Re: I need help

["Jon \"Sheer\" Pullen" <sheer@sheer.org>]

> I'm a student of mechanical-electric engineering
> and I'd like to build a EV as part of my tesis .
> I think to develop a bussines with the conversion to electric car in Peru
> But I want to get more Knowledge about EV's before to doing it.
> First I want to know What is the best DC or AC?

Unfortunately, this isn't a question that can be easily answered. It's like
asking 'Which is best, a car or a motorcycle'. The answer of course is that
it depends on what you're trying to do. ;-)

AC has several good points:

    1) easy to perform 'regenetarive braking', which extends range (10-30%
in stop and go driving) and the life of your brake pads.
    2) No brushes, hence no arc-overs, flare-outs, or brush replacement to
worry about
    3) most AC drives are high voltage, which has pros and cons [see below]
    4) AC drives are sometimes better built than DC drives. [in particular,
the one I've got is rock solid]
    5) because the inverter must be operational and performing decisions
several hundred times a second for the drive to be spinning, a runaway
condition is extremely unlikely
    6) AC motors are simpler, and therefore less likely to break. They can
also be easily sealed against the elements.
    7) AC motors can withstand higher RPMS.. and in fact put out torque at
higher RPMS.. AC motor power output can best be described as 'linear' ;-)

DC has several good points:

    1) DC drives are more common, and therefore cheaper
    2) EV machinists are familiar with DC motors, and can more easily build
adapters for them. [this may not even apply in your country, though]
    3) DC drives can have more low-end torque more easily..
    4) DC controllors are simpler, and therefore less likely to break.
[well, you'd like to think so anyway ;-)]. Actually, the simplest DC
controllor is just a collection of big relays!
    5) DC systems tend to be lower voltage, which has its pros and cons

Low voltage has a few good points

    1) A mistake probebly won't kill you. Don't underestimate the value of
this. My first EV was HV, but I'm used to working around high voltage, and
young and foolish besides. If you're not familiar with electronics, do
yourself a favor and build a LV EV first, because voltages > 200V can very
easily be lethal.
    2) It's the 'tried and true' way. You'll have lots of people to compare
notes with when you have problems, and lots of chargers, fuses, breakers,
battery level indicators, etc to choose from.

High voltage has a few good points

    1) The interconnects can be thinner, lowering the total weight of the
car. [by maybe 50 lbs ;-)]
    2) certain types of power electronics tend to be more efficient at
higher voltages.
    3) a high voltage series string of batteries is likely to give less
headaches than a buddy-paired low voltage string. [this is my opinion
anyway]
    4) in my case, my drive gives me a set number of amps no matter what the
voltage, so the higher the voltage I use, the more watts I can pump into the
load - i.e. the faster the car gets up to speed. If you've driven a 'lead
sled', you know that being able to dump watts in the drive to get out of the
way of other cars is really nice. ;-)

Now, the next question that will occur to you is what battery chemistry is
best.

There's _definately_ not a easy answer to that one. Read the list archives,
we spend most of our time discussing this question  ;-) [or moaning about
what peice of silicon we've let the smoke out of most recently..]