Original Motorola 90Amp Winding resistance values

[jcrubin]

Does anyone have any documentation with regard to the resistance values for the windings for the coils within this original Alternator. I'd like to know if possible.

Thanks,

Jordan

[dmcnc]

I'm not sure what the resistance value is supposed to be, but you might be able to derive it from the number of windings (36x).

[Bitsyncmaster]

I would guess each stator winding would be less than one ohm so not sure you will measure it accurately with a multimeter. Now the rotor windings may be a little higher resistance since they don't have to carry the 90 amps.

[David T]

If you are trying to measure resistance in an effort to determine if you have windings shorted to each other, that is not a good way. The only good reason to measure resistance would be from the windings to the case. If you have any significantly low reading that would indicate a winding (or windings) shorted to the case. The best measure is output. Anything less than spec would mean a shorted winding or a bad or leaking diode. To measure for shorted windings you could measure inductance but that is not always an accurate test. The best way to inspect the windings is to just look at them. If they look burnt they are done. The rotor can be tested on a tester by inducing a voltage into it and looking for a shorted winding.

[jcrubin]

If you are trying to measure resistance in an effort to determine if you have windings shorted to each other, that is not a good way. The only good reason to measure resistance would be from the windings to the case. If you have any significantly low reading that would indicate a winding (or windings) shorted to the case. The best measure is output. Anything less than spec would mean a shorted winding or a bad or leaking diode. To measure for shorted windings you could measure inductance but that is not always an accurate test. The best way to inspect the windings is to just look at them. If they look burnt they are done. The rotor can be tested on a tester by inducing a voltage into it and looking for a shorted winding.

I should rephrase...

I have the alternator completely disassembled. And am measuring across the rotor commutators for resistance.

[jcrubin]

[Bitsyncmaster]

Have not seen anyone using a vacuum tube voltmeter in many years.

[David T]

All that can tell you is continuity or short to ground. No good way to test any shorts turn-to-turn by just measuring resistance. If it doesn't look "cooked" (burnt) then it is probably OK. Clean up the commutators and it should be "good to go". New bearings, brushes and voltage regulator, test the stator for any shorts to ground, test the diode bridge and reassemble. You will find it is cheaper to just get a rebuilt unit but often they are not reliable, depends on the rebuilder. I have seen where they dip the whole thing assembled into a tank and then spray paint it, stick in new brushes and call it "rebuilt". The bearings fail shortly afterwards. The cheap Chinese bearings they may put in are not much better even though they are new.

[jcrubin]

All that can tell you is continuity or short to ground. No good way to test any shorts turn-to-turn by just measuring resistance. If it doesn't look "cooked" (burnt) then it is probably OK. Clean up the commutators and it should be "good to go". New bearings, brushes and voltage regulator, test the stator for any shorts to ground, test the diode bridge and reassemble. You will find it is cheaper to just get a rebuilt unit but often they are not reliable, depends on the rebuilder. I have seen where they dip the whole thing assembled into a tank and then spray paint it, stick in new brushes and call it "rebuilt". The bearings fail shortly afterwards. The cheap Chinese bearings they may put in are not much better even though they are new.

Resistance might have been a poor choice when I requested a measurement from such a coil, but its generally what everyone uses. A company's specifications would most commonly use inductance. i've read 25mH off of this rotor after cleanup.

[Ron]

Does anyone have any documentation with regard to the resistance values for the windings for the coils within this original Alternator. I'd like to know if possible.

Thanks,

Jordan

Generally, 2-4 Ohms is a good rule of thumb for most of the early auto alternators. As David mentioned, usually the way they look tells the story. But, back in the day, rebuild shops had charts showing the resistance for each rotor by Amp rating-manufacturer...I don't have 90 Amp but for Motorola 65/80Amp:

Motorola 65-80A.jpg
"Check rotor winding for breaks, short circuit to earth and shorted turns.
Resistance of coil 4 Ω ± 10 %. Max. eccentricity for slip rings is 0.03 mm.
BMW"

Have not seen anyone using a vacuum tube voltmeter in many years.

+1

HeathKit! ...we had to build one of those in class.