On Tue, Dec 04, 2007 at 09:30:40PM -0500, Bob Blick wrote:
> Byron Jeff wrote:
> 
> > I can do that. Any suggestions of where to start in terms of getting up to
> > speed on power electronics?
> 
> I haven't got any ready references. I did teach a short unit on motor
> control but that was many years ago and I don't have anything saved. The
> controllers I am familiar with have all used paralleled MOSFETs for the
> switch and fast recovery rectifiers for the freewheel diodes. Curtis PMC
> and Zapi are the two I'm most familiar with. Ones I have designed have
> the same topology. You do cycle-by-cycle current limiting because
> economics force you to design with little headroom. Operating frequency
> is typically 16KHz.

Any ideas about the economics? If it isn't cost effective to hand build,
then I probably won't waste time doing so. Say if the target is something
like 160V, 300A, 16 KHz. What would be a sample MOSFET that could be
paralleled to deliver that type of power? If it's going to be $500 in power
electronics, then buying a controller would be the right move. However if
the electronics are cheaper and you pay the rest with sweat equity, then it
may be worth looking at.

> > OK. Will re-evaluate the car I have in that light. I think I read that it's
> > close to 2000 lbs with the engine removed.
> 
> Pre-1995 Geo Metro hatchback is good for economy, but has poor handling.
> 1995-2001 Metro hatchback has much better handling and is still a light
> car. But the Metros require a fairly short motor because underhood space
> is limited.

There's about a 125 lbs difference in the curb weights of the two vehicles
(1993 Tercel (2002 lbs) vs. the Metro (1874 lbs for the 1997 hatchback)).
While I understand that every pound counts, from a cost standpoint it
simply doesn't make sense to me to get rid of a car that I already bought
and paid for in order to buy a car that's a little over 100 lbs lighter.

I must reiterate that cost is my primary concern. I'm not looking to invest
$15K, $10K, or even $5K into this project. Even more than trying to
minimize weight, I'm trying to minimize costs. And that's the costs of
everything. I need to find something other than an Advance 9 motor because
they run $1000 each for the low end model. I want to locate a cheap used
battery pack, even if half of its lifetime is gone. I plan to use the car
that I have. It's not a ultralight, but it isn't a boat anchor either. Like
I said above I'd be willing to put sweat equity into a controller because
commercial controllers are expensive.

Say I have a free donor car and a $1K budget. Should I forget about it? I
know this is a beg, borrow, scrounge project. I'm OK with foregoing pretty
much everything except for cost and performance adequate enough to be safe
on the roads. A vehicle that tops out at 10MPH, takes 20 minutes to
accelerate to top speed, and has a range of 1 mile doesn't work. However, a
bettery pack that'll only last for a year, a simple cheap controller, or an
adapted lower voltage motor because it's cheap is doable.

> You pretty much need to retain the clutch. Only a skilled
> driver can drive without one, and an electric motor doesn't spin down
> when you take your foot off the pedal like a gas engine would.

Now I'm confused. From what I've been reading I got the impression that it
was the exact opposite. Releasing the accelerator stops the motor and that
gears could be shifted without clutching. I absolutely don't mind retaining
the clutch and I planned to do so. But I'd like to be clear about the
situation you outline above.

> 
>> Since I'm not planning on using a high end DC or AC motor, regenerative braking 
>> probably won't be an option.
> 

> You can do regenerative braking with almost any motor,

I thought that it was simple with AC motors but that the controller had to
do a lot of work on series DC motors. High end Zapis do DC braking but that
the low end Curtis' didn't.

> but in practice a skilled driver will outperform it!

Good to know. Like I said I wasn't planning on it.

Thanks for the information. Hope to hear more soon.

BAJ
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