how to find motor rpms - Page 2 - RC Groups
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Apr 17, 2009, 03:30 PM
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i put it in my drill press and i put it to the belt to the slowest speed it has a ton of torque do you think it will be to slow for a tug with a 3 inch prop
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Apr 17, 2009, 06:06 PM
Mmm, tugs...
patmat2350's Avatar
Too slow, yes.
A good resource is the Raboesch prop catalog at
Max speeds given therein are good typical limits for any scale prop of a given size... max for a 3" (75mm) prop is around 4700rpm. You may not want to run that fast normally, but 1400 rpm is definitely too slow.

What is this beast of a motor?
Apr 17, 2009, 07:22 PM
Registered User
I have found that a model airplane tachometer is priceless on finding the true RPM of any spinning object. I cut a piece of white adhesive tape and put it across the center of whatever is spinning. Piece of cake.

I have found that slow high torque motors need a OVERDRIVE setup.
The PTB below is running 2 :1 stepup drive ratio. with 5.5" pitch props. Gives a very smooth acceleration to about 20 mph.
The motor is a MinnKota 41# trolling motor driving timing belts. About 60 lbs of boat.

A look at the first picture, lower photo shows the deck & hull of the Artisana Amsterdam.
It is running a 1 to 1 timing belt drive to get the prop shaft horizontal to the big Pittman motor. A Octura #1470 plastic racing prop of 2.76" x 3.8" of pitch gives a realistic boil of water that is almost the height of the fantail. Bow wave is white water.
Torque can always be made into speed. Heavy, slow boats
High RPM's can not always be made into enough torque. In most cases.

Last edited by cyclops2; Apr 17, 2009 at 07:40 PM.
Apr 18, 2009, 08:08 AM
Registered User
Originally Posted by patmat2350
mfr- au contraire, my friend. this method will give you the kV value of a DC brushed motor quite nicely.

dnch- you're measuring voltage, set it to one of the volt scales... probably something like "0-5v".
I will take your au contraire and raise you a think again.
Rotating the motor at a known speed and reading the voltage generated, then repeating at a different known speed will give useful figures. However, doing this takes no account of losses when used as a motor. Applying 6 volts will not give the same rotation speed as seen when generating 6 volts. The motor, even when not carrying an external load, still has its own internal inefficiences to cope with - these are taken care of by the driving device when used as a generator. Of course, an efficient motor does not necessarily equate to being an efficient generator, and vice-versa.

Being cautious, I would set the voltmeter to a higher value, then wind down. The needle tends to go curly when the meter sees too high a voltage.
Apr 18, 2009, 08:09 AM
Registered User
pat,it's a auto fan motor
Apr 18, 2009, 09:45 AM
Mmm, tugs...
patmat2350's Avatar
dnch- no clue, good luck!

mfr- I'll see your think, and call!
Bob Boucher is the brains behind Astroflight, and has written a number of papers in the field as well being a true pioneer in electric aviation. I have his book on motors, see below (now out of print, I think).

If Bob says the method works to determine a kV value, then I'm looking no further.
Apr 23, 2009, 03:44 AM
Big Boats take Big Bucks!
1Ironhorse's Avatar
If you really want to know motor RPM, then use a data recording unit. Eagle tree has some for RC. They have a RPM sensor (magnet & Hall effect switch) that works really slick. That's the next thing on my "Gotta Have" list.

By the way, RMS stands for Root-Mean-Square, an AC (alternating current) measurement. It is .707 of the peak voltage or current or power, i.e. the avarage of 1/2 cycle of a sinusoid (avarage of a full cycle is 0, half of the sinusoidal wave is positive and the other half is negative). It is used to express the avarage power of an AC circuit.
Apr 23, 2009, 09:04 AM
Registered User
I reckon that real life trumps any theory.
Determining what a generators KV is, however accurately, can never predict what its actual performance as a motor will be.
The method given does not state the electrical load that the generator is subjected to - this affects the output voltage. When the motor is being run, what mechanical load is being imposed? This affects the running speed. Even with an airscrew, fit a larger diameter or different pitch, and the results will vary. The same applies with boats, but we get the added bonus of a more complex drive arrangement, usually a prop shaft with at least two bearings and a flexible joint, each applying different loads at different speeds.
All the above assumes a constant voltage supply such as at full speed (i.e. straight DC) - a lower speed from a PWM ESC can give much more variable and differing results depending on the characteristics of the motor, pulse repetition rate and the dwell angle at any given time.
You can use a motor as a sensor head to indicate RPM, but expecting an accurate prediction of performance as a motor is asking too much.
Last edited by mfr02; Apr 23, 2009 at 09:05 AM. Reason: spelling
Apr 23, 2009, 09:21 AM
Mmm, tugs...
patmat2350's Avatar
mfr, you miss my point... the method gives a KV value which tells us what the FREE RUNNING (i.e., NO LOAD) speed is, and it does that quite well.

Nowhere did I address the issues of:
a) how fast will the motor spin with a load, and
b) how a powerful a motor is needed to drive a given prop at a given speed.

But elsewhere, I have mentioned that any load will slow down the motor, causing it to pull more current; and that with an appropriate match (how ever you get there), you want to avoid such a big load (i.e., big prop) that the motor is slowed to less than about 75% of the free running speed, else we have risk of overheating and may need to watercool everything (an abomination imho, except in b@lls-to-the-wall racing).

But, when dealing with "found" motors, knowing the no load speed is a good start for determining whether the motor is appropriate for a given application, especially when coupled with the use of recommended prop speed charts such as found at

Cheers, PM
Apr 23, 2009, 12:52 PM
Registered User
Fair enough - there is nothing harder to let go of than the wrong end of the stick once it is firmly grasped.
May 12, 2009, 05:17 PM
Got shenpa?
flieslikeabeagle's Avatar
Let's see if some lubricant can make that stick a bit more slippery, making it easier to let go of any firmly grasped ends.

mfr02 - you are right that Kv alone cannot tell you what to expect from a motor, for many of the reasons you mention. That is why a minimum of three different motor constants are needed to model the motor mathematically, and predict actual performance under load. The other two constants are Rm (winding resistance) and Io (no-load current). More sophisticated models add more constants to give a little more accuracy.

Patmat is right too - the method he outlines is one of the ways to measure a motors Kv. And I agree with him that knowing the no-load rpm is quite useful by itself, especially when you factor in that a good rule of thumb is to load a motor till the rpm drops by 20% - 25% from the no-load rpm. This much load will push the motor to the point on its operating curve where it is running at roughly 70% - 80% efficiency, which is a good starting point.

I know absolutely nothing about scale boats, but I do know a little about electric motors.


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