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[StevenageMBC]

So then, clever people of Mayhem, a brain teaser for you...

In my sub, I'm running an Mtroniks Msonik 15a controller, and a Graupner Speed 500e, on 12v. All fused to 15 amps.

On the water, it'll run fine for a minute or few, then it'll die. No motor, but still with control surface and pump power. If I leave it to sit there for a minute, all life will return and off it'll go again.

I have little sense of smell, but the mother informs me that the ESC doesn't smell of anything, and the motor smells 'horrible'.

Any ideas? Fried motor? If so, what do we reckon is the lowest current draw to power 500 sized 12v motor out there?

Cheers,

Andy

[andrewh]

Uselessat3d

Good news - the tiewrap round the motor has not melted!

It doesn't matter so much what the motor is - its more important the load you have on it (hence the amps drawn) 

As you describe it the problem is almost certainly the ESC cutting out the drive circuit because of either overheat (most likely) or just possibly undervoltage (the battery may have sagged in volts because of the load).  Other circuits are still running because the BEC in the ESC is still operating.

As soon as the ESC cools its drive circuit enough it resumes playing.

Solutions -

• ESC with bigger power capability
  smaller prop
  Lower pitch prop
  Less volts applied to ESC
  (cure friction in drive train?)

andrew

[StevenageMBC]

The odd thing is that after 5 mins of continuous full power in the bath, the motor is warm, but the ESC is stone cold. It's not going over current, as it's fused at the rating of the ESC. It's running a 50mm 4 blader with absolutely no pitch at all, which i should probably change.

[malcolmfrary]

If you can check the actual current being used, that will help a lot with the guesswork. 
Very few fuses actually blow rapidly at the stated rating, often the blow value is 50-100% more than what is says on the box for an instant blow.  Try it with a 10A fuse, see if you get the same problems and answers.  If the fuse does not blow, its not excess current.
The drive train friction suggestion could be the answer.  If the motor is working too hard, it will get hot.  The cooling fins on the ESC will now pick up this heat and warm the insides of the ESC (heat travels BOTH ways).

[StevenageMBC]

I'll stick an ammeter on it in a bit then

The esc is up the other end of the sub, in a separate compartment, so there's no chance of heat transfer.

[nick_75au]

from andrewwh

undervoltage (the battery may have sagged in volts because of the load)

the battery may well be the culprit, it may have failed, giving the impression of being charged but the above happening under load, measure voltage under moderate load on the battery, if it drops more than a volt or so then its probably time to get a new one.

this will give the symptoms you describe,

Nick

[Subculture]

Added to the above comments, a bit of simple deduction will unearth he culprit.

Try running the motor directly off the battery on the bench, measure the current if you want. It should pull a very modest currrent (500E right?) with no load on it.

Next try it hooked to the controller, if it starts to play up, you know the source of the problem.

[jabba]

i use one of those motors (500e) in my bow thruster,i use 12v battery and a 40x prop,
the  motor barely gets warm,i also use a mtronics 25amp esc,never no problems.
these motors are very low drain.
sounds like you have a dodgy esc,
 ive also had a mtronics 20amp esc
which kept cutting out even though i was only pulling 10amp,
 hope this helps.
 jabba.

[StevenageMBC]

Got to think about changing it at some point anyway, I'm going to need a motor with one hell of a lot of ooomph to swing a 80 odd mm 7 blader...

[Subculture]

You just need a 10:1 gearbox, which you can set up to run either in the wet, or in it's own little module forward of the main module.

[malcolmfrary]

How does the motor perform when not coupled to the shaft?  It might be having to work too hard just turning the McHenery.

[nick_75au]

Speed 500 E     12v     6-12v     12,000     0.4A     2A     10A     67%     50mm     35.8mm     8.5mm     3.17mm     158g

Now the prop size is available it becomes a little clearer. If we take the data from Raboesch props it suggests a maximum speed for a 80 mm prop as 4400 rpm, Ive found about 3/4 of that speed works well for most displacement boats. so say 3000 rpm would be a good target.
A 4:1 gearbox would allow the motor to rev closer to its most efficient speed and give it enough torque to spin that big prop.

or

Speed 720 BB TORQUE     12v     6-13v     4,100     0.75A     3.0A     13.5A     75%     72mm     44.5mm     6.5mm     5mm     380g

Direct drive

Nick

[StevenageMBC]

Doesn't mean a lot to me I'm afraid. If I could fit one of these new fangled motors that have a piston and a crankshaft and go bang a few hundred times a second then you'd be speaking my language...

[nick_75au]

Have a read through the very top thread in this section.

http://www.modelboatmayhem.co.uk/forum/index.php?topic=11713.0

All will be revealed

Nick

[Subculture]

Don't forget that the blade area on a submarine prop will be significantly higher, as they are multi blade. 4000RPM sounds too high to me, and I think the motor would struggle to pull it round.