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Understanding motor sizing?

Help for novice needed

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Bill Reed05/10/2017 11:41:04
80 forum posts
2 photos

Hello, I am a novice to the hobby and building my first kit. The kit was intended for IC but I want to run electric.

I know pretty much nothing about KV`s and Watts and realise there are sites which can give a recommended set up and if not a size and weight chart. I have done this and found a recommended setup.

what I would like to understand is...... say the kit is recommended to use 450w . I have seen motors of 450w with apx same KV and can run same 3 or 4 cells but the motors size varies from 28mm,30mm ,40mm 42mm and various lengths.

If all the specs are the same why would I use a 30mm over a 42mm. in my case I could fit a coke can in the bay so size is no concern.

I am probably missing something basic and would like to learn a bit.



Geoff S05/10/2017 12:16:52
3584 forum posts
14 photos

A big topic and you'll get lots of replies.

The first thing to think about when choosing the motor for a conversion from glow, oddly enough, is the propeller. What prop did the glow engine use and at what revs? You need to end up with something like that and there are a few rules of thumb that work reasonably but are not set in stone.

1: for a spritely performance you need roughly 100 watts/lb. However, my Ballerina flies very well on 90 watts/lb. The rule is an old one and dates from brushed motors and NiCads. Modern motors coupled with LiPos has made electric flight much more practical.

2: For a typical motor the usable power output is approximately 3 watts/gram. Ignore the quoted power for a motor it only works at the highest voltage. The important parameters for a brushless motor are the kv (rpm/volt) and the maximum current.

3: Electric motors are very flexible which is at the same time an advantage and and a problem. An advantage because by adjusting load (propeller size) you can get the power you need for a model. A problem because it means direct comparison with glow engines is difficult. Just bear in mind that an electric motor is lighter than a comparable glow engine and the energy source (LiPo battery) is also relatively light. That means it's often (always?) better to overspec the motor (ie have one much heavier than needed electrically) then under run it with a smaller prop. So fit the biggest motor you can sensibly accommodate.

If you give us some idea of your project someone will come up with a suitable power train - actually people will come up with several different options because of that flexibility I mentioned


Pete B - Moderator05/10/2017 12:39:21
7641 forum posts
733 photos

It's worth having a bit of a browse through the headings in this section of the forum, Bill. There's plenty there to stir some questions and the folks here will always do what they can to help...thumbs up


Bill Reed05/10/2017 12:43:52
80 forum posts
2 photos

Hi Geoff, Thanks for taking the time to help.The above was done very easily to understand and will help a lot.

But the thing I do not understand is,,, motors come in various can diameters and lengths , I have seen 2 motors with same specs BUT different can sizes, why would I choose one over the other if room is of no concern?

Geoff S05/10/2017 13:08:42
3584 forum posts
14 photos

All other things being equal always choose the biggest you can comfortably fit because it will be more mechanically robust (bigger, stronger shaft and bigger bearings. As I said above, the motor will always be lighter than the glow engine you're replacing.

The smaller motors are going to be lighter but may well suit a design intended for an electric drive train from the outset. So the whole structure may well be smaller and lighter than would be needed to withstand the harsher environment of a piston engine and the vibration. That's just my opinion. I'm not an expert on motor design but that seems to be a reasonable explanation.

Pete B's suggestion is also worth taking up. There was a recent article in the magazine which was an extension of a post/article on line here but I'm not sure who wrote it or its title. I know it was very clear and helpful. No doubt someone (the author?) will be along to point to its location.


Frank Skilbeck05/10/2017 13:12:51
4679 forum posts
101 photos

Bill, if the motor has the same Kv and max amps then there is no particular reason why you would chose one over the other. But you may find that the larger can size motor is at the bottom of the power range for motors in that diameter and the smaller one at the top end, i.e a 28mm dia motor is typically upto 400 watts and a 35mm dia motor 300 to 600 watts say.

John Emms 105/10/2017 21:34:56
237 forum posts


I think your core question that remains unanswered is about the difference between a 30mm motor of, perhaps, 1000kv, and a 42mm motor of 1000kv.

Core fact: A larger operating diameter (either of the stator of an outrunner, or the rotor of a conventional brushless) and/or greater length for the stator will provide you with greater torque.

Simply enlarging a motor design will result in a lower kv, so fewer thicker winds can be used to bring the kv back up, and the fewer thicker winds reduce resistance, which results in greater efficiency at higher currents.

The only real way to know how different motors react to having propellers put on them with different voltages is to check performance data, but in general, a 30mm 1000kv motor will be intended to operate around 15A, and a 42mm 1000kv motor would be intended to operate around 45 to 50A. Given the option go with the larger motor.

People will be shocked that I currently have a preference for the Mega 22/45/3, a 300g motor, when 150g motors would fly the same models using the same current consumed. The fact is that I get far greater torque at the prop, and greater efficiency with the larger motor.

Hope that is of some help,


Bill Reed05/10/2017 23:00:39
80 forum posts
2 photos

John, That answered my question on the physical size of motors. Thanks

Mike Blandford05/10/2017 23:14:01
618 forum posts
25 photos

Torque is directly proportional to current, and my understanding is that given two motors with the same kv value, they will provide exactly the same torque for the same current, a larger motor will not provide a greater torque for the same current.

A larger motor will, as you say, have a lower resistance, thus you would only need a lower voltage to obtain the same current. If you use the same voltage, then, due to the lower resistance, you will get a higher current and that gives a higher torque.

For the same mechanical output, the larger motor will, in general, have a higher efficiency and so be the better choice.

If you select a larger motor that has a lower winding resistance, then you may also choose to go for a (slightly) lower kv value. This will give a higher torque for the same current allowing you to use a larger propellor, which itself will tend to provide greater efficiency.


Erfolg06/10/2017 17:40:33
11706 forum posts
1309 photos

My great revelation is shaft size. I always look for a motor that gives me a shaft of 5mm dia or more. If i cannot get that with outrunners, I accept i will at some time suffer a motor shaft being bent.

For inrunnersI set up a spinner to nose plate of essentially zero, on the assumption that the spinner will move forward and create a clearance, yet on arrival there is no gap.

With respect to outrunner motors, I normally am looking for +12,000 revs, because most propellers still seem to have their pitch based on IC, of about 6". I look for a decent dia, for standard models, so invariably 9 or 10 dia on my std, 50 wing span.As for power yes, it is 100 watt per pound for me.

Do I always stick to this, defiantly not, I have motors turning at +20, 000 rev min. diameters down at 5 inch.

Although I am always looking for power, and I do get things wrong from time to time. Although I no longer worry. I just keep studying what there is of a manufacturers spec, and customer feed back, if available.

John Emms 107/10/2017 09:13:52
237 forum posts
Posted by Mike Blandford on 05/10/2017 23:14:01:

Torque is directly proportional to current, and my understanding is that given two motors with the same kv value, they will provide exactly the same torque for the same current, a larger motor will not provide a greater torque for the same current.

Hi Mike,

Trying to keep this in simple practical terms of the choice between a larger motor and smaller motor (I now have a large staff room full of MScs and Doctors who are only too happy to theorise on all things Engineering lol), I am sure you are right, and it explains why an electric car can have incredible acceleration using huge amounts of energy, yet have very good economy at low power levels, and why a hybrid bus can get up the same hill even more effectively using an engine of 2/3 the size and power (or rather torque) of a conventional diesel bus.

The danger for us of saying that any two motor of the same kv drawing the same current will have the same torque, is that if we take our typical 30mm 1000kv motor that would typically have a maximum current of 15A (and be most efficient around 8 to 10A), and simply load it down to draw 30A, the motor will not be able to turn the prop as fast as a larger motor, the efficiency will go right down (power in compared to power turning the prop), the waste energy builds up in the windings, the heat build up in the copper results in greater resistance, and that in turn results in further heat build up in the windings - it doesn't take long for a new retail opportunity, or a difficult conversation with your motor supplier! This is a very simple experiment: take a 28mm diameter 1000kv motor and a 36mm diameter 1000kv motor from the same range of motors. Take current and prop revs on the same batteries and ESC, and the results tell us a huge amount about the behaviour of our small motors being used at high currents.

There are two ways round using smaller diameter motors at higher currents:

1) Make the motor longer to be able to handle the higher current at the same kv. I did this during my early developments of EDF systems: when it was common to use 400 size motors, I started experimenting with longer 480 size motors, and replaced 500 size motors with 600 size motors. This was initiated by a comment from Oliver at Wemotec who said that fans need high torque motors, not high kv motors. This trend has continued with ever longer motors used in EDF systems - and the thrust for power consumed when comparing test results is telling (the longer motors are slightly heavier, but provide far greater efficiency).

2) We can simply put fewer winds of thicker copper in the same size of motor. This increases the current capability, but also the kv. We then reduce the output speed to turn a sensible size prop using (expensive) gearing. Kv can also be altered by changing the number of magnets or poles. The danger of going with a faster outrunner is that many of the lower cost controllers favoured by many modellers cannot keep up with the high switching rate that is need for the motor to get up to speed.

There is more about choosing a low kv and larger props, or high kv and smaller props in the same size of motor from the same range, but that is, perhaps, for another time!

Hope that is not too much information.


Erfolg07/10/2017 21:44:41
11706 forum posts
1309 photos

Well John, you have answered a question i posed many years back, what does a longer motor give, relative to a short motor of otherwise identical dimensions. Torque it would seem. Of course this leads to yet another question, typically what drop in performance would you expect between a motor operating at the max amps , with a short relative to a long rotor. I am guessing there is no easy answer, other than if turning a prop of the largest practical diameter, any heat build up with a short motor is a consequence of exceeding its viable torque.

I am now guessing that you will suggest as an idea, that with short motors do not expect to turn as big as prop continuously as a long motor.

I am getting the idea if this is how the argument goes, although for most of us it is a matter of judgement.

Although part of your description could answer why one of my models initially on take of flew fine, as the flight progresses the useful performance seemed to degrade. I put it down to voltage drop, now it seems that the answer is more complex, it could be that a smaller dia rotor was starting to get hot, as well as some voltage drop. The replacement bigger dia motor, being more tolerant and resistant to heating.

John Emms 107/10/2017 22:57:21
237 forum posts

Hi Erfolg,

I have been thinking more about Mike's all motors are equal (to paraphrase both Mike and George Orwell), and I think I should have initially said that the larger motor has a greater torque potential, because of course, there is no torque without current flow.

Mike's statement about the effect of the larger motors lower resistance for the same kv is backed up by testing. I was looking for a heavier replacement for the Mega 22/30/3 in my Okapi (to move the lighter battery back). The 22/30/3 was right on 40A with a Graupner CAM 10x6 and the Foxy 3S 3300s that I use. I put in a Mega 22/45/2 (same motor, but 50% longer, and 2/3 the number of winds, with a similar kv), and used exactly the same 10x6 prop. Testing using the same battery very shortly after (no further charge), the current was 47A, considerably more than I was expecting, and that can only be down to the lower resistance.

The effects of longer motors are equally surprising:

In the early days, the Kontronik Fun 400/36 (3600kv) was THE motor to use with 10 NiMh cells (equivalent of 3S LiPo) in the 70mm Wemotec Mini Fan. We wanted to settle on Mega for use in our fan systems, so I asked Mega to supply me with a longer 16/15/2 to reduce the kv from 4700 to 3600, and keep the system under 40A with 10 cells. Mega produced 4 prototypes, and I chose the 16/17/2 which became the 16 EDF. I asked Bob Partington to replace the Kontronik Fun 400/36 in his F15 with the prototype 16/17/2. Bob took this model and new motor to a BEFA event that weekend and won the technical achievement award because this was the first time that they had seen an EDF model appear to have unlimited vertical performance from a fast pass (they had seen this model a number of times with the original power system. So, same diameter motors, same kv, similar top quality materials and construction, but the higher performing Mega has 6 poles, not the 2 poles of the Kontronik, and it was 2mm longer.

Because customers were using the 16EDF on 4S and exceeding the manufacturer's limits (the motors DID hold up!), I started to spec the 16/20/2 with a lower kv to moderate the current, stay within limits, and give longer flight times on the same batteries (same diameter, same windings, but a further 3mm longer). I was sent a whole load of test data by a customer, including the 16/17/2 EDF and 16/20/2. All the tests, that had been automatically data logged, showed the 16/20/2 running below the power drawn by the 16/17/2, and the thrust produced by the 16/20/2 was always higher. The only change that could have provided the greater efficiency, was the length of the motor.

Edited By John Emms 1 on 07/10/2017 23:04:27

Mike Blandford07/10/2017 23:45:18
618 forum posts
25 photos

A general feature of changing to a longer motor is several parameters change at the same time.
If you just make the stator longer, but keep the same number of turns of wire, then the kv will reduce (longer wire in the same amgnetic field) and the resistance will increase (longer wire). So to keep to the same kv, the number of turns would be reduced (lower resistance), which then allows a larger diameter wire to be used (again reducing resistance).

Suppose you have a motor with a kv of 1000 and a resistance of 20mohms (milliohms), using 10 turns of wire. Now create a new motor with a stator of twice the length. The kv woud reduce to 500, so you would need to reduce the number of turns to 5. If you use the same diameter wire, the resistance would be about the same the first motor as you have half the number of turns, but each turn is about twice as long. However with fewer turns, you can fit a larger diameter wire in, one that gives twice the cross section in fact, and this would halve the resistance.

So you end up with a motor of the same kv (1000), but the resistance is now only 10mohms. For the same heating effect in the windings, this longer motor could handle a 41% increase in current, resulting in a (potential) 41% increase in torque.

To use John's example where the motor was 50% longer, this would reduce the winding resistance to 2/3.
Suppose the original resistance was 52mohm. At 40A, you would lose 2.08 volts across the windings which would heat at 83W, and bo operating at about 80% efficiency. The longer motor would have a resistance of 35mohm. At 47A, only 1.65 volts would be lost (giving an extra 0.43 volts towards turning the motor, 430 rpm with a kv of 1000).
Only 77W is now lost as heat, and the efficiency is 84%.

A note regarding using a longer motor. I use a "2832" (long) motor in a WOT4. Originally I had this mounted "backwards" so the stator was bolted to the firewall and the prop/spinner was mounted on the rotating part. This meant the prop/spinner was a large distance from the fixed mounting, and at high revs i got significant vibration and I couldn't get rid of it no matter how well I balanced the prop/spinner. Eventually I pushed the shaft through the motor, built a ply box onto the firewall and mounted the motor "forwards" with the rotor behind the mounting and the prop/spinner in front. This completely solved the vibration problem. If you wish to mount a motor "backwards" I would therefore suggest a shorter, but larger diameter motor would be better.


Edited By Mike Blandford on 08/10/2017 00:01:43

John Emms 108/10/2017 19:28:40
237 forum posts

As well as the current, we also know that manufacturers increase torque, among other things, by: Using stronger magnets. Using a thicker case to help contain, and increase, the magnetic flux. Minimising the gap between the rotor and stator.

So, keeping this very simple for the beginners forum:

1) The torque in a motor increases with an increase in the current.

2) The torque in a motor is the result of the current, number of windings, and the magnetic flux.

3) The magnetic flux measured in Webers, is the product of the magnetic flux density (how strong the magnetic force is) measured in Tesla, and the area of the magnetic flux.

We can roughly measure the area of the magnetic flux simply by calculating the surface area of the rotor on an inrunner, or the stator on an outrunner, and of course, that explains why (for the same quality of motor with similar winds) a larger motor will always provide more torque, why the increase in torque is greater with an increase in diameter than an increase in length, and how we can even achieve greater torque using a larger motor with less current draw (as we see in many examples of EDF power systems).

I hope that helps to provide an explanation of why (generally) it is better to choose the larger motor, given the choice of two suitable motors.

Should anyone want a copy of the full original Powerpoint presentation, I have been told I can have one to pass on to you. The full presentation includes all of the Formulae, which I am sure many will find particularly interesting.

John Emms 108/10/2017 19:44:58
237 forum posts

An issue with conventional LRK Outrunners , AXi etc is that the cantilevered tube that supports the bearings is quite light, and relatively weak. Model Motors were very reluctant to provide a means of radial mounting for this reason, but were forced to because the Chinese copies were being supplied with them. The real answer is to mount Outrunners with the motor front plate on the bulkhead with the shaft sticking through, and that will mean that the big (main) bearing is carrying the load of the propeller.

If anyone is ever tempted to simply knock a motor shaft through to the other end, please do not do it!

Quality motors have the shaft fitted to the rotor, and then the rotor and shaft assembly is checked for true rotation. The true rotation is needed to keep the rotor to stator gap even, and without that the motor will loose efficiency, and may even not run.

Some get away with it, and others don't. Who pays for the ones that don't, and ignore the instructions that say "do not alter this motor in any way", and "the warranty will be void if the motor has been altered or disassembled".

I hope the last two posts will be of help to someone.

Kind regards,


Geoff S08/10/2017 20:42:32
3584 forum posts
14 photos

Thanks, John, for some very interesting explanations. When I did all my electronic engineering courses back in the 1960s I tended to avoid the magnetism and electric motor aspects in favour of the semi conductor and logic which seemed irrelevant at the time (eg magnetic amplifiers etc). Now I regret that to some extent

I for one would like to be able to see the Power Point presentation if it were made available.

Geoff Sleath

Erfolg09/10/2017 20:37:01
11706 forum posts
1309 photos

In general terms I understand where you are coming from. In some respects I often regret this and that. Often after some thought I come to realise that the decisions made were made with the best intentions. That is passing an exam, usefulness in your career, dovetailing with your current knowledge. What may be relevant to a hobby I do not have, as yet, is just life.

In my case, one of what people would now call modules, then a subject, was "Instrumentation and control". I stuck with the maths side of defining system performance and operation. The ideas of those bits which I had less interest in, did stick to some extent, and awake distant memories, often in a form that is much less that perfect. I think this general rule, and will be true for most. I was training to be a "Mechanical Engineer", still, the subject was relevant, although maybe not immediately apparent.

I did think that the general presentations of John Emms is the sort of information and experiences, that would make a excellent article for RCM&E. In general John is indicating the relevance of certain geometric and electrical aspects of motors to the selection of appropriate solutions to motor selection. I did note that in some areas that their were theoretically no difference between a high cost motor and the sort i will typically use, yet John from experiences is able to indicate why some identical motors could be superior. It is the sort of discussion that i would like to see in the mag.

I only use low end motors, i do move and change shafts, which at my end of the market works really well. I will go further and say my Kontronics motor system, is now very poor compared to the cheap end gear.

I do seem to observe that the magazine is in general built around a few regular contributors. I guess for practical reasons. Yet I do feel we need to see more of the John Emms type of contributions, however infrequent. However reluctant. Which reminds me that the BEB contributions on subjects such as "aerodynamic forces experienced by control surfaces" or"Factors controlling looping" or " a bit more on Circulation Theory, and the relationship with Bernoulli etc", would be nice to see also. But for now i would settle for a bit more on Motors ESCs and how they relate to the real world.


Edited By Erfolg on 09/10/2017 20:38:58

Geoff S09/10/2017 20:56:46
3584 forum posts
14 photos

In my case, one of what people would now call modules, then a subject, was "Instrumentation and control".

I suppose it's all what you were interested in at the time. I ended up as an electronics designer in a department called Electronics and Instrumentation which was mainly concerned with the instrumentation and control of aero engine test beds and rigs. Electric motors just weren't part of my life except as drills etc in my workshop. I didn't go to university and ended full time education after 'O' levels at grammar school and studied at night school and day release. Sadly, that route seems not to exist now.

I just regard motors and esc as 'black boxes' you apply DC voltages to and get torque output from but I think it would help if I knew a bit more about the parameters of the 'black boxes' I don't suppose it would help my flying though - just my enjoyment which is all that matters now.


Erfolg09/10/2017 21:16:11
11706 forum posts
1309 photos

Geoff, from memory, the black boxes, with functions, and in arrangements where the the handle of the sausage machine( as described by the lecturer) would be cranked, and out pops an answer.

Perhaps what is less strange that many of the ideas, cross or did cross many disciplines, different set algorithms, functions, such as damping of systems. Although apparently not something recognised in our model world. I bet there is a full size aerodynamist on our forum, who could do a piece for the mag.

Although something from John would be a great start.

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