Capacitor precharging and capacitance/resistance measurement in electric vehicle drive system
An electric drive system for an electric vehicle has a DC power source and a contactor with an output coupled to a main bus and an input adapted to be connected to the DC power source. The contactor is selectably switched between an open state and a closed state. A link capacitor is coupled to the main bus. A precharge circuit is coupled between the DC power source and the link capacitor comprised of a controlled current source. The controlled current source is selectably activated with the contactor in the open state to charge the link capacitor to a predetermined voltage to before switching the contactor to the closed state.
1. An electric drive system for an electric vehicle with a DC power source, comprising:
a contactor having an input adapted to be connected to the DC power source and having an output, wherein the contactor is selectably switched between an open state and a closed state;
a link capacitor;
a main bus coupled to the output of the contactor and the link capacitor;
a precharge circuit adapted to be coupled between the DC power source and the link capacitor, wherein the precharge circuit is comprised of a controlled current source that is selectably activated with the contactor in the open state to charge the link capacitor to a predetermined voltage before switching the contactor to the closed state.
2. The drive system of claim 1 wherein the precharge circuit is comprised of:
an inductor;
a transistor switch connected in series with the inductor for selectably connecting the inductor to the DC power source; and
a control circuit for driving the transistor switch so that the inductor supplies a substantially constant current during precharging of the link capacitor.
3. The drive system of claim 2 wherein the control circuit is comprised of:
a current sensor measuring an inductor current; and
a trigger that turns on the transistor switch when the measured inductor current is below a first threshold and turns off the transistor switch when the measured inductor current is above a second threshold, wherein the second threshold is higher than the first threshold.
4. The drive system of claim 1 further comprising:
a voltage sensor measuring a link voltage across the link capacitor; and
a control circuit initiating precharging of the link capacitor by enabling the precharge circuit, monitoring the measured link voltage, and terminating the precharging by disabling the precharge circuit when the measured link voltage is greater than a voltage threshold.
5. The drive system of claim 4 wherein the control circuit determines a capacitance of the link capacitor in response to an elapsed time of precharging and a slope of the measured link voltage.
6. The drive system of claim 5 wherein the slope is comprised of a difference between a measured link voltage corresponding to a beginning of the elapsed time and a measured link voltage corresponding to an ending of the elapsed time.
7. The drive system of claim 5 wherein the controlled current is a constant current, and wherein the control circuit determines the capacitance according to the constant current divided by the slope.
8. The drive system of claim 4 wherein the controlled current is a constant current, and wherein the control circuit determines a resistance in parallel with the link capacitor in response to an elapsed time of precharging, a slope of the measured link voltage, and the constant current.
9. A method of precharging a link capacitor on a main bus in an electric vehicle drive, comprising:
activating a precharge circuit to supply a substantially constant current to the link capacitor with a main battery contactor in an open state;
deactivating the precharge circuit when the link capacitor voltage reaches a predetermined voltage; and
closing the contactor.
10. The method of claim 9 further comprising:
pulse-width modulating a transistor switch in series with an inductor for supplying the substantially constant current.
11. The method of claim 9 further comprising:
measuring a link voltage across the link capacitor; and
determining a capacitance of the link capacitor in response to an elapsed time of precharging and a slope of the measured link voltage.
12. The method of claim 11 wherein the slope is comprised of a difference between a measured link voltage corresponding to a beginning of the elapsed time and a measured link voltage corresponding to an ending of the elapsed time.
13. The method of claim 12 wherein the capacitance is determined according to the constant current divided by the slope.
14. The method of claim 9 further comprising:
measuring a link voltage across the link capacitor; and
determining a resistance in parallel with the link capacitor in response to an elapsed time of precharging, a slope of the measured link voltage, and the constant current.
15. A precharge circuit for charging a main bus link capacitor in an electric vehicle drive, comprising:
an inductor;
a transistor selectably coupling the inductor to a DC supply;
a current sensor measuring an inductor current; and
a trigger turning on the transistor when the inductor current is below a first threshold and turning off the transistor when the inductor current is above a second threshold which is higher than the first threshold.
16. The precharge circuit of claim 15 further comprising:
an optocoupler connecting the transistor to the DC supply, wherein the optocoupler is adapted to be controlled remotely by a control circuit of the electric vehicle drive.