Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - View Single Post - Anyone know how to figure out DC motor specs by measurements?

thingstodo · 10-12-2012, 12:11 AM

I bought a conversion - 1990 chev sprint, electrically less done that I thought - from someone and I'm trying to determine the ratings of the DC motor. The wiring was a bit odd, so I am checking out everything that I can. I'm looking for some rules of thumb to estimate the rated current of the field and of the armature. Estimating the rated voltage would be nice, too.

The motor is claimed to be a surplus brushed DC aircraft motor, 21 HP, rated 120 VDC, separately excited.

Field coil - posts are sized for about #12 wire.
Armature has 3/8 posts, looks like they are big enough for ... 150 - 200A, at a guess.
There are 6 brushes on the armature. The angle is hard to measure without complete dis-assembly. I'd guess maybe 12 degrees off vertical. The brushes are angled for clockwise rotation from the drive end.
The motor turns clockwise from the drive end, which is not the correct rotation to move the car forward. I'll post a separate question about how to reverse rotation (brush holders, etc).

Measurement with one 12V battery for both field and armature:

Unloaded - 200A surge draw when both Field coil and Armature are energized at the same time (120A surge on the field, 80A surge on the armature?) 770 RPM unloaded. It settles down to 7A for the field, 2.9A for the armature.

Measurement with one 12V battery for accessory (lights, contactors, field) and one for the armature so field is energized with the ignition, armature is energized with the accelerator pedal microswitch:

Motor partially loaded, coupled to the standard transmission (in neutral)
Field coil draws about 10A at 12VDC after it is warmed up (after about 60 seconds).
The Armature draws about 20A after it is warmed up.

Motor loaded, first gear or reverse, moving the car (and me). The motor and armature draw about 140 amps, after acceleration, moving steady state.

Motor loaded, first gear or reverse, brake on, motor stalled. The motor and armature draw about 200 amps (very short test).

No measurements of voltage sag during tests. I had one more meter but it didn't occur to me.

Any suggestions would be appreciated.

10-12-2012, 12:11 AM	#1 (permalink)
thingstodo Master EcoModder Join Date: Sep 2010 Location: Saskatoon, canada Posts: 1,488 Ford Prefect - '18 Ford F150 XLT XTR Tess - '22 Tesla Y LR Thanks: 749 Thanked 565 Times in 447 Posts	Anyone know how to figure out DC motor specs by measurements? I bought a conversion - 1990 chev sprint, electrically less done that I thought - from someone and I'm trying to determine the ratings of the DC motor. The wiring was a bit odd, so I am checking out everything that I can. I'm looking for some rules of thumb to estimate the rated current of the field and of the armature. Estimating the rated voltage would be nice, too. The motor is claimed to be a surplus brushed DC aircraft motor, 21 HP, rated 120 VDC, separately excited. Field coil - posts are sized for about #12 wire. Armature has 3/8 posts, looks like they are big enough for ... 150 - 200A, at a guess. There are 6 brushes on the armature. The angle is hard to measure without complete dis-assembly. I'd guess maybe 12 degrees off vertical. The brushes are angled for clockwise rotation from the drive end. The motor turns clockwise from the drive end, which is not the correct rotation to move the car forward. I'll post a separate question about how to reverse rotation (brush holders, etc). Measurement with one 12V battery for both field and armature: Unloaded - 200A surge draw when both Field coil and Armature are energized at the same time (120A surge on the field, 80A surge on the armature?) 770 RPM unloaded. It settles down to 7A for the field, 2.9A for the armature. Measurement with one 12V battery for accessory (lights, contactors, field) and one for the armature so field is energized with the ignition, armature is energized with the accelerator pedal microswitch: Motor partially loaded, coupled to the standard transmission (in neutral) Field coil draws about 10A at 12VDC after it is warmed up (after about 60 seconds). The Armature draws about 20A after it is warmed up. Motor loaded, first gear or reverse, moving the car (and me). The motor and armature draw about 140 amps, after acceleration, moving steady state. Motor loaded, first gear or reverse, brake on, motor stalled. The motor and armature draw about 200 amps (very short test). No measurements of voltage sag during tests. I had one more meter but it didn't occur to me. Any suggestions would be appreciated.