SDU parameter tuning - major failure - can't select drive gear

[Riwi]

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.

[Riwi]

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?

[johu]

God no DC bus!

[Riwi]

God no DC bus!

Check!

[Peakon28]

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?

[Peter]

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.

[Pete9008]

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!

[Peter]

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

[Pete9008]

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)!

[Peter]

Hi Pete9008. Its funny how destruction makes us think harder

[Pete9008]

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!

[EV_Builder]

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.

[phillipschip]

Would something like this (attached)work along with tripmode 1 wired at the HV inverter cables? Could just replace in case of failure? Only $35
Or this:
https://www.newark.com/littelfuse/v511d ... dp/74R2850

[jrbe]

Would something like this (attached)work..
...Or...
https://www.newark.com/littelfuse/v511d ... dp/74R2850

I don't know the overvoltage time. I believe this will be inconsistent and will depend on RPM, magnetization of the rotor, etc.

That very likely won't be anywhere near enough except at very low rpms. A MOV can handle huge currents for very short timeframes (too short for this) which makes the ratings look feasible. A combo mov / gas discharge tube with a slightly higher voltage rating on the mov could work if sized appropriately.

Adding a beefy resistor in line could help the overvoltage devices survive long enough to help. Just realize the voltage across the resistor will likely raise the clamping voltage, possibly by a lot.

I have mentioned turning on the cabin heater in other posts asking how to deal with this energy. It should be at least a few kw worth of load dump but needs to be fast enough to actually help.

You have to match the regen energy over time to the overvoltage devices ability to absorb it over time. It's tricky.

[phillipschip]

How would you wire the Varistor/gas discharge tube-parallel with Mov? Something like a 500v Mov and 600v tube? Also, it looks like you can get Mov’s with higher clamping rates-1.3kv etc. for not too much more. Would this help any or is the surge so high this value doesn’t matter too much?

[jrbe]

600 amps x 600 volts = 360,000 watts. But I'm not sure what current the motor would output, for how long, and what your clamping voltage would be.

This has a sample schematic at the bottom. https://www.bourns.com/docs/technical-d ... t_form.pdf

Tvs diodes are fast but weak and blow shorted. The T is for transient.
Mov are medium fast and medium strength. They can withstand a lightning strike in power lines. That's very fast though.
Gas discharge tubes are slow but fairly strong.

The idea of setting different voltages for the 3 types is the slower devices have a lower clamping voltage and take over doing the work from the faster & weaker ones when they start conducting.

TVS / MOV / GDT are all rated for fairly short duration. It will be challenging finding longer duration testing info to use them for this. It's not an easy or cheap thing to test.

There's also the direction of how much of a power dump would it need to keep the voltage below safe fet values. The heater being on may be completely insignificant thinking it through.

[phillipschip]

Having these two devices wired with a good estimated guess-would there be any downside besides the potential waste of money? If one were to do that, what ratings would you recommend? Also, would a snubber work-capacitor resistor at the motor?

[jrbe]

Having these two devices wired with a good estimated guess-would there be any downside besides the potential waste of money?

If youv undersize them they will likely fail spectacularly along with the typical overvoltage failure in the inverter. There's also the schrapnel potential landing in high voltage causing further failures.

If one were to do that, what ratings would you recommend? Also, would a snubber work-capacitor resistor at the motor?

Adding components like this isn't worth taking a guess at. Adding them means adding high voltage wiring which has some risk to it & could have it's own failures.

The only way I can think of testing this is with some controlled loads on a generator / dyno that you can spin up to higher and higher rpms and test. Not exactly diy.

Might be able to carefully do this opening the contactors at really low rpm, recording results, and working your way up. You might be able to plot the energy out below the threshold voltage and extrapolate some expected power and voltage levels from that. Testing at say 200v and adding lower voltage rated overvoltage protection might help work through the problem within the voltage range of the system.

Just realize this very likely will end up a huge amount of power, misses the vehicle coasting part of the equation, and will blow the drive if the overvoltage protections you're testing fail.
Note in the datasheets, these survive a few large events. It's a huge amount of energy to deal with, repeated hits damage them over time. Something to note if doing ramp up testing.
Short answer is lots of possibly expensive / time consuming testing is required to know for sure the size of the overvoltage protection required. I don't know how to calculate it, too many unknowns (at least for me..)

It's worth making sure the contactors stay closed and only open in an emergency. Extra time here is likely better time spent.