how to compare electric motors - RC Groups
Oct 21, 2011, 10:07 PM
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# how to compare electric motors

I am used to nitro motor sizes which was fairly easy to learn but all the numbers with brushless motors are beating me up. such as the kv how many poles the ##-##-#### stats and some motor pages dont even mention prop sizes, So what is the easiest way to compare them? for example
http://www.hobbypartz.com/96m601-big...20-1100kv.html
to
http://www.horizonhobby.com/Products...32A#quickSpecs
 Oct 21, 2011, 11:02 PM Registered User The part numbering schemes vary from one manufacturer to another and have meaning only within one brand or one series of motor. Usually they refer to motor diameter/length or number of turns in the windings. The essential specs are: 1. max current and/or watts 2. RPM per volt, otherwise known as Kv Second-order specs include motor case diameter, shaft diameter, etc. You'll find a very close correlation between item 1 (max current/max power) and the motor's weight. Most motors come in at around 3 watts per gram. Some of the brands (eg. E-Flite, OS, etc.) have denoted their motors as "25" size or "40" size or "15" size, suggesting equivalents to nitro engines. And then there's the other scheme of denoting motors as "250" size, "400" size, "480" size, which harks back to case sizes for brushed electric motors from a few years ago (pre-dating brushless motors.)
 Oct 21, 2011, 11:24 PM Registered User So the power32 has a max current of 60A the tacon has 51A the kv is 1100 for tacon and 770kv for power32. Which would be better? Which has more effect? Kv or current?
 Oct 21, 2011, 11:43 PM Registered User Power is the amount of work a motor can do. The more power you have, the more work you can do. Thus, you can fly a larger airplane with more power. Now for some more complicated information. Power = Voltage x Current A watt is a unit of power. It is defined as 1 joule per second. You can think of watts as the horsepower rating of the electric world(1 HP = 746 watts). Most motors should have maximum input watts listed, if not you can multiply the listed max amps and the max voltage. The Tacon Big Foot 10 is listed at 375 watts. The Power 32 is listed at 800 watts.
Oct 22, 2011, 12:13 AM
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Quote:
 Originally Posted by Eclipse_7 Power is the amount of work a motor can do. The more power you have, the more work you can do. Thus, you can fly a larger airplane with more power. Now for some more complicated information. Power = Voltage x Current A watt is a unit of power. It is defined as 1 joule per second. You can think of watts as the horsepower rating of the electric world(1 HP = 746 watts). Most motors should have maximum input watts listed, if not you can multiply the listed max amps and the max voltage. The Tacon Big Foot 10 is listed at 375 watts. The Power 32 is listed at 800 watts.
Then what does kv have to do with anything?
 Oct 22, 2011, 12:37 AM Registered User KV is RPM per Volt. Its useful when comparing motors of similar size, which the two you linked are not. A high KV is good for a fast prop speed, gear box set up, or when using a low voltage battery. Low KV motors will swing larger props at slower speeds, but you can generally get more static thrust out of them. There are several variables at play. What kind of performance do you want, what prop, what battery. Once again, to easily compare KV you need similar motors. The Tacon Big Foot 10 and the Power 32 are apples and oranges. It may be more useful to provide the situation where the motors will be used. Then we can give more specific comparisons. Edit: to be clear, power (watts) has the most direct affect on how your plane will fly Last edited by Eclipse_7; Oct 22, 2011 at 12:46 AM.
 Oct 22, 2011, 01:29 AM Registered User I've got a question: I purchased this motor: http://www.hobbyking.com/hobbyking/s...dProduct=18157 Specs. Turns: 8T Voltage: 2~3S Lipoly RPM/V: 1500kv Internal resistance: 0.026 Ohm Max Loading: 31A Max Power: 370W Shaft Dia: 3.175mm Weight: 80g It says 2-3S, and at 3S I measured 30amps with a 7.3 inch prop, and it spiked up to 70A with an 11.3 prop. What can I expect if I ran it with a 2S? Will I be able to use an 11.3 prop at lower kv if I use a 2S and stay near or under 30A? I'd like to be able to use this for a foamy 3D plane.
 Oct 22, 2011, 02:21 AM Registered User You can use any of the many electric motor calculators to give you a decent guess at what you can expect for specific set ups. There are also prop test charts out there as well. They can give you a good idea about real world setups. All that information can be found in the power systems forums. Dropping down to a 2S battery will let you swing a larger prop, but you would be hard pressed to match the power you would get from the 3S swinging a smaller prop. For a 3D plane, I would stick with the 3S. I literally have a box with about 40 different prop brands and sizes. I'll use prop charts or calculator programs to get me close, then do my own testing to find the right prop. Airplanes will fly on about 50 watts/lb Sport flying is around 120 watts/lb Hovering is about 150-160 watts/lb Most recommended 3D set ups will be around 200 watts/lb
 Oct 22, 2011, 06:06 AM Registered User Those two motors are not comparable in any way. The Tacon is the rough equivalent of a .10 size glow engine and the E-Flite 32 is equivalent to a .32. KV is a measure of maximum motor RPM (unloaded). Multiply the KV by the voltage to get RPM. The best way to do this is to start with the model you want to power and the flying style you want for it. Bring that to the forum and you can get some more useful recommendations.
 Oct 22, 2011, 08:41 AM Registered User I will be powering a pulse 25xt by eflite and would like to have plenty of power for hovering and pulling out of them, I will also be attaching the floats on occasion.
Oct 22, 2011, 09:09 AM
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Quote:
 Originally Posted by south.tx.dead.I I will be powering a pulse 25xt by eflite and would like to have plenty of power for hovering and pulling out of them, I will also be attaching the floats on occasion.
Given the naming of the plane, a "25" size motor should be adequate and a "32" size motor more than adequate.

But that's a bit sloppy. More commonly we go by watts per pound of flying weight (aka all-up weight.) For a medium-size sport plane like the Pulse 25, 100 watts/pound is "adequate" and 125-150 watts/pound starts to get exciting.

The Pulse 25 is around 4 lbs. AUW so you need... let's call it 500-600 watts.

The first E-Flite Power 25 motor I found via google says it weighs 190 grams... by the three-watt-per-gram rule, that would be 190*3 watts = 570 watts.

Going by the motor's specs (for the 870 Kv version): Max cells given as 3-4S, max current 32 amps. 4 Lipo cells is 4*3.7 volts 14.8 volts. 14.8 volts times 32 amps is 473 watts.

For the Power 32 motor: 42 Amps max, 3-5 Lipo cells. That works out to 3.7*5*42 = 777 watts. Personally, I don't believe that with 10 extra grams of mass, the Power 32 gives 200 more watts of output... but all these numbers are fungible.

Over time you'll get the hang of it. Vendors like E-FLite will almost always recommend a specific motor or a range of specs for the motor of any ARF they sell.
Last edited by rafe_b; Oct 22, 2011 at 10:10 AM. Reason: 3 watts per gram, no per pound. oops.
Oct 22, 2011, 09:10 AM
**** 37130 ****
Quote:
 Originally Posted by south.tx.dead.I I will be powering a pulse 25xt by eflite and would like to have plenty of power for hovering and pulling out of them
That is not what the Pulse is all about. It can be made to hover but you will not have the control throws of a 3D machine. Reasonably fast and smooth aerobatics are the Pulse's forté.
Oct 22, 2011, 09:26 AM
**** 37130 ****
Quote:
 Originally Posted by rafe_b For a medium-size sport plane like the Pulse 25, 100 watts/pound is "adequate" and 125-150 watts/pound starts to get exciting.
And 200W/lb is fun.

Quote:
 Originally Posted by rafe_b The first E-Flite Power 25 motor I found via google says it weighs 190 grams... by the three-watt-per-pound (gram) rule, that would be 190*3 watts = 570 watts.
The design of the Pulse's cowl means that good cooling flow is achievable, just so long as you open up a large air outlet behind the wing. In this instance, 4W/g is not excessive.
Oct 22, 2011, 11:11 AM
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Quote:
 Originally Posted by RobC(UK) And 200W/lb is fun.
Yep, on the right plane. And I've flown one or two that float nicely at 20-25 watts per pound.

Quote:
 The design of the Pulse's cowl means that good cooling flow is achievable, just so long as you open up a large air outlet behind the wing. In this instance, 4W/g is not excessive.
Yep, premium motors like Hyperion and Scorpion often come in closer to 4 W/g.

All the numbers are fungible.
Oct 22, 2011, 02:22 PM
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Quote:
 Originally Posted by RobC(UK) That is not what the Pulse is all about. It can be made to hover but you will not have the control throws of a 3D machine. Reasonably fast and smooth aerobatics are the Pulse's forté.
I just meant to use that as an example for power