How Safe is my Motor?

[Steve Hargreaves - Moderator]

I think you're right Jon......a few tenths here or there isn't going to hurt & we need to avoid getting bogged down. However the two basic points made by Timbo are relevant & important if you are to enjoy electric flight & keep the magic smoke at bay

1) You are putting two much power through that motor at WOT (equals smoke & a new motor!!)

2) The voltage of your LiPo seems a little high after charging (equals fire, smoke & a new shed if you're very unlucky!!!)

To solve 1) prop down a little......a 11x 5.5 or 10x6 maybe. For 2) not sure what to suggest as the charger should look after this bit.....which charger are you using?

[Jon]

A Big Thank you Steve for the solution to Prob 1)... re Prob 2) I have an IMAX 6 Lipo Balanced charger which I set to 4S at a 2.1A charge rate since Timbos valid comments I have tried 1.5A but no change.?

I did check the "current" reading of the charger with my multi and it was reading the same, its just the voltage reading thats seems a little odd... or could it be to with the Wattmeters count of mAmp's ? I dont know how the watt meter works out its sum's for the displayed power levels..just guessing!

jon

[Jon]

Just thinking... when I purchassed the motor/ESC & batteries the Prop was chosen by BRC the supplier. and that size is listed in the motor data..?

jon

[Myron Beaumont]

Jon

I dont know about steam power Too much pollution & a bit on the heavy side  HOWEVER  we could power hot air balloons via these threads 

[Steve Hargreaves - Moderator]

The Imax 6 is a pretty good charger from what I read/hear.....I don't have one so can't really comment. At least you're not using a horrible 20 quid charger...that might have been the problem.

The prop size dictates the current drawn & hence the power produced by the motor......I find it slightly odd that the 11x7 was recommended by BRC if it results in 620watts being developed by a 500watt motor.....are you sure this isn't the prop for a 3S set up?? How does the model fly with the 11x7? Does it seem to have an excess of power? If so then propping down would make life a little easier for the motor....we also have to remember that the prop unloads in the air so the max current will be lower in flight.

The problem here is that max power is used at take off & this is the last place you want the motor to fail......! That said the max current for the motor is listed at 45A & this is where you are. It's amps (current) that usually kills something rather than watts (combination of volts & amps) so you may be OK as you are.

Wattmeters work by placing a known "shunt" resistance (very low value) in series with the load, measuring the voltage across it & hence determining the current flowing from I=V/R. The voltage for the battery comes from..er..measuring it directly. Multiply the current by the voltage & you have the Watts....add time & you have Watts per hour.......add in the phase of the moon and (contd page 94)

[Tim Mackey]

Jon - I personally alays charge my Lipos at 1C - not any lower. Not saying its not good to do it lower....but never seen the point really ( I am impatient and an hour is too long - never mind any longer !)

Have you checked and recorded the ndividual cell voltages of the packs whilst you are charging /terminated.

Steves point about the specs for the motor could well be correct - they might specify that prop on 3s NOT 4s.

I have found ( with all due respect to BRC ) that quoted figures for motors and props and cell counts etc are on the very limits and sometimes beyond. I too have had a situ where a current consumption figure for a  motor prop battery combo was one figure....yet my results showed it as a fair bit higher. 

[Jon]

No Timbo I have never recorded cell voltages..... I have 5 red and 1 black 1 wire coming out of my 4s lipos... can you explain just which pair I should use to check the individual cell voltages?

So I also now need to re-run the tests with smaller props   (11x 5.5 or 10x6)  ...ok They are already on order!  ... in the meantime I could cut the ends off one of my 11x7s and rebalance it...but how much to cut off? any ideas... 

As for the performance I really haven’t been able to do much in the way of any vertical climbs, loops & rolls as when I'm flying I'm on still on a buddy link and still plodding around the sky practicing the "A" test flight.

However my instructor (an I/C type) has flown her and some of his comments have been  "I'm impressed with this lecy thing".... but there are times when I wish it had a bit more power but normally flying for me at the moment is just plodding about up there. It fly's mostly on about half throttle and will take off  on about 7/8 throttle......

 Jon

 

[Myron Beaumont]

You can go CAREFULLY  between the black & the nearest red then from that first red to the next & so on OR you can keep the probe on the black & go to each red and using arithmetics work out each cell voltage  If I'm wrong I;m sure Timbo will say so but I know I;m not .Be very careful I must emphasize  Dont do it after a bevvy

[Jon]

Oh I had not finished ....please see above

[Tim Mackey]

Sounds like a Flightpower pack Jon ? As Myron says....carefully touch negative probe on the black wire pin, and positive probing on each successive red will "read" each of the 4 cells cumulatively.

However....your charger should be giving you cell info as it does its stuff.

Cut off around .5" off each tip and end up with a 10 X 7 - at least that way you will see the difference it makes to current and watts .

[Jon]

Ok chaps... here's the next set of results

First I cut of .5" of the tips of the APC 11x7e Prop and rounded the ends to the apear the same as the previous shape, the prop was then rebalanced and refitted.

Before conecting the 18c 4s Batteries I checked the individule cell voltages. Both batteries had been fully charge 2 days ago... With the neg kept on the black lead each red read as follows

Battery "A" read 4.2v, 8.43v, 12.70v, 16.93v,16.93v all read on my multimeter

Battery "B" read 4.2v, 8.44v, 12.71v, 16.93v,16.93v all read on my multimeter

With the watt meter, ESC & Motor in circuit and each battery connected on its own, they read as follows

"A" 16.91v & "B" 16.91v the cold Wattmeter was reading "A" 17.5v & "B" 17.5v... ?

Now with both "A" & "B" batteries paired & connected plus the shortened prop turning I got the following readings

At 8.5A 140W 16.00v ( 16.40v on the Multi)

At 12.0A 188W 15.9v (15.42 on the Multi)

At 29.0A 408W 14.40v (14.03v on the Multi)

At 42.0A 590W 14.00v (13.60v on the Multi) Motor rated as 500W

Max current drain was 42A , I could not make it go any higher, motor is rated as 45A.

What say you? ok .....or should I go back to Kites

[Tim Mackey]

Well it looks as though your wattmeter is a little optimistic - which will also result in a higher than actual watt reading too. I think it all sounds "within tolerances" - go fly the darn thing

[Jon]

I have just been looking back over the previous readings I have posted and there is a pattern reflected there...

One thing that concerns me a little is that while taking the readings the meters are fluctuating all the time which could well account for many of the odd readings.

I found it quite difficult to record (on both meters) a number since it kept changing/jumping both sides of the number I finally recorded.... is that normal ? or could It be interference / faulty components or what?

Maybe I just need better test gear!<!--[if gte vml 1]-->   <!--[if !vml]-->

[Tim Mackey]

I should stoip worrying and just fly Jon - the differences are small and I often get erratic readings on some tests - if they are small like this...I just ignore 'em!

[Andy Gates]

Jon

 I agree with Timbo, just get out and fly your plane. If we worry about every little thing on the plane nd how it is working we would never fly at all.

I would guess the erratic readings are due to the chemistry and manufacturing tolerances.

Try this for an experiment, it is quite safe to do.

Connect a pack to your watt meter / DVM and read the voltage. Now GENTLY squeeze the pack and watch the voltage rise.

[Jon]

I was only a little taken a-back by the earlier comments about the motor running above its spec.... I began to think I had somthing very wrong... but now I know the test gear is Cr@p I'll just call "Taking Off" and shut my eyes if I see a second sun in the sky  !!!!

Thanks to all of you for all your comments its been a real good learning phase for me....

I will be building an electric Acro Wot soon, just started the ordering phase...so all this good stuff will come in handy

jon 

[Stephen Grigg]

jon,did you find that the correct start up ison hard on the program card