# Precharge relays



## ACEVS4US (Jul 21, 2011)

I've been building an ac controller for the last few months. I've finished the gate drive design for the IGBT's I have. I want to include the precharge within the inverter (rather than externally) so I'm now looking at the precharge circuit.

I've been looking at relays for switching on the precharge. The designed pack voltage is 320V. The issue I have come across is all standard relays can't handle dc at 320 volts even with no load at "make or break". Am I stuck with having to use an expensive contactor such as LEV100? 

Does anyone know of any cheaper relays/contactors suitable for precharge? 

To limited inrush I plan to use an NTC thermistor in series with my precharge resistor. This SHOULD allow me to reduce the precharge time by having a slightly smaller precharge resistor.

Thanks


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## dougingraham (Jul 26, 2011)

ACEVS4US said:


> I've been looking at relays for switching on the precharge. The designed pack voltage is 320V. The issue I have come across is all standard relays can't handle dc at 320 volts even with no load at "make or break". Am I stuck with having to use an expensive contactor such as LEV100?


Since precharge requirements are minimal why not use a 600v 10a igbt? A dollar or two at most even in single quantities. If you have to have isolated gate drive you will spend another $10 on the rest of the stuff but that is cheaper than any kind of contactor. You could probably do 10ohms for the resistor since the current is going to taper off so quickly. Spend a couple of more dollars and get a 90 amp rated device and go to a 3.6 ohm resistor. That should precharge plenty fast.


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## DJBecker (Nov 3, 2010)

You need a fail-safe circuit, and sometimes galvanic isolation. If the IGBT or MOSFET fails, it's most likely to fail shorted. And there is enough leakage current to potentially leave the capacitors energized.

If you are going to have a actively controlled fast device switching the precharge current, you might consider using a buck circuit instead of a resistor. This requires adding an inductor and diode, but would allows much faster precharging without heat build-up.


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## Coulomb (Apr 22, 2009)

ACEVS4US said:


> I've been looking at relays for switching on the precharge. The designed pack voltage is 320V. The issue I have come across is all standard relays can't handle dc at 320 volts even with no load at "make or break". Am I stuck with having to use an expensive contactor such as LEV100?


It's a pain. We've found it's hard to beat the US$65-70 EV200s on Ebay. Way overkill, but what can you do. It seems you need vacuum to get the good DC ratings, and it must be difficult to make small vacuum contactors, at least cheaply. Or maybe nobody's noticed the market opportunity.

The original and classic Prius models have 288 V and 274 V (nominal; typically around 350 V and 320 V respectively). I've been meaning to but haven't chased up to see where their "system main relays" are made, if they have a part number, and if they are available. Each Prius has three relays, one used for pre-charge and one on each leg of the hybrid battery pack.



DJBecker said:


> You need a fail-safe circuit, and sometimes galvanic isolation.


I think that galvanic isolation is a necessity. I don't want the only thing between 320 V and me being a semiconductor.


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## Coulomb (Apr 22, 2009)

Coulomb said:


> I've been meaning to but haven't chased up to see where their "system main relays" are made, if they have a part number, and if they are available.


Image here: http://birdbird.org/cars/prius/pics/dscn0385.jpg
The G3840 number seems to be a Toyota part number, so the AEV... number may be the Matsushita part number. A quick search didn't turn up anything useful.


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## ACEVS4US (Jul 21, 2011)

Thanks for the suggestions guys. I did think of an IGBT and a relay in series but the dc rating isn't really sufficient even with no load at make or break time. 

The idea for the Toyota Prius contactor is a good one. I'll look into the cost of these and post it on this thread.

US$65 - $70 is a good price for the EV200 contactor as well.

Chris


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## Tesseract (Sep 27, 2008)

An EV200 contactor for precharging the input capacitors??? Maybe if the precharge resistor is 1 ohm...

The whole point of precharge is to bring the input capacitors up to operating voltage in a controlled fashion by _limiting their charging current_... 

If you want to use a small relay for precharge then limit the initial charging current - it's as simple as that. The voltage rating of the relay is almost irrelevant as the relatively high value of the precharge resistance will prevent an arc from forming, and since the "load" is a capacitor, contact opening will always be benign.


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## Coulomb (Apr 22, 2009)

Tesseract said:


> An EV200 contactor for precharging the input capacitors???


It's overkill, but it works. EV200s can carry loads of just a few amps quite well, even if needed for less than a second per trip.



> Maybe if the precharge resistor is 1 ohm...


Or one hundred ohms.



> The whole point of precharge is to bring the input capacitors up to operating voltage in a controlled fashion by _limiting their charging current_...


Yes. A 100 ohm resistor limits the current nicely.



> If you want to use a small relay for precharge then limit the initial charging current - it's as simple as that. The voltage rating of the relay is almost irrelevant as the relatively high value of the precharge resistance will prevent an arc from forming, and since the "load" is a capacitor, contact opening will always be benign.


The high value will limit the current, but with a high voltage pack it will still be lethal. It would be cold comfort to me knowing that the relay whose voltage rating wasn't adequate didn't destroy itself in the act of killing me.

Granted, there should be other contactor(s) opening before the pre-charge relay, but I'd feel a lot safer if the pre-charge relay/contactor had full pack voltage rating.


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## EVfun (Mar 14, 2010)

Tesseract said:


> If you want to use a small relay for precharge then limit the initial charging current - it's as simple as that. The voltage rating of the relay is almost irrelevant as the relatively high value of the precharge resistance will prevent an arc from forming, and since the "load" is a capacitor, contact opening will always be benign.


So what happens if someone tries starting a car with a failed controller? The MOSFETs or IGBTs tend to fail on, the precharge circuit can't precharge because the caps are shorted across the motor, and at some point you will want to open that precharge relay. 

I was thinking that perhaps a PTC so if the precharge system doesn't do its thing pretty quick the current will drop quite low. From there a kuep-3d15-12 ($21 at Mouser), or perhaps lesser relay, could handle the break with a resistor and capacitor across the contacts. This seems like something that has to be tested for higher voltage systems.


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## Tesseract (Sep 27, 2008)

Coulomb said:


> The high value will limit the current, but with a high voltage pack it will still be lethal. It would be cold comfort to me knowing that the relay whose voltage rating wasn't adequate didn't destroy itself in the act of killing me....


Lethal to what or whom?

I suggest you reconsider the circuit - ie, a battery feeding a capacitor through a switch and relay - and then see if you can figure out how an arc across the switch - were it even possible to form - would affect the circuit's operation.

If anything, an arc between the relay contacts would simply add more "resistance" in series with the resistor and capacitor. 

But this is all very much theoretical, because the battery voltage will split across the resistor and the arc according to their ratio, and since the effective resistance of an arc is less than 1 ohm, even a 100 ohm precharge resistor will ensure that virtually all of the pack voltage will be dropped across it, leaving far too little to sustain an arc.

NB - it takes a voltage gradient of around 30kV/cm to strike an arc, so, technically speaking, a 0.1mm air gap will strike at 300V. It's not a perfectly linear relationship, but close enough to illustrate the point. Mains-powered equipment is required to have a much larger minimum air gap (called "clearance" in the argot) between conductors than the operating voltage might otherwise demand to withstand overvoltage from lightning strikes, load dumping, transformer tap-switching, etc... None of which are applicable to a battery-powered device like an EV.


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## ACEVS4US (Jul 21, 2011)

I tend to believe manufacturer's datasheets and am not going to willingly ignore manufacturer's recommendations. I have not yet found a conventional relay with a no-load dc voltage rating of >300V dc. Although if anyone can show me a normal power relay with suitable specifications I will use it.

P.S Although the breakdown voltage of air is 30KV/cm once the air has ionized the gap can be increased by a factor of at least 10 before the discharge will stop.


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## Coulomb (Apr 22, 2009)

EVfun said:


> So what happens if someone tries starting a car with a failed controller?


This sort of thing is somewhat dear to my heart at the moment:

http://forums.aeva.asn.au/forum_posts.asp?TID=980&PID=35630#35630

To fix our problem, we're thinking of a bimetal switch, which are commonly used in hot water systems, like this thermal circuit breaker. [ Edit: to go in series with the motor controller contactor coil, not the motor controller itself. ]

However, this probably won't help if the controller is shorted. So for that you also need a circuit that detects the voltage across the controller contactor, and doesn't energise its coil until the voltage drops below a threshold like 12 V or so. But with high voltage systems, this becomes tricky to do safely and reliably.

So maybe you just rely on your main fuse to clear the fault. After all, it's likely that the controller will fail while the vehicle is running, not between the last time it was turned off and when it is next pre-charged. Of course, it's always bad to assume these kinds of things.


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## EVfun (Mar 14, 2010)

Coulomb said:


> This sort of thing is somewhat dear to my heart at the moment:
> 
> [snip]
> 
> So maybe you just rely on your main fuse to clear the fault. After all, it's likely that the controller will fail while the vehicle is running, not between the last time it was turned off and when it is next pre-charged. Of course, it's always bad to assume these kinds of things.


Most bi-metal switches have rather small gaps and don't like clearing 300 volts DC. It is entirely possible for a controller to fail when off and take off like a rocket when turned back on. You hurt it with heat and it fails when it cools back down. That is how my Curtis failed. 

It took off like a rocket when I got back in it to drive it around the neighborhood to nose it into the garage. It cooled for a while from the previous (admittedly abusive) drive. The main pack fuse is a complete joke and will not blow in a reasonable amount of time. I left 4 wheel black marks (rear tires doing a burnout, front tires locked with the brakes) and I still switched it off before the fuse blew. 

You should check for precharge and NOT pull in the main contactor without it! But you then have to open the precharge circuit. The Zilla does this startup check and I'm sure a similar system is built into the Soliton controllers.


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## Coulomb (Apr 22, 2009)

EVfun said:


> Most bi-metal switches have rather small gaps and don't like clearing 300 volts DC.


Yes. So these would be in series with the "main" (we call it the motor controller) contactor coil.



> It is entirely possible for a controller to fail when off and take off like a rocket when turned back on.


Well, not for ours, since it's AC. But point taken: pairs IGBTs could fail and short the DC bus while the controller is off and cooling down from a thermal event.



> I left 4 wheel black marks (rear tires doing a burnout, front tires locked with the brakes) and I still switched it off before the fuse blew.


Eek. Could not have been fun.


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## Duncan (Dec 8, 2008)

Hi Guys

So if my controller fails - when I throw my pre-charge switch the pre-charge light will come on (using a light bulb as a precharge resister) 
BUT it will stay on and not go out after 20 seconds
So I should NOT throw the main contactor until the light goes out

Now I am going to do this manually but it would be easy to arrange the main contactor not to pull in while there is current going through the pre-charge resister


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## Coulomb (Apr 22, 2009)

Duncan said:


> Hi Guys
> 
> So if my controller fails - when I throw my pre-charge switch the pre-charge light will come on (using a light bulb as a precharge resister)
> BUT it will stay on and not go out after 20 seconds
> So I should NOT throw the main contactor until the light goes out


Yes. But also, you should not throw the main if the light doesn't come on at all, since it means the filament has blown, or something else has prevented the pre-charge from happening. (Or you've just turned off the controller, and the caps are still charged; that's OK).

Unless you use a light tube of some sort, this is impractical for long term use. But it should be fine temporarily.

I don't know if you mean the 20 seconds to be a typical pre-charge time, or way longer than expected so obviously something is wrong. I think it's important that the pre-charge be over within say 3 seconds, or someone is going to get impatient and connect too soon.


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## Elithion (Oct 6, 2009)

ACEVS4US said:


> I've been looking at relays for switching on the precharge.


The Precharge reference page has a discussion on components used for precharge , including precharge relays, which states: 

"The precharge relay needs to be rated for the full battery voltage, because, when the system is off, the full battery voltage appears across its contacts. An AC relay may be used because by the time it is turned off the current through it has gone to 0 A. 
The relay needs to be able to handle the peak of the inrush current; but, since the average current is low, and the breaking current is nearly zero, the current rating of the relay is not critical. "


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## Gary B (Jun 2, 2011)

ACEVS4US said:


> I tend to believe manufacturer's datasheets and am not going to willingly ignore manufacturer's recommendations. I have not yet found a conventional relay with a no-load dc voltage rating of >300V dc. Although if anyone can show me a normal power relay with suitable specifications I will use it.
> 
> P.S Although the breakdown voltage of air is 30KV/cm once the air has ionized the gap can be increased by a factor of at least 10 before the discharge will stop.


ACEV, - I tend to agree with your viewpoint. - If the relay contacts are not (yet) closed, there is NO current through the precharge resistor, and, therefor, no voltage drop from the full supply voltage that will be present across the relay contacts. There is voltage continuity right up to the contacts but NO voltage drop. - The separation of those contacts must be adequate to withstand the potential of arcing. That, most likely, is part of the design criteria and rating for those relay contacts. - Best to you. Gary B.


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## Elithion (Oct 6, 2009)

In a properly designed precharge system (emphasis on "properly"), the DC voltage rating of the precharge relay contacts is not an issue.

When the precharge relay is first turned on, the contactor on B- is open, and therefore you're not relying on the precharge relay's DC voltage rating.

When the precharge relay is on, there is no voltage across its contacts, of course.

Just before the precharge relay is turned off, it is bypassed by the positive contactor.

Once the precharge relay is turned off, again, it is bypassed by the positive contactor, and therefore there is no voltage across the precharge relay's contacts.

Even in case of emergency during precharge, the negative contactor can be opened to interrupt the DC current, before the precharge relay is opened.

At no time is the precharge relay required to interrupt DC current.

So, an AC rated relay (easy to find) will work just fine as a precharge relay.


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## Gary B (Jun 2, 2011)

Elithion said:


> In a properly designed precharge system (emphasis on "properly"), the DC voltage rating of the precharge relay contacts is not an issue.


El - Your reference is MUCH appreciated. Thank you. - That reference does say: "The precharge relay needs to be rated for the full battery voltage, because, when the system is off, the full battery voltage appears across its contacts."

It seems to me that this statement is NOT true (in accordance with your explanation). - (as long as the precharge relay contacts are closed before the K3 contacts.) - 

As you say:


> When the precharge relay is first turned on, the contactor on B- [K3] is open, and therefore you're not relying on the precharge relay's DC voltage rating.
> 
> When the precharge relay is on, there is no voltage across its contacts, of course.
> 
> ...


 Failure modes are not addressed in this discussion. - Best to you. Gary B.


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## Duncan (Dec 8, 2008)

Coulomb said:


> Yes. But also, you should not throw the main if the light doesn't come on at all, since it means the filament has blown, or something else has prevented the pre-charge from happening. (Or you've just turned off the controller, and the caps are still charged; that's OK).
> 
> Unless you use a light tube of some sort, this is impractical for long term use. But it should be fine temporarily.
> 
> I don't know if you mean the 20 seconds to be a typical pre-charge time, or way longer than expected so obviously something is wrong. I think it's important that the pre-charge be over within say 3 seconds, or someone is going to get impatient and connect too soon.



I was planning on using a light bulb as a precharge resister and another light bulb to discharge the capacitors

So I would have a 50watt bulb in the precharge and a 5 watt bulb in the discharge (permanently across the controller) - loses 5 watts all the time but shows me the unit is live

Start with both light dark, throw switch 
Pre-charge light goes bright then dims
Ready light starts dim and gets brighter
When precharge is real dim and ready is nice and bright throw second switch to bring in the contactor

Switch off pre-charge

Switch off main contactor - ready light will stay lit as it drains the capacitors

10,000 microF and 150v = 1.5 coulombs
50watts at 150v = 0.33 amps - about 10 seconds to charge

3 seconds to charge implies 150watts - is that not a bit high?


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## Coulomb (Apr 22, 2009)

Duncan said:


> 10,000 microF and 150v = 1.5 coulombs


A coulomb is a fair whack of charge. (Also, a Coulomb is very important, just ask me )



> 50watts at 150v = 0.33 amps - about 10 seconds to charge


It's hard to say how long a bulb will take to charge a capacitor. The hot resistance will be around 454 ohms if it's literally a 150 V bulb, but you'll probably use a 240 V part since it's readily available. So that's around 1152 ohms hot, and maybe 120 ohms cold and 900 ohms (wild guess) with 150 V across it for at least 100 ms. How rapidly it transitions from cold to hot depends on many things. When it's been hot but has 50 V across it... I can't guess. Basically, you just need to try it and see how it goes. For most of the pre-charge, the bulb will be relatively cold and therefore hopefully less resistance than 1152 ohms.



> 3 seconds to charge implies 150watts - is that not a bit high?


You won't get the full 150 W for long, just a few tens of milliseconds. But it's annoying to have to wire three 50 W bulbs in series or parallel, or use a huge incandescent (non-halogen, I mean) bulb.

Remember if you use the more compact halogen bulbs with integral reflector, most of them are dichroic so that in the normal hang-from-the-ceiling application, the heat rises but the light is reflected down. You probably don't want dichroic, so you need to find one with an aluminium reflector, or add a simple reflector (but aluminium foil or other uninsulated metal amongst high voltage presents its challenges).

We're luck with the Tritium controller; it has just 800 uF of film capacitors that do the same job as tens of thousands of uF of electrolytic capacitor, so we get less than 1 second pre-charge with a 100 ohm pre-charge resistor.

We were going to experiment with lamps, but decided that we couldn't spare the space. We'd need many lamps in series for our high voltage. Many lamps in series makes for short life, and of course, the consequences of the lamp going open circuit are not good. Vehicle vibrations and shocks are also not good for lamps (other automotive lamps seem to last OK most of the time, but if they blow, it's mostly nuisance value).


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## Coulomb (Apr 22, 2009)

Elithion said:


> "The precharge relay ... An AC relay may be used because by the time it is turned off the current through it has gone to 0 A. ..."


I would agree with that for the Elition-recommended configuration of two main contactors (one on each leg of the battery). If the motor controller is shorted and the main contactor doesn't come on (as it should not, since the voltage across it is too high), then the pre-charge contactor/relay has to break several to many amps at pack voltage. I was surprised to find that even with a 5 mm gap, an arc can sustain at less than 100 V and less than an amp.

(For example, see p11 of http://www.efcog.org/wg/esh_es/even..._Arc_Presentation_DOE_EFCOG_2008_Workshop.pdf ).

An AC rated relay would not be able to break the pre-charge current, so the pre-charge resistor will take full pack voltage, will be massively overloaded, and will become a fire hazard. If there is a second main contactor that can open in the case of the pre-charge resistor overheating, it will be DC rated and can easily break the pre-charge current. However, the logic for driving the three contactors/relays becomes a little more complex.


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## Duncan (Dec 8, 2008)

_so the pre-charge resistor will take full pack voltage, will be massively overloaded, and will become a fire hazard

_Surely the pre-charge resister should be sized for full pack voltage? - it effectively sees full pack voltage when it does it's pre-charge


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## Coulomb (Apr 22, 2009)

Duncan said:


> Surely the pre-charge resister should be sized for full pack voltage? - it effectively sees full pack voltage when it does it's pre-charge


It needs to not break down from the voltage, so yes its voltage rating needs to be higher than the highest pack voltage.

But in normal use, it will see a large pulse of power for a very short time, and the average power will be so low it typically doesn't need any extra heatsink over the package it comes in. You have to get a resistor with a high enough peak pulse power rating; some of them can't withstand even a 5x overload, even for less than a second.

Typically, if you sized the power of the resistor to withstand the pack voltage continuously, it would need to be massive and water cooled. On a 144 V nominal conversion, the pre-charge resistor might be 10 ohms. That reduces the peak current to a manageable 15 A or so, but 150 volts at 15 A is 2250 W. The largest reasonable resistor you can get is rated at about 300 W (which it can only handle with a large heatsink). You'd prefer to use a 50 W or 100 W unit with sufficient peak pulse power capability, and not bother with a heatsink. You protect the resistor (and hence the vehicle) by putting a thermal switch on the pre-charge resistor, opening the pre-charge contactor if the temperature exceeds about 70 degrees C.


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## Nabla_Operator (Aug 5, 2011)

*Re: Precharge relays: why not SOLID STATE ?*



In this DIY-page I describe a $5 mosfet relay for use in a 600V precharge circuit and want your critics/suggestions. Please take a look.


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## Madmac (Mar 14, 2008)

If you hunt around there are plenty of HV DC relays. This one from Tyco is designed for precharge..

http://www.te.com/content/dam/te/gl.../high-voltage-pre-charge-relays-mini-k-hv.pdf

A good choice is this one from Fujitsu with a 10 Amp rating (150 Amp inrush) and remember that contacts can be in series to increase voltage rating in DC applications

http://www.fujitsu.com/downloads/MICRO/fcai/relays/ftr-j2.pdf

Panasonic do a range from 10 Amps upward designed for EV's, but I found difficult to get hold of...

http://pewa.panasonic.com/assets/pcsd/catalog/aev-catalog.pdf

And another, power rated contactors

http://www.schaltbau-gmbh.com/files/b195_en.pdf

George McDuff


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