AC motors, multipoles, torque

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Post by Mesuge » Wed, 04 Feb 2009, 06:23

Thanks for the reply, we follow the same train of thought so far regarding the variable torque input as the master control unit for this setup. However, two very efficient(~95%) differentials working together will be <<90% efficient for the entire vehicle, right? Also, I doubt you can switch to 50:50 mode from say 20:80 driving mode abruptly, because of some potential traction instability issues, so the transition should proceed "slowly" hence some losses in comparison to pure FWD performance. Now, the added weight and complexity for double the traction components, in summary there will be some negative aspects to it as well.
Last edited by Mesuge on Tue, 03 Feb 2009, 19:33, edited 1 time in total.
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Post by Richo » Wed, 04 Feb 2009, 08:26

I would expect a newer FVD should be able to cope within 2 revolutions.
At 500RPM this is still 240ms.
Assuming the RPM encoder is sending enough info back.
But is hard to say what code each manufacturer uses.
Typically the torque loop might be updated 16kHz.
But variatons in RPM only updated 500Hz.
But who wants to test 10 different VFD's to find one that suits this setup?

Each motor should be around 60kg.
So 120kg for 2 motors which I'm sure is less than the original ICE alone.
So the extra ICE components that come out should allow for some batteries.

The issue I have is that the rear motor has no native mounting brackets.
So some engineer would have to design mounts to handle the 190Nm of counter torque on the rear of the car.
That is where, I believe, the complication for the dual motor lies.

Also batteries to produce 150kW won't be particularly cheap either.
Very possible just not cheap.

Each diff if 95% don't multiply together to get under 90%.
They are independant so the total stays at 95%.

The only time the balance between front/rear should change is acceleration/deceleration.
Brake harder the ratio diverts regen more to the front.
Accelerate hard and the ratio diverts more torque to the rear.
This is assuming that the torque value isn't maxed out...
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For more torque a bigger motor would be needed and this may be difficult to fit under the car.
Unless it was a Hilux with a lift kit Image
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Post by wombat6926 » Mon, 09 Feb 2009, 13:48

Hi

I have access to a Baldor M3313T Industrial AC motor configured for 1760 RPM at 60Hz. (possibly for free)

Baldor M3313T

I was wondering if I could use this for a EV motor.

I know these motors are designed to run at fixed frequency's and RPMs but can they be safely used as a variable speed motor with the right controller?

I am currently trying to source Ac motors and controllers and I am trying to co a conversion as cheaply as possible. I don't want to go the DC route and I think Re-generative Breaking is necessary to extend the range.

Comments and feedback appreciated.
Mark.

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Post by coulomb » Mon, 09 Feb 2009, 16:17

wombat6926 wrote:I have access to a Baldor M3313T Industrial AC motor configured for 1760 RPM at 60Hz. (possibly for free)

I was wondering if I could use this for a EV motor.
It's a 10HP motor, so ~7.5kW. That's a little small, but may be usable.

It also seems to be dual voltage, i.e. one winding with up to 230v, and another that allows operation at up to 460v. It is possible that you might be able to do star delta switching as well, which will allow you to get maximum torque at low speeds. The flexibility of windings may allow you to make up for the lack of nominal torque, by effectively giving you "contactor gears" if you want to go that way.

It seems a little wide (assuming 215T means 215mm from base to centre line; it might be different in the USA), but that's nowhere near as bad as a long motor, for fitting where the ICE has been taken out.

If you can get it cheap, I'd say grab it, if you are thinking of converting a small vehicle.
I know these motors are designed to run at fixed frequency's and RPMs but can they be safely used as a variable speed motor with the right controller?
Good question. Some motors (e.g. ABB industrial series) are "designed for VFD". I don't really know what that means. Certainly any induction motor will work with a suitable VFD, but perhaps not as well as one designed for VFD use. I'd be interested to hear of others' opinions on this.

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Post by woody » Mon, 09 Feb 2009, 17:35

Hi Wombat,

~1800rpm at 60Hz or ~1500rpm at 50Hz means 4 pole, which is good.

M3313T is a range of motors looking at that page, so they come in a large range of sizes. If you just googled M3313T to get that page then it may not be 10HP / 7.5kW. The nameplate of the motor will give good specs like output (HP / kW), efficiency, voltage etc. You'll also want to find out the breakdown ratio (about 3) or breakdown torque which probably won't be on the nameplate. The nominal torque looks to be 40Nm (7.5kw*9550/1765rpm). Depending on the breakdown ratio, the available torque should be about 120Nm which is about the same as a Charade.

Assuming it's a nominal 7.5 kW, and the voltage is 230V, then with a big enough VFD (415V 50kW) you should be able to get full torque to nearly 415/230 * 1800rpm = 3200rpm, which gives a peak power of about 40kW. Which will keep a little car up with the traffic.

The nominal 7.5 kW means it can do that all day from a cooling point of view, which for a small car will be about 80kph on the flat.

@Coulomb - my guess at "designed for VFD" is that they leave out the funky rotor designs which are designed to lower DOL starting currents.

cheers,
Woody
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Post by acmotor » Mon, 09 Feb 2009, 20:34

An induction motor 'designed for VFD' would be clearly labelled as VFD only operation as it would not function safely with DOL starting as Woody says. Namely the rotor resistance would too low and cause excessive start currents - mind you, it would be an ideal EV motor if you could resist the temptation to push 'till it broke.

Most induction motors are VFD 'rated'. This simply means that the insulation (particularly interphase) is higher rated because if the greater dv/dt that results from >50Hz (typically 5 to 20kHz) switching frequencies.
In today's market you could probably not buy a non VFD rated motor.

7.5kW 230V in delta on a gearbox would work quite well. A 30kW (nominal) controller would be more than enough to drive it I would have thought, since you will still be voltage limited at the top end. In fact even a 22kW with 160% overload would do it ?
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Post by EVLearner » Sun, 15 Feb 2009, 04:00

Would I be right in assuming that an AC motor rated at about 15 to 20 kW is about the right size for the average small/medium car?

Similarly an AC motor rated at about 20 - 25 kW is about right for the standard Falcon / Holden / Magna?

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Post by antiscab » Sun, 15 Feb 2009, 05:29

a 20kw motor would probably be overkill for a falcon or commodore, as far as far as continuous power requirements go.

remember when you increase the rpm, you increase the max continuous power aswell.
so a 20kw motor at 3x base speed will do around 60kw continuous and 180kw peak.

on the other hand, the bigger induction motors are more efficient (not sure if this is true at lighter loads though).

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Post by EVLearner » Sun, 15 Feb 2009, 06:09

Hi Antiscab

Understood, so about 10 - 12 kW (at 50 Hz) is about right for a small / medium car? yes? Or should I opt for about say 15 kW?

3x base speed is about 150 Hz, right, so could I increase the frequency to say 400 Hz and spin the rotor at say 8x the nominal rotor speed, so a nominal 1440 rpm motor would be spinning at say 11,520 rpm (192 rps), is that too fast?? (How fast is too fast?)

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Post by antiscab » Sun, 15 Feb 2009, 07:05

it varies from manufacturer to manufacturer, but the industrial motors tend to only be rated for around 4500rpm, or 3x base speed for 4 pole induction motors.

it is possible to go faster, its just at the higher rpm, rotor balance and strength become more important.

another factor is whether you intend to retain the gearbox.
on FWD cars, this is almost unavoidable (within reason, there are some exceptions where people have done direct to diff with fwd)

i would say a 15kw motor run at 3x base speed in a small car would run rather cold (this is usually a good thing)

you would either have to have a *very* high pack voltage (1800vdc) or you would have to rewind the motor for lower voltage, to make use of the power at higher rpms.

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Post by EVLearner » Mon, 16 Feb 2009, 04:00

Thanks antiscab, you have really cleared up an area that I have had trouble sleeping over in the last month or so - now I can sleep like a baby!!

BTW, in hindsight with the Torque curves provided by Woody (I think) it seems not that hard to switch from Wye to Delta while the motor is running (increasing revs), and this might be a way to get that extra torque in the high revs range. The switching does not look all that difficult :)

BTW2, I have just had alternate look at switching from '4 Pole' to '2 Pole' (or 6 pole to 4 pole etc) and this seems considerably more difficult - but - I am sure that with a bit of lateral thinking it might be almost too easy to step up / down the rotational rate of the magnetic field .

My notional thinking goes along the lines of producing a couple of torque optimised revs ranges in the one AC motor; that can be electrically switched and electronically speed controlled. This could get the best of both worlds with torque where you need it (and revs as you need them)!

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Post by woody » Mon, 16 Feb 2009, 07:13

EVLearner wrote: Would I be right in assuming that an AC motor rated at about 15 to 20 kW is about the right size for the average small/medium car?

Similarly an AC motor rated at about 20 - 25 kW is about right for the standard Falcon / Holden / Magna?
I look at it in two ways: Continuous Power + Maximum Torque

Power: These motors are rated for continuous power.
What continuous power do you need ?
e.g. small car - work out the drag at the maximum continuous speed you need.

e.g. my cortina needs 10 kW at the wheels for 110kph.

Add extra for drivetrain losses (I guess my diff will be 93%)
so about 11kW should be fine.

Torque: How much torque do you need / can you handle :-)

How much can I handle: e.g. cortina original engine peaks about 100Nm, gearbox approx 4:1 in first. I'm thinking direct drive, 400Nm is about the most I can expect to use before I run the risk of breaking things.

How much do I need: this is very subjective. Some rough calcs:
4 pole motors go to 1500rpm, which is about 60kph in my car.
So 400Nm to 1500rpm will give me acceleration like the best first gear can give me all the way to 60kph.
250Nm will be like second gear
200Nm will be like third gear
100Nm will be like 4th gear (yawn).

For me the new ABB 132-316 4 pole 15kW with 4.0 breakdown torque fits best:
392Nm breakdown torque
92 kg (lightish)
132 frame (smallish)

cheers,
Woody
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Post by antiscab » Mon, 16 Feb 2009, 07:49

by switching between 2 and 4 pole, do you mean you are deactivating (bypassing in the wiring) 2 of the poles for 2-poles?

wye/delta switching isn't too difficult, but only useful where you are controller limited.
ie, if your controller is capable to putting out enough amps to get the same peak torque in delta as with wye, then you may aswell just permanently wire in delta.
the increased losses in the controller would be offset by not having to hold 3 contactors closed (and of course the extra resistance in the contact).

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Post by woody » Mon, 16 Feb 2009, 08:12

There's two ways that ABB do it:
1. is separate windings - the motor is wound twice independantly, once with one speed, once with the other. Big, heavy motor. You are only using half the windings at a time.
2. is dahlander wiring - this is one way: Image
there are two ways : Fan dahlander and constant torque dahlander. I'm not sure which this is :-)
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Post by woody » Mon, 16 Feb 2009, 08:15

EVLearner wrote: Thanks antiscab, you have really cleared up an area that I have had trouble sleeping over in the last month or so - now I can sleep like a baby!!

BTW, in hindsight with the Torque curves provided by Woody (I think) it seems not that hard to switch from Wye to Delta while the motor is running (increasing revs), and this might be a way to get that extra torque in the high revs range. The switching does not look all that difficult :)

BTW2, I have just had alternate look at switching from '4 Pole' to '2 Pole' (or 6 pole to 4 pole etc) and this seems considerably more difficult - but - I am sure that with a bit of lateral thinking it might be almost too easy to step up / down the rotational rate of the magnetic field .

My notional thinking goes along the lines of producing a couple of torque optimised revs ranges in the one AC motor; that can be electrically switched and electronically speed controlled. This could get the best of both worlds with torque where you need it (and revs as you need them)!
I've come to the conclusion that with a powerful enough controller / batteries, none of the star/delta/series/parallel/pole/dahlander switching is helpful.

I.E. if your controller can put out enough amps for peak torque at the low voltage (in my case 174A @ 240V for 350Nm @ ~3000rpm), then star / delta or series / parallel switching can't help.

With pole count changing, it seemed to me that 2 pole @ 50 Hz @ 400V is much like 4 pole at 100Hz at 400V and 6 pole at 150Hz at 400V. Approximately same current and torque. The 4 poles seems to have more max torque per kg though.

There is research to be done though - an standard AC motor in one wiring configuration doesn't have a flat power band - it peaks when you run out of voltage and then drops off.

If you can work out a way to continuously switch those windings so that there's a flat power band, then you've changed things a lot.

I'd like to see the 400V windings of a motor split into 4 100V sets, so you could switch from 400V (4 windings in series) to 200V (2 strings of 2 windings in series) to 100V (4 sets in parallel). Makes for a huge terminal box (15 terminals) on the motor though.

If you can do all that switching inside the controller, it makes for a cleaner finished product :-)

cheers,
Woody

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Post by woody » Mon, 16 Feb 2009, 08:21

Johny wrote: I've priced shipping from the USA and it's prohibitive (the motors are cheap). I haven't tried Japan but the shipping from UK was also VERY expensive when I checked the CMG situation. That shipping price (400 Pounds) was from someone I knew over there as well.
Shipping is the killer. Strangely China isn't nearly as bad but the motors???
A number of motor distributors claim to handle special orders when you read the catalogue blurb - I wonder if they would consider 2 or 3 as worth while?
I priced shipping with an Aussie importer of US classic cars. They quoted AU$250 + import fees for a VFD (6062) if I had it delivered to their US depot with their paperwork attached.

I think a motor should be similar $$ (heavier but smaller).
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Post by Johny » Mon, 16 Feb 2009, 17:02

That sounds like a good price. The problem with motors is that once they are packed correctly they put on a few kilograms and lots of extra size.
A typical 90kg motor appears to have a shipping weight of around 120kg.

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Post by EVLearner » Mon, 16 Feb 2009, 17:04

Hi Woody

Something tells me that three of the magnetic fields are reversed in the parallel Star Dahlander arrangement and this could cause the rotor to abruptly spin in reverse if these were switched while in motion - that could be interesting!!

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Post by EVLearner » Mon, 16 Feb 2009, 17:16

In another look at the torque versus revs situation - and the current drawn by an AC motor, I was considering that the frequency is independent and that in the low frequency range (say 20 Hz to say 60 Hz) the motor would be Star (Wye) connected and from say 50 Hz to say 150 Hz the motor would be Delta connected!

Now as far as current goes, much of the current that you are speaking about will be 'reactive' current, and because the (star connected windings would have a) greater inductance in the lower frequency range, (in Star config), this reactive current should be reactively limited. Because with the upper frequency range the motor is in Delta structure the reactive current is again limited by the frequency!

At the higher frequencies, the motor should be equally efficient by being Delta connected - or even more than equal as you point out. So you get the best of the whole drive frequency range.

BTW I had this 'stroke of genius' about switching the winding configuration during motor use from winding several hundred different communications based transformers some decades ago. At the low frequency end of the spectrum, the transmission efficiency of these transformers is highly dependent on having enough self inductance so as not to noticably shunt the load resistance. At the high frequency end of the spectrum, the efficiency is limited by the amount of leakage inductance between the windings (so the windings need to be low inductance).

Star connected motor windings have a much higher self inductance making them ideal for the lower frequency band, and switching the same windings into Delta substantially reduces the self inductance, so the torque range should be considerably raised at these higher frequencies.

From my view, the Star - Delta switch would be like an electric overdrive gear! I believe that we would get the extra torque in the high revs range where the wind drag is the limiting factor.

(In testing this it might be worth having a 600 W incandescent lamps in series with each phase as visual fuses to make sure that one of the windings is not anti-phase.)

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Post by Johny » Mon, 16 Feb 2009, 17:24

If you use a 'Reversing Contactor" then the Star/Delta switch will be safe as these contactor pairs have a mechanical interlock that prevents them both being closed at the same time.

If you look back along this thread you will find Star/Delta switching gets a lot of discussion. Woody's speadsheet also has lots of support for examining the performance of Star/Delta or either alone.

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Post by Johny » Mon, 16 Feb 2009, 17:46

For anyone hanging out for prices on the new ABB Aluminium 132 frame motors - keep hanging!

I rang Clamp (ABB distributor) this morning and finally got an answer.
The answer started with a "the 11kW 4 pole 132 frame high output and rewind to 240 volts". Hang on - didn't I ask for prices on the NEW 132 frame motors. Yes, that was a little paragraph at the end of the email from ABB to Clamp. "The new 132 frame motors are still in production. Prices and availability are still unknown.".

Anyway, just to make it hurt - a rewound 11kW (the current 59kg motor) was $2116 (standard is $1041) - ouch! (The Trojan solution that I discarded a while back is starting to look good).

This smells a little bit of ICouldntBeBotheradom (acmotor terminology here) so I'm going back to ABB and give a gentle push for more info. I know it's been difficult for them - poor dears Image

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Post by weber » Mon, 16 Feb 2009, 18:52

I was talking to an electrical engineer friend on the weekend, about Coulomb's and my AC EV project and he mentioned that he'd worked on a lathe that had used an off-the-shelf high frequency (400 Hz) induction motor driven from a stock standard Control Techniques VF drive. So I googled "high frequency induction motor" and amongst a lot of sites with sub kilowatt motors intended for use in aircraft I found this monster:

150 kW continuous at 400 Vac, 400 Hz and it only has a mass of 39 kg!
http://www.400hertz.net/Products/ME-400-200-416.htm

The bad news is it's a four pole, so that's 12,000 rpm and you'd need a gear reduction of around 2 to 2.5 for it to be useful in driving a diff.

If you ran it at 50 Hz you'd presumably get a continuous 18.75 kW (150*50/400), but I note that that would be at 50 Vac and 300 A. The ABB 400 V, 50 Hz, 18.5 kW motor has a mass of 92 kg -- more than twice as much.

You could presumably use the Azure Dynamics DMOC445 VF drive (220 Vac, 400? A) and only run it up to 220 Hz (220 V) and still get 82 kW _continuous_ out of it at 6000 rpm. A 39 kilogram motor! Can this be for real?

Of course I have no idea of the cost.

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Post by woody » Mon, 16 Feb 2009, 19:10

Just quickly:
150kW @ 12,000rpm is ~120Nm.

The 150kW is a peak rating, normal motors are nominal.

18.75kW peak is more like 7.5kW Nominal

The Azure guys sell a 10:1 gearbox for FWD cars, so that looks like a nice match for this motor. Or just keep your gearbox.

On a peak Torque/kg measure, this is a bit over 3Nm/kg, which is good, but the new ABB 132-316 I won't shut up about is 4.25 Nm/kg.

cheers,
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Post by weber » Mon, 16 Feb 2009, 20:15

woody wrote: I've come to the conclusion that with a powerful enough controller / batteries, none of the star/delta/series/parallel/pole/dahlander switching is helpful.

I.E. if your controller can put out enough amps for peak torque at the low voltage (in my case 174A @ 240V for 350Nm @ ~3000rpm), then star / delta or series / parallel switching can't help.

That may or may not be true. [Edit: See viewtopic.php?p=37886&t=2435#p37886] But in any case VF drives are are not cheap (biggest cost after batteries) and tend to have cost proportional to kVA, and star/delta switching is practically free. So IMHO you'd be crazy not to do it if it lets you use a VF drive with sqrt(3) lower current rating, to get the same result. But I think you can forget all that other stuff (series/parallel/poles/dahlander switching).

All the terminals you need for star/delta are already in the junction box on almost every ordinary industrial motor. All that other stuff is either very rare or you have to take the motor apart and find it and figure out how to get it to the outside world, and we're playing with dangerous voltages and power levels here.

I learnt this stuff from Ross Pink of Electronic Innovations

http://www.electronicinnovations.com.au/ in Brisbane, who I have worked for on-and-off over the years.

About 4 years ago, apparently unaided by any who had gone before, he quietly converted an old Toyota LiteAce van (the "Lite" moniker is well deserved) to a no-gearbox AC EV using a VF-drive from Control Techniques (now obsolete), for which you could purchase a plug-in pod that gave you very low-level programming access to the drive's functions.

He implemented seamless automatic star-delta switching (but with manual override available via a 3 position switch) and an accellerator response that was a mixture between speed control and torque control and so felt more natural to those used to ICEs. I drove it once. He had regen when you backed off the accelerator, and good ideas about how to do more with the brake pedal. He planned to have the motor (132-frame, 100 kg, 15 kW 400 V, 50 Hz, 4-pole, 100 Nm) rewound for 240/415 V or lower. His 528 volt battery was made up of 44 used 17 Ah 12 V sealed lead-acids from UPS's.

Unfortunately he used a bad-boy charger which was just a diode-capacitor voltage multiplier off the mains. Not sure if it was a doubler or a tripler. And he had a soft-charge resistor (an electric kettle element) across the contacts of the main contactor between battery and VF drive. This lets the VF drive DC bus capacitors charge up sowly to the same voltage as the battery so you avoid blowing the contacts with a massive inrush current.

One day, about 3 years ago, he left the bad-boy on a bit too long without checking it and he heard a very loud bang from the carport. It was the VF drive's bus capacitors exploding! And possibly igniting a cloud of vented hydrogen from the batteries into the bargain. The capacitors had exploded because the batteries had gone way over voltage and this had been fed to the drive caps via the soft-charge resistor.

The lessons.

1. Don't use unregulated battery chargers. You _will_ forget one day.
2. Do switch your soft-charge resistor off automatically e.g. with a timer. You _will_ forget that one day too.

But I learnt many positive lessons from Ross Pink too. Such as the value of star/delta switching, lower voltage rewinds still being fed 415 V, and using the tail-casing and output-shaft off the original gearbox to adapt from motor to propshaft. He's been very helpful in advising Coulomb and I, and Electronic Innovations will probably supply our motor and drive.

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AC motors, multipoles, torque

Post by Johny » Mon, 16 Feb 2009, 20:44

Hi weber. Do you know how Ross overcame the problem of lubricating the rear oil seal (around the tail-shaft slip-yoke) without swamping the front motor bearing with oil? This is my current concern with using my tail-shaft housing. Sorry - a bit off topic here.

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