3 Power-up Sequence

The traction inverter motor control system must integrate a large bank of filter capacitors, typically called DC link capacitors, which generates a huge inrush current once the main contactors are closed while the capacitors are fully discharged. Therefore, a pre-charge function is mandatory to limit the inrush current during the power-up procedure. A contactor combined with a current-limiting resistor is the simplest implementation of the pre-charge function.

A process example during the power-up sequence follows:

1. Close the main negative contactor after power-up command is received.

2. Close the pre-charge contactor.

3. Close the main positive contactor until the voltage at the DC link capacitor reaches 90%–95% of battery pack voltage.

4. Open the pre-charge contactor after the main positive contactor is fully closed.

Figure 3-1 Power-up Sequence With Main Contactors and Pre-charge Contactor.

Additionally, no contact can be stuck and no insulation leakage failure is allowed before closing the contactors. Contactor status diagnosis and insulation detection must be implemented in the system-level design.