Motor 'stutter'

scottie2212 · 23-07-2006

Hi guys,

Can anyone help with this problem...
My motors (both) 'stutter' when on the water, i.e. they speed-up and slow down as if the trottle is being opened and closed very quickly. This does not happen when on the bench.

Motors - matched pair of 540 type with 3, 1uf (one microfarad) capacitors each, one between terminals and one from each terminal to case. Running at 12 volts and drawing 1.5 amps each at full throttle under load.
ESCs - Mtronic Viper 15 (pair)
Layout - (see picture) ESCs are behind battery (under it in photo) between motors. Receiver is to the right of the battery on the coaming. Aerial is routed (up) along coaming, across stern (top of pic) and then down on opposite side. Receiver has been replaced as I thought this was causing the problem, BEC's have been disconnected and battery pack installed. Only one servo lead runs anywhere near the aerial.

Has anyone had a similar problem? What was the solution?
Any help or suggestions would be appreciated.

TIA,

Scottie.

adzam · 23-07-2006

scottie
i`ve had a similiar problem, i found it to be a loose shaft coupling, it only happened in the water when there was a load on the shaft,
just check shaft uninons, props etc for tightness.

in fact anything metal which could be touching .

any loose terminal connections as well,

let meknow how you get on bud

Adz..

**Bunkerbarge** · 24-07-2006

Scottie, No real answers but just chucking some thoughts at you.

I too have had the slipping coupling effect but if yours speeds up and slows down as if the trottle is being opened and closed very quickly it seems more like an electrical effect.

If you can run the boat past yourself on the bank with the superstructure removed and see if it is one or both of the motors that are affected then it would help to narrow it down. Obviously if only one then you would be looking at that side only but if both then something common such as the receiver would be a possibility. Maybe put it in the bath and run it in the same way to see if you can identify which one. Another possibility would be to disconnect one side at a time and see if it persists with one or the other.

If only one side then I would suspect the speed controller but bear in mind Adz's purely mechanical suggestions. If it doesn't happen on the bench then it is load related and a slipping coupling is still a possibility.

If you are convinced it is both sides then you have to look at the receiver and its power supply. If you have already changed that and the problem persists then it is possibly only happening on one side.

Anyway, as I said just, some thoughts out loud that may be of some use. Let us know how it goes.

magpie · 24-07-2006

Yes I had that with my u-47. It then started going at half speed and making lots of noise. One of the props had fallen off!
Not enough loc-tight (I'm a dill)

scottie2212 · 24-07-2006

Thanks for your ideas. I've checked all mechanical and electrical couplings, there's nothing loose or dirty. I'm sure the fault is electrical but I can't think what it could be....

Since I'm only drawing about 20% of the current limit through the ESCs I don't see how they could be causing it... and why only on the water????
Yes Richard, it's both motors, I've had the top off and ran it against the bank so I can see what's happening, both motors are stuttering at the same time which is why I changed the receiver but I can't believe that two receivers are causing the same fault!

Still scatching my head,

Scottie

**Bunkerbarge** · 24-07-2006

Well at least we are eliminating things!!

I think we need to think outside the box a bit more now and I am certain that the next step isto try a differrent transmitter. I know it seems OK on the bench but not in the water but the water itself can play a part in the signal quality.

I have only recently been playing with a very cheap foam Cessna as a trainer without much success so far. Another story. One problem was a very erratic operation of the eirlerons which I couldn't resolve. We do tend to think of the radio transmitter as a solid state device that goes on forever but, of course the electro-mechanical transducer is a potentiometer on the end of the stick and these can fail. The Cessna one responded to a squirt of WD40 into the Pot, after taking the set apart, and working the stick for 5-10 minutes.

I would try the boat next with a differrent transmitter and see what happens.

scottie2212 · 24-07-2006

Thanks Richard but there's no problem with the transmitter. It's a Futaba 6EXA which I use for three different models including St Columba, I have no problems with any other, just the pusher tug.
In desperation I'm going to swap ESCs from St C and put them in the tug. Then if things don't improve I'll swap the 'genuine' futaba receiver from St C as well.
If it still fails there's always the lump hammer...:-)

Scottie

**Bunkerbarge** · 24-07-2006

The only remaining (lonely) thought I have are that the receivers and crystals are all compatible. Don't forget that conversion one transmitters must be used with conversion one receivers and the crystals must also be conversion one etc...

If any of those are not right it will cause signal problems.

**wonwinglo** · 24-07-2006

Try re-routing the antennae,also make sure the rx battery is isolated as far away as possible from the other electrics,you could be getting spurious signals from the most unlikely source.
It is not unknown for harmonics to set in at certain points,it could be a freak one this,also make sure there is no 'metal to metal' linkage giving out harmonics,it could just be.
Roll out some Bluetak with a rolling pin and encase the RX battery into it,sounds daft I know but it works.

ajg141 · 24-07-2006

I'm following this thread with great interest as I've exactly the same problem on my Krick U25 (see avatar). The sub has only a single ESC but twin motors and it works perfectly on the bench but 'pulses' on the water - and it seems to be getting worse. My suspicion is that there may be a bent shaft that causes binding under load - or am I clutching at straws. I seem to have tried most of the other remedies unsuccesfully.

Andrew