WHY USE HIGH VOLTAGE DRIVE SYSTEMS

[ChrisB]

This ones been doing the rounds for many years

But it really is (in my book) down to cutting current, this means many good things, smaller cables, faster speeds as paul says and lower currents which means smaller batteries, lower losses in cables/controllers etc
Thats one reason they transmit our household power at 400kv rather than 415 or 240 volts

The downside is you need truck loads of batteries

I think the main thing is make sure the batteries are discharged within spec , which is REALLY difficult to do with a EV as 9 times out of 10 you'll exceed their rating pulling away from a stand still.

But if you can discharge them within there C rating then they will last the distance.

Dare I mention things like milk floats and forlklifts they will opperate for years and years on one set of batts but then the discharge has been calculated and hence the size of the batts

ChrisB

[EVguru]

Peukert doesn't directly come into it. Doubling the battery voltage, generally means halving the size of the batteries, so the ratio of current drawn to battery capacity, the 'C' rate, stays the same.

Going to smaller batteries may allow you to fit more batteries (better packaging) or adopt a different battery type (AGM rather than flooded). In the first case you will reduce the battery current for a given power and gain a peukert advantage. In the later case an AGM is likely to have a lower Peukert co-efficient and you gain that way.

The real efficiency gains from high voltage come on the motor side.

A motor might be rated 24 volt, 200 Amp; 4.5hp @ 1500rpm.

Input power = 24 Volt * 200 Amp = 4800 Watt

4800 Watt / 746 (Watts per hp) = 6.43hp

4.5hp (shaft output) / 6.43hp (electrical input) = 70% efficiency, which is about right for a series wound motor at 24 volt.

The losses are 1440 Watt.

If you ran the same motor on 48 volt and the same current, the losses would be about the same, but the output would be much higher.

Input = 48 * 200 Amp = 9600 Watt = 12.86 hp.

Take away the losses.

9600 - 1440 = 8160 = 10.94 hp

10.94 / 12.86 = 85% efficiency

In reality it's not all gain. The frictional losses in the bearings will have gone up (more or less linear with rpm) and there will be additional windage losses. This is particularly true with an internal fan. Even so, the improvement in efficiency will be substantial.

You can usually see this if you get motor data showing performance curves at different voltages.

Since the current is the same, the torque will be the same (even with a series motor), so doubling the voltage will double the rpm. If you didn't want to double the vehicle speed you could then double the drive reduction. You would then have twice the torque and so acceleration, or you could get the original accelaeration, but at reduced current; More efficiency gain! (minus perhaps some extra losses in the drive system).

The motor would only be doing 3000 rpm and may well be safe to twice that and you could double the applied voltage again. You're only halving the losses each time so the improvement in efficiency wouldn't be as much this time.

On the Porsche 914 race car, we re-configured the battery from two parallel strings of Optimas (55 Ah) at 144 volt, to a single string at 288 volt.

We had hoped that the efficiency gains would allow us to circulate faster, for the same energy consumption. Much to our amazement, we went a lot faster with LOWER energy consumption. A more experienced driver helped, but the figures were so much better we wouldn't believe them until we'd been through them several times.

We estimated the street range of the vehicle to be around 100 miles and going from the 225/45-15 road legal race tires (basically slicks once a few laps had been done) back to LRR 185/60-15 tires would have given us a lot more. Quite a few people questioned this, but I pointed out that it was broadly similar to what AC Propulsion were achieving with a similar load of the same batteries.

[electricvehicles]

OK Paul,
Yes you are perfectly right regarding all your theories regarding motive power traction systems, thats both AC and DC drive systems. An no doubt you will one day put them all to the test and be the proud owner of perpetual motion car.
You really are light years ahead of all the major manufacturers, why not set your own development company up to assist in saving the planet from greenhouse gasses? And also offer development consultation to small manufacturers like ourselves ? OK we only produce small industrial vehicles -- But all positive input is greatly appereciated

[qdos]

OK Paul,
Yes you are perfectly right regarding all your theories regarding motive power traction systems, thats both AC and DC drive systems. An no doubt you will one day put them all to the test and be the proud owner of perpetual motion car.
You really are light years ahead of all the major manufacturers, why not set your own development company up to assist in saving the planet from greenhouse gasses? And also offer development consultation to small manufacturers like ourselves ? OK we only produce small industrial vehicles -- But all positive input is greatly appereciated

Indeed "positive input is greatly appreciated" so please guys calm down. We do appreciate both your inputs to the forum when they are helpful. Please shake hands and let's move on. Perpetual motion would be heaven for us if we all get along well but otherwise it could be purgatory.

[MalcolmB]

Good illustration Paul. You're right of course about the Peukert effect not being as important a consideration as I made it out to be.

(I edited the rest of this message due to some embarrassingly woolly thinking )

Next question: AC or DC?

[EVguru]

Wooly thinking is the cut price UK version of Fuzzy Logic.

AC or DC.

If you have the budget, go for an AC system, particularly at higher voltages.

Peak efficiency is usually only slightly higher (sometimes worse), but average effieciency across the speed/torque range is usually quite a bit better. You also get regen 'for free'.

AC systems tend to be expensive because for a given peak torque you need 6 times the amount of silicon in the power devices compared to an equivilent series motor system. A seperately excited system gets pretty close, whilst only going to 3-4 times. A sep-ex motor tends to be quite a bit more expensive though, particularly at higher voltages and/or rpm where interpole and compensating windings may be needed to stop destructive arcing at the brushes.

[MalcolmB]

Yes, fuzzy logic sounds much more high-tech

Personally I like to keep things as simple as possible, so I'd be willing to sacrifice a little efficiency if I can have a simpler (and less expensive) system. Having said that, I'm considering an AC system (or DC brushless system) for my next project. I've heard that in certain failure modes DC controllers can allow full current to the motor when they fail, which is obviously not a desirable feature. I'd be interested to hear what you think about the safety aspects. (Safety naturally becomes a more important consideration with higher voltage systems too.)

[TESLACOIL]

But we coud use hi current if we used Very thick cables - not realy an option for the electric grid - same reasom whywe dont run underground cables

even using hi voltage a fair amount of heat builds - hard to deal with underground and so would need thicker cables



However it would be possible to have an EV using 40v

by using paralel motors and packs and thick wires



if you used 4 low volt motors instead of 1 high volt - not much overall weight increase ( and made sure all the wiring was nice n thick ) you could close the gap between the two systems

the 4 low volt motors would be understressed - but would probably increase friction of the drive chain

silicone side of things may be cheaper - a modular sytem would be robust - you could lose a motor and a controller and just swap things about at the road side

justa thought



Im building a couple of 40v DC drive systems - mainly because AC is a killer esp at high volts - and i dont like messing around with it

so im going for a modular lick my terminals if you like - system

[Jeremy]

Paul's summed up things well, but it's worth remembering that cables are the really easy bit to reduce resistive loss. It's much, much harder to reduce the resistive losses in the motor, brushes, controller and most importantly the battery cells.

To reduce controller losses means, in essence, adding silicon (paralleling the output FETs to reduce on-resistance). This gets you back to the issue with AC drive at high voltage, in that more silicon is needed in the controller.

Reducing motor losses means using a big (and hence heavy) motor, running at a lower speed than a higher voltage system. The extra weight needs extra power to accelerate, so increases losses over a lighter solution.

Reducing battery internal resistance losses means either paralleling batteries up, or using bigger batteries. Bigger batteries mean more weight, parallel connections mean more weight and greater resistive losses in the connections. Either way efficiency is reduced.

It's perfectly possible to build low voltage systems that work - all those electric milk floats that worked for years are a testament to that. A low voltage system will not be a match for a high voltage one of the same on-the-road performance, in terms of overall efficiency, no matter what you do.

Jeremy

[arnolde]

I'm building an EV with a 660V pack (400V AC motor), and I'm a little worried about what kind of fuse and contactor to use. At upto 800VDC/20A (during regen) the fuse and contactor between batterypack and controller must be high-voltage, spark retarding type. Any ideas?

The best I could find so far is a solid-state switch 25A/800VDC which costs 150 EUR.

Unfortunately the controller (SEW MDV60A) has no power-off or standby mode, so I need to break the battery DC circuit manually when shutting down.

regards,
Ethan