350-400VDC vs 700-800VDC?

[cajunfj40]

Hello all,

I had posted a while back about trying to run two Leaf motors paralleled off a single inverter (short answer: not usually, but might be worth the experiment), and then asked a separate question about higher voltages that I think deserves to be split out on its own.

My questions are: what safety/regulatory implications arise from the increase, other than the vague "higher voltages are bad, m'kay?" I know from poking around at NFPA and similar regulations that it moves from a Level 2 risk to a Level 3 risk (for UPS battery systems), and that going to higher than 650V requires higher-rated tools like voltmeters and such to avoid internal tool damage. What extra risks are present on the EV itself?

Porsche recently unveiled an 800VDC EV concept, and I've found inverters/converters/etc. for 700-800VDC input continuous, 1000VDC transient, presumably for big transit buses, where the power levels make downsizing the current required really attractive.

I would intend to set it up so that the two packs only join up in either the inverter enclosure or a separate "high voltage control box" before going a short distance to the inverter, so no two wires in close proximity have 800VDC across them, and any single wire has at most 400VDC to ground or a nearby wire in case of a fault. I could re-use OEM plugs and sockets that way - no re-engineering needed. If the hazards aren't really any worse, nor a potential regulatory hurdle (for shops working on it? Dunno, I'd be DIY so not really subject to the regs) then that combiner box could be further away from the inverter box, potentially simplifying cable runs and parts layouts. Charge in parallel at "normal" OEM charger voltages, etc.

See, MPaulHolmes is working on his 200kW inverter, and to get that power (minus inefficiency) out of a single Leaf motor (or Volt motor, or other OEM motor of similar power ratings) without absurd phase currents, it would seem going to a higher voltage to allow extending the constant torque line up the RPM band would be "easier". Basically, take two OEM EV battery packs (Leaf, Volt, etc.) and connect in series, roughly doubling the RPM at which the constant torque can be maintained. I already want more range than one OEM pack can give me, so why not connect in series instead of parallel? 80kW is plenty for steady-state operation under most scenarios for my (still theoretical) planned build, and 100kW would not take much increase in cooling capacity either. Depending on how the efficienty islands act above the currently plotted ones for, say, the Leaf motor, running at higher power ratings at higher RPM's with the same phase current that it can handle 80kW at now shouldn't overheat it. I doubt I'd want to spin the motor much/any faster than stock RPM limit, though, because bearings/balance. (I wonder what an IPM rotor coming apart at 20kRPM would do? I think the stator and housing could contain it...) A Chevy Spark EV motor only spins 4500RPM now, so increasing that isn't too bad. (DC brushed motors with armatures that diameter spin up to 7K without too much extra work into them.)

Thoughts?

[freebeard]

No idea myself, but I need to learn. There is this:

http://ecomodder.com/forum/showthrea...ors-33661.html

Three pages of posts by oil pan 4, EVmetro and thingstodo. Check out EVmetro's build threads; he does really high-quality work.

[oil pan 4]

800 VDC presents an extreme arc flash hazard.
The other thing is can the motor insulation take double the voltage?
To me doubling the voltage and increasing the amount of heat put off by the windings is a recipe for short motor life.

Are the wound rotors brushless?

In industrial motors only specialized ones can be started on double voltage. And this is only allowed for a few seconds.
Any normal motor would be damaged and quickly ruined by double voltage.

Also you should take into consideration that it appears that all these motor drive bits and pieces were designed to prevent anyone from swapping parts around like this.
With electric vehicle motors and drives these motor and drive sets appear to be designed specifically for each vehicle.

Higher voltages are not inherently bad. Anything over about 90vdc is easily deadly.
It's once you start going over 600vdc that you go from deadly to spectacularly dead with bonus fireworks show when there is a fault.

[Piotrsko]

other than the propensity for arc flash at unexpected times, I see a chance of generating Gamma rays at 1 KV. shielding mitigates most of that, but expensive and heavy.

The other problem I see is physically maintaining insulation. lots of common and available stuff just isn't feasible above 600v. Hv stuff is expensive if you don't have space.

OTOH running 2 separate packs until just at the controller might work, kinda. isolating the 2 packs is an interesting thought puzzle. and extraneous leakage is just plain ugly.

[cajunfj40]

oil pan 4:

800 VDC presents an extreme arc flash hazard.
The other thing is can the motor insulation take double the voltage?
To me doubling the voltage and increasing the amount of heat put off by the windings is a recipe for short motor life.

Ah, there's the info: "extreme arc flash hazard." How much worse is it than at 400-650VDC? Is it that it can jump a longer distance to start the arc that causes the flash? In any case, there will be sufficient current behind whatever voltage chosen to vaporize wiring/busbar/terminal/etc and cause the resulting plasma to do very bad things. How much less dangerous is the common 144VDC system? In any case, I'll try to remember to invest in some fiberglass wrenches for dealing with battery terminals and the like. My shiny Craftsman wrenches will quite handily carry enough current to cause Serious Problems when dropped on top of some busbars.

I'll have to do some digging to figure out the insulation rating on the stators. Is there a relatively easy way to determine the peak transient voltages seen in a given motor/inverter system? That's the real issue - making sure a higher voltage system doesn't drive transients up to "blow a hole in the insulation every time you jam on the regen at freeway speeds" levels.

Doubling the voltage does not directly double the heat - generally heat is I^2R losses. I'm trying to go up in voltage to avoid going up in current. That said, without controls, doubling the voltage will likely double the current the motor will take (based on higher input voltage vs. BEMF at a given RPM). Either way, I am looking at more heating based on increased power output at a similar or slightly lower efficiency, depending on load and RPM. It'd be interesting to find out if that heating increase is more or less than mechanically paralleling two entire electrically separate motor/inverter systems. Using two systems would only double the total heating at any given operating point - though the two systems may run at a less efficient point lower on their operating curve than the one bigger system, thus increasing the nominal heating losses. I just want to make sure I keep the rotor below the point where the magnets get very unhappy. I'd prefer to go with an induction machine, but so far all the OEM's are still using IPM rotors to get the power density, IIRC.

Are the wound rotors brushless?

No, that was an analogy trying to say that spinning a Spark EV motor to near or above 7kRPM (vs. stock ~4,500RPM) ought not to be disastrous or even bad wear-wise if appropriate bearings are selected. DC brushed motors are often sped up quite a bit from design speeds in EV racing. Similar armature OD's exist to the Spark EV motor, and they run faster than the Spark EV motor does now. Failure is generally first at the commutator - which an OEM EV Motor of the type I am considering lacks - and banding strategies as applied to DC brushed armatures likely have similar applications on brushless. Most OEM motors now being IPM type where the magnets are fully contained makes banding probably un-neccessary.

In industrial motors only specialized ones can be started on double voltage. And this is only allowed for a few seconds.
Any normal motor would be damaged and quickly ruined by double voltage.

Is this true for VFD/speed controlled motors, or just direct-to-line motors? Is this for "squirrel cage" polyphase AC motors (usually 3 in industrial apps), permanent magnet brushless motors, or brushed DC motors? I can see a standard 208 3-phase motor being quite unhappy at 480V without the accompanying frequency change needed to make things "line up right electrically" inside. It would seem that having an inverter controlling the voltage/current/frequency that the motor "sees" would eliminate this problem, unless there are issues like the aforementioned insulation rating.

Also you should take into consideration that it appears that all these motor drive bits and pieces were designed to prevent anyone from swapping parts around like this.
With electric vehicle motors and drives these motor and drive sets appear to be designed specifically for each vehicle.

Well, yes. The OEM's have many reasons to want their kit to only work with their kit. Max efficiency can be obtained by designing the motor and inverter to work together, if the manufacturer takes the time to do so. Closer integration can eliminate redundant parts, having specialized plugs means making it harder for the wrong part to be installed in a repair shop, etc. DIY folks often have to work around this sort of thing when correct parts are no longer available, too costly, etc. There are many strides being made in cracking the CAN bus codes to be able to drive OEM parts outside their original settings - see the GEVCU, etc. MPaulHolmes' (and many other folks') inverter is a full-on removal of the usual CAN-bus controlled OEM inverter, bypassing all of that decoding. eldis over on the DIY Electric Car forum has a project he's working on for a "replacement brain" that will drive the OEM power stages in any given OEM inverter and thus also bypass the OEM CAN-bus stuff. People are figuring out how to swap new engines with full computer controls into older cars - hotrodding is not dead, just going high-tech out of necessity as people try to accomodate "wanting to have something cool" with "wanting decent fuel economy" and "not wanting to smog up the place".

Higher voltages are not inherently bad. Anything over about 90vdc is easily deadly.
It's once you start going over 600vdc that you go from deadly to spectacularly dead with bonus fireworks show when there is a fault.

Hence why I'm asking - I would like to try and plan ahead for how to prevent such accidents. If it requires larger spacing between components, I'll plan that in. If it needs a redundant extra layer of insulation, same. What I want to avoid is making it so hazardous that special precautions are needed for average service folks or emergency responders over and above what they are already trained for when dealing with electric/hybrid vehicles, when there is no clear way to make that knowledge generally available for a one-off custom.

[cajunfj40]

Piotrsko:

Whoops, didn't see yours until my previous posted.

other than the propensity for arc flash at unexpected times, I see a chance of generating Gamma rays at 1 KV. shielding mitigates most of that, but expensive and heavy.

Gamma, really? I thought x-rays came first. Odd. Source? If I can make Gamma rays with 1kV, the Mad Engineer wants to know...

The other problem I see is physically maintaining insulation. lots of common and available stuff just isn't feasible above 600v. Hv stuff is expensive if you don't have space.

This is a very legit concern - if I can't get parts without paying a hefty "high voltage" premium, then the whole thing isn't worth it. I'm trying to cut costs by re-using existing stuff, not give myself more expensive problems.

OTOH running 2 separate packs until just at the controller might work, kinda. isolating the 2 packs is an interesting thought puzzle. and extraneous leakage is just plain ugly.

A single pack already needs to be isolated from ground, so I don't see isolating a second pack as any harder. What extra issues need to be figured out for a series connection over and above running two packs in parallel? Now, properly disconnecting them from each other to allow charging with available OEM chargers is a different problem. I don't really want to cover new ground there, if I can avoid it. Series-parallel battery switching has been done with relays, but I don't believe it has been done at these voltage levels.

[freebeard]

Sounds to me like the trade-off is thinner wires with thicker insulation.

What vehicle is the 'one-off custom'? An FJ40? You could use two single-speed MGR instead of over-stressing one.

[cajunfj40]

freebeard

Sounds to me like the trade-off is thinner wires with thicker insulation.

That seems to be much of it, at least for the full voltage wires. Still need to figure out the insulation rating on the actual motor windings, though.

What vehicle is the 'one-off custom'? An FJ40? You could use two single-speed MGR instead of over-stressing one.

Yes, an FJ-40. I'm looking at using a Leaf motor because it will do 80kw continuous, based on ARNL(?) testing showing that rotor temperature seems to top out at 135C when driven at 7,000rpm at that power level. If it turns out that the high voltage is just is too much hassle, I may go the two-parallel-systems approach. That means two motors, but I'm not trying to run double the current through a stock motor. Maybe one of the "brain replacement" projects for OEM inverters will be available by the time I can do this build, which will lower the overall cost a bit.

I'm sticking with a transfercase and gearbox at the moment. Motors aren't quite there yet in power-density plus speed range for me to stick 1 per corner or 1 per axle and still have both rock-climbing ability and freeway speed capability without changing gears. With 35" tires, I need 576 RPM at 20kw continuous per wheel to do 60mph. To climb rock walls, I need 3250 ft-lbs per wheel (assumes full 6500 ft-lbs driveline torque in 1st-low with stock engine/gearing, and 2 tires in contact with the obstacle being climbed). Gimme a motor that can do that, at less than 100lbs per motor, and less than $2000 per motor/inverter set, and I'll seriously consider wires and cooling lines instead of driveshafts.

[oil pan 4]

DC jumps gaps much better than AC.
Higher voltage DC will also establish an arc and will maintain the arc until the power is turned off. When dealing with batteries this presents an obvious problem.

I have seen double voltage starting only on direct drive. But I have seen inverter duty motors that have the option for parallel or double voltage starting.
Double voltage starting on inverter motors is out there but I have not seen it.

My DC stick welder that I built runs up to 125VDC open current. It can throw a pretty good arc.

Also most wire is only rated to 600v.

For anything up to around 144vdc I would just use a normal wrench wrapped with electric tape and welding protective gear for making the energized connections.
For 100s upon 100s of volts NFP70 arc flash stuff should be uses which is pretty much like wearing a bomb disposal set.

If I were going to build an electric vehicle I would stay between 48 and 144vdc since it is relatively easy to work with and all the parts are off the shelf.

[Piotrsko]

oops didn't mean gamma rays.

look up typical magnet wire properties on Google. FWIW, I believe the higher voltage motors just use additional coats of varnish after winding.

Over on the DIYelectric site someone was commenting about some motor (honda?) has an inverter that doubled battery voltage to 500, so I would bet that 750 isn't unreasonable. You can also get motors rated for 408 vac which is almost 600 peak to peak. Some VFD's will run on DC, but make poor car controllers.

I still cant figure out a separation scheme for series batteries. based on my experiences, it is hard to not get full voltage leakage to somewhere. and high voltage high current stuff is $$$$$.