PDA

View Full Version : Missing something here?


hankg
04-01-2010, 10:59 PM
A friend and I were running up a motor and checking voltage, amps, and watts on my watt meter. We put a tachometer on the prop and found the relation between volts x rpm had no relationship to the Kv of the motor.

I was under the impression that input volts x Kv = RPM.

What am I missing?

Regards,

Hankg

crxmanpat
04-01-2010, 11:22 PM
Hank,

How far off is it? Most motor makers give a general Kv. Oftentimes it's found that the actual Kv is off a little, sometimes a lot.

I think it also has to do with the power of the lipo as well. I know that when I was running a 2212-6 2200kv on a 15C lipo, performance was OK. When I put a same mAh pack but 25C on it, I could definitely see an increase in RPM.

Twmaster
04-01-2010, 11:32 PM
Those kv number are no-load numbers.

mesh
04-01-2010, 11:52 PM
When I put a same mAh pack but 25C on it, I could definitely see an increase in RPM.

The higher C rating likely meant that the battery voltage did not sag as much under load, so the motor saw a higher voltage, and thus ran faster.

And yes, as Twmaster says, the specified kv is always a no-load number. When you put a propeller on it, you will see a lower rpm.

There should still be a relation though, a 2000 kv motor will turn an 8x4 propeller faster than a 1500 kv motor will, provided neither is overloaded by the propeller, and the power system can deliver the amps needed.
The 2000 kv motor will of course draw more amps than the 1500 kv one, with the same prop.

hankg
04-02-2010, 12:04 AM
I didn't realize the Kv is a no-load number. If I remember, the Kv is 1300 (Blue Wonder). I was using an 11.2 volt battery, .360 milliamps. I ran the RPM to around 400 with the voltage dropping to 10 volts from 11.4volts, 8x3 prop. I can't remember the wattage and amp draw because we were trying to figure out the inconsistency with 10 volts, 400 +/- RPM and Kv.

Thanks for the input,

Regards,

Hankg

Larry3215
04-02-2010, 01:18 AM
RPM was 400 at 10 volts?

What are you using to check rpm? Those numbers are not anywhere in the ball park.

Most tachs require you to select the correct blade count for the prop - 2 blade or 3 blade etc. The readout is also usually in either tens, hundreds or thousands of rpm.

So if the display says 400, then its probably actually reading 4000 rpm - but that isnt even close to where you should be.

Im wondering if you have the wrong blade count selected?

If you are using a direct phase reading tach - like the Eagle Tree - then you must enter the correct number of magnet poles for that motor into the Eagle Tree software.

Larry3215
04-02-2010, 01:21 AM
Oh wait - I think I know whats going on.

You can't tach your motor to check kV unless your running the esc at full throttle.


Other wise the numbers will never match. The esc lowers the average voltage going to the motor by switching the power on/off very rapidly at anything less that full throttle.

hankg
04-02-2010, 01:41 AM
Larry3215,

Thanks for the input. My friend and I are flyers. We're just getting into the more sophisticated technology of motor rewinding, using meters to measure various motor parameters etc. I'm going to have to talk to him and look into his tach a little further.

Again, thank you all for the input. The learning curve here is going to be as steep as the flying curve but I've learned that anything can be climbed slowly.

Regards,

hankg

kyleservicetech
04-02-2010, 01:54 AM
Larry3215,

Thanks for the input. My friend and I are flyers. We're just getting into the more sophisticated technology of motor rewinding, using meters to measure various motor parameters etc. I'm going to have to talk to him and look into his tach a little further.

Again, thank you all for the input. The learning curve here is going to be as steep as the flying curve but I've learned that anything can be climbed slowly.

Regards,

hankg

Ah Ha!

I've done my share of motor rewinding over the past 45 years, and actually got some of my brush type motors to fly an airplane. Then, I bought 6 of those www.gobrushless.com (http://www.gobrushless.com) motor setups, and got them up to about 160 watts per motor. Now, everything I own is built by Hacker, their A30, A40 and A50 sized models.

Just a note on these motors and their respective propellers. Increasing the RPM on a given size propeller results in the horsepower input raised to the RPM ratio raised to the third power. As an example, taking a prop from 8000 to 12000 RPM requires 3.3 times more horsepower. What appears to be minor increases in RPM can result in substantial differences in model performance. And with that, substantial increases in input wattage to the motor.

You can measure the winding resistance fairly accurately, take a look at my threads listed below:
http://www.wattflyer.com/forums/showthread.php?t=50740

And, "Dry Testing your brushless motor" per my thread below. Maybe you might find it useful :tc:
http://www.wattflyer.com/forums/showthread.php?t=35216