I am rebuilding a 601-101 diesel Pullmor motor, which uses a 600-125 armature – pretty common but reliable stuff from the 1950’s. As I am rebuilding, I noticed that the field windings appear to be made of finer wire than what I am used to seeing.

IMG_3678  Subject motor is "B" on left, "A" is one of 2 known good motors.

A quick check with a micrometer revealed the (magnet) wire used on this field is between 33 and 34 gauge as opposed to the 23 gauge wire which Robert Hannon lists for this motor. Also, my ohm-eater registers 3.0 ohms for this field winding as opposed to 0.9 to 1.0 ohms that Mr. Hannon specified for this motor. And just for comparison, I have two more identical but well-working Pullmors (motor "A" on right) that both measure 0.6 ohms on their field windings. It seems this field may have been rewound by another Pullmor lover.

In short, my question is what effect will this have on an otherwise fully restored motor?

For those of you who have torn one of these motors down to ALL of their individual pieces, including all the pieces of the aluminum frame, you know there’s a lot of careful work that goes into putting it all back together, and I really don’t want to waste my time recrimping the top plate to this motor if the final product will be deficient in some way. My suspicion is that it will be weaker than normal, so I need someone with some solid motor knowledge to help me understand if this thinner wire will actually work OK, or if I should be looking for a scrap motor somewhere as a donor for a proper field winding.

Thanks for your input




Images (1)
Original Post

The rewind clearly has more turns, which means a stronger magnetic field, which means higher counter-EMF, which means slower motor speed at any given voltage than a stock motor.

Well, yes and no Lew.  If there's less current because of the fine wire, then there will be less magnetic field.  We're talking ampere-turns here, and that fine a wire and the impedance offered will offset the greater number of turns.

I'd put it together and see how it performs on the bench before installing it in anything.

Heh. Yes, of course ampere-turns, John.  However, less current because higher impedance still means lower motor speed at any given voltage.

I concur that a bench test is simpler than the Calculus to cipher it would be.   

Could use a resistor to shunt the field and get the speed back up.  Be interesting to know how much of a difference the rewound field made on the operation of the loco.  

The resistor won't increase the current in the field, so the only thing the resistor would do is get VERY hot and increase the armature current.  However, that would reduce the already low current in the field, further impacting performance.

What finer windings would really do for a universal motor is to allow it to run on higher voltages.  However, the armature and field should be matched for the best results.  I'm 99% sure that a significant mismatch in the armature and field windings will result in a lower performance motor overall.

Last edited by gunrunnerjohn

I once (mid-1980's) bought a postwar 2360 GG-1 that had one field coil rewound by the original owner with fine wire.  The other coil was original.  He said he did it to make the engine more powerful.  I got it cheap because it "wouldn't run." 

In fact, when placed on the track the original motor would run but the fine wire one did not.  I disconnected the fine wire motor from the pickup rollers and tested it separately with alligator clips and it did turn but was sluggish.  I removed the fine wire and rewound the field coil with magnet wire from Radio Shack (remember them?) that matched the other motor.  The GG-1 then ran great.

Add Reply

OGR Publishing, Inc., 1310 Eastside Centre Ct, Suite 6, Mountain Home, AR 72653

Link copied to your clipboard.