Guide to motor selection - suitable power?

[Mike Blandford]

A brushless, electric motor is also an alternator. If you rotate the motor, it generates a voltage. A 1000kv motor will generate 1 volt if it rotating at 1000 RPM.

Here is an example (slightly simplified to ignore mechanical losses): use a 3S battery providing 11.0 volts on load with a 1000kv motor.

On testing you find the motor is spinning at 10000 RPM and the current is 20A.

At 10000 RPM, the motor is generating 10V, and this voltage opposes the voltage from the battery. This voltage is usually called the "back emf".

Power supplied by the battery is 11V times 20A = 220W.

The power used to drive the propellor is the back emf voltage times the current, so 10V times 20A = 200W.

The power lost as heat is (applied voltage - back emf) times current, so (11-10) times 20 = 20W.

 

Using both a wattmeter and a tachometer you may easily find out these values.

 

To find a good approximation for the mechanical losses, run the motor with no propellor. Measure the voltage and current and the resulting power used approximates the mechanical loss.

 

Rather than a wattmeter, these days I use telemetry, particularly I like the FrSky Neuron ESCs as they provide full telemetry (voltage, current, RPM, ESC temperature and BEC voltage and current).

 

Mike

A brushless, electric motor is also an alternator. If you rotate the motor, it generates a voltage. A 1000kv motor will generate 1 volt if it rotating at 1000 RPM.

Here is an example (slightly simplified to ignore mechanical losses): use a 3S battery providing 11.0 volts on load with a 1000kv motor.

On testing you find the motor is spinning at 10000 RPM and the current is 20A.

At 10000 RPM, the motor is generating 10V, and this voltage opposes the voltage from the battery. This voltage is usually called the "back emf".

Power supplied by the battery is 11V times 20A = 220W.

The power used to drive the propellor is the back emf voltage times the current, so 10V times 20A = 200W.

The power lost as heat is (applied voltage - back emf) times current, so (11-10) times 20 = 20W.

 

Using both a wattmeter and a tachometer you may easily find out these values.

 

To find a good approximation for the mechanical losses, run the motor with no propellor. Measure the voltage and current and the resulting power used approximates the mechanical loss.

 

Rather than a wattmeter, these days I use telemetry, particularly I like the FrSky Neuron ESCs as they provide full telemetry (voltage, current, RPM, ESC temperature and BEC voltage and current).

 

Mike

[Trevor]

Definitely worth saying twice, Mike ?

[Peter Christy]

Whist I accept it is necessary to correct incorrect assumptions, as other have said, this is not helping Tony much!

 

Watt meters are a guide to getting the results you want. Yes, there are losses in the system - in any system, come to that - but that does not invalidate the readings as a guide to the *relative* (not necessarily absolute) value of any changes made.

 

A watt meter will enable you to get as close as possible to the desired output within the other physical constaints (prop diameter, etc). It can also prevent you from inadvertently overloading a motor or esc.

 

I can also endorse what MikeB said about the Neuron escs, which I found invaluable when converting my DS-22 and Lark helicopters. Wish I'd had one when I did the Cobra...! (Its not easy to read a watt meter when a large helicopter is straining at the leash!)

 

So, Tony, you can safely ignore a lot of the nitty-gritty above. Just choose your motor and battery pack along the lines outlined initially, and see how close you are to the desired performance with a watt meter. Adjust the prop size if necessary.

 

Simple!

 

--

Pete

 

[MattyB]

2 hours ago, Graham Davies 3 said:

Gents, All this is very interesting but is doing nothing to help the OP...

 

2 hours ago, Dickw said:

Agreed, but once incorrect information is posted it has to be countered for the sake of anyone reading this later.

 

Dick

 

2 hours ago, Graham Davies 3 said:

Does it? The poor OP was confused, and 2 pages of Mansplaining has just made it all worse...

 

Based on his posts the OP got an answer he was happy with by the bottom of page one. Unfortunately at that point Peter chose to post his (I'm sorry, it has to be said) entirely unproven view that wattmeters only measure the heat disipated by the motor. It is that which has caused confusion and would continue to do so for any newcomer reading this thread in future if it were not challenged. 

Edited September 9, 2021 by MattyB

[Bob Cotsford]

Can I suggest that the mods move the last two pages to a new thread for those wishing to discuss motor efficiency, electric theory and what watts do wattmeters do?

Just leave those posts actually answering the OP.

[SIMON CRAGG]

Tony is going to contact me when he is back next week.

 

I hope to help him by putting all his data into e.calc and coming up with some options in laymans terms.

 

I'm afraid all the subsequent pontificating is mostly irrelevant at his stage, however well meaning it may be.

 

 

[Peter Beeney]

Very sorry for any distress caused by my posts folks, that’s definitely not what I intended. Indeed far from it, I’ve often said in the past that my ramblings should not be considered as any form of advice or instruction. I just thought it might be interesting to suggest another idea for checking prop speed etc. Now it seems that tachometers have gone out of fashion big time. Also I have no issues whatsoever with whatever anyone chooses to do their checking and testing with, it’s simply  a personal choice. I’m just an old dinosaur I guess, way back in the day when i/c was king in the modelling world everyone had a tacho; a quick rev check before flying could often be seen. But i/c also had it’s own set of problems as well…

 

However, I firmly reject the suggestion that the information I’ve posted is incorrect. I realise that won’t count for anything at all and I know I only tinker around with this stuff but as a for instance if anyone can show me with proof that the rotary action of a motor is not the result of mechanical interaction between two magnetic fields then I’d be pleased to read it and admit that I’ve got it all a bit wrong… Until then we shall have to gently agree to disagree on the whole subject… and I don’t think I can get any closer to Keeping It Strictly Simple than that…

 

Again humble apologies, I might well be looking at my toy tacho in a whole new light now; I’ll probably give it a good kicking for starters..

 

PB

[Tony Harrison 2]

9 hours ago, Graham Davies 3 said:

Does it? The poor OP was confused, and 2 pages of Mansplaining has just made it all worse...

 

Thanks Graham, but I must insist that I'm not confused, just slightly perplexed that what I thought was a fairrly simple query has morphed into a technical argument at what seems like postgrad electronics level! Speaking of which,I'm educated beyond first degree level myself, though certainly not in electronics, and reasonably proficient with practical stuff - but just as I don't need a degree in marine biology to go fishing, I suspect many of the finer points discussed here are not strictly relevant to getting my plane into the air... It's interesting in its way, but sort of reminiscent of medieval theologians arguing the toss about the precise nature of angels.

Still,I've learned a few things - thanks to one and all.

Tony

[Denis Watkins]

There is much sabre rattling on this site everyday Tony

Just use a meter, to avoid returning from the field with a bin bag full.

It is the simplest solution.

[Basil]

Just to say that 100% in/out will not be achevieved as there are always losses associated with the 'Machinery'.!!!!!!!

In simple terms, that is.

Bas

Edited September 10, 2021 by Basil
after thought

[Peter Beeney]

Before this thread finally disappears into a misty oblivion may I just take this opportunity to bump it up and say that there are still one or two questions I’d like to ask regarding the wattmeter and the useful information that can be given by it’s readouts. I’m not very familiar with it as I’ve never seen one, at least not the kind that modellers use anyway, so I’m thinking it may it may help to avoid making mistakes in any future posts.

 

To show the greatest respect to Tony’s Original Post and taking Bob C’s Good Idea firmly on board as well, would it then also be an alternative plan if maybe one of the moderators could be persuaded to fire up a new thread, say something along the lines of 'Watt difference does the odd volt or amp here and there make to anything’ perhaps, so that all this stuff can be discussed at leisure in an amicable fashion and any folks not interested needn’t even bother to open it. I”d be more than happy to include a disclaimer with any of my posts saying that these are just my ideas and may not necessarily be the true facts etc.  

                                                                         

PB

[john stones 1 - Moderator]

You don't need a Mod to start the thread for you Peter, you are free to start threads yourself, and discuss to/till ? your hearts content

[Peter Beeney]

Hi John,

 

Many thanks for your reply but I do think I would very much prefer to remain strictly in a neutral corner on this one. Earlier in the thread I was handed a few rather salty crisps simply just for posting what I thought was a simple might-be-of-some-interest idea. In the event I couldn’t have been more mistaken; and as I’ve not added any salt to my lunch as a seasoning flavour for many years now I couldn’t but help noticing. I don’t have any particular problems with this, rightly or wrongly  I’m more than prepared to defend what I said, but I do feel that some sort of explanation might not come amiss as well.

 

Although had I thought for one moment it would have resulted in that I definitely would not have bothered………… 

   

It would seem that no one has come back on my query about the motor's magnetic fields earlier so now I’ll take that as a yes then. Actually I think there is a little clue somewhere right in the middle very much related to the motor’s performance and it all just revolves around this. Sorry about the puny pun but it would just keep on going round and round….

 

PB

[john stones 1 - Moderator]

I'm keeping quiet Peter, lecky experts can be vicious if you rouse em. ?

[Dickw]

30 minutes ago, Peter Beeney said:

...........................................  

It would seem that no one has come back on my query about the motor's magnetic fields earlier so now I’ll take that as a yes then. ........................................

PB

Peter

I am not sure what point you are making, but I don't think anyone ever disagreed that a motor operates by the interaction of magnetic fields. It was your statement about watt meters that people were trying to correct.

 

Dick

[David Ovenden]

Personally I don't think this thread is the place for detailed discussion of electrical theory. It is not at all what the OP asked.  A new thread would be the place to air different views on watt meters and to debate principles of physics.

[leccyflyer]

A thread doesn't end just because the OP's question has been answered, IMO. Threads evolve and active discussion is what keeps a forum  alive. The forum would be a very sterile place if each question asked were just answered with no discussion and then closed.

 

When discussion moves on to principles underlying the advice given, the opportunity is there to learn something and to have one's assumptions tested and perhaps refined. Otherwise every motor selection question could be answered by the default "Just ask George at 4-Max". The old adage "Give a man a fish and you feed him fir a day, teach a man to fish and you feed him for life" springs to mind. Discussion is good and discussion of the fundamentals is a learning opportunity.

[EarlyBird]

Having flown electric for three years I have survived without a Watt Meter. I understand that a Watt Meter is used to check the max current and therefore the possibility of burning out the ESC and motor. This is the first time I have come across the use of a Taco on an electric motor to be honest I don't understand how knowing the prop speed tells me anything useful. 

 

Surely the ESC drives the motor at a speed dependent on battery voltage, Kv of the motor and throttle position. If the motor is over propped taking the amps over the maximum the motor/ESC will burn out but how does knowing the prop speed when it did help?. Puzzling ?

 

This is giving me a head ache. Time to bash some balsa.

[Shaun Walsh]

"This is the first time I have come across the use of a Taco on an electric motor"

WARNING, do not apply Mexican food to electric motors! ?

[EarlyBird]

Mexican food?

[Shaun Walsh]

9 minutes ago, EarlyBird said:

Mexican food?

Taco

[EarlyBird]

18 minutes ago, Shaun Walsh said:

Taco

Tachometer?

[MattyB]

 

On 21/09/2021 at 22:57, Peter Beeney said:

Before this thread finally disappears into a misty oblivion may I just take this opportunity to bump it up and say that there are still one or two questions I’d like to ask regarding the wattmeter and the useful information that can be given by it’s readouts. I’m not very familiar with it as I’ve never seen one, at least not the kind that modellers use anyway, so I’m thinking it may it may help to avoid making mistakes in any future posts.

 

To show the greatest respect to Tony’s Original Post and taking Bob C’s Good Idea firmly on board as well, would it then also be an alternative plan if maybe one of the moderators could be persuaded to fire up a new thread, say something along the lines of 'Watt difference does the odd volt or amp here and there make to anything’ perhaps, so that all this stuff can be discussed at leisure in an amicable fashion and any folks not interested needn’t even bother to open it. I”d be more than happy to include a disclaimer with any of my posts saying that these are just my ideas and may not necessarily be the true facts etc.                                                                      

 

If you have questions and want to keep them separate from this thread, start a new one. Starting a thread with a question is the very definition of being neutral on a subject, so I'm not sure what your statement (below) about is really about. Don't expect the mods to do it for you though, that isn't their job.

 

12 hours ago, Peter Beeney said:

Many thanks for your reply but I do think I would very much prefer to remain strictly in a neutral corner on this one. Earlier in the thread I was handed a few rather salty crisps simply just for posting what I thought was a simple might-be-of-some-interest idea. In the event I couldn’t have been more mistaken; and as I’ve not added any salt to my lunch as a seasoning flavour for many years now I couldn’t but help noticing. I don’t have any particular problems with this, rightly or wrongly  I’m more than prepared to defend what I said, but I do feel that some sort of explanation might not come amiss as well.

 

Although had I thought for one moment it would have resulted in that I definitely would not have bothered………… 

 

It would seem that no one has come back on my query about the motor's magnetic fields earlier so now I’ll take that as a yes then. Actually I think there is a little clue somewhere right in the middle very much related to the motor’s performance and it all just revolves around this. Sorry about the puny pun but it would just keep on going round and round….

 

The reason most of us are struggling with your posts is that you have stated you never seen or used a wattmeter, yet seem keen to explain those that do use and understand them how pretty much everything they know about them is wrong.... ?

 

Re: your point (which I have put in bold above), nobody has come back on that because nobody is contesting it is wrong. What we were pointing out is that your statement at the bottom of page 1 that wattmeters are "...just telling me how many volts and amps are being converted into heat due to the resistance of the power train" (exact quote) IS demonstrably incorrect. Mike Blandford's excellent post at the top of this page explains why that is, and there are lots of sources online that you can check that back up the theory in his post. Have a Google, it will undoubtedly further your understanding of this topic.

 

PS - 

Edited September 23, 2021 by MattyB

[MattyB]

46 minutes ago, Shaun Walsh said:

Taco

 

27 minutes ago, EarlyBird said:

Tachometer?

 

Tacho:

 

Taco:

 

 

?

[MattyB]

2 hours ago, EarlyBird said:

Having flown electric for three years I have survived without a Watt Meter. I understand that a Watt Meter is used to check the max current and therefore the possibility of burning out the ESC and motor. This is the first time I have come across the use of a Taco on an electric motor to be honest I don't understand how knowing the prop speed tells me anything useful. 

 

Surely the ESC drives the motor at a speed dependent on battery voltage, Kv of the motor and throttle position. If the motor is over propped taking the amps over the maximum the motor/ESC will burn out but how does knowing the prop speed when it did help?. Puzzling ?

 

Yes, you are quite correct that if the only thing you are looking to do is ensure your components are operating within their limits then a wattmeter alone is all that is necessary. As Mike B points out at the top of this page though, you can use a tachometer and a bit of maths instead if you wish, it's just a bit more work.

 

Where the tacho can give additional insights is if you have 2 or more props that you know are safe to run from the wattmeter, but aren't certain which will give the best flight performance for your goals (i.e. the best top speed for a pylon racer, or the most thrust for a short period in an e-glider). Taking the pylon racer example, it helps because a higher pitched prop will give a theoretically higher pitch speed, but the additional load it generates may mean the motor can't turn it as fast. A tacho will pick this up quickly and enable you to narrow down the props to a shortlist, though it still can't help with the unloading effect that occurs in the air (unless you have an onboard telemetry tacho of course).

Edited September 23, 2021 by MattyB