Re: (ET) E20 lacking power

[SteveS <ssawtelle fcc net>]

You didn't mention if the armature resistors were warm or hot - probably 
because they weren't, in which case never mind..

On the current measurement - use the shunt already in tractor- grab the 
power meter leads with a DVM set on 200mV scale . That should be 
accurate enough. The schematic I have shows the main shunt being 20" of  
#6 AWG  @ .4ohms/1000 ft =  667microhms, so your DVM will read 667 
microV / Amp - or 0.1V at 150A which I think is full scale on the panel 
meter. The 200mV scale on the DVM  will work up to 300A. This 
calculation may be off depending on the exact wire stranding the shunt 
wire is made of. The archives has some numbers on this.... And here it is:

doing a search in the list archives for "shunt wire resistance", the 5th 
result is from Tom Coate
Date: Tue, 04 Jun 2002

Click on that link and the answer is there:
"According to my wire table, standard annealed copper wire, 6 AWG, has a 
resistance of 0.3951 ohms per 1000 feet at 68 degrees F. So... the 20 
inch length in the tractor should have a nominal resistance of 0.0006585 
ohms. I don't know if they count on some resistance from the terminals 
on top of that. In any event, it's not a lot! So good connections make a 
huge difference."

so I think my numbers are about right..

On the motor shunt getting hot - others have reported bad connections to 
that shunt. You may want to clean them up.

Also poor motor performance and large currents is usually associated 
with a weak (or missing) field. You may want to hook up the voltmeter 
across the field terminals and check that it's 36V when in cruise mode - 
actually I guess anything below the field weakening setting should be 36V?.


SteveS
E20