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Old Feb 03, 2015, 03:12 AM
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REQ info on motors that pass external current to shaft

I hope i'm posting this in the right location- i have been looking for info for a few days, no doubt my lack of results is proportional to my lack of knowledge in electronics in general.

I have a project that needs to pass a dc current to the motor shaft- it's an external current that's not powering the motor itself. I see that some people have custom built motors for a similar purpose, but i was hoping that there might be motors already made.

For example, obviously, simply wiring power to a spinning device will just twist the wires until the wires break or prevent the motor from spinning. if a project needs to have something that's spinning powered, one way to do it is to pass the current via commutator, and i've seen motors that people design specifically to pass power separate from that powering the motor. I was hoping there might already be motors designed for that purpose.

I need it to pass up to 1.5 amps at 28 volts. I haven't seen any motors yet designed to pass an external power to the shaft, i was sure it must be a common enough need that they must exist? Maybe there is a particular name that identifies this kind of motor?

Thanks for any info,

Rob
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Old Feb 03, 2015, 07:34 AM
Condescending Pedant
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By "pass an external power to the shaft" do you intend to use the shaft as a conductor for some circuit in contact with the shaft? You are not very clear on what your intention is.

You mentioned a commutator. This is used in brushed DC motors. It is the method used to switch the current in the motor windings as the windings pass from north to south magnet poles on the motor.

It is quite common to use a brush on a motor shaft to ground the shaft, which keeps stray currents from passing through the motor bearings and ruining them. This is done on large industrial DC motors.

If you want to connect an electric circuit to a motor shaft you would use a brush in contact with a open section of the motor shaft. As far as this being common, I have never seen it in my 30 plus years of work in industrial electrical and electronics.

If you needed to get power to some circuit that is mounted on the rotor of a motor, you would have to use slip rings. Two if you are using single phase or DC power, three if you are using 3 phase power or even 4 if you use 2 phase power. Slip rings are generally used to supply current to the rotating field of an AC generator or synchronous motor. It is more common in modern machines to use a special winding on the rotor that rotates inside of a coil mounted around the rotating winding. A DC current is passed through the outer stationary coil and An AC voltage is generated on the rotating shaft. This AC voltage is then rectified and used to create the field for the synchronous machine.

I am sure that if you did a search for "slip rings" you would get a good description of their use and some pictures.
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Last edited by Mike Dubovsky; Feb 03, 2015 at 07:44 AM.
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Old Feb 03, 2015, 09:54 AM
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There are a few motors that have hollow shafts to allow wires to pass through. Perhaps that would be an easier option.
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Old Feb 03, 2015, 10:41 AM
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Thanks mike, the suggestion you had about slip rings sounds like what i need, i'll try using that search term again. I guess i could have said it that way- my hope is to find a motor that uses slip rings to pass power. Well, it probably is also possible that parts are also made that can just be added to any motor, i haven't been able to find it.

I'll do more searching with the slip string term tonight..

Ron thanks, but the connection needs to be wireless, otherwise the motor can't turn.
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Old Feb 03, 2015, 10:45 AM
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I design industrial motors. The way you would want to do this is probably with a slip ring. However I strongly advise you not to do this. The shaft is electrically connected to bearings which are electrically connected to the motor housing. Passing a current through the shaft means you will also see currents through the bearings and in the housing. Currents in bearings can rapidly break down bearings. Additionally, many motors have a ground connection to the motor housing, which would also cause issues for you.
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Old Feb 03, 2015, 04:29 PM
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Quote:
Originally Posted by RobFromPluto View Post
Thanks mike, the suggestion you had about slip rings sounds like what i need, i'll try using that search term again. I guess i could have said it that way- my hope is to find a motor that uses slip rings to pass power. Well, it probably is also possible that parts are also made that can just be added to any motor, i haven't been able to find it.

I'll do more searching with the slip string term tonight..

Ron thanks, but the connection needs to be wireless, otherwise the motor can't turn.
Have a look at the indoor contras. With enough effort, the design could be modified for your use.
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Old Feb 03, 2015, 09:43 PM
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Quote:
Originally Posted by RobFromPluto View Post
I hope i'm posting this in the right location- i have been looking for info for a few days, no doubt my lack of results is proportional to my lack of knowledge in electronics in general.

I have a project that needs to pass a dc current to the motor shaft- it's an external current that's not powering the motor itself. I see that some people have custom built motors for a similar purpose, but i was hoping that there might be motors already made.

For example, obviously, simply wiring power to a spinning device will just twist the wires until the wires break or prevent the motor from spinning. if a project needs to have something that's spinning powered, one way to do it is to pass the current via commutator, and i've seen motors that people design specifically to pass power separate from that powering the motor. I was hoping there might already be motors designed for that purpose.

I need it to pass up to 1.5 amps at 28 volts. I haven't seen any motors yet designed to pass an external power to the shaft, i was sure it must be a common enough need that they must exist? Maybe there is a particular name that identifies this kind of motor?

Thanks for any info,

Rob
Guess we need more information on exactly what you're trying to do. Are you locking the shaft, and spinning the motor???

Methinks the only way of doing what you're after is to pick up an old automotive alternator from a junk yard, pull the slip rings and brushes out of it, and machine it to fit on your project. Those slip rings and brushes will easily handle an amp or three.
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Old Feb 03, 2015, 11:24 PM
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Motoreng- that is exactly what i was thinking about, and why i was hoping there might be motors specifically designed for it- a shaft that was made for this would basically have a shaft inside of a shaft, separated by an insulator to keep the motor's current separate from the current being passed. You haven't heard of anything like this? I suppose that's why people have been designing their own motors from scratch.

Thanks ron, i think that's an r/c style copter? I'll look more into it, but seems expensive to buy something premade that needs to be torn down and rebuilt, but for sure worth a look if it's all there is.

Vollrathd, i'll look at that too, the equipment would add far too much weight but also worth the look to see how it's made. To describe what i meant a little better i hope, if you imagine a bar fastened to the end of a motor spindle so that it rotates similar to a propeller, that would be how a motor is normally used. But then imagine that mounted on top of the metal bar is a light bulb or electrical circuit. The light bulb needs its own power, but how to power it? Wires would just get tangled when the motor spins.

But like motoreng said, the shaft isn't electrically isolated from the motors current, so it would need to be a special shaft that can have an electrically isolated current pass without messing with the motor's function. I'm not a complete newbie with electronics, on the other hand it's a project that keeps coming up over the past 10 years or so, every time i revisit it i run into the same roadblocks, i'm sure someone smarter than me has thought it through already. I have endless pages of sites and notes, but so far i haven't been able to get past this problem.
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Old Feb 03, 2015, 11:35 PM
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I should add, i've seen that people have used a 1/4" or 1/8" stereo jack:

https://encrypted-tbn0.gstatic.com/i...BNAE85__zNJDYF

The plug part is welded onto the spindle (and it seems a big project to make it so it spins on center) and when it plugs into the jack on the other side it can still spin, so it functions more or less as like slip rings (i read a lot about slip rings, thanks for that!). But the parts aren't made for that and so they wear our really fast from the metal parts touching while the motor spins. A special shaft with two or more cores (one for each contact point) that are isolated from the motor's current is the only thing i can think of, the more isolated cores, the more useful- i have another project that would need as many as five separate electrical paths (four minus, one common), but i'd be happy just getting two (+ and -) to work.
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Old Feb 04, 2015, 07:10 AM
homo ludens modellisticus
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The Netherlands, GE, Nijmegen
Joined Feb 2001
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Add a coil(s) to the shaft, and stationary magnets nearby? Will add drag, and voltage will depend on rpm though. Not a problem when operating at constant rpm.
-> axial flux motor design

Vriendelijke groeten Ron
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