Re: field weakening

[Lee Hart <leeahart@earthlink.net>]

Gary Flo wrote:
> Forgive me if I missed this before: I'm going to do this with my
> 800 lb City-El.  How much resistance do I need?

I would try 1/2 field. That means shunting the field with a equal
resistance, so half the current goes through the field and half through
the external resistance.

You need to know (or measure) the field resistance. It will be very low;
something like 0.001 to 0.05 ohms. The best place to get it is from the
motor's data sheet. If that's not available, use a micro-ohmmeter. And
if that's unavailable, you'll have to measure it yourself.

To do this, power the field winding with a 6v or 12v battery, resistor,
and ammeter in series. The lower the resistance, the higher the current,
and the more accurate it can be. You have to use a battery; not a
battery charger or PWM controller, or AC ripple will confuse the meter.
Connect it up, measure the current and voltage across the field, and
calculate the field resistance.

For example, you have a 6v battery and 1 ohm resistor. The ammeter reads
6 amps. The voltmeter reads 0.03 volts across the field. R = 0.03v / 6a
= 0.005 ohms. (Now you see why you want a high current; the voltage is
so small that it is hard to measure accurately).

Now you need a big contactor and a equal-value resistor. An automotive
type intermittent-duty starting solenoid is adequate, because it will
never be on for long, and won't see over 1v when it switches.

Resistances this low can just be a piece of wire, steel strap, etc.
Measure its resistance as above. Be sure to include any connectors,
crimps, etc. They have a noticeable resistance, too.

I suggest using a fuse. Fuses are in fact low-value resistors. It is
easy to change to different current ratings to change resistance values.
A fuse also has the advantage that it will blow if you switch in field
weakening when motor rpm is too low and current is thus already high. 

Don't energize the field weakening contactor unless the controller is
already full on, and motor current is less than rated current.

As an example of what should happen: My ComutaVan has a GE 60v 160a
motor. Wide open in high gear is 60 mph, 150a. 1/2 field weakening
increases current to 250 amps, and increases speed to 70 mph.
-- 
Lee A. Hart                     Ring the bells that you can ring
4209 France Ave. N.             Forget the perfect offering
Robbinsdale, MN 55422 USA       There is a crack in everything
phone (612) 533-3226            That's how the light gets in
leeahart@earthlink.net                  Leonard Cohen