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Author Topic: Strange motor behaviour  (Read 855 times)

Terry

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Strange motor behaviour
« on: February 22, 2019, 02:08:29 PM »


Have just completed my winter project, an Aeronaut Jenny. Have been bench running the motor in the finished model and can hear what I can only describe as a "2 stroke motor 4 stroking", strange sound for an electric motor! The driveline is a Johnson Race 650 running on 8.4v, MtronicsViper Marine 40 esc, a Huco type coupling, also tried a solid coupling, a 4mm propshaft 36cm long and not bent.
Slow speed running is fine, but as the revs are increased a point is reached where thing start to go wrong. Any ideas guys.


Cheers, Terry
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Netleyned

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Re: Strange motor behaviour
« Reply #1 on: February 22, 2019, 02:25:48 PM »

Motor bearings/bushes worn or motor shaft out of alignment. ???
Ned
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Tug Fanatic

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Re: Strange motor behaviour
« Reply #2 on: February 22, 2019, 03:45:55 PM »


36cm is longer than I would go with 4mm propshaft unless there is a centre bearing but does it make the same noise with the prop submerged?


I am guessing that you have tested the motor not connected to the propshaft and also complete less the prop?
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chas

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Re: Strange motor behaviour
« Reply #3 on: February 22, 2019, 04:17:10 PM »

Eliminate 1 thing at a time. Try the motor on its own, if OK connect the coupling and shaft, if OK that only leaves the prop. If the motor is noisy on its own, connect directly to a battery, if its still noisy, its the motor, if not check the ESC.
  I agree that 36 cm is very long for a 4 mm shaft, unless it's turning very slowly, say less than 2000 rpm.

Chas

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bj

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Re: Strange motor behaviour
« Reply #4 on: February 22, 2019, 08:40:20 PM »

Have you balanced the prop? Do not assume that a new prop is "good to go".
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malcolmfrary

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Re: Strange motor behaviour
« Reply #5 on: February 23, 2019, 09:26:03 AM »

Have you balanced the prop? Do not assume that a new prop is "good to go".
But first do it like chas suggests, incrementally eliminating things. 
You get to the prop eventually, and because other possible problems have been found, you are not trying to balance a prop on a system that has other problems.  While the shaft appears straight, a long thin one only needs to be a thousandth of an inch or so out of true to start thrashing about as revs increase.  A center bearing about half way up the shaft reduces this by effectively making it into two shorter shafts.
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john44

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Re: Strange motor behaviour
« Reply #6 on: February 23, 2019, 10:05:01 AM »

I must agree a 36cm - 4mm shaft at high speed would possibly be whipping inside the prop tube
and rubbing the inside of the tube causing both noise and vibration.
check the coupling brass insert are drilled dead centre, side to side movement at the tube bushes.
What is the speed of the 650 race Johnson motor at 8.4volts? As already stated you may need centre
bearing, I had the same problem on a huntsman I built years ago. After removing the inner shaft
I cut through the brass tube with my Dremel cleaned the end of the tube and refitted the bush
Fitted a bearing/bush support, for the bare shaft to run through, lined everything up and glued in
that solved my problem. I had easy access on my Huntsman so the job was simple to do.


John
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Tug Fanatic

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Re: Strange motor behaviour
« Reply #7 on: February 23, 2019, 11:40:32 AM »


There is the identical question with largely the same answers on the Model Boats magazine forum.


https://www.modelboats.co.uk/forums/postings.asp?th=140889


What is the forum view on this type of multiple posting?

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Terry

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Re: Strange motor behaviour
« Reply #8 on: February 23, 2019, 12:11:33 PM »

The boat is kit built and I used the prop shaft as supplied, I checked the shaft for straightness very carefully, it was 100%. The sound I can only describe as previously written, a 2 stoke engine 4 stroking, I have run the motor connected directly to the battery, smooth as silk, held in the hand. Then ran it using the esc and could feel the motor twitching in my hand !! So next step, the esc will be substituted.
I do sometimes post to both forums, I see no harm in it. Surely two Forums are better than one.


Thanks everybody for your replies
Terry
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Tug Fanatic

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Re: Strange motor behaviour
« Reply #9 on: February 23, 2019, 01:38:40 PM »


They might be except that members of both forums have spent time putting together answers where someone else on a different forum has already done exactly the same.


Personally I would let one forum exhaust its ideas without success before I approached the second forum & then provide a link to the answers that I had already received.


Just my two penny worth.
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malcolmfrary

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Re: Strange motor behaviour
« Reply #10 on: February 23, 2019, 06:25:05 PM »

There is the identical question with largely the same answers on the Model Boats magazine forum.


https://www.modelboats.co.uk/forums/postings.asp?th=140889


What is the forum view on this type of multiple posting?
The OP is asking what he imagines is two different sets of people the same question in the hope of getting a different answer, or a wider based agreement with the same answer.  He just wants the noise to go away.  I don't see any problem.
The motor twitching might just be a result of it being pulsed, albeit at a high frequency. The pulses are varied in duration, at some settings, particularly at low power settings, the pulses can get out of sync (or into sync, depends) and give lumpy running as the pulses arrive and stop at different parts of the motor turning and offering differednt combinations of commutator segment to the brushes, but this usually vanishes with a different speed setting.  Since it runs smooth on straight battery, carry on the incremental testing adding the extra bits down the drive train.  If the noise never shows up on battery, the villan must be the component that has been missed.  Not a problem that I have had, but if the ESC is causing a rapid variation in power it might transmit as a vibration into that long thin unsupported shaft.
I did say somewhere that problems like this might not have a single source, there might be more than one thing giving the trouble, it might be a combination.
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