Reversing permanent magnet dc motor polarity for reverse

Audioguru again

Joined Oct 21, 2019
1,908
A permanent magnet DC brushless motor turns forwards or backwards the same "forever".
A motor with brushes might be built for the brushes to last longer when turning in one direction but brushes wear out fairly soon anyway.
In addition to wear on the brushes, the brushes and their springs burn out when the motor is stalled and draws a very high current.
 

MaxHeadRoom

Joined Jul 18, 2013
21,402
Some who obtain T.M. DC brushed motors for other purposes are often concerned by the notation on the plate of CW or CCW for direction, this is generally because the heavy flywheel is screwed on to the threaded shaft and running it in reverse spins the flywheel OFF!
Otherwise, with the flywheel removed, there is no problem for either direction.
Max.
 

Thread Starter

Hutch2793

Joined Jul 7, 2020
133
Some who obtain T.M. DC brushed motors for other purposes are often concerned by the notation on the plate of CW or CCW for direction, this is generally because the heavy flywheel is screwed on to the threaded shaft and running it in reverse spins the flywheel OFF!
Otherwise, with the flywheel removed, there is no problem for either direction.
Max.
Haha good guess. I have a treadmill motor without the flywheel. I attached a v belt pulley to drive one of my projects. I have tried to find info about it. I gave up because I read something about having to take the motor apart because of some sort of guard to keep it from spinning the wrong direction. Please forgive my technical jargon, I don’t remember what they called it. I know it had to do with minimizing sparking and brush wear. Sounds like your confident it is safe to do though?

What is cw and ccw?
 

MaxHeadRoom

Joined Jul 18, 2013
21,402
At one very rare instance there may have been a T.M. motor that had these brush characteristics, but it has largely remained as an urban myth.
CW=clock-wise, CCW=Counter clock-wise. ;)
Max.
 

Thread Starter

Hutch2793

Joined Jul 7, 2020
133
At one very rare instance there may have been a T.M. motor that had these brush characteristics, but it has largely remained as an urban myth.
CW=clock-wise, CCW=Counter clock-wise. ;)
Max.
You just saved me a lot of money. Stupidly I built my project from start to finish without checking every issue that could occur. Sure enough I needed to drive the belt in the opposite direction to make it run efficiently. My options were buy another longer belt to drive an opposite roller, or put the positive to the negative input on the motor.
 

MaxHeadRoom

Joined Jul 18, 2013
21,402
If you are using the flywheel that is attached to the motor with threaded shaft and need to reverse it, you could try drilling a hole axially that includes both flywheel and shaft and then thread said hole in order to prevent spin off of the flywheel when spun in the opposite direction.
This way you can run in both directions.
Do you really need the flywheel? Or can you not fit a regular pulley in some fashion?
Max.
 

KeithWalker

Joined Jul 10, 2017
1,216
High quality DC motors have the brushes advanced a few degrees to achieve maximum output efficiency. If you run them in reverse, the efficiency will be lower and there will be a lot of sparking at the brushes. This will eventually cause to brush and commutator damage.
If you wish to run the motor in reverse, you will need to re-adjust the brushes. On most good motors, this can be done by rotating the end cap that contains the brushes. Connect an ammeter in series with the motor and adjust the brush position for minimum current, with the motor running under normal load.
Regards,
Keith
 

MaxHeadRoom

Joined Jul 18, 2013
21,402
I have yet to come across a current T.M. motor with brushes off centre.
If it is, it is more for optimizing for uni-direction.rather than quality issues.
A definite sign of high quality is a skewed rotor lamination's as was often done on high quality DC servo motors
@Hutch2793 Is there a name on the motor?
Max.
 

Tonyr1084

Joined Sep 24, 2015
5,039
Do you really need the flywheel?
Isn't the flywheel a part of the fan too? Those I've pulled apart have fins on the back side of the flywheel. Without air flow heating can become an issue. Magnets don't like to be overheated. They lose their permeability.

Some of the TM motors I've pulled out have sufficient shaft on the back end that might facilitate placing a fan there to move air either into or out of the motor body. I'm only bringing up the cooling issue so you don't burn up your motor under heavier (normal) loads.
 

Thread Starter

Hutch2793

Joined Jul 7, 2020
133
I have yet to come across a current T.M. motor with brushes off centre.
If it is, it is more for optimizing for uni-direction.rather than quality issues.
A definite sign of high quality is a skewed rotor lamination's as was often done on high quality DC servo motors
@Hutch2793 Is there a name on the motor?
Max.
Cambridge motor works permanent magnet motor direct current.
 

Thread Starter

Hutch2793

Joined Jul 7, 2020
133
Isn't the flywheel a part of the fan too? Those I've pulled apart have fins on the back side of the flywheel. Without air flow heating can become an issue. Magnets don't like to be overheated. They lose their permeability.

Some of the TM motors I've pulled out have sufficient shaft on the back end that might facilitate placing a fan there to move air either into or out of the motor body. I'm only bringing up the cooling issue so you don't burn up your motor under heavier (normal) loads.
Yes, you are correct. It has the fins and is for cooling, but it really didn’t provide any air flow. I think it being outside or having a fan near while running would cool it much more than the flywheel. I could be wrong though.
 

Thread Starter

Hutch2793

Joined Jul 7, 2020
133
If you are using the flywheel that is attached to the motor with threaded shaft and need to reverse it, you could try drilling a hole axially that includes both flywheel and shaft and then thread said hole in order to prevent spin off of the flywheel when spun in the opposite direction.
This way you can run in both directions.
Do you really need the flywheel? Or can you not fit a regular pulley in some fashion?
Max.
I don’t need the flywheel. I already have a pulley on the shaft.
 

Thread Starter

Hutch2793

Joined Jul 7, 2020
133
High quality DC motors have the brushes advanced a few degrees to achieve maximum output efficiency. If you run them in reverse, the efficiency will be lower and there will be a lot of sparking at the brushes. This will eventually cause to brush and commutator damage.
If you wish to run the motor in reverse, you will need to re-adjust the brushes. On most good motors, this can be done by rotating the end cap that contains the brushes. Connect an ammeter in series with the motor and adjust the brush position for minimum current, with the motor running under normal load.
Regards,
Keith
Thanks, I will try that. My motor is 2 hp overkill, so I don’t need full power anyways.
 

MaxHeadRoom

Joined Jul 18, 2013
21,402
Magnets don't like to be overheated. They lose their permeability.
Generally the reason for the fan is high loads at low RPM's in a T.M. application, when run in a general purpose mode such as a drill press etc Especially without the flywheel, these motors run at fairly low currents,
In a DC brushed P.M. motor, all the heat is generated in the rotor, which is radiated out to the P.M.'s By the time these magnets reach their absolute heat limit, the armature would be cooked!
In a T.M., the motor has to keep a smooth rotation, when subject to the varying loads it sees, hence the flywheel.
Max. ,
 

Tonyr1084

Joined Sep 24, 2015
5,039
Have seen fans on motors for many years. My sander/grinder rig has an AC motor with an internal fan. That fan came off the shaft and had to be repaired. Only when the motor got smoking hot did I notice the fan problem. So air flow may be important to how the motor is being used. I said "MAY BE important". If not damaging the Perm Mag, the potential for igniting flammable materials nearby. So I raise the caution about the fan mostly for concern of fire prevention. Though a magnet can degrade due to heat. But yes, Max, it does take a lot of heat before it loses its permeability.
 

nsaspook

Joined Aug 27, 2009
7,587
Have seen fans on motors for many years. My sander/grinder rig has an AC motor with an internal fan. That fan came off the shaft and had to be repaired. Only when the motor got smoking hot did I notice the fan problem. So air flow may be important to how the motor is being used. I said "MAY BE important". If not damaging the Perm Mag, the potential for igniting flammable materials nearby. So I raise the caution about the fan mostly for concern of fire prevention. Though a magnet can degrade due to heat. But yes, Max, it does take a lot of heat before it loses its permeability.
Just to be clear if you indeed have a AC motor.
Your AC motor uses induction to generate the opposing/attracting magnetic field. The stator magnetic field originates from the stator currents, while the rotor magnetic field originates from the induced rotor currents. There is no permanent magnet to lose anything.
 
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Tonyr1084

Joined Sep 24, 2015
5,039
Motor types and functionality aside - the fan cools the motor. That's the point I'm trying to make. If the motor gets warm, even hot it can scorch surrounding materials, which MAY lead to fire danger. Let's not get hung up on different types of motors and how they function. I'm focused on the flywheel of the TM Motor which has cooling fins. Without them the motor COULD run warmer than expected.
 
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