Thread: Help needed in reversing direction of a DC motor

Hi all,

Hi have a 12V hydraulic pump. It used to live on my press until one day I asked too much of it and burned out the motor. I have since replaced the pump on the press with a hand operated one but I kept the pump, minus the burnt out motor which I have since sent to the tip (stupid move, would have been worth getting it rewired)

Anyway, I chanced apon a 12V motor on ebay that also once powered a hydraulic pump, albeit from a different manufacturer. I offered the seller $15 and he accepted. Bargain!

Get it home and all is good BUT the motor runs in the opposite direction to the needs of the pump I wish to adapt it to. Reversing the polarity does nothing, runs in same direction. A quick search of google has educated me on the fact that only permenant magnet DC motors can be resversed by that method.

Further reading indicates that this motor needs to be treated in a similar way to a universal motor in that you either change the polarity of the field or the brushes, but not both. No problems, I have done that before.

I take the motor apart and it becomes apparent that my knowledge in DC motors is still somewhat wanting. It has 4 fields and 2 pair of brushes. Opposing brushes are physically connected together which indicates that "maybe" I need to rotate the entire brush assembly by 90 degrees. This could be incorrect as well.

The one thing that really has me stumped is that of the 4 field windings inside the motor casing, 3 are series wound, as in the end of one connects to the start of the next etc. BUT one winding is all on it's own and it's winding is connected directly across 12V. Hopefully the pics show what I am trying to say. The "odd" winding connected by itself across 12V also has a different coloured insulation material.

Based on what you can see (pic will be in the next post) and what I have explained, any ideas on how I change the direction of rotation of this motor?

Thanks in advance,

Simon

Pics below...

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Originally Posted by simonl

Pics below...

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I was wondering if it was as simple as just reversing the polarity of that single individual field winding. Somehow i dont think life would be that simple though!

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that looks like the car starter motor. I think current goes into the armature in stator in such the way that they maintain the north/south. You might need to change either stator or armature only but not both to reverse?

Is it some sort of DC compound wound motor.

https://www.electrical4u.com/compound-wound-dc-motor-or-dc-compound-motor/

http://www.industrial-electronics.com/images/elec4_3-5.jpg

Originally Posted by .RC.

Is it some sort of DC compound wound motor.

https://www.electrical4u.com/compound-wound-dc-motor-or-dc-compound-motor/

http://www.industrial-electronics.com/images/elec4_3-5.jpg

Thanks guys. I have done some more searching.

The fields coils and armature windings are wound in parallel which according to info found on the net means that its a type of shunt wound motor.

I have since reversed the windings on that individual field winding and tested but it had no affect.

Im thinking i have to rotate the entire brush assembly by 90 degrees. This is relatively straight forward but at least one brush will need its lead lengthened and since the assembly is held with 3 screws (120 degrees apart) i will need to drill 3 new holes.

Ill have a bit more of a think....

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I tried doing this on a similar motor a couple of years ago.

When I reversed things around "every which way" it either worked in the same direction as before, or it didn't move at all plus lots of sparks and some smoke
Then I sort of worked out I had to pull apart the compound wiring (thick copper connections) and reconnect the coils in a different order.
On my motor the thick copper inter-coil connections were brazed to the coils and after breaking them apart I had problem with one of the coils and was not be able to fix it so it ended up in the scrap bin.

But please let not this experience stop you - I would be really glad if you can work it out.

Hi Bob,

Thanks for the info and insight. I may well find the same thing happens to me. At $15 I'm not overly fussed and in any case my backup plan is to use a 24V motor I already have and tested on it which works OK. However, the 24V motor runs too slow on 12V and so will need a 12 to 24V converter. A 480W 12 to 24V converter can be bought on ebay for about $45 but I would prefer the simplicity of a 12V motor if I can.

What does not make sense to me is that all four field coils are permanently connected to 12V in series except for one coild which is still connected to 12V but not in series. Also, the wiring to that individual coil is not as heavy.

Also, with the brushes, the current path is not from one brush through the armature and then out to the opposing brush, it runs out to the brush perpendicular. Opposing brushes are connected in parallel. This would indicate to me that if I swapped the brush locations by 180 degrees, it would have no affect on the direction of motion. I think I need to rotate the brushes by 90 degrees.

Edit: one theory I have is simply this; the "odd" field coild is simply a repair job sometime in the past (hence the different colour) and instead of reconnecting to the other field coils, it was just easier to connect in parrallel instead of re-soldering to the other coils which ultimately would do the same job. Not too sure about that theory, but it is a theory.

I'll keep you all posted.

Simon

OK. I have since had another look at this motor. One thing I failed to notice was the 3 field windings that are series connected, then pass on and are connected to 2 of the brushes. Anyway, I have drawn a basic sketch to help people visualise what I am trying to say.

20180427_080435.jpg

Cheers,

20180427_080435.jpg


Simon

Hi RC,

you are correct. I think this motor is what is called a "Long Shunt Compound Wound DC motor"

Thanks.

Simon

Simon,

You appear to me to have a classic compound wound DC motor. One series winding (made of 3 sections) and one parallel winding. Like has been said, you need to reverse one or other of coils or brushes, but not both.

It looks like the easiest way to do this is to leave the + to the coils as it is, leave the earthed end of the parallel coil as it is. Move the connection from the end of the series coils which currently goes to the pair of brush connections to the other pair of brush connections (the one that is earthed), and earth the end of the brush connections that used to go to the series coil.

In this way, you'll leave the current flowing through the coils in the same direction, but reverse the flow through the armature windings. This will reverse the direction of rotation. There may be some subtle field leading going on in the built-in geometry of the motor, but given that this is fixed into the design, the above is the best you can do.

Hope this helps.

Graham.

Originally Posted by Oldbikerider

Simon,

You appear to me to have a classic compound wound DC motor. One series winding (made of 3 sections) and one parallel winding. Like has been said, you need to reverse one or other of coils or brushes, but not both.

It looks like the easiest way to do this is to leave the + to the coils as it is, leave the earthed end of the parallel coil as it is. Move the connection from the end of the series coils which currently goes to the pair of brush connections to the other pair of brush connections (the one that is earthed), and earth the end of the brush connections that used to go to the series coil.

In this way, you'll leave the current flowing through the coils in the same direction, but reverse the flow through the armature windings. This will reverse the direction of rotation. There may be some subtle field leading going on in the built-in geometry of the motor, but given that this is fixed into the design, the above is the best you can do.

Hope this helps.

Graham.

Hi Graham,

Thanks for your comments and idea. Unfortunately the idea of swapping the wiring to the brushes is very difficult. If you have a close look at the brush assembly you will see that the brushes that are series wound to the field coils and held at 12v have small insulation tabs which keeps them electrically isolated from the metal assembly, which is ground via the mounting screws.

Cheers

Simon



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OK, so you have to reverse the coils instead. Can you do that?

Graham.

G/day Simon, if it is a gear pump can you just pipe it up in the other direction and use the motor as it is ?

Hi Graham,

I think the easiest way out of this is to reverse the brushes BUT by physically rotating the entire brush assembly and making new mounting holes for the screws. This is what I would consider to be the least painful way of doing it. One of the series connected brushes will end up with a lead that is too short and so will need to be lengthened.

Hi Shed,

Not that easy I'm afraid. The pump is a self contained pump and resevoir all in one and also has 2 solenoid valves that can change the direction of flow. It's basically a solenoid version of a spool valve. The pump only runs when it's being used. ie a push of a button for up and a push of another button for down etc. Reversing the flow would be near impossible.

Cheers,

Simon