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True kv

Old 09-18-2014, 03:12 PM
  #1  
ChuckD
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Default True kv

What do I need to be able to determine True kv rating of a motor. I have read that some guys post some motors don't have the advertised kv?
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Old 09-18-2014, 05:49 PM
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fhhuber
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kV is theoretical. You can expect 20% to 30% less under reasonable load.

kV is how fast the motor would turn if frictionless with no load, calculated based on the winding pattern and number of magnets.

You should be able to mount a popsicle stick as if it was a prop (very low load... a short piece will be better) and use a wattmeter/voltmeter and a tach to check you kV. This works best at the lowest voltage your ESC can use, because some loads go up by the square and others by the cube of rpm.
You'll still get about 5% less than advertised.
If you try to compare static with a prop appropriate to the motor and tach you won't ever get close to rpm = voltage * kV

You can play with it .... check assorted loads on a motor. As the load changes the rpm/volt will change and the watt draw will change. If you have one of the logging ESCs that records rpm, then you can also try no load other than the motor's bearings.

Note that some motors don't cool well with no prop on to aid in blowing air over them.... Even with no load if there's inadequate cooling you can overheat it.
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Old 09-18-2014, 06:48 PM
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Wildflyer
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For outrunners, I have checked no load rpm, by using 2 strips of white tape, or narrow cut off strips of postit notes, stuck to the outside housing of the motor. It's less load than a flat stick, and very easy to do.

For inrunners, I used a circle of stiff paper, with a dark stripe marked across it with a felt pen. Judging by how little air was moving, it also seemed like very little load.

With a wattmeter and an optical tach. note the RPM and the voltage applied at that time. Then divide RPM by Volts.

My tests came out reasonably close to the advertized KV, at least close enough to identify a motor. I buy used outrunners (that do not smell burned!) when I can, so I had to figure out what I had.
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Old 09-19-2014, 01:12 AM
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ron_van_sommeren
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Originally Posted by ChuckD View Post
... Kv rating of a motor ...
  • The Kv constant is not a rating, not a figure of merit, it's just a characteristic. More windings will give lower Kv, less windings will give higher Kv, that's all there is to it. No big deal, anyone can do that.
  • A motor's Kv constant says nothing about max.power, max.current, efficiency, rpm, quality etc.
  • Motors have just one Kv, not e.g. 1450Kv. It is physical quantity (length, weight, time, current, ..,.), not a physical unit (meter, kg, s, ampère, ... ).
    Therefore: Kv = 1450rpm/volt.
  • Motor/battery-current wants to go up with voltage squared and with Kv cubed.

Originally Posted by fhhuber View Post
Kv is theoretical. You can expect 20% to 30% less under reasonable load. ...
Kv does not drop under load Fred, after all, it's a motor property. It's the rpm that drops under load due to voltage losses in the battery, wires, ESC and motorcoils.

Originally Posted by ChuckD View Post
... What do I need to be able to determine true Kv rating of a motor ...
Determining Kv, several methods. But which one is used by the manufacturer? This may already account for the reported differences. Same goes for ESC timing. The most consistent test is the generatortest. (In fact, in the non-RC world 1/Kv is known as generator-constant)

Last edited by ron_van_sommeren; 09-19-2014 at 01:33 AM.
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Old 09-19-2014, 01:39 AM
  #5  
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Sometimes you criticize a fact not stated perfectly in your engineering languange...

I said Kv is theoretical (the theoretical rpm per volt if there is no friction or load on the motor) in actual practice you should see less rpm per volt under load.

This is fact.

If you have an ESC which will deliver 1 volt and you don't load the motor at all... its still going to turn a little less because there's friction in the bearings.

I get tired of you "correcting" fact.
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Old 09-19-2014, 01:55 AM
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ron_van_sommeren
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Originally Posted by fhhuber View Post
kV is theoretical. You can expect 20% to 30% less under reasonable load. ...
Sorry Fred, I did not mean do belittle you (or other people in the past), but how would someone less motor-knowledgable then you and I interpret this quote? I don't want TS to get the wrong idea, that's all.

That's why I referred to the generator test, electrical and mechanical resistance have no influence on the measurement.

Last edited by ron_van_sommeren; 09-19-2014 at 06:41 PM. Reason: + this quote
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Old 09-19-2014, 05:39 AM
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ChuckD
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Thanks for the info guys. From my own experience I have had varying performance from motors with similar specs. Also have read posts relating the advertised kv to the actual kv. When considering a motor for specific type of flying, knowing the kv has been helpful. Kinda new to the e-stuff and wanted to be able to verify these numbers on my own.
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Old 09-19-2014, 06:11 AM
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kyleservicetech
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Originally Posted by ChuckD View Post
Thanks for the info guys. From my own experience I have had varying performance from motors with similar specs. Also have read posts relating the advertised kv to the actual kv. When considering a motor for specific type of flying, knowing the kv has been helpful. Kinda new to the e-stuff and wanted to be able to verify these numbers on my own.
Yeah
Those KV numbers and associated maximum current/max voltage/max watts values specified by the mfg sometimes are not even close to the real world. Makes me think that those numbers are provided by the Sales Department, not the Engineering department.

One useful value that give some indication of the ability of the motor to provide power is its "Watts per Ounce of Motor" value. This isn't a standard specification by motor supplier. But a reasonable value is about 100 Watts per ounce of motor weight. As an example, a motor weighing in at 6.8 ounces should be able to handle 680 Watts input. And, as usual, higher quality $$$$ motors may be able to handle a bit more, and el-cheapo motors a little less.

A real red flag is a motor claiming to handle 200 Watts per ounce. Unless its a very high RPM ducted fan motor anyhow.

One useful program for calculating this stuff is www.motocalc.com, free for $30 days, then $39. Just input or select your motor, put in the battery and prop size. Motocalc has an "opinions" feature that will let you know if you're pushing the motor beyond safe limits. But again, motocalc is only as accurate as the specs provided by the mfg.

There are also a number of other similar programs, some of which are free.

If you have a "No name" motor, programs such as Motocalc require the motor's KV number, no load amperes, winding resistance, and motor weight. The KV, no load amps, and motor weight can be measured with fairly inexpensive metering equipment. Measuring the motors resistance does take a bit more work. Take a look:
Measuring Brushless Motor Resistance
http://www.wattflyer.com/forums/showthread.php?t=50740

Hope this helps.
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Old 09-19-2014, 06:20 AM
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Originally Posted by Wildflyer View Post
For outrunners, I have checked no load rpm, by using 2 strips of white tape, or narrow cut off strips of postit notes, stuck to the outside housing of the motor. It's less load than a flat stick, and very easy to do.
Yup
I've been using that white tape for years along with a photo RPM tach. Much safer and easier to do than sticking some sort of dummy prop on the motor shaft.

If your tach doesn't respond, just shine a flashlight on it. If you do this inside, just make certain that all indoor lights, especially florescent types are turned off, or your tach will read 3600 RPM.
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Old 09-19-2014, 06:32 AM
  #10  
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The white tape and watt meter tip is the one bit of info that will stick in my head. All the technical stuff is fun to read too.
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Old 09-19-2014, 09:32 AM
  #11  
Larry3215
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If you do get around to testing motors, one thing you might notice is that even two "identical" motors can often show significant differences in kV.

This is something I found back when I was doing lots of motor testing. Its not all that unusual to find that even name brand motors vary significantly from one to another - 5% - 10% variations were not unusual.

The cheaper you go , the larger the variance can run into. Ive seen up to 50% or higher differences in cheap motors with the exact same model number, same winds, same everthing that were purchased at the same time from the same dealer.

In some cases it was obvious they were not wound the same way and in others it was hard to tell what it was. I'm assuming poor quality control and sloppy manufacturing techniques. It could also just be they slap any old label on a motor and sell it as what ever they want.
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Old 09-19-2014, 11:53 PM
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ron_van_sommeren
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Originally Posted by kyleservicetech View Post
... There are also a number of other similar programs, some of which are free. ...
Sticky: e-flight calculators (compilation)

Notably Drive Calculator, Scorpion Calc and derivatives, E-Calc. E-Calc is not freeware.

FAQ eCalc: Q&A for propCalc, fanCalc, heliCalc, xcopterCalc
FAQ Scorpion Calculator (SC) (not only for Scorpion motors)
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Old 09-20-2014, 02:08 AM
  #13  
ChuckD
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Larry, strange you should mention this. Had a rough landing and bent the shaft on my ST10. Replaced it with a new ST10 that was a few months older. No other changes. It brought the plane alive.
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Old 09-20-2014, 03:49 AM
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Originally Posted by ChuckD View Post
Larry, strange you should mention this. Had a rough landing and bent the shaft on my ST10. Replaced it with a new ST10 that was a few months older. No other changes. It brought the plane alive.
Usually, the $$$$ motors are fairly close in performance. My eight Hacker motors, ranging from an A30 to two A60 series motors are pretty close to the specifications provided by the Mfg. Same with my one Hyperion motor rated about 1KW.

My two A50-12S motors were purchased two years apart, and turn the same prop at the same identical RPM. Now, Hacker went and updated them
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