On and Off
High Voltage Contactors are an important component of any vehicle’s High Voltage (HV) system. They are there to switch the HV battery on and off, but they are also there to do lots of things during their operation to check they are working properly but also the introduction of HV from the HV battery is done in a way that won’t harm HV components externally or within the HV battery itself.
In almost all cases when a relay (contactor) is supplied with a voltage to switch it on, the electromagnet field created from the coil operates the relay to close the main contacts and completes the circuit to a higher load.
Conversely, when the relay is switched off and the voltage supply is interrupted, the magnet field collapses, and the contacts are opened through spring pressure. Also, during this process, the magnetic field in the relay around the coil produces a reverse electrical voltage due to the magnetic induction. This creates a back voltage to the controlling circuit and can be more than the supply voltage.
Graph / Oscilloscope
Take a look at the example below from a Pico Oscilloscope capture of a HV Battery relay switching off.
As it can be seen the negative voltage measured amounts to -20.13 volts and in this range, goes off the scale, so actually the negative voltage peaks much lower, or higher whichever way you look at it.
This phenonium is not unusual and can be expected.
In some cases, to prevent damage to the controlling circuit a diode is placed in the circuit to prevent this reverse voltage.
But, if we look at this second example below of an HV battery relay switching off, it can be seen that the reverse voltage stops at -11.19 volts, it doesn’t peak as a spike as in the previous example but also continues for a period of 19.18ms (milliseconds) in time.
So why?
To answer the question we need to appreciate the reason, we need to understand what the engineer is trying to achieve first.
Contactor service life is dependent on the prevention of electrical arcing occurring within the relay when being switched on or off between the contact surfaces within the relay and whenever there is a voltage potential present. If arcing is not controlled, then the life of the relay will be depleted. Also, there could be a risk of welding the contacts together if the arcing is allowed to go unchecked.
The less arcing within the relay the longer the life of the relay. So, the engineer, if within design budgets can lessen this effect.
In this example, what is being recorded is the relay drive circuit is being reversed! This creates an opposite magnetic field and forces the contacts to open instead of using spring pressure to do the job. This effect results in a faster switching off of the relay and lessens the arcing across the contact surfaces.
Not easy to do and more expensive to have the drive circuit to the relay do this but probably less cost in service of a failure of the HV Contactors!
Not all HV Battery contactors operation is the same