Timing, Inductance & Bypass, oh my

Hi Jeff etc,

I measured the overall motor inductance amr + field with my B&K LCR meter.
I found that adding an additional 150uH doubled my acceleration rate from a
dead stop *independent* of the timing test. I always use a bypass (and
engage start) contactor that shorts the control *and* the chokes when fully
depressed so the batteries are connected *directly* to the motor for best
efficiency. The trick is to set the fully mashed position at the nominal
speed your going, for me it's 60mph in 3rd gear at 96V. Yes field weakening
is a thought but would it be more efficient than just shifting into 4th
gear?

The best thing to do is *test* your acceleration and operating RPM's
performance at neutral and then at an advanced timing setting. That way
you'll know for sure (like me).

In general it appears from testing and general consensus for lowly commuters
operating around 96V or below that neutral is best overall since RPM's
aren't that high <4k rpm's. When above 96 , certainly at 144V, the voltage
*and* rpm range is higher making advanced timing required for performance
and flashover.

Cheers,
Mark

Date: Tue, 31 Jul 2007 05:37:49 -0700 (PDT)
From: Jeff Major <[email protected]>
Subject: Re: Additional Motor Inductance/better acceleration
To: [email protected]
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Message-ID: <[email protected]>


Hi Mark,

You run a by-pass? Not many do, that I am aware of.
This is with the MTC motor, right?

Anyway, regarding additional motor inductance. Yea,
sometimes these high speed motors would be called
rotating short circuits by those involved in control.
Increased PWM frequency helps. Adding inductance to
the motor circuit with chokes, inductors or reactors
or whatever you call them is a fix when you're stuck
with existing components. From the get-go, it would
have been better to put the additional steel and
copper (used in the choke) into the motor. A bigger,
higher inductance motor motor makes more sense than
external inductance. But working with what you have,
nice way to raise the average current when the peak
current is limiting.

You know, the MTC motor was designed with a little
heavier field just to get the inductance up to where
it's at. This makes it a good candidate for field
weakening. Once you're in by-pass, you can switch in
a field diverter resistor and increase motor speed.
You'd want to use some smarts so that the control
never attempts to chop with the field weakened. A 19
milliOhm diverter resistor is good for the MTC. Would
give you another 500 RPM or so.

Also, how did you get the inductance of the motor?
You know that value is current dependent. Probably
doesn't make a big difference in what you're doing.
Which BTW, I love to see. Keep playing and let us
know results.

Jeff

--- Mark Hanson <[email protected]> wrote:

>Hi,
>
>Since I have a 400A 2kHz ye-ol Curtis that battery
>amps are limited (noticed 50A starting). I decided to try an additional
>choke
>in series with the motor but inside the bypass contactor loop. Since
>T=L/R with the motor inductance at 150uh and 58 milli-ohms, the time
>constant to pulse by pulse current limit is quick thus limiting the duty
>cycle
>on start to 2.5ms. By doubling the inductance or adding a 150uH choke, the
>start pulse width would then be limited to 5ms. So I took a uWave oven
>transformer, angle ground off the I bar on the E core and removed the coild
>and magnetic shunt. I then put 6 turns of #2 welding wire on and MIG
>welded the I back on the E for a 72uH choke measured. I then started the
>car
>again from a dead stop and noticed 80A instead of 50 battery amps and it
>moved quicker into operating range. I'll get another dead uWave and
>add another choke for a total of 150uH. So if you have acceleration
>problems and you're duty cycle limiting on start, try a choke or 2.
>
>Best Regards,
>Mark
>
>



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