hub motor design & torque

coops

Esteemed Pedelecer
Jan 18, 2007
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It seems to me hub motors generate torque in two ways: one "electrical" (more copper windings give greater effective current flow past magnets & slow down the motor's ungeared rotation speed while increasing torque) and one "mechanical": using internal cogs & rings to gear down the motor speed from thousands to hundreds of rpm.

Question is, since motors vary in the proportion of torque derived from each means, what difference (if any) does it make to motor performance how its torque is generated?

For example, two motors, same voltage & current limit, magnets layout etc. one has a higher winding number (and so higher "electrical torque") runs at around 2000 rpm and is mechanically geared down by around 7:1 to around 280rpm; another has less windings, runs at about 3300 rpm but has a higher mechanical geared reduction ratio of ~12:1 to give the same 280rpm approx. What performance difference could be expected, in principle (if any), between these two motors, in the same size wheel eg 20"?

One thing occurs to me, that "mechanical" gearing is more voltage-related & so may require less current and hence less heating - could that make it more efficient: lower losses as heat? But then, would torque originating from current be somehow inherently more "torquey" & better for hills, say?

Please feel free to correct if my terminology is incorrect or inaccurate: I'm only just starting to find out about these things and I realise the picture is incomplete and probably vastly over-simplified! :) having said that, I was rather satisfied the other day when I grasped the relationship of "torque" to the rpm and power output of the motor, and the turning force of the wheel on the road :) (though torque graphs don't get mentioned here, so I'll not go into it, but will post what I found if anyone's interested :D)

Stuart.
 

Flying Kiwi

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For example, two motors, same voltage & current limit, magnets layout etc. one has a higher winding number (and so higher "electrical torque") runs at around 2000 rpm and is mechanically geared down by around 7:1 to around 280rpm; another has less windings, runs at about 3300 rpm but has a higher mechanical geared reduction ratio of ~12:1 to give the same 280rpm approx. What performance difference could be expected, in principle (if any), between these two motors, in the same size wheel eg 20"?
For a given power output a higher voltage motor will tend to have a higher number of windings but what matters is the strength of magnetic field and although the lower voltage motor will have fewer windings, those windings will tend to be from thicker wire so the field strength is still there, it's just like with transformers. There are other differences depending whether the motor is unbrushed or brushed and if it has brushes, how it's wired ie, shunt, series or compound, these will all affect its performance and electrical loading. Fortunately (for reasons of low maintenance as well as better efficiency) many electric bikes these days rely on induction motors without any brushes.

Please feel free to correct if my terminology is incorrect or inaccurate: I'm only just starting to find out about these things and I realise the picture is incomplete and probably vastly over-simplified! :) having said that, I was rather satisfied the other day when I grasped the relationship of "torque" to the rpm and power output of the motor, and the turning force of the wheel on the road :) (though torque graphs don't get mentioned here, so I'll not go into it, but will post what I found if anyone's interested :D)

Stuart.
Torque is an interesting one because although we're limited here to a 200 or 250 watt motor power (depending whether you go by UK or EU laws), there's no limit on torque itself so it is possible to have a motor with excellent starting characteristics that doesn't go over that power limit. The problem is that complicated electrical switching controls are required especially for induction motors to achieve this. The number of poles in an induction motor determines its rotational speed for a given power supply frequency. There's also worries about electrical efficiency and as efficiency is of prime importance in determining range, it's important manufacturers get this right rather than just throwing in a bigger battery to compensate for less ideal electrical motor designs.

Although different mechanical gearing designs come with different efficiency figures because some methods are better at multiplying torque while putting out less waste heat than others, mechanical transmissions can be designed to be very efficient. It's much easier to provide a highly efficient complete drive train by incorporating a well designed electric motor with a fairly narrow power band coupled to a multigear transmission rather than using a fixed gear arrangement and relying on switching via a controller to achieve speed flexibility. I found this Electric motor - Wikipedia, the free encyclopedia about electric motors generally but as alot of it's not relevant to whats in our electric bikes, perhaps other forum participants may have more relevant links. It really is a very involved topic!
 

Ian

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Hello Stuart,

as you've guessed the whole thing is very complicated. Basically the torque of a motor is proportional to The strength of the magnetic field x The current in the windings x The number of turns of the windings x The number of poles, and the maximum rpm at a given voltage is inversely proportional to the above.

But to complicate matters achieving a uniform magnetic field is not easy, it requires tight mechanical tolerances to minimise the air gap between rotor and stator as well as good design and materials. The windings have to have as low resistance as possible to allow high currents and prevent over heating, the controller has to switch the windings quickly and accurately in response the signals from the hall effect sensors.

In short, there is much more to a good motor than rpm and gear ratio, correct design and construction being critical.
 

flecc

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Oct 25, 2006
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As Flying Kiwi says, mechanical gearing is generally very efficient in a given circumstance, and this is illustrated in the comparison between the F series hub motor which is internally geared, and the equivalent power US BionX hub motor. The latter is a direct drive where all torque is generated electrically, and it's starting torque is very much lower than the geared F series motor.

The same relationship is evident in the direct drive Sparta Ion motor.

The lack of precise legal specification of torque and where power is generated is well illustrated by the Lynch motor that Cycles Maximus use on their 5 cwt goods trike and Taxi trike. That's a nominal 200 watt motor, but at very low revs can hit almost 5 kilowatts! :eek:

Needless to say almost, it can pull a total of 500 kilos up a 1 in 10.
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coops

Esteemed Pedelecer
Jan 18, 2007
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Thanks for all the replies

Flying Kiwi said:
It really is a very involved topic!
Well, that's for sure by the sounds of it! I was afraid it would be, but nothing ventured, nothing gained I guess! :D

To narrow down any discussion, I'm thinking brushless motors with high quality & strength magnets.

Have I got it right, Ian, that you were referring to what I called the purely "electrical" aspect (I suppose I should say "electromagnetic"?) of torque, the "direct drive" due to electromagnetic effects in the motor?

I guess between the two motor examples I gave there would be much more difference in performance due to design & construction quality than any theoretical advantage of one torque type over the other, then :D. For me, although I know its sadly too rare, such good design & build must be a given! :rolleyes:

What you say, flecc, shows the spectrum of hub motor torque: from "direct drive" only with no mechanical gearing at all like the bionx, to internally geared motors like the ezees: heinzmann would also fall into that geared category I take it? Actually, having re-read the ebikes.ca hub motor information page for the umpteenth time, its starting to sink in, and indeed they also say that geared motors can produce superior torque to direct drive ones, and often weigh much less too!

So, is there a simple but fairly accurate analogy that can be made to illustrate how electromagnetic & mechanical torque differ qualitatively & in what circumstances (e.g. rpm level) they are delivered, in an "idealized" brushless etc. hub motor, to help to determine a design for ebike hub motor which makes best use of each? And furthermore, a simple explanation of the main limitations of turning such a concept into reality e.g. excessive weight, friction losses, overheating dangers etc.? EDIT: though I appreciate you've already given much information about design & construction issues, I'm just trying to narrow the remit to simplify it a little :D.

I rather naively conceive of a geared hub motor in cycling terms as being the ultimate "spinner" (one who pedals at a high frequency/cadence in a lower gear) - I know its the opposite of the usual pedalpower-gearing which is almost always geared up rather than down, but thats why i say "ultimate"! I suppose the direct drive torque could be said to be analogous to a "pushing" cyclist (one who pedals with low frequency/cadence in a higher gear, but with greater torque and effort needed): I'm aware that analogy is the weakest form of argument and as such is ever likely to break down, but since spinning is more efficient than pushing, does that say anything useful about efficiency of geared vs EM torque, or if both should work together how the two aspects can be best balanced and incorporated into a well-built hub motor?

Stuart.
 
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flecc

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since spinning is more efficient than pushing, does that say anything useful about efficiency of geared vs EM torque, or if both should work together how the two aspects can be best balanced and incorporated into a well-built hub motor?

Stuart.
Yes, spinning is more efficient in the sense that a geared down hub motor is able to spin and deliver earlier power than a direct drive one which is being asked to work the second it starts to revolve. Here's a fuller explanation of the subject of gearing and torque, which is what I think you may be looking for in part.

It's not a case of ever faster being better, since there is a point at which the electrical efficiency reaches it's maximum, and that's where the wastage of current reaches it's minimum and therefore the maximum amount possible is converted into drive power. That point is the peak of the torque curve, where the maximum pulling power is available, and on an e-bike geared to reach 15 mph at the peak motor revs, that point corresponds to about 8 mph on a Hall effect motor, and often on brush motors too.

Obviously that's the best speed at which to aim to climb a hill, but on a hill that's too steep for that to be achieved, and speed then drops, the motor climb power falls away rapidly. Fortunately, as the speed drops, the power needed to climb also drops, but unfortunately, the power consumed remains constant at all points below the point of maximum torque, so the slower the climb, the higher the proportion of wasted power.

Above the point of maximum torque, the power consumed in relation to speed declines, and that continues until the point of maximum efficiency, which is just short of the top speed, about 14 mph in this example.

When a hub motor is over geared as the Quando motor is by 40% in the Torq bike, those speed points move up, maximum speed 22 mph, maximum efficiency at 20 mph and maximum torque at 12 mph. That's why the Torq has trouble climbing, since the maximum torque that was available in the Quando at 8 mph is only now available at 12 mph, where 50% more power is needed to climb a given hill.

In a reverse case, the eZee Chopper which has the Sprint motor geared down by 23% through being in a smaller wheel, has the maximum torque at 6.2 mph. Since climbing at that speed needs less power, it has a steeper hill capability than the Sprint.
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coops

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Jan 18, 2007
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Thanks flecc, yes thats a part of what I'm trying to find out.

To try to put it briefly and clearly, what I'm trying to grasp is whether there is a qualitative and/or quantitative difference between geared and EM torque, and how different proportions of each contributing to the same maximum torque, would influence performance of a well-made motor, with respect to hill-climbing (especially speed at which maximum torque occurs) and acceleration from a standstill, in particular.

With further thought, I'm surmising that a higher EM torque component gives better hill climbing and starting-off "grunt" whereas geared torque would be more effective for maintaining speed under minor load changes such as minor slopes: the speed dropoff under load may be initially slower for geared, but maximum torque would then occur at a higher relative speed/rpm than for higher EM torque bikes, which would fair better on steeper hills since it kicks in at lower rpm.

I'm probably chasing more red herrings :D since if there was a big advantage from one or the other it would probably be well publicised, but theres always a chance its underacknowledged, that it does play a part however small in hub motor performance, and that if this information was known to buyers, they may be able to make hub motor choices more appropriate to their needs :).

Stuart.
 

flecc

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I think I've given the answer Stuart, and as you say with "red herrings", it's probably that you're looking for something that isn't relevant or existing.

The answer for hill climbing lay in the point of maximum torque, and as I said, a climb is ideally at that speed. Therefore gearing comes in there, to put the point of maximum electrical torque at the speed which uses every watt of available climb power, therefore equalling the steepest hill possible at the lowest current wastage.

The answer on the flat is as said, best travelling speed is at the point of maximum efficiency, when consumption is lowest and the range therefore longest.

Since the hill climbing is the important one that must be fixed first, that determines the gearing, which in turn determines the speed at which maximum efficiency occurs and sets the flat riding parameter.

That's it with the basic design, nothing more to be said or relevant. If the above is departed from, there's a cost, as in the loss of hill climb ability in the case of the Torq departing from the correct Quando configuration, or in the case of the Chopper where the speed is lost.

There are the matters of technical advances, such as magnet strengths asnd low resistance wire developments, but that's a completely different subject.
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coops

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Jan 18, 2007
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Thanks again for your answers flecc

I think I've grasped the principles of designing the point of maximum torque for hills at a speed/rpm such that the power output is matched to that needed for the maximum gradient attainable, and the other of maximum motor efficiency for the flats.

If its not too technical, since it is a critical point of design I'd like to know what determines the rpm at the point of maximum torque? You said that this point occurs "where the wastage of current reaches it's minimum and therefore the maximum amount possible is converted into drive power"; does the rpm when that occurs depend on the number of turns of the windings, for instance?

EDIT: The other factors Ian stated being equal i.e. The strength of the magnetic field x The current in the windings x The number of poles [x The number of turns of the windings].

Stuart.
 
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coops

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Jan 18, 2007
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Or, to put it another way... what might cause the maximum torque point rpm of two motors with very similar maximum torque to differ?

Out of interest, and I know this information is hard to come by, is anyone aware of the maximum torque and rpm it occurs at of the Sprint, for instance? (The Sprint motor is definitely internally geared for ~15mph in a 26" wheel i.e. around 200-215rpm max under normal load, isn't it?)

Stuart.
 

flecc

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If its not too technical, since it is a critical point of design I'd like to know what determines the rpm at the point of maximum torque? You said that this point occurs "where the wastage of current reaches it's minimum and therefore the maximum amount possible is converted into drive power"; does the rpm when that occurs depend on the number of turns of the windings, for instance?

Stuart.
Yes, it is too technical Stuart! This is properly the field of the motor designer and includes so many complex factors that it's beyond our province.

That said, it's not worth us bothering with it, simply because the variations are so small. That's why I can confidently say near enough where the maximums are, since they are predictable for the Hall effect motors our bikes use.

Indeed all electric motor types are predictable and confined in a way that internal combustion engines aren't. The type of AC motor that's commonly used in fan heaters and the like runs at about 1450 rpm for example, so if you're designing a fan heater, the motor's internal complexities you don't need to bother with.

Our Hall effect motors also run at a similar speed, and trying to juggle with that is pointless, there's nothing to be gained. Any gains can only be through technical advances like the ones I mentioned, lower resistance wires, stronger magnets etc.

They can have strange sources. The considerable advances in magnets in the 1960's and '70s were due to research for loudspeaker improvements, later becoming valuable for other technologies like ours. Incidentally, the voice coil and magnet of a loudspeaker is called it's motor.

You'll have something else to get involved in shortly, the Torq Radical project is almost ready, ending the guessing game! Plenty of surprises. :)
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Flying Kiwi

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If its not too technical, since it is a critical point of design I'd like to know what determines the rpm at the point of maximum torque? You said that this point occurs "where the wastage of current reaches it's minimum and therefore the maximum amount possible is converted into drive power"; does the rpm when that occurs depend on the number of turns of the windings, for instance?

EDIT: The other factors Ian stated being equal i.e. The strength of the magnetic field x The current in the windings x The number of poles [x The number of turns of the windings].

Stuart.
Those are some of the factors (although its not a case of multiplying them altogether, it's just they're relevant). Other factors which jump to mind are the gearing ratio of the hub (assuming it is geared) along with the frequency of the electrical supply from the controller. You'll see from all of this discussion that hub motors are very much a compromise solution which only offer peak efficiency under one fixed set of circumstances. As your thread title mentioned *hub* motor design, I assume you're not interested in considering the much more flexible motor drive through the gears system as used on bikes like the classic Twist and Swiss Flyer - even though they offer peak efficiency at different speeds depending on the gear selected.....
 

flecc

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Just seen your supplementary post while I was typing the answer above Stuart.

I think I've answered that as you'll see, the question not being relevant since these rpm are largely predictable constants in each motor design.

There's a simple rule for Hall effect hub motors in our bikes. The point of maximum torque is just over half the maximum speed the motor is geared to achieve.

For a 15 mph maximum bike it's about 8 mph, but in the case of the Sprint and Quando which run to more like 17 mph, it's about 8.7 mph. These are roughly right and produce very accurate hill climb calculations.

Brush motors on our bikes are very little different, but sometimes a touch lower. For example the Powabyke 15 mph brush motor has it's point of maximum torque at just over 7 mph, though brush motors have higher but narrower peaks, Matterhorn shaped, rather than the more useful humpback hill shape of a Hall effect motor torque curve.
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coops

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Jan 18, 2007
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flecc said:
Yes, it is too technical Stuart! This is properly the field of the motor designer and includes so many complex factors that it's beyond our province.
Glad I added the proviso then in that case :)

Just read your last post, thanks flecc, and you'll see below (in the post I was writing meantime), I hope, the reason for my query, since the information seems to conflict with what we both are saying about rpm at maximum torque, and I want to know why!:

Well, following from my last post, I have to say I think i've made a rather circuitous approach to the question on my mind, to try to understand what the underlying causes might be rather than ask directly, so here's the question plain and simple, I hope, and possibly the context of my other enquiries here may become clear:

Most hub motors seem to give their peak torque rpm at around (just over) half their peak speed rpm e.g. quando peak torque ~8.6mph, sprint ~ 8mph chopper ~ 6mph, delimited Torq ~12mph etc. (I derived the sprint rpm @ max. torque from the chopper rpm you gave flecc).

What then would cause a hub motor to appear to give maximum torque at around 13.8mph when it is normally geared for 15mph in a 26" wheel (200 or so rpm) and how might this affect its performance on hills for example, where peak torque at lower speed might be more desirable to assist climbing in a lower power range?

@ Flying Kiwi: Thanks for your input; although this thread is specifically asking a question about the torque mechanics of hub motors, I'm not at all disinterested in transmission drive bikes, and I may seriously look at them more closely as they seem to be in Europe, if the Italian ebike scene is anything to go by, if they can supply the power to climb moderate to fairly steep gradients at hub motor speeds, or if I find I'm regularly needing to tackle gradients beyond the limit of hub motor, or if I find the extra efficiency is necessary to extend trip range where extra battery power instead may become too heavy & cumbersome: I'd have to justify to myself the extra expense of buying a non-hub bike by such necessities, when a hub bike may do the job for me (I don't have too many hills, but it would be nice to be able to reach and explore them too) - however if, as it seems, one must spend large amounts to get a hub bike for the job, I will surely look along that particular road to see what it offers for a decreasingly small amount more dosh :D.

flecc said:
You'll have something else to get involved in shortly, the Torq Radical project is almost ready, ending the guessing game! Plenty of surprises. :)
Can't wait to hear more flecc :D

Stuart.
 

flecc

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Oct 25, 2006
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What then would cause a hub motor to appear to give maximum torque at around 13.8mph when it is normally geared for 15mph in a 26" wheel (200 or so rpm)

Stuart.
Short answer, it can't, unless you want a useless motor. As I explained, if the motor is geared for 15 mph, the maximum torque will be at about 8 mph. That's it, final, a fact of these electric motors. That's why there is a torque curve, and not a torque ramp on a graph. From zero revs there's a constant demand for current, which is increasing used usefully as the motor revs rise until the maximum possible is used at which the wastage is minimal. Thereafter, the current demanded falls with increasing revs until the point of maximum efficiency. This is what I've already explained in various ways, but you don't seem to want to accept it, possibly because you're bringing in irrelevant factors in the motor context, like the hub speed, wheel size and gears.

The motor has largely set characteristics and asking for those to change is like asking objects with mass to fall upwards. :)
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coops

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Don't get me wrong flecc: I admit I've asked the question in a roundabout way (no change there then... :rolleyes:), but if "thems the way things are" with motors, then I can only assume the information I've seen on that one i.e. with peak torque rpm very close to peak efficiency rpm (under a load of about 20Nm or so) and then current/power falling off steeply with a small rpm increase towards a point of maximum efficiency & minimum power, suggests either its not useful, as you said, or the data is somehow in error? :rolleyes: :)

Stuart.
 

flecc

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Probably some element of both Stuart. There's some very suspect info out there, like efficiencies of 95% or more. A motor can be designed for a specific aim, for example a high speed, but that doesn't make it very useful for us in bikes.

A bike using such a motor would be ok for speed fun, but point it at a hill that demands to be climbed at a lower speed than the maximum torque falls at, and then in comes the "useless" I mentioned.

Going back to the gearing you mentioned, that motor could be geared down to produce that torque at the 13.8 mph you asked for, but what would be the point of using this necessarily less efficient motor to do that when a conventionally designed one would do that job very much more efficiently, with a much greater range, and in addition have a better low speed performance?

Electric motors are a bit like that, infinitely less flexible than petrol engines which can have huge variations in characteristics while remaining efficient.

Been having the greatest difficulty staying online at present, exchange problems I think, and the post has turned up so I'm out of here in a moment for you know what. :)
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coops

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flecc said:
ok for speed fun, but point it at a hill that demands to be climbed at a lower speed than the maximum torque falls at, and then in comes the "useless" I mentioned.
yes, exactly what I was afraid of.

flecc said:
...that motor could be geared down to produce that torque at the 13.8 mph you asked for, but what would be the point of using this necessarily less efficient motor to do that when a conventionally designed one would do that job very much more efficiently, with a much greater range, and in addition have a better low speed performance?
Yes, again, exactly... I feared it would affect hill performance negatively and you seem to have confirmed that.

flecc said:
Electric motors are a bit like that, infinitely less flexible than petrol engines which can have huge variations in characteristics while remaining efficient.
Hence threads like this one, to try to sort the useful from the not :D.

flecc said:
the post has turned up so I'm out of here in a moment for you know what. :)
:D well, good luck & best speed! Thanks again for the information flecc & hope you beat the weather & that we have some good news as soon as you're done ;).

Stuart.
 
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flecc

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well, good luck & best speed! Thanks again for the information flecc & hope you beat the weather & that we have some good news as soon as you're done.

Stuart.
Just broke my test route record by exactly 2.5 mph, and not in still air either. An average of 21.6 mph, up from 19.1 mph, for 9.53 miles of hilly country could be a first for a Torq motored bike. :) :) :) :)
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