Motor Kv - the truth

[Tim Mackey]

Lousy weather found me doing some testing of Motor Kv etc - comparing figures for loaded versus unloaded RPM, and also bench test static data versus real time in flight data. All tests utilised the eagle tree data logger V3 with latest firmware and GUI. Battery was freshly charged for each test.

The motor used is the KMS 41/20/07  which has a stated Kv of 480.

 

First off, some theory.  The rather excellent but pricey Thunder Power 4s battery used has an on load voltage of around 16.5 V for the first few minutes, so I used this figure - 16.5  X  480 is....7920

             

           therefore 7920, is the specified theoretical no load speed.

 

All the following data is averaged from a 10 second static bench run.

 

Unloaded with no prop at all showed  9460 . This is over 1500 RPM more than spec, almost 20% higher.

 

GWS slow fly 10 x 6 ( smallest I had that could be fitted ) showed 8890 which is over the specified no load speed by 970 RPM, and under the actual by 570

 

Zinger beech 15 x 8  shows 6706 which is under specified no load speed by 1214 RPM, and under the actual no load speed by  2750.

 

APC E 15 x 10  shows 6762 which is under specified no load speed by 1158, and under actual no load speed by 2698.

 

 Interesting.....firstly because of the difference between specified no load RPM and actual ( 20% higher ) and secondly the larger pitch 15 x 10 actually revved slightly higher than the 15 x 8. I put this down to perhaps the extra weight of the APC versus the lighter wooden Zinger prop - more flywheel effect ?

 

Leaving aside the first anomoly - and using the actual no load speed - fitting the typical prop for that sized motor, 15 x 10 showed the revs dropping by 2968 which is around 30% lower.

 

Now I usually work on a drop in revs of around 10 - 20%  so allowing for the difference in revs as the motor unloads in the air ( higher ) it will be interesting to see how near or far off that mark I am, when I compare the flight data, which will be in the next post.

[Tim Mackey]

Right, part two of the testing was to compare static data with inflight data, Now this is not all that easy, as real flying requires much variation in throttle settings - impossible to replicate faithfully in the workshop.   The parameters logged are......

1) Pack Volts

2) Pack Amps

3) Watts

4) RPM

5) Temperature of ESC ( and also occasionally the motor )

6) Cum mAh

 

I always use the same system for bench testing my powertrains, for no particular reason other than it seems reasonable for the most demanding type of model such as an EDF jet which is mainly at full throttle most of the time.

The test consists of

WOT from the off for 30 seconds

50% throttle for 30 seconds

WOT for 30 seconds

50% throttle for 30 seconds

WOT for 2 minutes until stop at 4 minutes total elapsed time.

 

I then replay the data via the eagle tree GUI and also convert some of the more crucial data to a JPEG image which shows things like maximum, minimum and averages.

 

Unless you have an eagle tree inteface, then the "raw data" is no use , but the two JPEG images shown below should give an idea of what is happening.

Clicking the image should enlarge it.

 

First image here is of the 4 minute bench static run as outlined above.

 

 

This next image is of a recent actual flight, which lasted around 12 minutes or so, the data captured is longer, as the recorder doesnt stop until the model is attended to and the power disconnected, which in this case was a few minutes after landing due to the photograher wanting ground run shots. I have therefore averaged the data captured during the first 4 minutes or so of the flight, to try to get reasonable comparison with the static data. There is typo on the image, - the prop is marked as 16 x 10 but in fact it was the same APC e 15 x 10

 

                I'll discuss the results in the next post.

Edited By Timbo - Administrator on 05/05/2010 23:57:38

[Tim Mackey]

I have noted some other parameters not shown in the JPEG image, these being RPM, and ESC temperature. Firstly, the maximum RPM reached on the bench was 6762, and in flight this was 6858. Only around 1.5% higher. The temperature of the 60A ESC climbed steadily and quickly to 29 degrees C on the static test, and within 1 further minute reached a maximum of 49 degrees C after the run was stopped. I have some reasonable cooling holes in and out of the cowl, and expected that some prop wash would waft through , but this seems to not be working too well I was half expecting the ESC to shutdown, but it carried on just fine ( cheap turnigy unit ).   In flight the maximum temperature reached was only 19, but climbing again after landing - to around 32 degrees C. This suggests that the cooling holes are more effective in actual flight, probaly the exit holes literally drawing the warm out through "suction" from forward motion.

I was interested to see the effect of unloading in flight on the peak static current drawn, and also to see the RPM.

Static maximum current at WOT was almost 47A.

In flight it was 43A, so a reduction of just 4A, or in round figures about 10%.

I have ignored one or two high "spikes" in the data log.

The peak RPM reached in static testing was 6762, and in flight this was 6900, an increase of some 2%.

As mentioned earlier, the theoretical no load motor speed should be 7920 RPM therefore the 6900 achieved when loaded with a decent prop is 15% lower.

This is bang in the middle of the range I usually estimate.

The ACTUAL no load WOT speed of this motor showed as 9460 RPM, so the 6900 in  flight is over 35% lower !

[Tim Mackey]

Not sure where this leaves me...... apart from wondering why the actual no load rpm is a lot higher than the specified rpm

However, it does show that the prop RPM unloads in the air - but not that much really, and also that current draw ( on this particular powertrain at least ) reduces by around 10% roughly. The other interesting thing is the average wattage for a typical flight...the model weighs around 6.5 lb, and can be classed as a general sport flyer I guess.

Average power for the flight? Around 200 Watts !

And for full power spells, around 600.

Finally, total mAh consumed for a twelve minute flight....just 2660, just over half the total capacity of the battery.

[Brian Parker]

Timbo,
I would have expected the no load RPM to be slightly lower than ‘spec’ as you are calculating from pack volts rather than ESC output volts.

 We can (?) assume that the wire gauge and number of windings are standard, so either the stated Kv figure is incorrect or perhaps tolerance in the ESC's back EMF sensing is allowing higher RPM.

[Tim Mackey]

I think the most likely explanation is the ESC - I havent tried altering the timing or PWM settings etc. I have it set to default which is hard timing ( normal for outrunners )

[Tim Mackey]

Ok, for those still following this ( not many it seems ) I did some more static testing  of claimed KV of another motor today. First off, let me explain the test rig. First photo.

 

A brand new Scorpion 515Kv  2kw motor is mounted with no prop. A simple 60A OPTO ESC is used, and powered up via a 4 cell NiMh pack and servo tester for throttle.  Data was logged in "live mode" on an  "Eagle Tree" logger - data from which was displayed on the PC screen, and also saved to file and JPEGs.

 

This user interface is shown here.

 To get a regulated and accurate voltage input to the ESC, I used my IC106B charger set to "motor drive function" with a maximum of 10A available for 10 minutes  and the charger is powered by a T/P 4s Lipo....because it was to hand

 

This is shown here

 

 

Two tests were done, one at 10V input, and the other at 15V.

This gave easy to calculate figures, as 10V X 515Kv is simply 5150 RPM

...and 15 V input should show around 50% more at 7725 RPM

Furthermore the motor data sheet shows the Io ( no load current consumption ) for a 10V input. The claimed figure was 1.48A at WOT.

 

First test was 10 V input to assess the claimed no load current and RPM.

Thiswas spot on target with the 106B unit displaying 1.4A @ 10V, and the eagle tree unit showing only slightly differently at 1.57A average....with 9.95V average input voltage over the 10 seconds. This can be seen in the following two pictures

First the 106b display...

 

Then the logger data display

And the 2d Chart of the same data but showing average figures over the 10 seconds.

  

Next up was checking the actual Kv ( RPM ) at 10V input - which specs say should be 5150 RPM. Given that the input voltage was to the ESC rather than at the actual motor windings, the results were pretty darn close @ 5292 average over 10 seconds.

 

The RPM average is shown in the same picture above -  just 2% higher than spec.

 

This is certainly a lot better than those obtained with the earlier KMS motor

In the next post I will show the results obtained on 15V input.

[Tim Mackey]

Heres the RPM results when run at 15V input.

 

 

The specs would give 15 x 515 = 7725 RPM and again the results are pretty good with the actual average being 7942 RPM.

 

Interestingly, when using both results ( at 10V and 15v input ) calculations show an actual Kv of 529 in both cases.

5292 / 10V = 529

7942 / 15V = 529

 

This is around 2.7% higher than spec, so again, considering the input volts are to the ESC not the motor windings, and potential meter errors and so on..... pretty good I think

[Bruce Richards]

I wounder why motor manufactures quote KV to such a high level of accuracy? Surly if they has just sail 520 instead of 515 that would have been sufficient. Perhaps they are using some formula which gives them an accurate estimate of what the Kv should be given the design parameters.

[Chris Bott - Moderator]

Hi Timbo

 

All very interesting stuff, I suspect you've had few replies while people still read and re-read to try to keep up! 

 

When I first got my eagle Tree the one stand out result that I hadn't expected was just how low average current is over a whole flight.

 

This is pretty much the whole reason that A123's work so well for us in 4 to 6lb models. They are capable of large bursts of current for nice impressive climbs etc etc, but we need our average to be low as they hold onl 2300 mAh.  (Bet yr surprised how soon I got that plug in )

 

Unloading: from tests I've done, it can vary quite a lot. It seems to depend on the model type and how you have matched the pitch to the model's airspeed. With a slippery model and/or a low pitch prop, it may be hardly pulling at all as the model gets up towards pitch speed. But with a draggy model and/or a high pitch prop, the prop could still be pulling quite hard even when the model reaches top speed.

 

The prop tests are interesting, appearing to show that on static tests, the APC-E 10" pitch slows down the motor and pulls less current than the Zinger 8" pitch. This is counter intuitive. Suggesting that the APC-E is a more efficient design.

I guess to complete the picture we'd need thrust measurements and in flight measurements for both including airspeed??

 

Temperature climbing after stopping indicates that things are getting warm inside and the outside is getting some cooling. Which is fine if things are kept cool enough. Interesting to see your thoughts about suction due to forward motion, I just opened up the vac formed exhausts on a Funfighter P51 hoping to get just that effect. I guess I should tuck the Eagle Tree in there to check just that. 

 

I've just had a problem with me Eagle Tree, a few times when I've plugged it in, I got no power through it. All soldered joints look fine, a hard tug on the wires and nothing stretches but a waggle or reconnect and it all comes good. I assume the bar through the sensor is a solid piece of metal so at the moment I'm a bit puzzled. At the moment is is not going in any BEC powered models. I'm suspecting the surface or springs of one of the 4mm connectors, but that really is clutching straws. I think I'll replace all connectors and remake the soldered connections to see if I can get some confidence back.

 

See you at the funfly.
Cheers
Chris 

[Tim Mackey]

Thanks Chris....

Yes the average current draw, and tot capacity used during a flight is surprisingly low isnt it. The 10" pitch prop actually allowed the motor to rev HIGHER - not slow it down - as well as you correctly say, pulled less current. I dont have an airspeed measuringh gizmo for the eagle tree, but I would say from observations at the field over two flights with each prop, there was no discernable difference in speed or apparant "power".

This was confirmed by the photographer. Not very scientific I know

 The Eaglke tree logger problem sounds a bit worrying...maybe you should contact supplier ? Good move on not using it for ESC BEC models me thinks! In nearly all of the models that I fit it to, I use a seperate UBEC direct off the main battery, so no worries there.

 

I have just done a few more tests on Kvs of a couple more motors..... posting soon.

[Tim Mackey]

The Turnigy 5055 400Kv outrunner, and then the Purple Power 1200 Kv Inrunner

First the Turnigy.

With a Kv of 400, that should produce 4000 RPM on 10 Volts and 6000 on 15v

Actual averaged results were  4231, and 6344 respectively.

Not great but not too far out, at about 5% high.

 

Then the Inrunner Purple power 1200 Kv

This obviously should show 12000 on 10V and 18000 on 15V

Actuals....   12965    and 19600 respectively.

This is almost 9% high, and shows a true Kv of 1300 rather than 1200.

Incidentally, I also found on previous testing that this motor consumes WAY more than the stated figures for particular cell counts and props. Beware.

OK thats all for now....and no more from me until Sunday night as I am away for the weekend you will be glad to know

 

 

Edited By Timbo - Administrator on 07/05/2010 20:04:28

[David Ashby - Moderator]

Very impressive Tim.

 

As you know, I've got an identical Scorpion motor sitting on the shelf, waiting for a model to come along - looks like you're impressed with it.  

[Tim Mackey]

Yep, its a nice bit of kit and is likewise...waiting for the right model to arrive in the hangar.

Rated at 2kW and 100A continuos it should prove a goodly beast at hauling something along very well - I might even convert my SA Spitfire if the .90 FS engine ever dies.

Should suit around 10-12s A123 cells I reckon.

Edited By Timbo - Administrator on 07/05/2010 20:19:16

[David Ashby - Moderator]

So a model in the 10lb region?

[Tony Smith 7]

Posted by Timbo - Administrator on 06/05/2010 00:26:43:

The peak RPM reached in static testing was 6762, and in flight this was 6900, an increase of some 2%.

 

I'm surprised that's so little, with my electric glider I can clearly hear the pitch rising as the plane speeds up. 

[Tim Mackey]

Posted by David Ashby - RCME Administrator on 07/05/2010 20:36:51:

So a model in the 10lb region?

 

Assuming 100WPlb is suitable for the model, and fairly high pitch speed is acceptable then yes. For 1000 watts of course, one really needs fairly high input voltage to keep the Amps manageable, and this high input voltage means high RPM.

For example, if we used just 4s LiPo and managed to hold 15V under load, then 1000 Watts needs a hefty 66A.

Thats do-able but the pack would be under heavy load, and duration wouldnt be too good!

On 4s the pitch speed would probably be around 6500 -7000 RPM with a decent sized prop. 

Moving up to a 6s we see maybe 21V so current would drop ( assuming the same prop of course ) to around 47A....more manageable, but pitch speed now increases to 9400+

As its a fairly high Kv for such a large motor, fast models would be better for it I guess.

[Tim Mackey]

Posted by Tony Smith 7 on 07/05/2010 21:10:29:

Posted by Timbo - Administrator on 06/05/2010 00:26:43:

The peak RPM reached in static testing was 6762, and in flight this was 6900, an increase of some 2%.

 

I'm surprised that's so little, with my electric glider I can clearly hear the pitch rising as the plane speeds up. 

 

Yes but do you know that it is actually exceeding by much, the static RPM that would be achieved on the bench ?

[Chris Bott - Moderator]

We are using a 50-65  350Kv Turnigy in the Seagull Sea Furys.

That's with a 17" or 18" prop and 6S2P  A123.

 

I think we are at about 800W and the models are around 8.5lb. 

Edited By Chris Bott on 07/05/2010 22:13:44

[Tim Mackey]

Hi Chris

Thats a lot of cells / weight / cost there.

Assuming 2.75 per cell under load, you are inputting around 16V then, and with an estimated unloaded Kv of about 390 thats 6240 RPM. Loaded with a big prop, I reckon thats dropping by maybe 15% or more...to around 5300. Add in some revs as it actually unloads in the air, and my final stab is circa 5500 RPM WOT.

Do I win a sticky bun?

Incidentally, I was viewing thta rather excellent film that you posted about the YT hurri and Eagle treee data overlayed...that was 12s cells...I assume not 12s 2P

 

Your Fury is pulling maybe 50A at WOT, and the hurri was higher at times...so have you considered using just 6S in the fury, or is flight duration an issue, and if so, how did the Hurri do on duration then ?

Edited By Timbo - Administrator on 07/05/2010 22:23:46

[Chris Bott - Moderator]

We started with the Furys on 8S1P with a different motor. 8 ish Lb, 800W, 8 Cells..

It worked fine but was down on duration so we came up with a way to go to 12 Cells rather than 8S2P. We also have some 6S packs for other models, so they now have multiple uses.

 

Warbirds seem to need some power on at all times so the average is a bit higher than a sports plane. Hence needing a bit more capacity.

 

For the Hurri we need Danny to answer..  where is he when you need him

 

As for costs of A123's have a look here. One of our members has had individual cells from this seller and they appear to be genuine. No one has bought a pack yet.

A123 packs on Ebay  I've just bought a couple of 8S Dewalt packs for £49 each too. 

[Chris Bott - Moderator]

Oh and the Fury needed the weight up front, and it has what appears to be a purpose built battery box that these would go straight in.

[Danny Fenton]

Hi Timbo, fascinating results there matey, still struggling to wrap the grey matter around it all.

The Hurricane will be at the RCM&E fun fly so you can see for yourself what the flight is like. The last few flight I had with it were around 10 minutes, but I do fly with a very low throttle setting and always have an eye on conserving innertia through manouvres. I have as you rightly said gone to 12S1P A123 which if not flown carefully and if prop selection is wrong would result in just a four minute flight, I know this because the first flight i got it  wrong and it went deadstick after just four minutes.

 

It seems to me that if you can match the prop pitch to the sweet spot that the model wants to fly at the duration will be much better. If you reduce the pitch so the motor speed has to increase to maintain the same airspeed you will draw more current and the prop is less efficient. If you increase the pitch too far then the prop can never unload properly and the model will be sluggish and you will be forced to use more power again.

The 11lb Hurricane is very happy on an 18 x 12 and as you have seen on the eagle tree figures pootles about at a very low average current.

 

We needed the weight in the nose of the Fury's because we chopped over 2" off the length of the cowl! the Seagull version had IMO an overlong cowl and it didn't look right to me. By mounting the motor directly on the firewall and shortening the cowl it balanced beautifully with 12 cells. The extra cells give it a little more weight to penetrate rough air. I am surprised by Scotts findings with his that he says it doesn't like windy conditions, mine seems okay.

 

Anyway as Chris says we need thrust numbers to sort out what is going on between the zinger and apc props.

 

 

Keep up the good work

 

Cheers

Danny

[Tim Mackey]

Cheers guys.... all adds to the knowledge base eh!

Those cells from the Ebay seller are pretty good VFM too - might have to get me some more soon, in readiness for whatever the big scorpion is going to go in

[Tim Mackey]

OK something odd has happened to this thread...a clone seems to have appeared , and one or two peeps have posted on that version - tech support will have to get back to me on why this has happened, but THIS is the proper and original thread, so if you posted and it hasnt appeared, you need to do it again on this thread below this notice.

The other duffer has been locked off until we find the issue.