> Yup, that was quite the brain fart sorry about that. Since nobody knows
> me here I'll risk further imbarrasment. It would be V/R wouldn't it?
> I calculated 120 amps from a measurement of .3ohms and 36 volts. When
> I locked the rotor and applied voltage, from a good distance away, I
> measured 643 amps. The armature and feild are connected in series.
> It was very exciting to actually measure the current but I don't think
> I'll ever do that again.
> How can I make a quick estimate of lock rotor amps for a series motor?

Hi Pete,

It's hard to use a multimeter to measure such low resistance. You will do
better to apply some small voltage and measure the current. Even that is
not going to be accurate because the armature/commutator is not
predictable at zero speed. But having designed quite a few motor
controllers, some over 1000 Amps, I will impart a piece of design
information that you should consider.

Cycle by cycle current limiting.

All commercial controllers pretty much have it. It's a good thing. It
doesn't need to be particularly accurate, but it should be part of your
design. Even if you don't care about protecting your motor's armature, you
need to protect your mosfets every microsecond (or at least every 10 or 20
microseconds).

One reason is simply that it's impossible to predict how much current your
motor may draw under some unusual circumstances. Let's say you roll
backwards on a hill and then give it full throttle. You could draw 1400
amps. Or if your potbox gets a loose connection and the throttle toggles
on and off 10 times in a second (and you didn't filter the potbox inputs
properly).

Another thing you need to look at is gate drive. Where does the supply for
that come from? Do you have a good undervoltage lockout circuit that
doesn't turn into a dirty oscillator? That can toast your mosfets in a
hurry.

But let's say you're serious about hotrodding, you think you want as much
current as possible. Fine, but there is a limit. You've measured 643 amps
already. Maybe you should design around limiting to 450 amps. Now how do
you make it into a hotrod? More voltage. Your 36 volt motor will not draw
anywhere near 450 amps at cruising speed if you only give it 36 volts.
Think more like 60 volts (but watch your RPM - don't goose it in neutral).
You can get 30 to 40 horsepower that way, which is a respectable amount.

60 Volts * 450 Amps = 27000 Watts. This is no beginner project.

Do you have a  radiator fan motor or something like that you can mate up
to a bicycle or scooter and then design a controller for that first?
You'll learn a lot, especially if you try to scale it as you would your
27KW controller (in other words, in the 27KW model you won't be able to do
massive overkill, it needs to be smart and protect itself. Use those same
principles to guide you in a 1/4 horsepower version).

I haven't been following this thread so I don't know where it's gone
before, I hope I haven't treaded old ground or anyone's toes.

Cheerful regards,

Bob




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