# Electric Motor Design/Performance Limitations BDC BLDC AC



## Tesseract (Sep 27, 2008)

Whitehawk said:


> I....
> Apples to Apples, keeping the same windings, wire diameter and PM strength, in order to handle this "nitrous-oxide" amp boost, could you just increase the performance by improving cooling the wires better, improving wire insulation, improving to higher temp PMs, improving PM cooling, or a combination of those?


No - the ultimate limit is the coercive strength of the permanent magnets. If you push too many amps through a PM motor (AC or DC) you will literally demagnetize them.


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## subcooledheatpump (Mar 5, 2012)

Yes, with any motor there is a point where no matter how strong the magnetic flux is, the magnets, or in the case of the induction motor, the rotor will not be magnetic anymore. 

do a google search: "induction heater" and you'll learn quite alot about what happens when you try that


In my personal EV experience with induction motors, (sorry I don't know much about PM motors) it seems you can put about 4 times the nominal current through the windings before the motor simply won't make torque anymore. It might be possible to get more with a vector mode drive though. (I wouldn't try that though, too much heat)


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## PStechPaul (May 1, 2012)

I don't have experience with large motors as used in EVs used for passenger cars and trucks, but I have done extensive reading and I have these books:
http://www.amazon.com/Build-Your-Own-Electric-Vehicle/dp/0830642315
http://www.amazon.com/Electric-Motors-Control-Techniques-Gottlieb/dp/0070240124/ref=pd_sim_b_5
and one or two others.

After learning about the pros and cons of various motor types, especially for vehicle application, I decided that the three phase induction motor was the best overall, for reasons of cost, ruggedness, low maintenance, and ease of control. They can be tweaked for maximum efficiency at low torque, and can operate at up to 3 or 4 times rated torque before stalling. They can be used for regeneration and dynamic braking, and can be overclocked to obtain perhaps 2-4 times the HP at higher speeds and voltages.

I know that BLDCs are extremely popular, but they tend to be more expensive, and relatively fragile, with catastrophic failure modes. The advantages are small size, high efficiency, and high peak power and torque.

For raw power and torque, the series wound motor can be pushed as high as 10x rating, but only for short durations and at the expense of fast wear and damage to brushes and commutators. They are also difficult to control and don't make good generators.

So, yes, there are ways to "boost" the output from any motor. The limitations are as you have noted, based on temperature, conductivity of windings, voltage limitations of insulation, demagnetization, and mechanical stress and failure at high RPMs and torques beyond the original design specifications.


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## ruckus (Apr 15, 2009)

Tesseract said:


> No - the ultimate limit is the coercive strength of the permanent magnets. If you push too many amps through a PM motor (AC or DC) you will literally demagnetize them.


Are we talking permanent demagnetization?  Those are $3000 magnets.. should I carry spares in the glove box?

I thought the limit on most BLDC motors was heat buildup. 

How do you know what the limit is? The Chinese spec sheets seem to have settled on 2x the continuous duty, but I assumed this was conservative. 

What percentage of continuous amperage would you dare run for a (10-15 sec) drag race? 2x, 3x, 4x?


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## PStechPaul (May 1, 2012)

I don't have much experience with PM and BLDC motors, so I tried searching for information on demagnetization problems. There is definitely a problem with high temperatures, but otherwise I could not find much about it. Here are some resources that may help. But from my understanding, it does not seem to be a very common problem, and some higher performance magnet materials may improve the performance, reliability, and longevity. But rare earths are expensive and, well, rare, so they may not always be readily available. Here are a few links to what I found:

http://www.yaskawa.com/site/dmdrive.nsf/link2/NKOE-7U9HJG/$file/WP.AFD.05.pdf
http://circuitelec.blogspot.com/2009/07/brushless-dc-motors-theory-and-driver.html
http://www.advancedenergy.org/md/knowledge_library/resources/permanent_magnet_motors.html
http://lib.tkk.fi/Diss/2011/isbn9789526040011/article1.pdf
http://e-futures.group.shef.ac.uk/publications/pdf/56_full_paper_ISG.pdf
http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=5530366&url=http%3A%2F%2Fieeexplore.ieee.org%2Fxpls%2Fabs_all.jsp%3Farnumber%3D5530366


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## Salty9 (Jul 13, 2009)

ruckus said:


> Are we talking permanent demagnetization?  Those are $3000 magnets.. should I carry spares in the glove box?


Not permanent. Post #30 and below in the link below discusses the remagnetization procedure. If you like fireworks you will love this process.
http://www.rcgroups.com/forums/showthread.php?t=307133


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## Tesseract (Sep 27, 2008)

ruckus said:


> Are we talking permanent demagnetization?  Those are $3000 magnets.. should I carry spares in the glove box?


Yes, despite what you might read in some forum post that starts with the disclaimer "Do not try this at home," the demagnetization is effectively permanent. It is technically possible to remagnetize the PMs, but the procedure is difficult and dangerous. The difficulty is ensuring that each magnet is remagnetized to the same level of coercive force while the danger is in trying to put the freshly-remagnetized rotor back into the stator housing where just being a millimeter off center will cause the stator to slam into housing and stick.

Note that I am talking about the more common surface PM rotor construction; interior PM motors - such as the Remy HVH250 - are truly beyond repair if demagnetized.

So, don't bother about carrying spares, especially since the PMs used in traction motors are so strong that if you put one on either side of your hand it would first crush it then rip the iron out of your red blood cells*. 



ruckus said:


> I thought the limit on most BLDC motors was heat buildup.


It is. but that's a different problem altogether. The OP asked, essentially, how far a BLDC motor can be pushed. The answer is: not nearly as far as a series DC motor. Different motor designs have different strengths and weaknesses. You take the bad with the good in every case, no exceptions.



ruckus said:


> How do you know what the limit is? The Chinese spec sheets seem to have settled on 2x the continuous duty, but I assumed this was conservative.


Nothing on a Chinese datasheet is conservative. If anything it's the exact opposite.

Note that in addition to catastrophic demagnetization from extreme overcurrent, it is also possible to partially demagnetize, or weaken, the rotor PMs through a combination of more modest overcurrent AND high operating temperature. Not as hot as the so-called Curie temp, which is when the magnets will spontaneously demagnetize. I don't have a lot of experience with this personally, but I've seen some anecdotal reports here and elsewhere about this. It shows up as a permanent loss of torque in the motor over time. Insidious, that.



ruckus said:


> What percentage of continuous amperage would you dare run for a (10-15 sec) drag race? 2x, 3x, 4x?


I have no idea, but BLDC isn't such a hot choice for drag racing on account of the above limitations. I mean, BLDC may be the most efficient type of motor, but it's no match against series DC in sheer peak power, both in the motor itself and in the "controller" required to drive it.



* - I made that last bit up, but whether it is true or not it sure sounds dramatic, doesn't it?


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## subcooledheatpump (Mar 5, 2012)

Not to get off topic but these sound like perfectly good reasons to not go with BLDC for use in EVs. 

Mind you, in most cases you wouldn't (shouldn't) be pushing any motor to it's limit during normal driving, but just in the off chance that you did, it would be a little unnerveing knowing you could potentially destroy a multi-thousand dollar motor. Plus, since the field is always excited, that could lead to fires in the case that the motor windings accidentally short circuited while the vehicle was still moving. 

Even with an induction motor, the field atleast would be de-energized in case of an emergency


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