I just rewired the coils of an old style generator to act as a motor. I'm running it on 48 volts. In every situation listed below the stator (driven through brushes and commutator) is at 48v. My question is what field coil voltage will create less heat. It overheats quickly (10 minutes to 160 degrees F) with no load on 48 volts both coils. If I drop the field coil only to 24v, I lose about 15% of my torque and gain about 20% speed. Will this lower field coil voltage create more or less heat? On 12v the motor won't spin up properly, and at 36v it's about half way between 24 and 48 volts. I'm trying to get max usable power without burning the motor up. I have not tried running it on lower field coil voltage for more than a few seconds.

 

It may not be possible to optimize this old generator as a motor. One obvious problem will be the positioning of the brushes. Which are rotated one way away from the centerline for generator action and should be rotated the other way from centerline for motor operation.

 

It may not be possible to optimize this old generator as a motor. One obvious problem will be the positioning of the brushes. Which are rotated one way away from the centerline for generator action and should be rotated the other way from centerline for motor operation.

The generator brushes appear to be in line with the center of the magnets. Also I'm running it backwards in my current setup. At 12 volts both coils it can run for 2 hours under load spinning a 12" fan blade and be just barely warm to the touch, cold if not loaded, so I'm assuming that the excess voltage is creating lots of heat somewhere. I'm hoping its a simple fix because I can push more power out of this motor than any DC motor I've worked with in the past.

 

The higher the voltage on the field, the less the torque (current) so you will get higher RPM at the expense of torque.
Torque is directly related to current, no matter what the rpm.
Be also aware that if the field goes open while running the rpm can take off at an uncontrolled rpm.
Max.

 

It may not be possible to optimize this old generator as a motor. One obvious problem will be the positioning of the brushes. Which are rotated one way away from the centerline for generator action and should be rotated the other way from centerline for motor operation.

Reminds me of the Yamaha RD200 I had years ago, it had what they called a; "Dynastart" - the dynamo also served as the starter motor, it had 2 sets of brushes and some extra solenoids in the electromechanical regulator box.

Being a 2-stroke twin it didn't take much torque to turn it over - it could've been started by flicking it round by hand, if it had an exposed flywheel to get hold of.

 

The higher the voltage on the field, the less the torque (current) so you will get higher RPM at the expense of torque.
Torque is directly related to current, no matter what the rpm.
Be also aware that if the field goes open while running the rpm can take off at an uncontrolled rpm.
Max.

In my tests it appears the opposite. Field coil at 48v spins slower and has better torque than 24v with 48v on stator both times. I did try powering it up without the field coils, it got moving really fast but when I stopped it and put a toothpick in the blades of the fan on the pulley and tried again the tiny toothpick kept it from moving at all. Since this is going on an electric scooter that's not a problem because it won't have the torque to even spin the rear wheel.

I'm willing to sacrifice some of that torque for a lower field coil voltage if that will reduce the heat generated.

 

Reminds me of the Yamaha RD200 I had years ago, it had what they called a; "Dynastart" - the dynamo also served as the starter motor, it had 2 sets of brushes and some extra solenoids in the electromechanical regulator box.

Being a 2-stroke twin it didn't take much torque to turn it over - it could've been started by flicking it round by hand, if it had an exposed flywheel to get hold of.

I got the idea of using a generator as a motor from a golf cart I had a few years back. It had a different kind of generator that also served as the starter. I didn't realize these old car generators were different until I had already bought one at a scrap yard.

 

Max, I think you posted the opposite of what you were thinking. Raising field strength slows down the armature and will increase torque (assuming fixed armature voltage) due to increasing back EMF.

To the OP, do you know what kind of car the generator came out of? A lot of the 1950 cars were still operating on 6 volt systems. 48 Volts would be a very high increase above design so I would expect a lot of heat to be generated. The old voltage regulators had several relays inside them. One was the CUTOUT relay that disconnected the armature from the system when the engine and ignition switch was off. Another was the voltage sense relay. It had the job of monitoring the battery voltage and by opening and closing, regulated the output of the generator.

 

In my tests it appears the opposite. Field coil at 48v spins slower and has better torque than 24v with 48v on stator both times.
.

Sorry I meant to say lower voltage on field increases the rpm at the expense of torque.
This is what machine tool spindles do, the operator can lower the field voltage to get higher rpm.
Max.

 

Sorry I meant to say lower voltage on field increases the rpm at the expense of torque.
This is what machine tool spindles do, the operator can lower the field voltage to get higher rpm.
Max.

That's fine I kind of figured you just got mixed up. Happens to everyone.

Max, I think you posted the opposite of what you were thinking. Raising field strength slows down the armature and will increase torque (assuming fixed armature voltage) due to increasing back EMF.

To the OP, do you know what kind of car the generator came out of? A lot of the 1950 cars were still operating on 6 volt systems. 48 Volts would be a very high increase above design so I would expect a lot of heat to be generated. The old voltage regulators had several relays inside them. One was the CUTOUT relay that disconnected the armature from the system when the engine and ignition switch was off. Another was the voltage sense relay. It had the job of monitoring the battery voltage and by opening and closing, regulated the output of the generator.

Its a 12v generator from some import car. Its a bit smaller than the old 6 volt ones.

So would running 24 volts on the field coils generate less heat? Most of the heat appears to be coming from the field coils, the motor case is very hot but I can touch the end of the stator and its still hot but not nearly as bad as the case.

 

One way to accomplish what you want would be to remove the field coils and rewind them with smaller wire. More turns with less current could create the same field strength. Most of those generators had removable field coils. Look for two square or hex female holes on the outside of the generator. Those will hold the pole pieces which, in turn, hold the filed coils. Of course that fix wouldn't do much for the armature current going into windings designed for 12V.

 

One way to accomplish what you want would be to remove the field coils and rewind them with smaller wire. More turns with less current could create the same field strength. Most of those generators had removable field coils. Look for two square or hex female holes on the outside of the generator. Those will hold the pole pieces which, in turn, hold the filed coils. Of course that fix wouldn't do much for the armature current going into windings designed for 12V.

The 2 flathead bolts holding the field coils on are rusted in. I broke 2 heavy duty screwdrivers (grabbed the handle with pliers to get leverage, one handle broke and the other's end got bent up) and in the end stripped both bolts. I was trying to remove them so I could attempt to find permanent magnets to fit in my pile of old motor parts.

 

Sounds like time for some good penetrating oil and maybe a little heat.

 

Sounds like time for some good penetrating oil and maybe a little heat.

AFAIK: they peen the end of the bolts so thay can't vibrate loose - penetrating oil as such won't help much, but the PTFE enhanced variety might help a little. Heat may destroy the windings insulation. An impact driver might do it, but not without risk!

Its 100% vital to chose the correct driver bit right from the start - any damage to the bolt head is yet more odds stacked against getting it out.

 

If the heads have been staked, a small drill bit can remove the staking. If all else fails, a larger drill bit can remove the head and the remaining stud removed after pole piece removal.

 

The old gen/starter repair shops had a press for removing the screws. A V-block held the body of starter/generator. The ram of the press had a screw driver bit with a long handle( kind of an L shaped thing) and a thrst bearing on the press ram. One guy put pressure on the ram ,to keep driver in place and other guy turned the driver. When helping my grandpa I got the press handle. If I remember correct the screw threads were/are an interference fit.

 

Well if you are trying to feed 48 volts to the stock field coils that were wound to never see much over 12 volts that would be why they overheat so bad. At 24 volts you are pushing 4x the power though them and at 48 volts you are pushing 16x the power they were designed to handle.

The only solution I know of for doing that is to with rewind them for 48 volt use or use a simple PWM circuit to limit the average power on them to a level below their overheating point.

 

I may have found the source of the heating. When I cut power to the motor it brakes like a permanent magnet motor shorted out. It does not do this if I disconnect the field coil from the stator when I cut power. I think the stator may be shorting into the field coils, which drive each other until the motor stops. I think if I separate them with diodes this may go away, and if they were somehow doing this while running it would explain the massive amount of heat. I hope I don't have to use a diode on the stator, the field coils draw about 10 amps max (when stator is not powered or motor stalled, about 2 when spinning without load) but the stator is pulling about 30 under heavy load (climbing a steep hill at the park at full power) so finding a big enough diode will be hard.

 

Also without load the motor spins up to a higher RPM then quickly slows down to a stable RPM that sounds about 20% slower. I'll try to get some pictures and possibly a video of the speed change tomorrow. This has turned out to be a very interesting project.

 

In normal generator operation, the field is fed from the battery by some sort of voltage regulator and the armature then supplies power to the battery. The regulator provides isolation between the two. Since the device you are using was designed to be a generator, when you have armature connected to the field, and power is removed from the motor it will become a self feeding generator with a fairly good sized load which is the field coil. That would explain the quick slowdown. I think an automotive relay to disconnect the field when power is removed would be effective at eliminating the dynamic braking situation, but would not answer the high heat problem. OOOPS -- You are using 48V, so a standard 12V automotive relay would need some form of coil protection. I'm still convinced that the only answer forteh excessive heat is to rewind the field coils with smaller wire, but even that would not be a total solution since you are applying 48 volts to an armature designed for 12.


Removing the power to the field will cause an increase in RPM due to lack of back EMF being generated. The armature is reacting to residual magnetism of the field pole pieces. As the residual magnetism fades away, the torque will be reduced to the point that it matches the various friction elements, ie brushes, bearings, fan and the speed will stabilize at that point