is in the undervoltage condition; and
operating the pre-charge relay in the on state if the measured DC link voltage is in the normal voltage condition.

14 . The method of claim 11 wherein the rectifier comprises a silicon-controlled rectifier (SCR) bridge and the one or more pre-charge relays in the pre-charge circuit comprises three pre-charge relays; and
wherein controlling the pre-charge circuit comprises:
operating the overvoltage relay in the off state and operate each of the three pre-charge relays in the on state if the DC link voltage is in the overvoltage condition;
operating the overvoltage relay and the three pre-charge relays in the on state if the measured DC link voltage is in the undervoltage condition; and
operating operate the three pre-charge relays in the off state if the measured DC link voltage is in the normal voltage condition.

15 . The method of claim 11 w

Assignees

• Eaton Corp

Inventors

• Li Huaqiang
• He Youdong
• Li Xiaoling

Classifications

• H02M1/44 Primary

  Circuits or arrangements for compensating for electromagnetic interference in converters or inverters · CPC title

• H02M5/4585

  having a rectifier with controlled elements · CPC title

• H02H3/20 Primary

  responsive to excess voltage · CPC title

• H02M1/36

  Means for starting or stopping converters · CPC title

• H02M5/458

  using semiconductor devices only · CPC title