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Need for Speed: AC vs. DC motors

by Tim
(Chandler, AZ)

I'm considering converting a Mazda Miata to an electric car, and I've learned a lot reading your site. I really liked the page you have concerning AC vs. DC motors. You make some very convincing arguments for AC. I stated doing research, specifically looking at the data entered for AC conversions on the EVAlbum online.

I was surprised to see that so many of them list the acceleration as slow or "not what they'd hoped." All of the points that you've made for why an AC system is better are quite good, but can you tell me is the trade off that you have a car which is slow off the line? I'm not looking to build a dragster, but 0 to 60 in 25 seconds just isn't acceptable.

All things being equal (batteries, volts, amp/hours etc) is an AC system simply going to be slower than a DC system? If I want an AC system is the price of that a slow car?

Hi, Tim!
You know, I've been pondering this myself for quite a while.
I thought...Hey, why not have two motors, one AC (for efficient use of your batteries on the freeway) and the other a series-wound DC (for brisk acceleration from a standstill)?
Turns out I wasn't the only one thinking along these lines. I'll quote "Build Your Own Electric Vehicle" (p. 57, second edition) once again: Darwin Gross (EV engineer) says "the ideal EV drive for tomorrow has...DC (motor) to get started, AC (motor) to run over 30 mph."

Sounds like a whole new hybrid to me!


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Need Some Help
by: Ed C.

I am new in the art of building an all electric vehicle. Is it possible to run an ac motor powered from the converter which is powered from the batteries? Once the vehicle is moving, Can the ac motor be powered through another sourse directly? Like a wind mill? Or can the batteries be charge fast enough to maintain its charge?

Hi, Ed -
A. Yes.
B. Yes, but it takes a big (mobile) source to provide enough power for acceleration.
C. More or less. Most people prefer to plug in at night anyway, and just use an alternative power source for range extension.

- Lynne

by: Anonymous

Within some assumptions AC and DC motors will perform equally well. ASSUME same RPM and HP and torque demanded by load is constant
DISREGARD wind resistance for now.

STARTING nothing beats a series DC for starting torque. If you have a bulldozer or stump puller or winch this is the choice. High start torque can be your worst enemy in snow ice or mud. However series has very poor speed regulation ie droop (RPM drop with increase torque ie hill) which must be overcome by speed control (either controller or foot on throttle). Torque is proportinal to amps squared - must have high current components or suffer meltdown AND battery must be able to deliver high current without too much voltage droop.

Trading starting torque for speed control is the shunt motor. Mainstay of industrial DC variable speed drive.

COMPOUND DC is a compromise of series and shunt. IF I were to go DC drive this would be my choice.

Industry generally will go with AC drive and for variable speed especially so on new equipment will go with VFD.

Wind resistance must be added to the torque demanded by vehicle. This is a square function of the relative velocity and is not a factor below 35. Double the speed and the HP needed goes up by 4 so it is appreciable at 60.

HILL CLIMBING takes a lot of HP more than that demanded by vehicle and windage.

ACCELERATION is basically a matter of HP. More HP more acceleration capability. Bigger motor bigger battery - there is some point of diminishing return. MORE MONEY too.

IF you design for the max grade you will need to climb (here in Seattle steepest is 20%) then you should have plenty of reserve HP to get quick acceleration.

IF you must jack rabbit out of a light so you can be first at next light and wait 2 minutes then maybe electric is not for you???
IF you want 9 sec quarter mile electric is not for you. There are tradeoffs.
Dan Bentler

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