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Author Topic: Wondering if the currents I am measuring make sense....  (Read 340 times)

Offline Teodorico Terry

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Wondering if the currents I am measuring make sense....
« on: April 25, 2021, 12:22:32 PM »
Hello All,

I am trying to set-up an E-flite power 25 to run on 4S while turning a 12x6 prop.  The desired in flight speed is of 53.5 mph; did a quick calculation and I need to turn the prop at about 9400-9500 RPM.  I am using a Castle ESC in governor mode where you set desired RPM.  The motor has a Kv of 870 and 14 poles.

Tested the set-up and the motor is turning at about 9450 RPM but the current is the 40 amp range which is not what I had expected.  I had expected the current to be around 25 amps.  I did a quick check with one of the online calculators and for the Power 32 (770KV) it estimated 22 amps at 10500 rpm and 33 amps for a Power 15 (950kv) at 11400 rpm.  In both cases the RPM's are higher than my target number and the current draws lower which makes me think that I goofed up something.

So I am wondering a few things:

1)  The newer firmware for the Castle provides pulse width for the throttle input, do I need to adjust the timer?  (It came from Brodak and it uses a pot to adjust RPM, in my previous model I did not worry about the throttle pulse, I just saw it as either on or off regarding of pulse).
2)  Could the motor have been wound to a different KV?

I was thinking about changing the ESC back to airplane mode and just running it through the input from the throttle - Get the RPM I am looking for and measure the current.  If the results are different from those using governor mode I know I made a mistake in the programing, if not, something might be wrong with the hardware (I am guessing).

Any suggestions?

Thank you,

Teo

Online Crist Rigotti

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Re: Wondering if the currents I am measuring make sense....
« Reply #1 on: April 25, 2021, 08:35:41 PM »
Static runs will always be higher than when you actually fly the model.  Be very careful running a flight "set-up" on the ground for any length of time.
Crist
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Offline Teodorico Terry

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Re: Wondering if the currents I am measuring make sense....
« Reply #2 on: April 27, 2021, 06:01:31 AM »
Crist

Thank you for the suggestion; the tests I have been running have been of short duration, just long enough to allow the motor to spool up and reach the desired RPM and then confirm the RPM and current draws.

I ran some additional tests yesterday and came to the conclusion that the current draw for the 12x6 prop is probably correct;  but I am surprised by the differences between the predicted and measured current draws:

1)  Hooked up a Hacker A30-M14 motor that I had sitting around.  This motor also has 14 poles but a slightly lower Kv of 800.  With the 12x6 prop at 9400 RPM the current draw was in the 34 amp range; the Eflite 25 was pulling 39 amps at 9500 RPM.  I would expect some differences between the two; given the 100 RPM difference between the 2 runs the 5 amp difference seems reasonable.

2)  The CC ESC I am using is older so when I hooked it up to the computer to program the governor I also updated the firmware.  This got me wondering if the issue was with the firmware given the age of the ESC so updated the firmware once again to an older version.  It did not change the results so the firmware does not appear to be the issue.

3) I tested a 12x8 prop on the Hacker to see what would happen.  My desired flight speed is in the 53.5 mph range so given the higher pitch (8 vs. 6) the RMP went down from about 9400 to 7100.  During the test the motor pulled about 18 amps from the 4S pack which was more inline with what I expected.  I still need to test the Eflite with that prop.

4)  I will also test a 11x5.5 prop which appears to be quite popular.  The desired RPM will be in the 10,200 range which is only about 700 more that I was attempting to use while testing the 12x6 prop.  The 1" reduction in diameter should reduce the current draw significantly.  We will see.

I am aware that once I start flying I will have to adjust the RPM once again to account for the drag of pulling the model around the circle.  The results are interesting in that they indicate that a slower turning, higher pitch prop is generally more efficient in achieving a desired flight speed.  This is consistent with my experience flying R/C models.  Larger diameter props are also more efficient at overcoming drag as well.

I am also aware that turning a prop in the 7000 RPM range is not ideal for the ESC in governing mode but my current model flies on a 13x8 prop with no apparent ill effects.  You will see significant variation in the currents depending on the maneuver being flown (+/- 8 amps).

Teo
« Last Edit: April 27, 2021, 02:11:06 PM by Teodorico Terry »

Offline Teodorico Terry

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Re: Wondering if the currents I am measuring make sense....
« Reply #3 on: May 03, 2021, 11:43:25 AM »
Well, as it turns out I made a rookie mistake.  It appears that the RPM that I would like to use on the 12x6 and 11x5.5 are higher than what people normally use, but more importantly, for the 11x5.5, using the 4S pack with a 870Kv motor is not a good idea if you are trying to reach over 10000 RPM.  In essence, the max RPM is limited by the voltage (Kv * Volts) and ideally you want to run at about 80% of your no load RPM for a given voltage.  In essence, for the 11x5.5, I was pushing the motor pretty close to the limit of what it can do on 4S.  Changed the pack to 5S and the current dropped to 25 amps or so rather than the nearly 40 amp needed when using 4S.

In the end it appears that the Hacker motor is about 10% more efficient than the E-flite which is something that I will keep in mind for the future.  Right now, the Eflite will go into the model I am trying to finish as it was build around it.  The thing that was interesting is that the Hacker was meant for a largish parkflier and in principle it should not have the power of the Eflite.  Go figure.

Teo

Offline ericrule

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Re: Wondering if the currents I am measuring make sense....
« Reply #4 on: May 07, 2021, 11:01:07 AM »
Hacker A30 has curved magnets. That allows the rotating portion to be placed closer to the stator. This gives the Hacker better efficiency. Better efficiency will usually mean lover amp draw.

Easy way to check type of magnets a motor has without needing to take it apart is to look at the specs. If the motor (A30 Hacker) is 30 mm vs (Eflite Power 25) 36 mm that usually means the 30mm has curved magnets while the 36 mm has flat magnets (which have to located further away so they will clear the stationary flat magnets of the stator).

Hope this info helps.

Offline Teodorico Terry

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Re: Wondering if the currents I am measuring make sense....
« Reply #5 on: Yesterday at 12:55:34 PM »
You made me take a look at the two motors using a flashlight.  It is hart to tell if the magnets in the Hacker are curved, but the outer case does have a slightly larger diameter which allows for slightly wider magnets.  Either way, know that I know now, I might favor the Hackers over the E-Flites.

Thanks,

Teo


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