openinverter forum

Peter wrote: ↑Wed Feb 22, 2023 9:15 pm Hi Rikard. If you are thinking of adding TVS I would use bidirectional as spikes could happen with drive and regen possibly so current flow is in either direction.
Jump in anyone if you have thoughts please.

Thanks! I will try to guesstimate the amount of energy that needs to be absorbed. I’ll keep you posted.

johu wrote: ↑Thu Feb 23, 2023 8:44 am unidirectional is fine. Spikes in reverse direction, if even possible, would be absorbed by the diodes contained in the phases. Remember, stick your multimeter on there the wrong way round it will measure like 0.5V forward voltage.

Ok, I’m placing my tvs+resistor on the dc side. If possible, would it be better to connect the between phases?

God no DC bus!

johu wrote: ↑Fri Feb 24, 2023 5:06 pmGod no DC bus!

Check!

Peter wrote: ↑Sun Sep 18, 2022 7:52 pm Tripmode 1 is the best idea as Catphish suggests.
As an added path to clamp spikes I use suitably voltage rated paralleled TVS in series with a high wattage 25R resistor across the inverter HV input cables as close to the inverter as possible.

Hi Peter could you share the details of your connection. When you say TVS I guess you mean TVS - Transient Voltage suppressor diode?

Hi Peakon28. Suggest a high power Varistor in series with a resistor. My max pack voltage is 330v so in my case the Varistor must be able to handle 330v DC continuous without problems. The Varistor will start conducting above that value and I am doing some tests again with it. The type I have has a max voltage of 590v DC. Its all a matter of trial and test but as mentioned before Tripmode 1 is a MUST. Not sure how high regen voltage can become so consider that also.

Not sure whether I've misunderstood something, and don't want to seem negative, but I can't see the varistor/resistor arrangement doing much.

On varistors and transzorbs the slope resistance is a fairly critical metric and adding a 25R series resistor just ruins it. Say you are trying to dump just 50A though the two, the resistor will drop 50A x 25R = 1250V (with the varistor voltage on top of that), still more than enough to kill the inverter.

The best protection is the tripmode, except for emergencies the breaker must NOT open while there is still current flowing in the motor!

Hi Pete9008. Tripmode is the best solution agreed. But as a second line of defence my aim is to catch a spike voltage before it gets over 600v and kills the SDU power stage. I tried with just varistors before and as the spike voltage increased above the varistor continuous DC voltage the varistors went into self destruct mode, quite a dramatic event. Saved my inverter though My thought (rightly or wrongly) was to have the series resistor act as a load so the power would be dissipated across that too and save the varistors doing all the dissipation and hopefully survive to work another day. As long as there is a method to absorb a spike should the worse case scenario appear that would be my goal. However I would gladly bow to your greater knowledge and appreciate any advice please

A varistor alone would work as it will clamp the voltage but as you say if it can not handle the power it will self destruct. It may save the inverter though.

The trouble is that if you add a series resistor the inverter sees the voltage across the combination of the varistor and the resistor. The varistor will top out at say 590V but you have to add to that whatever voltage drops across the resistor. If you have too much current through them (which is likely if the motor was taking current as it has nowhere else to go) the added voltage across the resistor (1250V in the above example) will easily put you back in the inverter killing range. The varistor may survive though!

Edit - unfortunately most of my knowledge on how not to destroy things has been gained by accidentally destroying things (in this particular case a few industrial measurement boards, followed by a scope while investigating, were the unlucky pieces of equipment)!

Hi Pete9008. Its funny how destruction makes us think harder

Always say you learn a lot more when you fail than you do when you succeed - still not nice at the time though!

Remember also having to build a custom rig to reproduce the several 1000A surges so that the new improved protection circuit could be tested - and that self destructed to when we pushed it to the limit while trying to find the limits of the new protection circuit.

I'm afraid I can't think of a guaranteed circuit to protect the SDU if the breaker opens, there is just too much stored energy to deal with. Your straight varistor approach is probably the only real option but with the caveat that if it ever has to operate it will likely fail violently!

Maybe the solution lies in a current sensor and a relais and HVIL style circuitry. So we don't open a breaker but we do limb the engine and thus the current flow with out opening a breaker.