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  • Writer's pictureProject Fig-Leaf

What the HVJB is pre-charging?

Unless you have an electrical engineering degree, you might be forgiven for wondering what on earth terms like High Voltage Junction Box (HVJB) and pre-charging mean.

Electric motors turn by the power of magnetism. This is achieved with electromagnetic coils and in the Leaf motor’s case, permanent magnets. The coils of wire fire to pull the magnets in a precisely timed sequence using a resolver which reads the position of the motor (a bit like the timing sequence in a petrol car).

These coils of wire use enormous bursts of static electricity stored in a capacitor within the inverter. Capacitors are great at storing and emitting large burst of energy, but they can be tricky to control. When you connect an uncharged capacitor to an energy source, it wants to soak up that energy as quickly as possible. If this surge is not controlled carefully, some of the more delicate components within the inverter can end up being damaged.

Before an inverter is used, most systems will throttle the flow of electrons through a resistor until the capacitors are pretty much full. This is called pre-charging. Once the capacitors are full, the pre-charge resistor becomes an obstacle for the electrons to move freely when you want to drive so a new route (or junction) needs to be made. This is done using a High Voltage Junction Box (HVJB). The HVJB uses contactors (big switches) to control and change the flow of electrons in the car. Cue terrible schematic drawing...

Initially, when we switch the car on, the ground contactor will engage to complete the negative side of the circuit. When the ignition switch is started, the pre-charge contactor will engage, completing the positive side of the circuit and allowing electrons to flow (slowly) through the pre-charge resistor. When the inverter’s capacitor is full, the Positive contactor engages allowing electrons to flow to the inverter, unimpeded by the pre-charge resistor.

All of this is controlled by the VCU board which we talked about in our previous post Board to Tears.


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