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Author Topic: Very hot m400 motor  (Read 4740 times)

ferny

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Very hot m400 motor
« on: July 16, 2016, 06:24:28 pm »

Hi all

Have just built a small launch she is about 15 inches long and weighs just over a 1kg, has an m400 motor and 28mm sport type  prop running on a 3s lipo. Any way took her for her maiden voyage and after 10 mins the motor was to hot to touch and smoking

So is the motor to small/under powered for size of boat or could it be the prop?

Thanks in advance

Jon
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malcolmfrary

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Re: Very hot m400 motor
« Reply #1 on: July 16, 2016, 09:33:38 pm »

Both. 
In all probability, it is probably a racing type motor with a very low wind count intended to run at incredibly high revs.  Too much prop will not allow the motor to rev high enough, causing high current and a lot of heat generation.  In one boat that we saw recently, where something very similar happened.  When it drifted back in, it was noted that the motor had got hot enough to melt the solder, which allowed the wires to disconnect.
On a 15" boat I would be looking at a 20mm or thereabouts prop.  With the right prop, the right 400 should go very well.  It is one that is intended for 11.1 volts?
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ferny

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Re: Very hot m400 motor
« Reply #2 on: July 16, 2016, 11:19:19 pm »

That has confirmed what I thought, yes motor is rated to 12v will get my self a 20mm prop and see how it works

Thank you for your help
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tigertiger

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Re: Very hot m400 motor
« Reply #3 on: July 17, 2016, 02:11:46 am »


A question from the unknowing.I don't know fast boats, but would water cooling be something to consider?
I have seen water coils/pipes around motors advertised somewhere.
If not, where would these coils be used?
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ferny

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Re: Very hot m400 motor
« Reply #4 on: July 17, 2016, 08:40:20 pm »

Well ran her again today on 7.2 volts not as hot so definitely  going for  a smaller prop.
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andrewh

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Re: Very hot m400 motor
« Reply #5 on: July 18, 2016, 02:09:20 am »

Hi, Jon


The real question is which M400?  This means only 3-pole, armature is 40mm long and made (maybe) by Mabuchi


For boats around this size I used (when I had 400 motors) a 23mm dia racing prop and let it really spin (this was a Speed400 7.2 on (Amazingly) 7.2 nicads.


Motor hotter than hand hot is , as you surmised, death


The other limit (in an aircraft) is 10 Amps continuous, so probably a little lower in a boat (less fresh air)


fwiw I have now gone brushless  - and use a desperately cheap and simple bell motor (generic 2808-15T of about 1500kV) as replacement 


regards,  andrew

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ferny

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Re: Very hot m400 motor
« Reply #6 on: July 19, 2016, 08:48:21 am »

Andrew


Its an Mtroniks M400 rated at 12v so i guess only really a 380? I may go brushless in the future but at the moment it has been built to a budget hence the second had hobby wing 60 amp esc for a fiver!





Jon
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andrewh

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Re: Very hot m400 motor
« Reply #7 on: July 19, 2016, 09:48:43 am »

Hi, Jon


Not familiar with the MTronics but if it is happy at 12V then your prop is too large


I am now remembering that I took advice from George Turner and used a Graupner racing prop about 23mm diameter - presumably no longer available easily. 


Just find something smaller - or stick that one in a dremel and sand it down with coarse sandpaper (on a backing, of course)


andrew
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malcolmfrary

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Re: Very hot m400 motor
« Reply #8 on: July 19, 2016, 09:49:59 am »

Looking at the datasheet on http://www.mtroniks.net/prod/Motors-For-RC-Boats/M400.htm it claims 27000 rpm unloaded on 12 vols, or 20000 loaded.  Wattage rating is 20 watts, so this would imply a very light loading to avoid cooking the motor.  Small, 2 blade prop.  Anything too big (too much diameter, too many blades, too much pitch, any combination) will take it over its wattage rating.
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ferny

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Re: Very hot m400 motor
« Reply #9 on: July 19, 2016, 06:03:20 pm »

Thanks for all the advice

Andrew what esc would you recommend with the brushless  motor you mentioned?
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andrewh

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Re: Very hot m400 motor
« Reply #10 on: July 21, 2016, 08:07:41 am »

Jon


it won't help very much, but I used a 10A forwards only brushless ESC - now readily available for virtually no money.  we let the smoke out of it at Wicksteed Mayhem sail in many years ago and replaced it with an 18A  fwd and reverse car ESC from Giantcod (when it existed)


Just going back to the ratings you mention 20watts.  At 12V this is about 1.5Amps, or a mere trickle of current.  The rule of thumb for Speed400s is 10A and at say, 7V this would be 
70watts.  Note Speed400s had metal backplates so they cooled their brushes much better than a plastic-backed 400


What would I use with a simple brushless? - well I would look for a fwd/reverse ESC of about 20Amps (if I trusted the vendor) or 50A (from unknown sources).  I have never bothered with programming cards, and in fact have never even attempted to reprogramme and ESC, so the default settings are acceptable (to me)


Incidentally mounting my BL motor might be of interest.  The 400 motor it replaced fitted into a half-round trough of the right diameter and was held down by rubber bands.
So I took an old, fried S400 can and twisted it into dense blue foam to produce a foam cylinder of the right diameter.  Then drilled the foam cylinder (through the can) and opened the hole out for the 6mm diameter motor tube on the brushless. 
Poked the foam out of the can, glued in the motor into it with with silicone and rubber-banded the assembly in place


hope this helps
andrew









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ferny

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Re: Very hot m400 motor
« Reply #11 on: July 21, 2016, 06:26:21 pm »

Thanks Andrew very useful


have now changed the prop, same diameter but different pitch.


Before :



After:



Have the same prop on another boat and at 12v it pulls around 1.2 amp at max chat so if my maths is correct about 14watts so should run cooler in theory!


this is what it is driving:






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Stan

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Re: Very hot m400 motor
« Reply #12 on: July 21, 2016, 10:45:29 pm »

Hi the M F A 380  motor revs at 26000 at 7.2 volts and the stall current is around 26 amps.I think the 380 only rated up to 7.2 volts.Is the m400  a speed 400 ?In your model a M F A 385  would  have been fine  get rid of the plastic prop  and if finances allow a small prop from prop shop.Course prop,large diameter,direct drive may give problems.

Hope this helps
My thoughts on this matter only

Stan
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malcolmfrary

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Re: Very hot m400 motor
« Reply #13 on: July 21, 2016, 11:36:05 pm »

On general principles, any given can size for a motor will be capable of handling a given number of watts as heat to be got rid of before it cooks its surroundings and/or itself because the dissipation ability is limited by the area of the outside of the can.  It is possible to increase this by adding extra area (cooling coils, fins etc). 
In that size of motor there is room for just so much.  This includes the magnets, the brushes and commutator and the armature assembly with the wire wound onto it.  On the armature, looked at from the end, there is the metal of the armature arms and the space between, which gets filled as far as possible with wire, either a lot of turns of thin wire (high voltage, low current) or a few turns of thick wire (less volts, lots of amps).  The more amps wanted (therefore power at any given voltage), the more room the brush/comm part should take up and the less space is available for the rest.  All a question of balance and compromise.
To make a 15" boat go as fast as possible as long as possible with that physical size of motor, I would be looking for the smallest diameter 2 blade prop I could find to let the motor spin up while minimizing the loading.  This would compromise acceleration, but the top speed that the hull could allow would be available along with a hopefully acceptable run time without melting the motor.  Pitch and blade shape are two bonus cans of worms that either increase fascination or build frustration, depending.

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ferny

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Re: Very hot m400 motor
« Reply #14 on: July 22, 2016, 10:19:15 am »

Thank tbh the reason I went with the current prop was because it was less than a fiver! But I fully understand what you mean and it makes total sense.
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Stan

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Re: Very hot m400 motor
« Reply #15 on: July 22, 2016, 10:31:26 am »

HI Ferny have you now sorted out the problems? I can be pain at times getting things right I am sure you will get it sorted hope all goes well.

Stan
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ferny

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Re: Very hot m400 motor
« Reply #16 on: July 22, 2016, 11:09:06 am »

Im working on it, did a test in the kids pool at with the new prop she was pulling between .98 and 1amp on 7.2v. Hope to test the speed and endurance on open water today. Am also looking in to bladed props but not found one yet.


Jon
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andrewh

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Re: Very hot m400 motor
« Reply #17 on: July 25, 2016, 02:13:45 am »

Hi, Jon


Glad you are seeing light at the end of the whatsit.
The current you measure with the boat stationary is greater than it will draw when moving - my guess is about 20 to 30% higher, so in actual running you will probably be pulling 2/3 Amp at full throttle.  But even at 1A that is 12Watts , which seems to be within the motor specification


andrew

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ferny

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Re: Very hot m400 motor
« Reply #18 on: July 26, 2016, 11:52:46 am »

Hope to test her tonight fingers crossed!
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ferny

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Re: Very hot m400 motor
« Reply #19 on: August 01, 2016, 04:36:49 pm »

Have fitted a 20mm prop will report back after a test run.
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ferny

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Re: Very hot m400 motor
« Reply #20 on: August 09, 2016, 07:56:44 pm »

Problem solved!

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