I am currently getting into the topic of electric vehicles and I often come across the terms link voltage and link capacitor.

simulate this circuit – Schematic created using CircuitLab

As far as I know is that the fully charged battery must not be switched suddendly to the system (AC/DC and Motor) because of the link capacitor. The high inrush current could damage this capacitor and the link voltage should be raised steadily using a pre-charge switch which is in series to a pre-charge resistor. After pre-charging the intermediate curcuit, the main switch (without the pre-charge resistor) of the battery can be closed. Therfore without power loss.

First of all I don't know to which component this capacitor belongs. Is it built in the AC/DC converter? If so why is it needed? Or is it just because of the capacitive behavior of the cable?

• Where do the terms link voltage/capacitor belong to? To the field of power electronics?

• Could someone plase give me more information on the terms link voltage and link capacitor regarding charging and discharging of the battery system?

• How long does it usually take to load the capacitor and what are the damages that could happen if no pre-charging happens?

• Does the link voltage has to be the same as the current battery voltage?

• Where do you measure the link voltage? Across the link capacitor?

Are there any online materials or books available you can recommend?

• Link caps in a mains powered VFD are used to smooth the rectified mains supply (noisy DC, even from a 3 phase supply). They also provide a bit of a smoothing function in a regeneration mode. How much depends on the caps and the design. Commented Sep 20, 2015 at 12:49

You're right about the precharge circuit. It's needed to limit the current when turning on the drive after the link capacitors have discharged. They're designed to have a very low series resistance, so initially they'll appear as a short circuit and could damage the battery/cabling/etc.

The capacitors are physically in the inverter (DC/AC converter), they're as close to the transistors (likely IGBT's) as they can be, they exist to minimize the ripple in the DC link from switching in the drive. So, in terms, it's actually because of the inductive nature of the cable that they're needed.

I've most often heard these terms in power electronics, around motor drives like this.

Link Voltage simply refers to the voltage across those capacitors, in this case it's also the battery voltage. It's crucial to know in the control system so it can control the behavior of the motor. The capacitors probably don't have much to do with the charging or discharging of the battery system, they have much less capacitance than the super caps that are sometimes used in place of batteries.

The precharge time depends on the size of the drive, my experience is with ~2000HP VFDs and they take upwards of 30 seconds to precharge. If you don't precharge you might blow some capacitors or burn up cabling because of the inrush.

A typical EV has 300-400 Volt battery rather than 48V although some mild-hybrids do use 48V.

The converter is DC to AC not AC to DC and referred to as the inverter. The link capacitor is part of the inverter and would not be externally accessible

The Link capacitor may have a value of a few hundred microfarads. It will take a second or so to precharge through a separate relay and resistor to limit the current. When it is charged the main contactor engages and shorts out the resistor and precharge relay to provide full power to the inverter.

If the capacitor was not precharged the main contactor inside the battery could suffer damage to its contacts through excessive current - burning or welding them together.

The popular power electronic motor drive topologies for electric traction draw current with lots of ripple .This is true for DC or AC motor systems.The powerelectronic circuits that dont draw significant ripple current tend to have a Link Inductor which sounds like it would be heavier and lossier than the link capacitor you are asking about .The link capacitor is best closest to the motor drive for EMC reasons and to minimise voltage spikes caused by breaking large currents quickly thruogh parasitic inductance .Some battery technologies dont like ripple ,and if the effect of ripple on battery life isnt well known then its better to be safe than sorry.In fact the power losses in the internal resistance of the battery will go down when the link capacitor is present.Inrush current issues have been covered by others.