Charging currents (single phase)

After the startup sequence described in the previous posts it is now time to move on to the actual charging. Again, all the measurements come from SpeakEV user ElectricBeagle and tons of valuable info has come from user arg. If you want to check out the discussion that lead to all of this, here it is.

Let’s set the stage first. We assume single phase charging, the earth check has been performed, the mains has already been connected and, since it’s single phase charging, the relay between N and L3 has already closed. More about that later. Charging has started and as shown in the previous post, the current has ramped up to the desired level. The ramping is gradual, taking about one second.

The rather massive 100 uF capacitor is now creating that 6 A reactive current. It’s sole purpose seems to be to dampen the huge current spikes created by the switch mode power supply. Note that the capacitors are rated for 63 A. The worsening power factor is simply a side effect of that. However, at 16 A, the PF is already above 0.9, so no worries. Bottom line is: at decent currents, say at or above 16 A, the power factor is great and they do clever things with the switching to follow the voltage, see next picture, taken at a current setting of 30 A, with a very small resistor in the N line to measure current without phase shift.

At significantly lower power levels things worsen very quickly. For instance an off grid PV inverter would need to be very potent in coping with that reactive power. But I digress. Because what you really want to know is the effect of the switching. So let’s zoom in on the raggedness of the current curve.

What I wrote here before was way off, as arg argued in private…….An earlier picture suggests that the current coil produces roughly 1 volt peak-to-peak per 4 A wire current (RMS). That suggests a 10 kHz ripple current of just over 200 mA RMS. On a 10 A charge current that is give or take 2%. But this measurement was done with a current coil on the L wire at 16 A, while the previous picture was done with a resistor in N, at 30 A. I suspect the current coil is quite frequency dependent. Going back one picture, one-and-a half-dot ripple on a 26 dot amplitude is roughly 6%. At least ball park same.

Another interesting thing to look at is earth leakage, given the relatively high capacitance between the stator coils and the motor housing. One “blob” is a half-cycle, lasting 10 ms. The pattern is pretty symetrical, with a small (about 6 dots) peak to peak 50 Hz component, and a decent (about 15 dots) 10 Khz signal. The scale is 30mA/div. My rough estimate is that the 50 Hz component is 13 mA RMS and the 10 kHz part a bit above 30 mA RMS. Note that a consumer grade RCD will trip at 30 mA but as arg argued, it should not care too much about the higher frequencies (though old ones may!). All in all the stray capacitance to earth seems to be about 2nF. Note that the leakage current should not go significantly up with power, as it’s a capacitive coupling.

And this is the reason why ZOE sings. Ramping up the frequency further would progressively increase leak current up to the point where dedicated, uncompromised coils would be needed, adding quite a bit of weight if one would want to do 22 or 43 kW rectifying.

All in all I would say: not too bad really! The only thing I am not really sure about is what is meant with HF and EHF frequency in the leak currents the car can report. I always assumed 150 Hz (3rd harmonics) and 10 kHz (the switching frequency), but I am not 100% sure. If anyone knows or suspects more, I’d love to know.

Oh and about the single phase relay. It can’t close before the chargepoint contactor is closed, because as long as it is open, the car doesn’t know if the chargepoint is single or three phase. However, assuming no contactor in the filter module (and I haven’t found any in the pictures of the filter and charging module), once power is switched on and it is single phase, the capacitor between L3 and N is idle, so it’s no problem to bridge it with a relay at that moment in time.

Leave a Reply