Efficiency of PM DC Motor Used as Generator

As long as you don't draw more than what its amps rating is for extended times it should follow a similar efficiency to what it was as a motor. Excessive overloading will cause overheating of the windings with will increase the internal resistance which in turn will reduce the effective efficiency.

However do expect that the output voltage when spun at its rated RPM's and loaded to its rated amp draw will be less. The overall efficiency will still be similar though.

Thanks a lot!

I am only drawing about 10% of its rated power. Would you say that the efficiency at this point is (much) lower than the rated efficiency. since the efficiency curve peaks at the rated power!

Hope it makes sense!

It would not be surprising if the motor efficiency peaks at about 30%-40% load, and falls off slightly as it approaches 100% load. At 10% load, it is likely to be somewhat lower (around 70% eff.) At 5% load it ls likely to be a lot lower (50%, or even less).

since the efficiency curve peaks at the rated power!

This is probably not the case. The curve is more likely to be as described above, peaking at about 30-40%.

If you do not realise that a motor and a generator are each specifically designed for a particular job, then we may not be able to help you much......

Ask yourself, if they were the same, why would we have both????

A PM motor used as a generator is VERY inefficient as the brush position is wrong and the brush assy will need to be rotated for the optimal pickup position......also there will be a lot of sparking at anywhere near max load, which may burn the commutator/brushes.

Do not put a lot of load on such a motor when used as a generator for that reason alone.....and make sure that you observe when sparking occurs and avoid it like the plague.

The output voltage will be a product of the RPM, more RPM = Higher voltage.....

If you need an efficient generator, take a car alternator and replace the rotor coil and slip rings with a permanent magnet........no brushes!!! and it generates from 100 RPM or so.......

If you want proper efficiency, buy a generator......

A PM motor used as a generator is VERY inefficient as the brush position is wrong and the brush assy will need to be rotated for the optimal pickup position......

I use a PM motor as a generator in a small plug-in hybrid vehicle. While it is correct that the brush position should be different for use as a generator vs motor, that is 1. often easily changed, but 2. not an issue if you change the direction of rotation -- which accomplishes the same thing, more easily.

Sorry, I disagree on that last point completely.....

Actually changing the direction of movement of a permanent magnet motor only needs the polarity to be reversed, but it does not change the magnetic flux position correctly for when using it as a generator.......it does change of course, but not correctly.....

If the brushes are adjustable for position, which is not always true/possible, then they need to be moved to the point of least sparking.......at the load needed, as even changing the load will change that position again. Its best NOT to have brushes.....

In a bigger style of DC generator, there are extra poles to bend the flux always back to the correct position and keep sparking to a minimum......with a PM motor, that is very difficult to organize....

That is why a car alternator with a PM installed is rather effective in many cases= no brushes is a real plus point!!

Also the diodes are already in place to perform rectification of the 3 phase output (usually, but could be single phase I am told) produced......DC voltage will then be a product of the rpm......

For small loads it probably does not matter too much......simply ignore it!!

If the brushes are adjustable for position, which is not always true/possible, then they need to be moved to the point of least sparking.......at the load needed, as even changing the load will change that position again. Its best NOT to have brushes.....

But all this is true of motors as well. The standard brush position in a motor is a compromise, just as is the brush position in a generator. In both cases, you position the brushes for least sparking at the intended load, but in both cases the actual load will probably vary. For motors that are intended for routine operation in both directions, brush position is centered, and has to be a compromise for both directions.

Perm from Germany makes good PM motors, and advertise the same unit as a motor and generator. They say:

"Application of the motor as a generator results in the same determining features" (such as the 90% eff they just mentioned in the sentence before.)

If perm says that, I would expect that they have these extra "poles" to correct the path/position of the maximum flux.......

I looked intensively on their web sites and could find no reference to that either in German or English.

I did find another firm making similar products and they clearly showed the Interpoles, they also mentioned being a generator or a motor, AND THEY CONNECTED THE INTERPOLES DIFFERENTLY FOR EITHER ONE!!!! It was not wired the same for both generator and motor usage.........

This I expect it to be the same for Perm motors.....they follow the same laws of physics......

This supports exactly what I said, either its a motor or its a generator, but not both without any changes to its wiring...the interpoles are needed to correct and hold the flux path in the correct position for minimum sparking.....

The document can be found here as a pdf:-

http://www.ineltrade.ru/katalog/abb/02/14/04/02.pdf

Perhaps it is a language problem, but in most of the english speaking world, permanent magnet motors do not have interpoles. Here, in the US, interpoles are auxiliary field coils. The Perm motors do not have interpoles.

This supports exactly what I said, either its a motor or its a generator, but not both without any changes to its wiring

You'll have to take that up with Perm -- they are the ones saying that no change is required. I've communicated with them many times, and they have never suggested that any rewiring was required. The people who use the Perm and other PM motors in windmills don't rewire them.

BTW: Your link does not apply to PM motors. The ABB motors described in your link have field coils.

I agree that it was a different style of DC motor, but I simply could not substantiate any of your claims about Perm motor, perhaps you can do that for us?

But I do feel that the .pdf I linked to showed a "need" for the changes to stop/reduce sparking and to allow the same physical unit to be used in two different modes.....

Having a PM does not change the physics, it probably means that the company Perm accept a certain level of sparking as being part of the deal.......and do not forget, it will only spark less at around one physical load as a motor or at one load amps as a generator......any more load and sparking will get substantially worse.....the load is probably the max load amps as a motor and probably considerably less as a generator. Videos at the end of this post will show exactly why I say this.....

But the main point from the original poster's question, from which we have strayed rather far for a moment or two, was the efficiency of using a motor as a generator.

Efficient it is not to use the same physical object as one or the other.....as the .pdf I linked, showed quite clearly a need for a different configuration (impossible for a PM motor/generator to my mind, but I am willing to be shown different - maybe a PM AND Interpoles!!) to allow efficient usage as one or the other......

Of course any high school kid knows that some/most (but not all!) PM DC motors (and a lot of non PM DC motors) can be used as generators, but nobody I have heard, seen or read about has ever said that they were efficient when so used!!! Till this blog anyway....!!!!

I maintain that they are not efficient as a purpose built (and FAR more expensive) DC generator (I wonder why they are so expensive if not a standard car alternator?), and as previously mentioned, if not set up as well as possible with regard to sparking at the brushes, probably (depending upon loading) a relatively short life.....

I believe that I have clearly demonstrated these points in several good links and documents already previously posted.....

By the way, I personally would never use a brushed DC motor in this manner ever, the work involved does not reflect an efficient usage. Far better to take a purpose built generator and if required, remove the slip rings and wound rotor and replace it with a permanent magnet......a car alternator is a good example of this and many have already done this as can be seen on many blogs and webs.

If they are doing this, simply ask yourself why? simply the answer lies in the "lack" of brushes.

Commutator (as against slip ring) brushes and commutators simply do not like the outdoor life, nor do they like dust or any other form of dirt (even indoors!)......

Slip rings, due to the minor design differences, though not perfect in this area still wear far far slower. Naturally having neither is the best solution of all!!

Here are a few interesting links on this subject:-

http://www.youtube.com/watch?v=z6AdPgM8dXY

http://www.youtube.com/watch?v=JO_ByukKyPA

http://www.youtube.com/watch?v=Kx187qlNkkw&feature=fvw

There are many, many more.......

But one in particular really shows the problems of using motors far far better than the demonstrator was intending I feel.

1) He does not tell you to check if the motor (because of the brush holders) is unidirectional or not. Very important!

2) The motor he uses is rated at 130 volts at 4500 RPM. He neglects to mention either wattage or amps!!!!See what he gets as voltage output at two different but lower RPMs!! Simply laughable. Getting this to spin at anywhere near the needed RPM will be VERY difficult in real life.....

3) He is measuring with NO ELECTRICAL LOAD ON THE GENERATOR!!! The voltage will probably drop to less than half of the no load level, if you are lucky that is!!!

4) Although just measuring volts is equivalent to a "no-load" condition", he should have mentioned also the need to observe the possible level of sparking when being used under load.......he forgot!!

5) The fact that he was quite happy with the demonstrated results is also an indication of a complete lack of knowledge in this area........efficiency it is certainly not!!!

This post (particularly this next video) I feel demonstrates quite clearly that most DC motor characteristics are simply NOT an indication of the possible useful Voltage/current/power outputs when a motor is used as a Generator.....

Link:-

http://www.youtube.com/watch?v=si-pHmMQv_U

I found a further video that demonstrates many of the problems in this area. (By the way, Denise, the Lady in the video, has seemingly ended up with a heavy pair of very muscled legs while making these videos it would seem.......!!). But it does show just how difficult it is to generate power in a usable manner from a DC motor......

http://www.youtube.com/watch?v=a-7iSmC4s5A

I hope this clears up any possible misunderstandings about the efficiency of using DC motors as generators.....

I wish you all a really great day. Enjoy the videos.......

I agree that it was a different style of DC motor, but I simply could not substantiate any of your claims about Perm motor, perhaps you can do that for us?

Sorry, no, I can't do a full investigation to see if the Perm people are lying to the public. (I've already provided a link to their literature, which states that their brushed PM motors work equally well as a generator, without any "rewiring" -- which is effectively impossible on their motors). I have no reason to think that they are lying and know that they are very highly-regarded in the electric motorcycle world. All the people who have used their motors as generators seem well-satisfied, and I can say from personal experience with the Etek motors (which are very similar, but not as well-built) that the efficiency as a motor and generator are essentially identical (at least as closely as I can measure -- within a percent or so). The Eteks have long been used in wind power installations. Yes, of course, they have brush drag, but they have that same drag when used as motors. Brushed motors are not dead, by any stretch. They have the advantage over brushless motors that no electronics are required for commutation.

If you believe that the Perm people are lying, then give them a call and accuse them. Don't ask me to further spoon feed info -- I already provided the link to their data. They are nice folks and will probably patiently explain to you how motors work.

Do you think Rhabe is lying too?

I have spoken at length with people from Perm, and have no reason to believe they are liars. All these pancake motors that are used in electric motorcycles have very high efficiencies and light weight, and those with which I have experimented live up to the manufacturer's claims (and, in fact, many come with the actual dyno curves generated by that individual motor during QC.) In my experience, the assembly line dyno curves are essentially perfect -- they are not fictitious. So I can't help with your conspiracy theories -- and given that Perm is in your country not mine, would it not make sense for you to call them to become acquainted with their motors in general and the basic physics* that make them operate as they do?

If you want to make a claim like "F does not equal MA", don't ask me to supply the reasons for my belief that F=MA. It is you, not I, making the extraordinary claim.

If there were other conspiracy theorists here, then yes, I'd supply more data on Perm motors and the others. But I have no interest in spoon-feeding you data that is commonly understood about PM motors. Sometimes you have to simply accept that the patient is brain dead. I don't have endless time available.

* Here is a is a good site for help with understanding physics.

I did not accuse anyone of lying, I just said that "I could not substantiate your comments". You may not simply understand the difference......the Perm websites I found did not have the infos as you mentioned....

I really tried to find the details as well from Perm..........and as you appear not to be able/want to substantiate them for one reason or another yourself, maybe we should drop the name Perm off the list!!!!

Unsubstantiated claims are easy to make, but much harder to set in stone.......as they (Perm) were your comment, then I see you to be the one to substantiate them first and foremost.....of us both!!

As someone who has just said "I have spoken at length with people from Perm", surely you could do more to bolster your comments further I feel....

Certainly if I was in your position, I am sure that I would easily be able to find such infos somewhere......call one of your friends at Perm please.....I am personally really interested, even if you are not!!!maybe I am not alone either!!

Furthermore I certainly did not make any claims with regard to "F=MA", that you appear to have plucked out of the air from somewhere.....or at least not from my posts....

The physics site you mentioned is really nice for school children, but I don't feel that anyone here needs such a level of education at this time, not even you!!!

Sorry, your comments on this subject therefore appear to be "bl****r" and the lack of interest in substantiating them yourself, just a way to try and leave the blog gracefully.......

I call it how I see it.......you still may be fully correct, but who knows that? No evidence to the contrary I am afraid....

Seriously, please try and find some stronger evidence for us from Perm or even a similar company might prove acceptable too.....I am truly interested in learning something more.....

See you shortly.....stay cool!!

Furthermore I certainly did not make any claims with regard to "F=MA", that you appear to have plucked out of the air from somewhere.....

Sorry, I jumped to the conclusion that you were quick enough to understand the analogy.

Andy, the reason I mention F=MA is by way of analogy. You are claiming crazy stuff: that PM DC motors have interpoles, that these interpoles need to be rewired for use as a generator, that the DC motors from ABB are the same as those from Perm, (despite their not being PM motors) that Perm is likely to be lying in their literature (can't be "substantiated," in your words) etc. If you make outlandish claims and assertions then you need to do your own research. (In other words, in line with the analogy: if you think M = FA, then I can't help you.) It is not up to me to spend time to bring you up to a fifth grader's understanding of how motors work. (Having taught 5th grade robotics students, I can assure you that many 5th graders know that the PM motors in small robots work equally well as generators, without any changes whatsoever.)

The very first sentence of the first response (TCMtech's) in this thread is correct. As long as you don't draw more than what its amps rating is for extended times it should follow a similar efficiency to what it was as a motor. Since then you only served to drag the thread into completely irrelevant gibberish. And I keep trying to drag it back to relevance, but you persist.

In Rhabes second post, he says:

PM-motors have such a strong flux-density and so low rotor magnetic field (if iron-less rotor!) that the magnetic field from the rotor will not much change the stator field So no change necessary.

I know that Rhabe knows his stuff re motors -- because what he says concurs with the basic physics involved, and because he has a history here of correct answers. What you say does not concur with the physics, and further does not even reflect the big pieces (the physical construction) inside a motor -- you are imagining there are things that are not there.

The burden of proof falls on the person making the outlandish claims. I'll go along with what TCM and Rhabe have written, and with what Perm, Etek, Mars and others have published, and with the basic physics of PM DC motors.

I've got to quit this discussion: TCM provided the correct answer right off the bat, and no more elucidation is required for the OP.

Here's a site where you can buy PM DC motors. You will see the Perm listed (and see it listed for use as a generator) and will also see several similar brushed PM DC motors. Manufacturers site links are provided. A small amount of effort on your part, to read up on how these motors work, will help you gain a basic understanding of motors in general and PM motors in particular.

I am not at this point annoyed. Children, even clever ones, I always find interesting,

Although you pretend to be unaware of the subtleties of language, this is intended to be insulting. You are strongly implying that I am childish because I will not accept your bad advice to the OP, namely that PM DC motors must be rewired to function as generators -- a notion for which you have not provided a shred of evidence.

The fact that you are (were) annoyed, would seem to be supported by the number of exclamation points in your post.

Probably that annoyance is a good thing, because perhaps it will help restrain you from being insulting and belligerent to others in the future.

Do you have a browser that does not not support links like this one?

I provided such a link to the Perm literature, and provided such a link to the video that described the operation and construction of the Lynch motors. I also provided a link to a vendor of several similar motors (and the Perm) all of which have been used in windmills. The "substantiation" you seem to require is in the Perm literature, and that has been provided. They say that their units work equally well as generators. This is not an unusual claim or outlandish claim, because all PM DC motors work well as generators, (Rhabe correctly states that they can be over 90% efficient as such and the ones I have here are 89% efficient as such). If the motor happens to be shipped with centralized brushes, there is no difference in efficiency motoring forward or reverse, generating forward or reverse, or generating vs motoring. If, as I wrote long ago, the motor is shipped with the brushes optimized for one direction over the other, then it is sometimes more convenient to reverse the generating direction than to move the brushes: but either accomplishes the same thing: optimizing the machine for use in one direction vs the other. (This is very clearly explained in the video to which I provided the link.)

If you are saying that this is incorrect, namely that the well-accepted physics of DC PM motors is wrong, and that Perm is lying -- that their motors do not work well as generators, then (as I have written 'til I am blue in the face) the burden of proof is on you: take that up with them, not me. I can only say that my own motors (which are of slightly lower quality than the Perm motors) obey the laws of physics in the way that TCMtech, Rhabe, I, Perm, and many others have described.

To make a believer out of any of us, you would need to provide something other than insults. You need to provide a link showing that PM DC motors are equipped with interploes, or a manual for a DC PM motor stating that the motor must be rewired to act as a generator. The OP is well-aware that PM DC motors work just fine as generators, and your advice takes him in the wrong direction. I hope he is not taking his motor apart in an attempt to find interpoles, or ripping the wires off his armature to rewire it in some magic way. (There is no "wiring" in a DC PM motor, other than the armature windings and the brush pigtails. Changing the brush pigtails does nothing that can't be done by swithing the outside leads, and there is no rewinding of an armature that makes a DC PM motor more efficient as a motor than as a generator. Clearly, suggesting rewinding of a small cheap motor from China would be bad advice -- better to buy a different motor.)

You are asking me to prove (substantiate) that Perm and all the others are not lying. Was TCMtech lying too? Was Rhabe lying too? What conspiracy do you imagine? I cannot find any support for your assertion that PM DC motors have to be rewired to function efficiently as a generator. Why would I lie about my own experience with these motors? If you believe that such rewiring is required then provide the data instead of insults. Providing a link to a wired-field DC motor is not such data -- those are much different machines.

You are coming across as belligerent here, Andy. Put up of shut up. You need to provide links to PM DC manuals that say that interpoles must be changed to make a PM DC motor efficient as a generator. I am not going to call Perm again to have them reverify the same question I asked them 2 years ago: I don't want to come across as a nitwit. And if I did call them again you would not believe what I write. You would simply hurl additional insults. Your illogic defies comprehension, Andy. They are in your country. If you want to accuse them, then pick up the phone. Get the info from them, not from me, your childish hippo.

So please calm down and discuss here in an honorable CR4 fashion, otherwise I will just leave you to your own little muddy pool!!!!! Hippos generally like that!!!

That would suit me fine, I don't need your insults. There is nothing to discuss here, Andy: the physics are simple, and there are no interpoles in DC PM motors. No amount of "discussion" as you call it, will change that. I am not "discussing' this with TCM tech or Rhabe, because they know how motors work. If you do not, that's fine, you can learn. No need for endless exclamation points and what you call discussion in which you provide no evidence at all, only innuendo.

As I sad, TCM provided the correct answer right off the bat, in one sentence. I see no hope of convincing you that PM DC motors do not have interpole windings, nor am I inclined to think that you will believe any assertions that I make. That's fine: I will try to direct my comments to people who have a basic knowledge of motors in the future, and will put you in my list of belligerent, intentionally insulting people to whom I do not respond. Simple. Consider it done.

Again, attack, attack, attack and no supply of proper relevant data again.......I do not expect or get any better from you ever........

None of the links you have provided up to now have had anything to do with the detail of the discussion in hand.

Again, if you read carefully, which you obviously don't (I have mentioned this already, you may not have seen it fully with your speed reading), I never said on any of my posts that I had ever seen a PM motor with interpoles (learn the spelling by the way of interpoles!!), I just did not completely say "never" because you never know. I personally have never seen one........

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You have imagined something again that you WANTED to see.......its made up again.........by your mind.......look through all my posts for that, you won't find it - anywhere except in YOUR OWN Posts....

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I am of the opinion now that you have several seeming strong character faults and a normal person like myself cannot (or wants to) penetrate them.....I have learnt that now....you have achieved that much....

And by the way, I was not the first one to be sarcastic or rude, I just followed your example a little (probably better too I feel!!).

PS. Let me remind you that you still have not (cannot?) provide the links requested several times......my link, if you had read it to the end showed convention motors being rewired (re-linked?) to be both a generator and a motor........but perhaps you could not speed read through such a document and get to the diagrams showing this near the end.....

I hereby refute all the wrong "quotes" of what you might think I said (wrote) as you clearly have problems with staying on page one with the rest of us.

I have a life and you do not form even a single atom of it!!! Maybe its envy on your part, I really cannot say for certain.......simply too far away (thank God). I'm off for better fish to fry.....I will unlink from this blog now as its getting boring to read the same boorish texts each time and nothing new and interesting......

Bye.

Andy, the very first sentence in your very first post (to the OP) in this thread is an insult. Perhaps you are so habituated to being insulting that you can no longer see that you are being insulting.

The OP, from the beginning, showed that he has a good understanding of PM DC motors. If you are uncomfortable with his knowing more than you, then even the score: learn. No amount of insults will help you.

Insulting him or me just makes you appear to be a bully. Look at all the exclamation points you use... the mock incredulous ????... the ALL CAPS...

What were you thinking?

You wrote:

You have imagined something again that you WANTED to see.......its made up again.........by your mind.......look through all my posts for that, you won't find it - anywhere except in YOUR OWN Posts....

Most of us read this on computer screens where the quotes can be seen to be correct or not. The whole thread is right here. I cut and paste quotes. Simple. Misquoting is effectively impossible.

You wrote:

- If you do not realise that a motor and a generator are each specifically designed for a particular job, then we may not be able to help you much......

- Actually changing the direction of movement of a permanent magnet motor only needs the polarity to be reversed, but it does not change the magnetic flux position correctly for when using it as a generator.......it does change of course, but not correctly.....

- either its a motor or its a generator, but not both without any changes to its wiring...the interpoles are needed to correct and hold the flux path in the correct position for minimum sparking.....

- AND THEY CONNECTED THE INTERPOLES DIFFERENTLY FOR EITHER ONE!!!! It was not wired the same for both generator and motor usage.........

This I expect it to be the same for Perm motors.....they follow the same laws of physics......

- A PM motor used as a generator is VERY inefficient as the brush position is wrong and the brush assy will need to be rotated for the optimal pickup position......also there will be a lot of sparking at anywhere near max load, which may burn the commutator/brushes.

- and as you appear not to be able/want to substantiate them for one reason or another yourself, maybe we should drop the name Perm off the list!!!!

- but I simply could not substantiate any of your claims about Perm motor, perhaps you can do that for us?

If you neglect friction from bearings, commutator and drag

then the torque divided by current (Nm/A) is named the motor-constant K. (T/I=K)

This is identical to the generated voltage divided by the turning speed measured in rad/s.

(Uemf/Φ^.=K)

So from this you see that the voltage is directly proportional to speed.

To use this voltage you need some circuit, either a battery to be charged or a resistor to heat or both.

The internal resistance (coil) of your generator is in series with these external resistors.

So you divide your no-load-voltage between the internal and the external resistor if extracting energy.

The maximum energy you can get from a generator is delivered if the external resistor equals the internal resistor - but most generators will burn at much lower current values! So look for the current limitation, in most applications this is in reality derived from a thermal limitation as heating up the coil with limited cooling will raise the temperature until the insulation will fail.

So look at your maximum allowed current and the internal resistance if you can operate near this energy-optimum. Usually you cannot, then set your external system so that you operate near the current limit or below.

As power in an electrical circuit equals R*I² you can write output power/(internal plus output power) = efficiency = R_ext/(R_ext+R_int). So efficiency is not varying with speed!

Caution: if rotation is fast then above assumptions of no drag are no longer valid.

One further neglected term in above relation is the cold to hot resistance-change of copper with 0.4%/K will result in an efficiency drop if the coil is allowed to be hot: cool is cool.

With an oversized generator (motor) you may get efficiencies of above 90%.

The bigger problem is the control system that is varying the load so that it is matched to the available energy: operating the system near the current limit at any useful turning speed requires for a variable resistor if heating or converter to get a wanted output voltage.

Have a look at the motors efficiency/speed diagram!

RHABE

Excellent post.

My only quibble is with this:

So efficiency is not varying with speed!

Caution: if rotation is fast then above assumptions of no drag are no longer valid.

The no drag assumption can also be incorrect for low speeds. Actual motor dyno test curves fall off dramatically at low loads (<10%) even at low speeds. Brush drag can be significant.

But as always, a very good post.

Perm motors is a newcomer so more choice is available from Heinzmann or Mattke or Baumueller.

Auxiliary field coils or inter-poles: used in non-PM motors, as the stator windings are generating a magnetic flux density near the saturation density (in the poles).

With rotor-flux-density added this will saturate locally the poles and thus derivate the location - so requiring change of commutator location with load. (Negative load is generator region).

PM-motors have such a strong flux-density and so low rotor magnetic field (if iron-less rotor!) that the magnetic field from the rotor will not much change the stator field So no change necessary.

Arcing: with higher operating voltage theses motors/generators as any electrical machine need more turns in the windings, this more inductance as inductance is proportional to n². More inductance is more difficult to switch to zero current: where is the energy dissipated? So arcing is strongly influenced by design voltage.

One more influence: eddy-currents at fast changing currents. As modern magnets (FeNdB and SmCo sometimes also Ferrites) are electrically conducting they inhibit too fast current changes by high losses.

There was once a small motor of iron-less rotor design with capacitors (many) across the windings to suppress arcing. Does anybody know if something similar is still in use?

RHABE