How do you compare electric motors

[Nigel Heather]

Returning to the hobby after many years, and whilst I still find IC engines very familiar, electric motors are a little puzzling.

In particular, how do I compare motors or look for alternatives. It is fairly clear cut with glow but not so much with electric.

For example if I wanted to consider alternatives for the Quantum II 25 how would I go about that.

**LINK**

I thought it would be a matter of loking at the KV value but I am not convinced, because I can find other motors with the same KV but their other parameters such as power, amps, required ESC are very different.

Cheers,

Nigel

[Simon Chaddock]

Nigel

The kV simply determines the speed that the motor will want to run at for a given battery voltage and by itself has nothing to do with the power it generates.

In the simplest terms motor power is determined by the amount of its copper, iron and magnets.i e its weight..

This does not give the full picture so really it is best to decide the power you need in Watts and then start browsing the motor specifications.

Within reason the same motor can run a bigger prop slowly but from a lower voltage battery or faster with a smaller prop from a higher voltage but any motor is ultimately limited by the maximum amps (heat) it can handle.

It is this very flexibility that means there is likely to be more than one 'workable' alternative so the best solution to start with is to use the recommended motor/prop/battery set up.

Failing that post the full details of the intended plane, how you want it to fly and just ask for a recommendation on this forum.

There is no substitute for experience.

Edited By Simon Chaddock on 31/08/2018 22:36:03

[Biggles' Elder Brother - Moderator]

The two main factors are amps and kV - taken together. Power is related to max amps and is fixed for any particular cell count of battery. Some examples:

a 3s battery is noimally 11.1v, if your motor can handle 40Amps hen then the max power available would be 11.1x40 or 444W.

As a general rule 100W/lb AUW gives good all round performance. So this motor battery combination could fly a plane weighing around 4-5lb.

kV required can be estimated by saying we want 10,000 rpm at full throttle. So with 11.1V we would need a kV of around 10,000/11.1 or approx 900. In practice this a very rough and ready estimate so anything from about 750 to 1000 would do.

To find an equivalant to your Quantum then look at its spec and find the max amps and kV - and look for something in the same range for both. You'll need to be quite close with the amps but can allow a bit more leeway wit the kV as I suggest above.

Personally I wouldn't go that route - I'd take the AUW of the model, estimate the power required on the 100W/lb basis. Then decide if you want to use 3s, 4s or what ever batteries and use their nominal voltage and the power to determine the amps needed. Knowing the amps you can then search for a motor that will handle those amps with suitable kV depending on the battery choice.

BEB

[MattyB]

There’s a whole section of the forum that should help you get up to speed on electric flight, with sticky threads on all the elements of an electric power train including the motor. I suggest you put the kettle on and wade through that thread, all the info you need should be there. 

Edited By MattyB on 01/09/2018 00:39:28

[Piers Bowlan]

As BEB says, a systematic approach is best, starting with the model's weight. Clearly, how much power you will need will be influenced by what sort of model it is. An aerobat with good vertical performance, or alternatively a 'hotliner' will need need more power than 100W/lb (150W/lb?), whilst a vintage job, perhaps only 70W/lb.

To risk making some generalisations about kv. A fast pylon-racer type model will tend to need a higher KV (smaller prop/turning faster) than an aerobatic ship where a larger slower turning prop is more appropriate (more torque). Another generalisation about kv is that if you are planning to use a 3s LiPo for example, a 1000v motor may be appropriate in your particular sport model. However, if you decide to use a 4s Lipo instead, a lower kv may be more appropriate (e.g. 800kv) in a motor of the same physical size (or use a smaller prop). Also, be wary of manufacturers claimed max power (W) figures, as this may involve running the motor near to the max Amps figure, which will be inefficient. Also, a hot motor is not a happy motor . Instead, go for a slightly bigger and heavier motor, running comfortably within its spec.

Just my 2p worth.

Alternatively, speak to George Worley of 4-Max, as he will be able to make a sensible recommendation.

 

Edited By Piers Bowlan on 01/09/2018 06:57:00

[Keith Berriman]

Hi Nigel Just out of interest the Quantum motor mentioned is equivilent to a 25 2 stroke

Just been building a Easy Street 2 and doing some web searching on motors specs etc

[Dave Hess]

Choosing an electric motor is more complicated than for an IC one because there are more variables, so it's not easy to make rules for selection.

As said, current handling capability will give some idea of how much power you can get from the motor, but you still have the choice of battery voltage, which significantly affects the motor speed current and power. Then you have the propeller, which also affects the motor speed and current. That means that you need to have kV, propeller size/pitch and battery voltage in one side of you head while you figure out what the actual motor speed and current are likely to be, then compare those with the motor's current-handling capability. That's not easy. You need data sheets and spread-sheets.

The easiest thing is to copy what works for somebody else. I guess when you've done it 100 times, you get a feel for how all these things work together, so you can make your own choices.

On our electric bike forum, somebody made a simulator. You could feed in all the parameters and it would produce graphs of all the performance characteristics including how long to melt-down. That contributed more to the understanding of how things worked than anything else. I've seen this one for R/C motors. It looks quite thorough, but I'm still trying to get my head around which parameters I need to enter and the graphs are a bit back to front. What i'd prefer to see is current vs RPM. Are there any other ones:

**LINK**

Edited By Dave Hess on 01/09/2018 10:13:11

Edited By Dave Hess on 01/09/2018 10:52:06

[Nigel Heather]

Many thanks for the info.

I understand KV now.

And I get the power per pound guidelines - they are vey useful.

But it still seems over complicated because the manufacturers often don’t quote the specifications I need. For example if I look at a plane model, sometimes the weight isn’t specified at all, or when it is it is not clear wwhether that is all up including the battery or not, or even if it includes the motor, ESC, servos and receiver.

And then when I look at motors manufacturers don’t quote the same specifications. Some will give the power (that’s easy), where some will say maximum current, but not clear whether that is the normal operating current to the safe overload, and some just specify the size of ESC required.

Cheers,

Nigel

[Bob Cotsford]

It's almost as much a minefield as picking an ic engine where capacity only gives a rough guide - some engines are low power sloggers, some high output but rev their nuts off to get that power.

With electrics it's worth checking the can size too, as a rule of thumb motors with the same diameter, length and kv will have a similar current rating, within reason. I try not to run a motor near it's rated limit anyway so I can accept small differences in ratings. In any case I think manufacturers ratings can often be taken with a pinch of salt and just used as a rough guide. I think that if a manufacturer only specifies an esc size then I'd probably look elsewhere.

[Biggles' Elder Brother - Moderator]

Posted by Nigel Heather on 01/09/2018 12:08:36:

But it still seems over complicated because the manufacturers often don’t quote the specifications I need. For example if I look at a plane model, sometimes the weight isn’t specified at all, or when it is it is not clear wwhether that is all up including the battery or not, or even if it includes the motor, ESC, servos and receiver.

And then when I look at motors manufacturers don’t quote the same specifications. Some will give the power (that’s easy), where some will say maximum current, but not clear whether that is the normal operating current to the safe overload, and some just specify the size of ESC required.

Yeap - ain't life a swine sometimes .

The reality the specs may not tell you everything you want but you can often work things out or make an educated guess.

For example, if they give only the power, then provided you have an idea what battery you intend to use you can work out the amps - that is the max amps. Suppose they say 900W, you fancy using 3s, so the amps would be the power divided by the battery voltage (11.1v) that will give us 81amps. Oh,that's too high! Anything above about 60amps or so starts to get expensive - cost of ESCs goes up etc. So lets use a bigger battery, let's try a 5s, that would have a nominal voltage of 18.5v, so that gives 900/18.5= 49amps max current - much better. See what I mean?

If they tell you the weight of the plane is 5lb and it has a 60" span running a 60 2-stroke - look up the weight of a couple of 60 2-strokes (OS always list the weight of their engines for example on their web site). Subtract that weight off. Now go to Hobby King - look up some 5s batteries - HK also always give weight. Add that on. Allow say 100g for the ESC and wiring etc. and you have educated guess at the all up weight.

Electric power is not so "plug and play" as IC - you have use your noggin a bit more! Some find that a chore, I actually quite enjoy the detective hunt for info to use as the basis for my design!

BEB

[Denis Watkins]

My dad gave me a ballpark figure years ago BEB for I/C of 0.06 per lb

E.g, 6 x 0.06 = 0.46 for a 6lb model ( ballpark )

Therefore approx 100W = 0.06cuin I/C for rough conversions

[Nigel Heather]

I get the electronics, but using the Quantum II 25 specs that I linked.

Max Current = 50A

Suggested ESC = 40A

LiPo = 2S to 4S = 7.4v, 11.1v or 14.8v

So I assume that max amps would be on 4S which would make the resistance = 3.37 ohms

That would mean

4S = 740W

3S = 415W

2S = 184W

Does that sound right?

Cheers,

Nigel

[Former Member]

[This posting has been removed]

[PatMc]

Posted by Nigel Heather on 01/09/2018 15:05:45:

 

I get the electronics, but using the Quantum II 25 specs that I linked.

Max Current = 50A

Suggested ESC = 40A

LiPo = 2S to 4S = 7.4v, 11.1v or 14.8v

So I assume that max amps would be on 4S which would make the resistance = 3.37 ohms

That would mean

4S = 740W

3S = 415W

2S = 184W

 

Does that sound right?

Cheers,

Nigel

Your calculations are correct, except for the resistance. The motor doesn't run as strict resistive circuit, in fact the windings resistance of this motor is around .031 ohms but explaining that at this stage is probably best left until later.

Unfortunately the stated specifications of the motor are also suspect, in fact there's an obvious contradiction in that the advised ESC rating is 40A whilst the max current is given as 50A.
IMO the realistic max continuous current is probably more like 35A and the 50A is the figure for 10 seconds running. TBH I don't think I'd rely on running it continuously above 30A.

The motor appears to be the same as an old version Turnigy 3542/1000 which sold with various labels depending on the retailer. Here's the 4-Max 1450kv version (formerly Purple Power 3542, I believe) which is alslo an old version. Here's the latest 3542 1000kv Turnigy version which has a 5mm shaft instead of the 4mm shaft of the older versions. 4-Max also sell a range of 3541 motors with 5mm shafts with various kv ratings.
The same motors are also sold elsewhere with different labels "Thumper" springs to mind.
My choice would be one with a 5mm shaft.

It might be worth mentioning that I've been using an original version Turnigy 3542/1000 (labeled XYH 3542/1000) in an Easyglider Pro for about 9 years. On 3s lipo using a 11x6.6 folding prop current drawn is 26.5A - 295W. Model weighs 1kg, so that's 134w/lb. My usual practice is a 30 sec motor run climbing near vertical to 200mtrs then glide.

 

 

 

 

Edited By PatMc on 01/09/2018 16:48:22

[Dave Hess]

Posted by Nigel Heather on 01/09/2018 15:05:45:

I get the electronics, but using the Quantum II 25 specs that I linked.

Max Current = 50A

Suggested ESC = 40A

LiPo = 2S to 4S = 7.4v, 11.1v or 14.8v

So I assume that max amps would be on 4S which would make the resistance = 3.37 ohms

That would mean

4S = 740W

3S = 415W

2S = 184W

Does that sound right?

Cheers,

Nigel

Unfortunately, it's not nearly as simple as that because the speed that the motor maxes out at has a big affect on current and efficiency. The max speed is more or less controlled by the choice of propeller. A critical characteristic for all that is the motor's kV. You therefore have kV, battery voltage, propeller size/pitch and motor winding resistance all affecting the current significantly.

Also, you have to bear in mind the efficiency because it's the difference between the power consumed and the power output that will burn the motor. In theory, if you had a motor with 100% efficiency, it wouldn't burn at any current, but it would break if you went too high. Basically efficiency accelerates downwards as the motor slows down, which is why you need to choose the right propeller to keep the speed up in the efficient zone. Increasing the voltage shifts the whole efficiency curve up along the RPM axis, which is why you need a smaller propeller for the same motor when you increase the voltage, so you can get the same current and efficiency at higher rpm, which means that you get more power because torque is proportional to current and power is torque times RPM.

It's difficult to explain in words because of all the different variables working together at the same time. Did you look at the simulator I linked above. It takes some figuring out because the graphs are back to front from what we need to get a clear picture of what's happening, but if you move the cursor up and down, you can see the results changing in the table at the top right. that should help with your understanding if you spend some time studying what happens when you change things.

[Nigel Heather]

Posted by Dave Hess on 01/09/2018 20:42:34:

Posted by Nigel Heather on 01/09/2018 15:05:45:

I get the electronics, but using the Quantum II 25 specs that I linked.

Max Current = 50A

Suggested ESC = 40A

LiPo = 2S to 4S = 7.4v, 11.1v or 14.8v

So I assume that max amps would be on 4S which would make the resistance = 3.37 ohms

That would mean

4S = 740W

3S = 415W

2S = 184W

Does that sound right?

Cheers,

Nigel

Unfortunately, it's not nearly as simple as that because the speed that the motor maxes out at has a big affect on current and efficiency. The max speed is more or less controlled by the choice of propeller. A critical characteristic for all that is the motor's kV. You therefore have kV, battery voltage, propeller size/pitch and motor winding resistance all affecting the current significantly.

Also, you have to bear in mind the efficiency because it's the difference between the power consumed and the power output that will burn the motor. In theory, if you had a motor with 100% efficiency, it wouldn't burn at any current, but it would break if you went too high. Basically efficiency accelerates downwards as the motor slows down, which is why you need to choose the right propeller to keep the speed up in the efficient zone. Increasing the voltage shifts the whole efficiency curve up along the RPM axis, which is why you need a smaller propeller for the same motor when you increase the voltage, so you can get the same current and efficiency at higher rpm, which means that you get more power because torque is proportional to current and power is torque times RPM.

It's difficult to explain in words because of all the different variables working together at the same time. Did you look at the simulator I linked above. It takes some figuring out because the graphs are back to front from what we need to get a clear picture of what's happening, but if you move the cursor up and down, you can see the results changing in the table at the top right. that should help with your understanding if you spend some time studying what happens when you change things.

Which gets back to my point.

The advice is x watts per pound. But how do you know what motor/ESC/battery will do x watts per pound before you buy if you need to connect it all up and try a range of propellers.

All the motor that I have quoted gives is that it can handle a maxium of 50A and can support 2S, 3S or 4S batteries.

Cheers,

Nigel

[Biggles' Elder Brother - Moderator]

The reality is Nigel that this is very complex stuff with a lot of interactions. To be honest most experienced electric power modellers can make an educated guess using theory at the sort of size prop they will need to extract the power they want from any particular set up - but its just that,...an educated guess. Theory can only take us so far here because in reality all the theories are approximations and simplifications of the true situation.

So yes we tend to have good stocks of props for models of around the size and power we like to fly, just like we have good stocks of particular batteries. So for me, for example, I have a lot of models that use 6s, 5000mAh batts, set up to give in the range of 1kW to about 1.5kW and I have a selection of props that tend to work in that range with that battery. In the end the Watt meter decides - even there I have take into consideration that in the air the current pull will be less than in a static test on the ground.

BEB

[PatMc]

Posted by PatMc on 01/09/2018 16:44:10:

Posted by Nigel Heather on 01/09/2018 15:05:45:

 

I get the electronics, but using the Quantum II 25 specs that I linked.

Max Current = 50A

Suggested ESC = 40A

LiPo = 2S to 4S = 7.4v, 11.1v or 14.8v

So I assume that max amps would be on 4S which would make the resistance = 3.37 ohms

That would mean

4S = 740W

3S = 415W

2S = 184W

 

Does that sound right?

Cheers,

Nigel

Your calculations are correct, except for the resistance.

Oops, I'm afraid I was guilty of mis-reading your post.
In fact most of your calculations are wrong because they are based on incorrect assumptions.

The max current is a fixed parameter regardless of voltage.

As previously said the motor doesn't run as a strict resistive circuit, in fact the windings resistance of this motor is around .031 ohms but explaining that at this stage is probably best left until later.

The max watts is calculated simply as applied Volts x max current.

If we take the max current to be 50A then :

2s = 370W

3s = 555W

4s = 740W

Note I'd still take the 50A figure with a pinch of salt, IMO 35A is more realistic, even a little optimistic as a continuous max current.  

The rest of my previous post stands.

Edited By PatMc on 01/09/2018 21:33:30

[Dave Hess]

Posted by Nigel Heather on 01/09/2018 21:11:14:

The advice is x watts per pound. But how do you know what motor/ESC/battery will do x watts per pound before you buy if you need to connect it all up and try a range of propellers.

All the motor that I have quoted gives is that it can handle a maxium of 50A and can support 2S, 3S or 4S batteries.

Cheers,

Nigel

That's an easy question to answer, but not to give the answer you want. The answer is that you can't because that's not enough information to figure out anything useful. You need the kV.

[PatMc]

Posted by Dave Hess on 01/09/2018 21:28:47:

Posted by Nigel Heather on 01/09/2018 21:11:14:

The advice is x watts per pound. But how do you know what motor/ESC/battery will do x watts per pound before you buy if you need to connect it all up and try a range of propellers.

All the motor that I have quoted gives is that it can handle a maxium of 50A and can support 2S, 3S or 4S batteries.

Cheers,

Nigel

That's an easy question to answer, but not to give the answer you want. The answer is that you can't because that's not enough information to figure out anything useful. You need the kV.

Dave, the KV is given as 1000.

[PatMc]

Posted by Nigel Heather on 01/09/2018 21:11:14:

All the motor that I have quoted gives is that it can handle a maxium of 50A and can support 2S, 3S or 4S batteries.

Cheers,

Nigel

Nigel, the specification 2-4s lipo is misleading. The only electrical limiting factor for the motor is the max current.
However the current is dictated by the input voltage & applied load (the prop), therefore depending on the model to be powered there's a practical minimum size prop to be considered. Also a suitable ESC, which is voltage as well as current limited, must be used.

[Nigel Heather]

Posted by Biggles' Elder Brother - Moderator on 01/09/2018 21:24:14:

The reality is Nigel that this is very complex stuff with a lot of interactions. To be honest most experienced electric power modellers can make an educated guess using theory at the sort of size prop they will need to extract the power they want from any particular set up - but its just that,...an educated guess. Theory can only take us so far here because in reality all the theories are approximations and simplifications of the true situation.

So yes we tend to have good stocks of props for models of around the size and power we like to fly, just like we have good stocks of particular batteries. So for me, for example, I have a lot of models that use 6s, 5000mAh batts, set up to give in the range of 1kW to about 1.5kW and I have a selection of props that tend to work in that range with that battery. In the end the Watt meter decides - even there I have take into consideration that in the air the current pull will be less than in a static test on the ground.

BEB

I get all the complications but still puzzled.

I see a plane I like, but not sure how much it weighs - the specifications include a weight but does not make it clear whether that includes battery, motor, ESC, servos and receiver.

And when I look at motors the specifications are often limited. For example, the one I linked just gives the maximum current - which I assume is the absolute maximum current it can draw before it breaks/burns up.

So how do I say, right I’ll have that plane, with that motor, that ESC and that prop. I get the fact that it is complicated, there are lots of factors and you need to experiement but I don’t think I will get far going into a store saying “I’ll take those three motors, that ESC, those two batteries and those five props. I’ll play with them at home to see which works the best and bring back what I don’t want”.

An example, say I am looking at the Ripmax Esay Street 2. It says use the Ripmax Quantum II 25 and a 60A ESC. It gives a weight but not clear whether that is the all up weight. Now say I don’t want to use (or can’t get hold of) the Quantum II 25, how do I determine what alternatives I could use.

Cheers,

Nigel

[PatMc]

Posted by Nigel Heather on 01/09/2018 22:02:16:

An example, say I am looking at the Ripmax Esay Street 2. It says use the Ripmax Quantum II 25 and a 60A ESC. It gives a weight but not clear whether that is the all up weight. Now say I don’t want to use (or can’t get hold of) the Quantum II 25, how do I determine what alternatives I could use.

Cheers,

Nigel

Any motor that's the same weight & kv will deliver similar power on the same prop range.

The Turnigy motor I linked to previously for instance.

[Nigel Heather]

Posted by PatMc on 01/09/2018 22:12:33:

Posted by Nigel Heather on 01/09/2018 22:02:16:

An example, say I am looking at the Ripmax Esay Street 2. It says use the Ripmax Quantum II 25 and a 60A ESC. It gives a weight but not clear whether that is the all up weight. Now say I don’t want to use (or can’t get hold of) the Quantum II 25, how do I determine what alternatives I could use.

Cheers,

Nigel

Any motor that's the same weight & kv will deliver similar power on the same prop range.

The Turnigy motor I linked to previously for instance.

Thanks for the info - sorry missed your previous link.

Cheers,

Nigel

[PatMc]

Actually there's another factor to be considered. How is the motor to be mounted in the model ?

It's most common in an aerobatic model that the motor is to be mounted on a the firewall type former using an "X" mount. In which case this version of the Turnigy 3542 is more suitable as there will be no shaft protruding backwards through the former.