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Electric Motor failed ?

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alan barnstable10/08/2020 11:04:23
19 forum posts
2 photos

Have a Funfly with Turnigy SK3 3542 800kv with 5S battery, 60amp ESC, 12/6 prop. Has run fine for several flights but yesterday motor appeared to fail midflight. It just jerks when activating the throttle. Have tried a different ESC, does the same. Motor smells.

Do motors burn out, was it an unsuitable combination, what more can I do to pinpoint the problem ?

thanks

malcolm woodcock 110/08/2020 11:08:19
409 forum posts

Check against the manufacturers ratings then drop them by a healthy margin. Check your connections from esc to motor, whenever I've had a 'stuttering' problem it usually turns out to be a bad soldering joint either on the motor or the esc.

kc10/08/2020 11:26:41
6646 forum posts
173 photos

It is possible one of the motor 'bullet' connectors has come loose or unsoldered. I had a similar problem that turned out to be a connector that had become slightly loose and therefore arc'ed and then unsoldered itself. Held together by the heatshrink it wasn't obvious.......

Allan Bennett10/08/2020 11:31:21
1733 forum posts
53 photos

Usually a comment that a motor is jerking indicates that one of the three wires from the ESC to it is disconnected or only partly connected. The problem is, if you don't realise this quickly, and therefore fail to shut the throttle, it can cause the motor to burn out. The bad connection is often in the soldered bullet connectors, but it can be actually inside the motor, which makes it difficult to deal with.

A smelly motor sound like it's gone. If you look at the windings, you should be able to see if they're shiny copper, as they should be, or darkened or even black. If you have a digital multimeter you can check the condition of the windings by checking resistance -- between each of the three pairs of windings the resistance should be in low single digits, and the same for each pair; between each of the leads and the body of the motor, the resistance should be infinite (i.e. open-circuit).

Did you check with a wattmeter what your actual amps are when running at full throttle? The motor is rated for 42A or 625W maximum. Unfortunately, if you prop it to achieve 42A with a 5S LiPo pack you're generating at least 770 watts (maybe 880W with fully-charged pack), so you need to keep an eye on both figures when testing.

Bob Cotsford10/08/2020 12:19:50
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8746 forum posts
489 photos

According to E-calc that combination is trying to absorb 900W at 54A, well outside the motors specs 625W for 15 seconds. A 10*6 drops the power down to a more sustainable 570W.

If you want to run that motor/cell count/prop then you need a motor with a much higher current rating, maybe more like a 4258 size one?  Personally if I wanted a 900W setup on 5S I'd be looking at 6-700kv and 13 or 14" props.

Edited By Bob Cotsford on 10/08/2020 12:27:26

Geoff S10/08/2020 12:28:19
3769 forum posts
36 photos

I suspect that on 5S with a 12x6 prop you are overloading the motor as others have said. Did you check the current? The motor is rated at 42 amps but it would be better to keep it well below that. The rated power isn't as significant as the current which what causes the most of the heat and potentially damages the windings.

Geoff

MattyB10/08/2020 12:34:54
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2211 forum posts
32 photos
Posted by Bob Cotsford on 10/08/2020 12:19:50:

According to E-calc that combination is trying to absorb 900W at 54A, well outside the motors specs 625W for 15 seconds. A 10*6 drops the power down to a more sustainable 570W.

If you want to run that motor/cell count/prop then you need a motor with a much higher current rating, maybe more like a 4258 size one?

Completely agree - that size motor is one I would probably install in something needs 350-500W on a 3S or 4S pack. 5S on a 12x6 will cook it very quickly!

The 350-500W max recommended power above is based on the "3W/g" rule of thumb you can use for most outrunner motors (i.e. max power in Watts should be no more than 3x the weight of the motor in Grams). Of course if you buy a higher end motor you can bend this rule, and it doesn't apply for specialist applications like EDFs, but it's a good guide for anything mainstream from vendors like HK.

Alan - If you want to learn more about electric flight start with the stickies in the Electric Flight for Beginners forum on this very site...

Edited By MattyB on 10/08/2020 12:37:34

alan barnstable10/08/2020 13:27:10
19 forum posts
2 photos

Many thanks for the responses, I think it's very likely I've cooked the motor by over stressing it with 5s battery and big prop - it did fly very fast, but not very long !!!

I've got to decide now whether to buy a better motor, e.g. a 4258 as suggested by Bob, or I do have an Irvine 46 I could convert it to, decisions, decisions.

Thanks again.

Andy4810/08/2020 15:49:54
1550 forum posts
1 photos
Posted by alan barnstable on 10/08/2020 13:27:10:

Many thanks for the responses, I think it's very likely I've cooked the motor by over stressing it with 5s battery and big prop - it did fly very fast, but not very long !!!

I've got to decide now whether to buy a better motor, e.g. a 4258 as suggested by Bob, or I do have an Irvine 46 I could convert it to, decisions, decisions.

Thanks again.

Don't think its the "big prop", that's not large with electric setups, but a Funfly certainly doesn't need a 5S battery. My electric Funfly has more than enough power with an SK3 3548 840kv and a 4S 3000 battery. Easily get a good 10 minutes out of that on a 12x6 prop with 700+ watts. Flown more gently and you will get considerably in excess of this.

Don't forget that the heavier weight of a 5S battery will have a negative effect on the performance.

 

Edited By Andy48 on 10/08/2020 15:51:27

Allan Bennett10/08/2020 20:46:36
1733 forum posts
53 photos
Posted by Andy48 on 10/08/2020 15:49:54:

Don't think its the "big prop", that's not large with electric setups, but a Funfly certainly doesn't need a 5S battery. My electric Funfly has more than enough power with an SK3 3548 840kv and a 4S 3000 battery. Easily get a good 10 minutes out of that on a 12x6 prop with 700+ watts. Flown more gently and you will get considerably in excess of this.

Don't forget that the heavier weight of a 5S battery will have a negative effect on the performance.

Given the choice of a 5S LiPo (which the motor is rated for), the"big prop" is the problem surprise. Amps draw, or consumption (depending on which way you want to look at it) for any particular motor is dependent principally on supply voltage and prop diameter and pitch.

PatMc10/08/2020 21:34:25
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4471 forum posts
548 photos
Posted by Andy48 on 10/08/2020 15:49:54:

Don't forget that the heavier weight of a 5S battery will have a negative effect on the performance.

There is no reason a 5S battery should be heavier than a 4S (or any nS) if they have the same energy storage capacity.

Ben B10/08/2020 21:48:14
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1453 forum posts
4 photos
Posted by Allan Bennett on 10/08/2020 20:46:36:
Posted by Andy48 on 10/08/2020 15:49:54:

Don't think its the "big prop", that's not large with electric setups, but a Funfly certainly doesn't need a 5S battery. My electric Funfly has more than enough power with an SK3 3548 840kv and a 4S 3000 battery. Easily get a good 10 minutes out of that on a 12x6 prop with 700+ watts. Flown more gently and you will get considerably in excess of this.

Don't forget that the heavier weight of a 5S battery will have a negative effect on the performance.

Given the choice of a 5S LiPo (which the motor is rated for), the"big prop" is the problem surprise. Amps draw, or consumption (depending on which way you want to look at it) for any particular motor is dependent principally on supply voltage and prop diameter and pitch.

One of the big differences between IC and electric is how they respond to different props. If you put no prop on an IC engine you'll get a shaft run and it will kill itself. Conversely an electric engine is happiest when not being stressed but will cook itself with a big prop that it will try to turn at the kV*volts RPM. When it comes to props they're basically the opposite of each other.

J D 810/08/2020 22:07:04
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1586 forum posts
87 photos

Nothing like a shaft run to scare the bejeebers out of all around.face 7

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