AC Drive

[ken.simmermon]

Is anyone planing an AC drive conversion? If so, what motor, controller and pack voltage are you planing to use?

[tom.gralewicz]

I'm planning on an AC system.  My primary goal is to build it off scavenged parts.

What I have currently is:

30HP Baldor AC Drive 230V
20HP Baldor 1750 RPM motor
1999 Dodge Neon
18 12V 75AH AGM batteries - I expect to need 26 total   240V AC  * 1.414 RMS to peak = 339V pack voltage;
                                 339V / 13V per battery = 26 batteries
If you are interested in more, we hold open build sessions the 1st and 3rd Sunday of the month, send me your e-mail address if you want to be added to the mailing list or watch the parallel posts on FVEAA Forum titled "Milwaukee Electric Car Build"

Tom.

[ken.simmermon]

Hi Tom,
That sounds great, where did you get the motor and drive? I like you high pack voltage, what do you think the weight of the vehicle will be when finished? I would like to do an AC drive system too. Here's my email blast321off@yahoo.com but I can watch for it on the forums to.Have Fun, Ken S.

[tom.gralewicz]

I've added your e-mail to the list, if you are ever in the Milwaukee area drop me a line and I'll show how its going.

The motor was easy, I have a few friends in the junk yard and industrial scrapping business.  The challenge was finding one that didn't weigh too much.  Most of the 10HP and up motors are built in cast iron frames.  I spent some time looking for a lighter model.  There are aluminum versions of the cast motors but what I found were general purpose motors.  They are sheet steel frames and the lightest ones I found.  I have a M2515, you can find good information on baldor.com.  They also make a 200V version of the same motor, this would let you run a lower pack voltage or get a longer constant torque range with a higher voltage.  I'm still watching.  In general with a few months looking I can find a usable motor from my scrap buddies.

The drive was another story.  Most of the used AC Drives I have found over 10HP are 480V.  You can buy new 230V version for under $2000 - I found some on e-bay.  I got lucky and hooked up with a small company that does industrial automation, the owner was interested in what I was doing and had an old drive sitting on the shelf - he gave it me!  He's in line for the first "look it runs" visit.

I have been told by a few experts that the ideal motor is an AC Servo or Spindle motor.  You find these on newer CNC lathes and mills.  My scrapper buddy got me 2 a 20 and 30HP units, with the controllers.  The only problem is they are 480V - meaning a pack voltage of 600.  These might be a good choice for an S10 where you can put a larger pack under the bed, something I would consider for a 2nd project.  I have also toyed with the idea of using an isolated DC to DC converter to generate the 2nd 300V off the main pack to run the motor.  You would be loosing some efficiency here but should gain some back in the motor.

There is a lot more involved in motor selection poles, rpm, etc. and I am still learning more each day.

[ken.simmermon]

Sounds like you've gotten lucky so far for components. The motor your using, is it rated for inverter duty. I've heard this term when using inverters but I'm not sure if it makes a difference. Also why would you have to run a 600V pack with a 480 inverter? I personally think 480V would be the way to go, just use fifty 12V 30A batteries split the pack for charging. 480V might be the magic number too all the trains use it, the current would be nice and low.You have me thinking now, but maybe 50A 12v batteries would be needed. Yes please keep in touch and if I get up there I'll contact you. I hope to be starting something in a couple of months. There is a company making a diesel hydro motorcycle. Since hub motors of any decent size are not available yet, do you think putting a couple of 50HP hydraulic motors on wheels driven with a motor pump arrangement would be efficient? John J. from the club was working with this principal a few years ago, Hi John. But I guess you would be hard to get regen braking then. Thanks for the good info and it looks like you'll be the first in the club to do an AC conversion, I'm jealous. Talk to you later, Ken

[tom.gralewicz]

I have been told different things about Inverter duty motors, from it doesn't matter, to add and LC filter to turn the high frequency component to low frequency, to its just a matter of insulation.  I plan on experimenting with the filters once I get the car running.   If I burn out the motor there are others I can try, this car will be an educating experience.

The voltage puzzle looks like this:

AC voltage is listed as RMS value - think of it as average.  If you look at the sine wave, the peaks will be 1.414 times the RMS voltage.  Peak voltage for a 230V power line is 325V (for 460 AC its 650V).  Inside the inverter the capacitor bank charges up to this max voltage and the inverter uses it to create the peaks coming out so if you want to just provide DC you need the DC voltages.  The inverter I have will run down to 200V without shutting down but the motor won't have as long of a constant torque curve.

The batteries I have are 75AH AGM (C&D  UPS 12 270) the motor is rated at 53A at full load - at 50A I should get 40 to 60 minutes out of the batteries, at freeway speeds that means 60miles - yes, thats best case and I don't expect to see that.

Wheel motors are a neat idea, but the real problem is unsprung weight.  If you look at formula 1 cars they even move the brake rotors inboard to keep the unsprung mass at a minimum.  I have been thinking about configurations with 1 motor per wheel and if I was going to make one I would get a small SUV like a Honda CRV or similar and use the 4 wheel drive mechanism to put 1 motor on each wheel but keep them on the chassis not in the wheel.  I actually have some rather large brushless DC ring motors that might be useful for this.

As for Hydraulics they don't have a reputation as being efficient, you tend to generate a lot of heat pushing fluid around - notice the transmission coolers and industrial radiators on hydraulic systems.  I would expect you might have a speed problem as well, to get a hydraulic motor spinning fast you need lots of flow, big hoses and a big pump.  Regenerative braking would be possible with a hydraulic system, you just need some big tanks that can hold 2000psi and enough fluid to pump into them against a gas - I suppose that could get a little heavy :-)