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-   -   Motor specs, what do they mean? (http://www.rcuniverse.com/forum/brushed-brushless-motors-speed-controls-gear-drives-123/1269595-motor-specs-what-do-they-mean.html)

basmntdweller 11-11-2003 05:12 PM

Motor specs, what do they mean?
I'm assuming Kv is rpm per volt with no load
What is Rm and Io?
With those three known is the watt capacity calculatable?
If I have a motor rated 2500 Kv, .75 Io and .12 Rm, can it handle 1000 watts or do you need more info?
Just taking a guess, Rm is measured resistance of the motor and Io is the amp draw per volt no load. If there is a formula to figure max watts a motor can handle, let me know. Most of the power requirements are given in watts/pound yet nearly no motors give watt handling capability. I'm trying to learn how to better select a motor/battery combo to fit a given airframe.

Matt Kirsch 11-12-2003 08:32 AM

RE: Motor specs, what do they mean?
I'm pretty sure you're correct in your guesses at what Io and Rm mean, especially if Io is measured in Amps, and Rm is measured in Ohms.

Unfortunately, I'm pretty sure that isn't enough info to do the calculations. Don't take my word for it though... I'm sometimes wrong.

Most motors on the market give you a maximum cell count and a maximum current that they can handle. THIS is what you need to calculate maximum Watts. Basically, it's <# of cells> * <1.2 Volts> * <max Amps> = max Watts.

The calculations I've seen and done, however, boil it down differently. You start with your Watts needed to fly the plane, then you assume a reasonable peak current for the plane. I like 40 Amps for glow conversions, but some people like 25, 30, 35, 50, 60 or higher. The lower the Amps value you choose, the more cells your plane needs to carry. You take the Watts, divide by the Amps, and get Volts. Divide Volts by 1.0 Volts per cell to simulate the cell's performance under load, and you have your cell count. With the cell count and peak current, you can go pick a motor from a catalog that can handle it.

Let's say you have a plane that needs 400 Watts. I'm picking 40 Amps as my peak current. Doing the math, this means I need a 10-cell pack. Now I go to a catalog and look for a motor that's rated to handle 40 Amps and 10 cells.

I actually picked this up at a symposium hosted by Matthew Orme, the designer of the famous (and hard to find) Razor motors, a couple of years ago at the WRAM show in White Plains, NY.

basmntdweller 11-12-2003 07:07 PM

RE: Motor specs, what do they mean?
I knew given amp and volt limitations I could figure max watts. Looking through several of my catalogs though very few list max amp draw. Most will give cell count info but that's about it. For example, I looked on the Hacker site and saw only the specs I mentioned in my first post. If I was looking for a Hacker motor that could handle 35 volts and 30 amps, there is nothing on their site that tells me that. That is why I asked the question in the first place. I thought maybe there was a way to calculate it given those specs. Kv and max amps or volts would have more value to me as a consumer than Io or Rm.

Steve Neu 11-12-2003 09:45 PM

RE: Motor specs, what do they mean?
I doubt you will get anywhere near 1000 watts with that motor--the resistance is way too high. A motor with that high of resistance likely should be kept under 20 amps. Taking that you would need well in excess of 50 volts to get to 1000 watts. RPM would be over 125,000. Spin again and try a different motor:)


basmntdweller 11-12-2003 10:15 PM

RE: Motor specs, what do they mean?
You are on the right track for the info I want. How do I tell what a motor can handle from those specs or how do I decide what the specs need to be for a specific application. As an example, lets say I want to electrify my Raptor 50 heli. In order to make it a simple conversion, I would want a motor with a similar operating rpm to an OS50. Say 17500rpm. I figure I want to run 24 cells. That would be a loaded voltage of about 26volts. That would give a Kv of .673. To get a decent performance level, I have been told I need about 150 watts/lb. I'm guessing AUL of 7lb so I need 1050watts. 1050watts/26 volts=about 40 amps. These are the figures I think I know. Now, how do I figure what the Io and Rm need to be to select a appropriate motor? Or do I need more info than specs given. I realize that based on Kv alone Hacker doesn't list anything suitable even if one could handle the watts.
I have thought about this conversion previously and that is why I used it as an example.
I really just want to get a good grasp of the specs and parameters for selecting a proper motor.

rmh 11-13-2003 09:16 AM

RE: Motor specs, what do they mean?
You are not among the "truly lost" -but even a calculated setup can miss the mark -
I remember very well all of the calculated setups years back -for going fastest in the 1/4 mile.
We read, then ignored the calculations and ran the fastest.
No tricks - just do good scientific tests .--actual use beats calcs every time .
I built a little Dyno for my tiny electrics -
You could easily do one for any size electric motor.
Aside from a Astro Whatt meter --all you need is a pivoted, L shaped arm - with a mount for the motor on top and the lower leg setup to rest on a ounce scale.
Simple geometry shows that if each leg is same length - then any forward thrust will read in the same force on the scale.
Make sure the Dyno is solidly mounted -and can't flip over backward -if the motor runs backward - (I had this one happen!)
When running the setup with your Tx - stand behind the motor and place the scale facing you and the Whatt meter along side.
In testing various motor batt /prop combos - it is instantly apparant that many combos will all produce the same thrust -and the total watts are similar.
the best performance setups for your own tastes are easily read -to be recorded.
I find that more voltage and lesser amperage setups, usually work best -for me- also the prop combo for best thrust -thru a wide range of power, is easily seen.
I frankly would not be without it.

azfrench 11-13-2003 10:00 AM

RE: Motor specs, what do they mean?
Dick, I would love to see a pic of your little dyno if you have one. Maybe a simple diagram would do the trick.


basmntdweller 11-13-2003 06:37 PM

RE: Motor specs, what do they mean?
Dick, I agree the testing is the best way to go. The down side is you have to have the motor and batteries to test! When you are scraping your pennies together to buy just one setup, You want to be sure you get the best bang for your buck! The test stand may be a good weekend project. I had just ordered the Astro super whatt meter last night. Hopefully it will be in house tomorrow or saturday.

mglavin 11-14-2003 03:10 AM

RE: Motor specs, what do they mean?
I have read some books and garnered some info on motor specific math. Hope this helps more than it confuses.

Speed Constant "Kv" is defined as the voltage constant and is expressed in units of RPM/VOLT.

Ideal no load motor speed is equal to the product of the motor voltage multiplied by the voltage constant (Kv) or RPM = Kv x Volts.

RPM is constant regardless of load if the voltage remains constant. Unfortunately this is the crux of our problems as the voltage falls off under load and in real time. Additionally there is another factor, otherwise known as the electrical resistance or Thermal Resistance "Rm" (Ohms) of the motor windings. When current moves through the motor, there are voltage losses due in part to the electrical resistance of the motors windings. In the real world when a load is applied, the motor will consume current, the voltage on the circuit will drop and the motor will loose RPM.

Copper Loss:
Net Voltage or Effective Voltage = "Vm".
Vm = Vin - Im x Rm
RPM = Kv x Vm, RPM = Kv x (V - Im x Rm)

Torque Constant "Kt" is defined as the torque constant and is usually expressed with inch-ounces per amp for our small motors. For all motors, the voltage constant multiplied by the torque constant is 1355. Kv x Kt = 1355. Kt = 1355/ Kv (in-oz/amp)

Torque = Kt x Amps, BUT there are variables, such as NET average current (amps) the motor actually realizes.

To calculate motor performance one can assume that there is a leakage current or loss current shunting the ideal motor by an amount equal to the measured no load current or Io. The net effective current and the torque it produces are decreased to the values

Inet = Iin –Io
Torque = Kt x inet

Iin is known as input current (amps).
Io is known as the loss current (amps), or no load current.
Net average current, (Inet). Inet = Iin - Io.

Real world examples for TORQUE and RPM would factor losses associated with Iron Loss. There are several contributing factors here.

Stall current is defined by the locking the motor shaft and allowing the current to increase to high levels. This is not generally a good thing and can damage a motor in short order. When the motor is stalled no Mechanical Power is delivered to the load, thus the electrical power (Watts) is converted to Heat.

Stall Current, Istall = Vin / Rm
Heat @ Stall = Vin x Istall = Vin / Rm

Brushed motors are typically 70% efficient.
Hi performance brushed Cobalt motors are typically 80% efficient.
Brushless motors are typically 85% efficient.

Electrical Power in WATTS, Watts = Volts x Amps
Mechanical Power out WATTS, Watts = Speed x Torque
One Horsepower = 746 Watts
One Kilowatt (KW) = 1.34 Horsepower

Volts and Amps required for 1 KW:
1.25V @ 800A
6.0V @ 167A
12V @ 83A
25V @ 40A

Power Out (Watts), Pout = (Iin - Io) x (Vin - Iin x Rm)

Eff = (In - Io) x (Vin - Iin x Rm) / Vin x In

Highest efficiency is achieved when the Iron Loss equals the Copper Loss, this occurs at a load current of: I maxeff = SQRT (Io - Istall),

Efficiency (Eff) is defined as the ratio of Pout to Pin. Efficiency is zero at no load and stall. Max power is achieved when RPM is reduced to 50% of free no load speed and input current is half of the stall current spec... Motors operating in the realm are subject to overheating and damage in a very short time.

A typical .50 size glow engine develops around 1.5hp I believe. This equates to a power output (Pout) need of 1119 Watts or thereabouts… So if you need an 1119 Watts, motor efficiency is 85%; 1119 x 115% = 1286Watts input needed.

Check on Aveox’s website there’s a great deal of info and many motors listed.


rmh 11-14-2003 10:13 AM

RE: Motor specs, what do they mean?
1 Attachment(s)
So Brother Glavin - you see why I whipped up the little Dyno - all of the calculations are about as good as calculating how long it will take to get a girl "in the mood". If one does the dyno correctly --all of the equipment from the model simply lifts off and quickly sets up on the dyno

Steve Neu 11-14-2003 11:43 AM

RE: Motor specs, what do they mean?
Unless I am missing something what you have is a thrust measuring stand. A dyno measures power into the motor and torque and RPM out--from that you can get power out.


Steve Neu 11-14-2003 11:52 AM

RE: Motor specs, what do they mean?
The Aveox web site has some good info on motors in the power range you are looking for"http://aveox.com/hobby.htm" You want a motor in the 700-800RPM/volt range with a resistance in the 40-50 milliohm range. a Aveox 3638/3 or a 4630/3 would be close. Also Kontronik and Hacker have suitable motors too. For what you want stick with the larger frame motors as heat is a problem in helicopters.


mglavin 11-14-2003 12:04 PM

RE: Motor specs, what do they mean?

I here you. But what are the helo guys gonna do? The math will help them get close. I suppose the helo becomes the dyno and pinion changes are the key to dialing in a specifc machine.

I like your EXTREME dyno setup. I built one as well. Similar to yours, simply invaluable, IMO... What I like best is the basic math required to figure things out! :D

rmh 11-14-2003 02:44 PM

RE: Motor specs, what do they mean?
Call the little "thrust measuring stand" anyting you like -
Calculations are only that -- calculations
They never have and never will replace actual data from operation.
I do know what a dyno is - we used em many years ago for racing engines .
On aircraft - the power plant has only one criteria
---delivered thrust from the prop .
calculated thrust is simply another calulation.
When full scale aircraft engine is rebuilt - you place a calibrated prop on it and read actual rpm - this tells you if the engine is to spec.
Extrapolating "power out" from calculations gives more data --on paper .
what it doesn't tell --is if all the units input into the calc - actually work well together.
Fair enough?

Steve Neu 11-14-2003 05:43 PM

RE: Motor specs, what do they mean?
Static thrust can be increased by simply using a larger diameter lower pitch prop. As a result it is not very useful in helping to derermine airplane or motor perfromance.

If you really want to know what is going on with a motor you have to measure the inputs (volts and amps) and outputs (rpm and torque). Thrust does not come into the discussion.

BTW your Whatt meter calculates watts based on volts and amps--do you trust it? A dyno does the same thing with RPM and torque.

Steve--who has a "real dyno"

rmh 11-14-2003 07:24 PM

RE: Motor specs, what do they mean?
Do I trust the Astro Whattmeter?
What should I trust ? Some Calc chart?
If there is a faulty device somewhere in the setup - does the calc catch it?
A water brake or an electric motor load and readout is fine - if one wants to read how much a motor will produce under a given load --but for the hobbiest - who wants a low cost direct look at what he is getting - itis not a handy setup .
I gather you are Poo pooing this type setup N/Y?
Here is a test I just ran --
Hackers B20S - in both 22 and 26 turn winds
same gearing (D GWS gearing)
Same prop (same prop adapter -
motors switched back and forth on the stand
Same battery pack - Kokam 1200 3 cell HD
Thrust measured - full throttle and held for 5 sec each time
the 22 wind pulls aprox 2- 3 more amps -even as voltage droops
the actual thrust - (not extrapolated figure)
is 3 ounces difference.
from aprox 20 ozs --to aprox 23 ozs.
So I know that the 22 wind on THAT prop is just bout maxed out -if the motor rating is 9 amps and the battery is rated close to the same.

I am just trying to achieve a real time look at thrust - which is the real expression of what the motor is for in the 3D setups..
Do you read thrust?-or calculate it?
Static thrust as a COMPARATIVE -is relevant.
Changing props to change thrust is -of course of little value ----unless we know how much electrical energy need is involved.
For my aerobatic setup - I will reduce static thrust a bit and use a prop more efficient at flying speed. less power consumed
If I was doing a race plane - the static thrust would not be of much value.
many other factors in a speed setup- airframe setup is another game.
But why do you not see value in a thrust comparison - when very low speed performance is the issue?

mglavin 11-14-2003 10:44 PM

RE: Motor specs, what do they mean?

Maybe I did not make it clear enough in regard to my comment "BASIC MATH" requirements... I have to agree that real world test data is the most valuabe. There are to many variables with calculations, WAY to many!!!

Let me give you an example of MY basic math scenario:

9A - 11A = 3A difference or maybe 20 ozs verse 23 ozs = 3oz difference... Lets see this one has 22 winds and that one has 26 winds, 22-26=4. I can leave the calculator on the desk. BUT I do have to take notes...

rmh 11-14-2003 11:07 PM

RE: Motor specs, what do they mean?
My training from the University of Hard Knocks --drove home the idea that if one can not explain something to a
jury of 12 "peers" (?)--then one truly does not understand the problem.
Therefor, I avoid the use formulas when trying to explain or make a point.
I also tend to get a bit sharp with others who see the avoidance of the use of formulae, as a lack of understanding.
So I guess I am just an ol curmudgen--

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