# Converting a series motor to permanent magent



## Greenflight (Sep 13, 2007)

So, I'm sure a lot of other people have thought about this too, so I might as well throw it out there for discussion.

The idea is to replace the field coils in a series motor with permanent magents. You can get some pretty intense neo magnets for pretty cheap these days, for example these. Mounting them in the motor wouldn't be too difficult. Air gap might be a bit of a concern, but nothing that you couldn't take care of with a little ingenuity. Degaussing might be a problem, especially considering the heat inside a motor like this.

What do you guys think? Would the neo magnets be strong enough to effectively replace the field coils?


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## major (Apr 4, 2008)

Greenflight said:


> The idea is to replace the field coils in a series motor with permanent magents.
> What do you guys think?


Why would you want to screw up a perfectly good series motor?

Seriously, it wouldn't work out so well. First off, you would not be able to get the correct shape magnets for less than a fortune. Then spend another fortune for a magnetizer. Then you'd have a mismatched armature and field. Yeah, after all the time, trouble and money, it might just work. But I don't think any better than the series motor did before you started messing with it. Probably worse.

Regards,

major


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## Greenflight (Sep 13, 2007)

Haha thanks for the input, good points. Honestly I've always been intrigued by PM motors so it just got me thinking.


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## Bingo Sun Noon (Mar 13, 2009)

What a great idea!
That's what I thought when I did exactly what you suggest.
Don't waste your time. I put NdFeB magnets in a series wound motor, spent a lot of time and money, and got almost the same performance. Not quite as good, but close.
The magnets lost their mojo real quick like. These were Neo42 with Hci of over 14000, about the best there is.
Data: new mags 3.5 Tesla, after 10 hours 1.4 Tesla


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## major (Apr 4, 2008)

Bingo Sun Noon said:


> What a great idea!
> That's what I thought when I did exactly what you suggest.
> Don't waste your time. I put NdFeB magnets in a series wound motor, spent a lot of time and money, and got almost the same performance. Not quite as good, but close.
> The magnets lost their mojo real quick like. These were Neo42 with Hci of over 14000, about the best there is.
> Data: new mags 3.5 Tesla, after 10 hours 1.4 Tesla


Hi Bingo,

Looks like you confirm what I told him, post #2.

1.4T I might believe, but 3.5T? Come on

major


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## Tesseract (Sep 27, 2008)

major said:


> Hi Bingo,
> 
> Looks like you confirm what I told him, post #2.
> 
> ...


According to Technologic there are magnets out there that put out 5T... I wonder if they have to be cooled in liquid helium, or if just liquid nitrogen will do?


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## Bingo Sun Noon (Mar 13, 2009)

Hey guys. 3.5 Tesla is easy but not common. Simply adjusting the recipe gives you more energy. Trouble is, the very high flux densities tend to shatter easily and are not ready for prime time, mostly stuck in the lab until some day. Five tesla has also been demonstrated and at room temp, although I'm not sure what temperature has to do with flux density. Is there something I'm missing? If so, please don't tell my boss, he thinks I'm a smart SOB, but then again, aren't we all?
thanks


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## Tesseract (Sep 27, 2008)

Bingo Sun Noon said:


> ...Five tesla has also been demonstrated and at room temp, although I'm not sure what temperature has to do with flux density....


I thought that was a rather obvious reference to superconducting electromagnets; the kind commonly used in NMR machines, etc... If there were a way to generate a 5T field without resorting to superconductors don't you think GE, for example, would use that instead?

As far as 5T permanent magnets - it sounds like they are still the province of materials science experiments. If you can't buy them commercially then you certainly can't use them to replace the field electromagnet in a series wound motor.

EDIT: Like major implied, the highest field strength permanent magnets I could find have a field strength of 1.38T (Neodymium-Iron-Boron). Maybe there are lab curiosities with double, triple, or more the field strength, but it seems they are just that: lab curiosities. If you need 5T, you pretty much need a superconducting electromagnet (10T seems to be the practical upper limit for them).

So, bingo, not saying these 5T supermagnets don't exist, just that I couldn't find any sources.... Like major, I'm also very curious as to where you obtained 3.5T permanent magnets.


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## Bingo Sun Noon (Mar 13, 2009)

Ah yes, doubters are good

My workmate tells me Google is making us all stupid. Maybe so.

I'll check the calibration on my Gauss meter and get back to you.


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## Dennis (Feb 25, 2008)

I am confused, but why would anyone waste funds on PM's that loss their magnetic strength from strong opposing magnetic fields caused by the armature and also lose their magnetic strength from high temperatures. An electromagnet is superior and cheaper to implement in the power levels that we deal with here when converting to electric. Maybe for a <=15 Hp at the most it would make sense.


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## major (Apr 4, 2008)

Dennis said:


> I am confused, but why would anyone waste funds on PM's


Hi Dennis,

Permanent magnets can work well in motors (and generators) of reasonable size for EVs. The electric machines and control systems need to be designed for PM use. This overcomes demagnetization issues. Benefits from using PMs are increased efficiency among others. UQM has some excellent PM (BLDC) products going up to 150 kW. And Toyota uses PM machines in their hybrids and has done a great job with the design. I saw an efficiency map of the larger electric motor used in the Prius and it was very impressive. Toyota didn't waste funds, IMO, they made a wise investment.

With that said, I still like the wound field and induction motor drives for EVs, unless you have a Toyota R&D department in your garage.

Regards,

major


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## Dennis (Feb 25, 2008)

> Hi Dennis,
> 
> Permanent magnets can work well in motors (and generators) of reasonable size for EVs. The electric machines and control systems need to be designed for PM use. This overcomes demagnetization issues. Benefits from using PMs are increased efficiency among others. UQM has some excellent PM (BLDC) products going up to 150 kW. And Toyota uses PM machines in their hybrids and has done a great job with the design. I saw an efficiency map of the larger electric motor used in the Prius and it was very impressive. Toyota didn't waste funds, IMO, they made a wise investment.
> 
> ...


BLDC motors make sense in something like hybrids where the motor is only asked to take over at low RPM's or to supplement the gas engine for acceleratation. For a pure electric car perspective though they are a poor choice because although they are more efficient they cannot match the reliability of an AC induction motor for example because in the end the permanent magnets are the weak link and will eventually lose their magnetic field strength from aging and heat. Also, to limit demagnetization the current has to be limited to a low value which again is another negative in my book.

So I think only pure electromagnetic based motors should be used. They have been used in industry for many years without much issue. I even seen Raymond power jacks which now use 3 HP 3-phase induction motors instead of brushed DC motors because the demanding appilcation the power jack has to endure. The motors get HOT after about an hour use because I took the cover off a Raymond power jack and touched the motor frame when I use to work for C&S Wholesale in my college days. You could imagine how much damage the heat would cause to permanent magnets' field strength.


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## methanolcat (Sep 14, 2008)

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## Dennis (Feb 25, 2008)

You still have to solve the heat problem that effects neo magnets. Until this is solved I think I'll stick with what the industry has been using for many years and that would be AC induction motors. Reliability just is not there for PM based motors because of heat sensitivity of the magnets. They cost more too and require better cooling then a pure electromagnetic based motor.


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## methanolcat (Sep 14, 2008)

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## methanolcat (Sep 14, 2008)

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## Dennis (Feb 25, 2008)

Generally traction duty motors are insulated with a enamel coating that can take temps well in excess of 300+ F. Usually any motor to be designed for severe service will have class H (356F) or higher insulation rated wire and resin dip, and if you want to know where temps routinely go above 176 F then a fine example of that are traction motors used in Diesel-Electric trains and electric trains. Those motors are spec'ed to work at 230F temp rise in an ambient temp environment of 113F. PM's don't stand a chance. I hope whatever it is you guys are researching can take those temps. Until then, I will remain skeptical of their performance in hot environments.


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## methanolcat (Sep 14, 2008)

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http://www.electronenergy.com/


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## Dennis (Feb 25, 2008)

Those are *samarium cobalt magnets *big difference bud... Those magnets I understand can take high temps, but you do realize that those are many times weaker power unit volume in magnetic field strength compared to *Neodymium*. The magnets you linked too have been around for ages and are used in industrial PM motors up to about 3 HP. *Neodymium *has a much lower temp rating that samarium cobalt. Sorry you did not prove anything to me...


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## methanolcat (Sep 14, 2008)

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## methanolcat (Sep 14, 2008)

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## Induced (Apr 15, 2009)

Hello, new poster long time lurker, I'm working on building an electric motorcycle and have also converted a series wound motor to permanent magnet. 

I used neodymium magnets I think similar to what methanolcat used, got them from here. http://www.ndfebmagnets.com/permanent148363-ndfebdiscmagnet.htm

I bought the 33EH grade which are good up to 392 degrees F. they also have them in 2 other grades that maintain their strength up to 464 degrees F. I bought mine early last year so there may be better grades out by now but what I bought is I think more than sufficient as far as heat goes.

From the tests I have run I have proven to myself that the conversion is much stronger than the series wound ever was, I hooked my converted motor and a stock series wound to a riding mower, one at a time, both at 36 volts. The only thing I changed was the 2 motors everything else remained the same. The converted motor out pulled the series wound by far.

Testing in 20 minute increments the converted motor was warm but not hot but the series wound did get hotter, most likely because of the current flowing through the fields which are touching the case itself and are quite large.

I did not do any current comparisons mainly because I think that is a gimmie, the series wound would obviously use more current, my main reason for doing a conversion in the first place. I want to go as far as possible on the batteries I have.


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## major (Apr 4, 2008)

Induced said:


> I did not do any current comparisons mainly because I think that is a gimmie,


Hi there, Induced,

Rather a subjective test, don't you think? No measurements, just your impressions after a 20 minute comparison. And after the time and money you put into the PM conversion, you wanted it to be better.



> the series wound would obviously use more current,


I would not think this is true for all load conditions. It is likely the case for light loads, but unlikely for heavier loads and definitely untrue for stall.

If you have a controller and are working the motor at the light load end of the curve, you will have brought the speed vs load curve down significantly, accounting for a large reduction in current vs torque. But couldn't the same result have been accomplished by rewinding the coils to shunt? Sure, that would require a fraction of an amp for field excitation, but might yield some advantage over the PM field, depending on the application.

And you mention heat. You probably were judging that on case temperature. What was the actual armature temperature? Unlikely a 20 minute test was sufficient to stabilize the entire motor. Depending on the ventilation and/or cooling method, PM motors can actually insulate the armature causing thermal runaway in the armature. The steel poles in the wound field motor will conduct the heat from the armature better.

Lastly, if you are satisfied with your series to PM conversion for your application, more power to you. Great! Good going. But for those following this thread, one or two success stories with subjective results should not be taken for global proof that series to PM conversion is a smart idea.

Regards,

major


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## blackpanther-st (Apr 4, 2009)

methanolcat said:


> Hey all, new poster here, been here for some time though. To all those who wonder why spend money on R+D, lol, it has to be done to come up with new products, besides I will get my R+D money back many many times again. All that aside and no harm meant to anyone, I as well as many others are here to help the future of efficient, affordable electric cycles and cars.
> 
> To answer this question "What do you guys think? Would the neo magnets be strong enough to effectively replace the field coils?
> 
> ...


What I think would really be beneficial would be a PM / series wound hybrid design, wind the fields coil around the PM and fit a proper field shoe over the PM. this would alllow regen and increased eficency, and should have even more tourque capability than ither alone. And yes it would nead to be cooled, ither air fins on the body with forced air, or beter yet water jacket it.


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## Induced (Apr 15, 2009)

Major,

Yeah I would say my test method is rather crude and I didn't really get into the complete testing method. What I did was hooked the riding mower to a fullsize truck with the e brake slightly depressed for some drag then timed the distance I pulled it with each motor in 20 minute intervals, once the e brake was tensioned some I never released it until I was done (kinda hillbilly I know).

Like I said I did time tests from both and the permanent magnet motor times were always shorter by 25%, not a huge difference but enough that I think it made a difference, just testing for now.

Once I get the bike done I will run both motors and do a distance test as thats what I am really interested in. I don't have any means of loading these motors for more scientific method.

The only reason I chose 20 minutes runs was because it only takes me 10 minutes to get to work and I figured if it doesn't heat up badly in 20 minutes then it shouldn't in 10 minutes either. I'm pretty sure I loaded the motors as hard or harder than they will be on the motorcycle. They were working hard I made sure of that.

I'm not a motor expert by any means but I figured if I had to power field coils then I was using (losing) energy in them and I thought the temperature of the series wound kinda proved that. 

No I did not check the temperature of the armature what I really wanted to know was the temperature of the case because thats what the magnets are mounted to and I wanted to make sure I didn't overheat them. I would have thought the armature might be about the same temp on each motor and not have much effect on the case of the motor since the only place it touches the case is through the bearings.

"PM motors can actually insulate the armature causing thermal runaway in the armature. The steel poles in the wound field motor will conduct the heat from the armature better."

I'm not sure what you mean here or how this could be possible but like I said I'm not a motor expert. Could you go further into what you mean here?

I would say at this point I am satisfied but will be sure once the motorcycle is complete, I will try both motors and use which ever works best. I guess my main point in posting in the first place was not to say that PM motors are better but that there are neo magnets capable of high temperatures. 

Hope you reply back and I will continue reading this forum as it is full of valuable information. Thanks


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## major (Apr 4, 2008)

Induced said:


> "PM motors can actually insulate the armature causing thermal runaway in the armature. The steel poles in the wound field motor will conduct the heat from the armature better."
> 
> I'm not sure what you mean here or how this could be possible but like I said I'm not a motor expert. Could you go further into what you mean here?


Hi Induced,

Mostly from back in the days using ferrite or ceramic magnets, they were thermally poor conductors. And they would surround the armature. So the heat from the armature had a tough time getting to the frame to radiate to the atmosphere. This was especially a problem in totally enclosed motors.

Whereas the wound field motor had steel pole shoes about .020 inch from the armature surface which could conduct the heat to the frame much easier. Sure, there was heat from the field coils, but not that much and that frame had a large radiating surface.

I don't know the thermal conductivity of these newer rare dirt magnets. Maybe this isn't a problem. But what you really need to look at is the armature temperature; maybe an IR thermometer pointed at the commutator after your drag the truck tests. Even if the magnets survive, a burned out armature isn't going to be good.

Regards,

major


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## Induced (Apr 15, 2009)

HI Major,

That makes since, I get what your saying now, I just wasn't seeing it until you explained it. I'll get one of those IR thermometers and do some more checking, probably not until I get the motorcycle a lot further along though, I do have 2 motors still one converted one not so I will do more checking before any final decision is made, I do plan on ducting air through the motor though and hopefully that will help, both ends of the motor are open, I have read that the comm end puts off quite a bit heat in any brushed motor.

Thank you fellow ev'er


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## major (Apr 4, 2008)

methanolcat said:


> ____________


I wonder why he erased his messages on this thread.


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## Dennis (Feb 25, 2008)

> I wonder why he erased his messages on this thread.


Because the troll got owned by the fact that his "super magnet motor" is not practical or competitive in the high wattage range against what has been proven reliable and cheaper, which are electromagnetic based motors. You must have struck a nerve when you said flat out that PM's trap in heat which was another negative to using PM's. Guess he could not take the fact that he was wrong. He was probably some marketing person for those magnets..


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