Detection of welded contactor using AC coupling

1. A system for detecting a condition of a contactor configured to connect a battery string to a direct-current (DC) bus in a vehicle, the system comprising:

a controller configured to open or close the contactor;

an alternating-current (AC) signal source configured to provide a first AC test signal when the controller opens the contactor;

an AC coupling circuit having a primary side and a secondary side, the primary side being connected to the AC test signal source, the secondary side connected to the contactor;

a detection circuit configured to:

receive a first AC return signal corresponding to the first AC test signal, and

detect a defective condition of the contactor based on the first AC return signal; and

at least one capacitor coupled between the secondary side of the coupling circuit and the contactor.

2. The system of claim 1 , wherein the detection circuit is further configured to:

determine an amplitude of the first AC return signal; and

detect the defective condition of the contactor if the amplitude of the first AC return signal exceeds a threshold.

3. The system of claim 1 , wherein:

the AC test signal source is further configured to provide a second AC test signal when the controller closes the contactor; and

the detection circuit is further configured to:

receive a second AC return signal corresponding to the second AC test signal, and

detect the defective condition of the contactor based on the first and second AC return signals.

4. The system of claim 3 , wherein the detection circuit is further configured to:

determine a difference between an amplitude of the first AC return signal and an amplitude of the second AC return signal; and

detect the defective condition of the contactor if the determined difference is less than a threshold.

5. The system of claim 1 , wherein the controller is further configured to disconnect the battery string from the DC power bus if the contactor is defective.

6. The system of claim 1 , wherein the first AC test signal has an amplitude ranging from about 10 μA to 25 mA and a frequency ranging from about 100 kHz to about 10 MHz.

7. The system of claim 1 , wherein the coupling circuit is a transformer.

8. The system of claim 1 , further comprising a resistor connected in parallel with the contactor.

9. The system of claim 1 , wherein the detection circuit is further configured to provide a warning signal if the defective condition of the contactor is detected.

10. A vehicle, comprising:

a battery string;

a direct-current (DC) power bus;

a contactor configured to connect the battery string to the DC power bus;

a controller configured to open or close the contactor;

an alternating-current (AC) signal source configured to provide an AC test signal when the controller opens the contactor;

an AC coupling circuit having a primary side and a secondary side, the primary side connected to the AC test signal source, the secondary side connected to the contactor;

a detection circuit configured to:

detect an AC return signal corresponding to the AC test signal, and

determine a defective condition of the contactor based on the AC return signal; and

at least one capacitor coupled between the secondary side of the coupling circuit and the contactor.

11. A system for detecting a condition of a contactor configured to connect a battery string to a direct-current (DC) bus in a vehicle, the system comprising:

a controller configured to open or close the contactor;

an alternating-current (AC) signal source configured to provide a first AC test signal when the controller opens the contactor;

an AC coupling circuit having a primary side and a secondary side, the primary side being connected to the AC test signal source, the secondary side connected to the contactor;

a detection circuit configured to:

receive a first AC return signal corresponding to the first AC test signal, and

detect a defective condition of the contactor based on the first AC return signal; and

at least one capacitor coupled between the primary side of the coupling circuit and the AC test signal source.

12. The system of claim 11 , wherein the detection circuit is further configured to:

determine an amplitude of the first AC return signal; and

detect the defective condition of the contactor if the amplitude of the first AC return signal exceeds a threshold.

13. The system of claim 11 , wherein:

the AC test signal source is further configured to provide a second AC test signal when the controller closes the contactor; and

the detection circuit is further configured to:

receive a second AC return signal corresponding to the second AC test signal, and

detect the defective condition of the contactor based on the first and second AC return signals.

14. The system of claim 13 , wherein the detection circuit is further configured to:

determine a difference between an amplitude of the first AC return signal and an amplitude of the second AC return signal; and

detect the defective condition of the contactor if the determined difference is less than a threshold.

15. The system of claim 11 , wherein the controller is further configured to disconnect the battery string from the DC power bus if the contactor is defective.

16. The system of claim 11 , wherein the first AC test signal has an amplitude ranging from about 10 μA to 25 mA and a frequency ranging from about 100 kHz to about 10 MHz.

17. The system of claim 11 , wherein the coupling circuit is a transformer.

18. The system of claim 11 , further comprising a resistor connected in parallel with the contactor.

19. The system of claim 11 , wherein the detection circuit is further configured to provide a warning signal if the defective condition of the contactor is detected.