Two Motor Issue

[David Butler 3Participant @davidbutler3]

As a quick Xmas holiday project I've built a Tamiya USS Missouri 1:350 scale plastic kit. It is fitted with 2 x Graupner 280 motors wired in series to a Mtroniks Viper Marine 15A controller back to an OrangeRx receiver (all spare parts I had in my collection of bits). Powered from a 7.2V battery everything works fine – but one motor always starts up later than the other and seems to run a lot slower. Only at about 3/4 throttle do both run the same speed. This is a bench test and I'm not sure what it will be like in the water and frankly I'm not that concerned given the 'devil may care' approach I've adopted with this quick RC conversion (I've even glued in the motors and servos with epoxy). I might add that this lack of concern is only for the RC conversion and I've been very careful with the main kit build given that it is likely to be a static model with only rare trips to the lake. I suspect that given the motors cannot be made exactly the same the different speeds are inevitable.

[David Butler 3Participant @davidbutler3]

A quick RC conversion with electric motor issues

[David Butler 3Participant @davidbutler3]

And here are some photos.

Note the naughty use of epoxy resin everywhere which means that if it breask then I'm sorta stuffed. Note that is not the sort of bad behaviour I usually get up to.

[shipwrightParticipant @shipwright]

Hello David,

I was slightly surprised to read that you had wired the motors in series – usual practice is to wire in parallel to the ESC if operating from a single ESC. Was there a special reason for the series connection for the motors ?

Ian

[Charles OatesParticipant @charlesoates31738]

I agree that the series wireing is the likely problem. When wired in series, one motor is acting a bit like a variable resistance, which changes the powrr availble to the second motor.
Can you change the connections to wire the motors in paralel? This should solve the issue.
My only reservation is that you will have a lot of volts going to the 280s.
Good luck,
Charlie.

[David Butler 3Participant @davidbutler3]

Thanks for the prompt replies. I'd seen the motor configuration and some pictures on a website forum and therefore assumed it was correct. The theory is that the motors are only 6V and hence wiring them in series would prevent any damage from the ESC. However, I can see that a parallel configuration and the interaction between the motors is the main issue. Hopefully the motro specification can cope with a little more volts than stated.

[Charles OatesParticipant @charlesoates31738]

The 280s are fairly forgiving, and you will probably need to keep them throtled right back to stop the boat taking off like a scalded cat. I think you will enjoy sailing this one.
Charlie.

[shipwrightParticipant @shipwright]

If you have a digital multimeter with an adequate amp rating you could do a test in the bath (assuming that your model will fit in the bath). Graupner states that the 280 operates most efficiently when the current is about 1.6 amps with a 6 volt supply. Note that the current can rise to about 7 amps when the motor is stalled. Given that the motor will operate in excess of 10,000 revs even when loaded, you will need a very small prop to avoid overloading the motor. From your photos is looks like you have 3 bladed plastic props – I assume that they are small diameter (eg 20mm) ?

If you find that the motors are consuming too much current (ie they get too hot) you can insert a wirewound power resistor in series with each motor though this is obviously at the expense of efficiency. Let's assume that at full throttle each motor has an armature current of 2 amps. Then a 0.5 ohm resistor will reduce the voltage by 1 volt and will dissipate 2 watts (so you need a resistor with a power rating well in excess of 2 watts in order to keep the resistor temperature low).

Ian

[ashley needhamParticipant @ashleyneedham69188]

David. You are in no danger of burning out your ESC. IF the motors get hot, it will be from over use of full throttle if you have too big a prop fitted.

I have brass 3 blade 25mm props on a converted Severn with twin 280`s and a 7.2v pack and it goes like a rocket. Aslim light ship like yours should be fantastic fun at speed. It will need hardly any throttle to go quite fast in any case

Below, one speed 280, very brisk, runs all day at full throttle

Ashley (courtesy Argos, £20 incl tanks and aircraft)

[David Butler 3Participant @davidbutler3]

An excellent model Ashley!

Did you have to add a keel to your ship? The Tamiya Missouri has a flat hull so that it fits on the stand and I was wondering whether this will be an issue once on the water.

[David Butler 3Participant @davidbutler3]

It uses a three-bladed 25mm prop.

I had great trouble sourcing a 2mm prop shaft as they were out of stock and ended up buying a prop shaft / blade set from ebay. The quality is not so good as the shaft ends have plastic inserts but for about £5 a pair it should perform ok.

[ashley needhamParticipant @ashleyneedham69188]

David. thank you. I had to glue the "tank" draw shut, cuttoff and fill the wheel wells and modify the innards to provide some room for the bits. The only work above water was spraying the whole thing grey and painting the deck (using a very small brush) brown and blacking up the bollards here and there. The detailing on this, as a kids toy is very good. It needs no ballasting and is very stable. HOWEVER, it is much heavier that yours will be, and a bit wider compared to its width.

Try Marks model Bits for parts, propshafts.. he will make to order all at the same price. I happen to have some 3mm shafts and 25mm props left over from something else. I would have used a 4mm shaft if not, due to the difficulty (or expense) of finding props at this size. Graupner I thing may do soem at 2mm. An alternative would be to cut down a larger prop. I am aware that this will do nothing for the effiiency of the prop, but I think it will not make much difference considering the power available etc.

Ashley

[David Butler 3Participant @davidbutler3]

Motor configuration now changed to parallel wiring rather than series – and everything works just fine.

Thanks for everyone contributing to this thread – and I'll post some photos of the first sailing very soon.

[David Butler 3Participant @davidbutler3]

Well now the Missouri is nearly complete:

But the aerials remain incomplete between the various masts.

I can't manage to follow the Tamiya instructions and melt the black sprue to make the thing threads. OK so I did manage to do one length but failed to glue the threads onto the masts. Then I tried thing rigging thread but failed to get the right tension, then I got cross and gave up. I might try again….

[SoupParticipant @soup]

David try this as an alternative to the usual 'stretching over a candle flame' method.

**LINK**

I have not tried this method, the 'stretching over a candle flame' just seems to work for me.

[Kimosubby ShipyardsParticipant @kimosubbyshipyards]

Hi David,

for really small scale threads, ropes etc try the copper windings from a very small electric motor. Ok they'll be copper coloured, but they will be very small diameter and can be very lightly sprayed black with airbrush or black u/c. The thing is, being wire you do not need to tension, but you need to strectch on a former prior to use to get nice and straight.

I've used on small scale card models.

Kimosubby

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