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Author Topic: MFA 850 - Brushless replacement.  (Read 7183 times)

Mankster

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MFA 850 - Brushless replacement.
« on: July 19, 2010, 11:51:28 AM »

I have 38" fast fishing boat hull ( Models By Design -Revenge 1/12 scale http://www.modelsbydesign.co.uk/model_boats.aspx   ) thats fitted with 2x MFA 850's on direct drive to 4 blade 40mm scale brass props. What would be a suitable brushless replacement for these motors on 3SLipo or 12v NiMH to give similar or better speed/performance?
Cheers

nick_75au

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Re: MFA 850 - Brushless replacement.
« Reply #1 on: July 19, 2010, 12:20:51 PM »

Operating Voltage 12 volts.
Current approx. 10.82A at max efficiency.
RPM at 12.0v - 9778 at max efficiency.

9788 / 12 gives a Kv of around 815 need slightly more for Lipo due to lower voltage
and about 120 watts.

Do you know what  current the MFA motors are drawing now

 I have run the 1000 Kv in my springer with a 40 mm prop on 12 volts and it handles the current Ok, A lower kv will reduce the current. For a bit of head room a 28mm diameter motor in the same Kv range would keep the power lower in the motors range for cooler running First link. A 36 mm diameter is probably the best bet, second link.

any motor with power handling greater than the MFA and similar Kv will give you the performance you want

http://www.hobbycity.com/hobbyking/store/uh_viewItem.asp?idProduct=8139&Product_Name=Turnigy2836_Brushless_Outrunner_1000kv

http://www.hobbycity.com/hobbyking/store/uh_viewItem.asp?idProduct=3888&Product_Name=TR_35-48-B_900kv_Brushless_Outrunner_Eq:_AXi_2826

Regards
nick

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Mankster

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Re: MFA 850 - Brushless replacement.
« Reply #3 on: July 19, 2010, 01:56:57 PM »

Operating Voltage 12 volts.
Current approx. 10.82A at max efficiency.
RPM at 12.0v - 9778 at max efficiency.

9788 / 12 gives a Kv of around 815 need slightly more for Lipo due to lower voltage
and about 120 watts.

Do you know what  current the MFA motors are drawing now

 I have run the 1000 Kv in my springer with a 40 mm prop on 12 volts and it handles the current Ok, A lower kv will reduce the current. For a bit of head room a 28mm diameter motor in the same Kv range would keep the power lower in the motors range for cooler running First link. A 36 mm diameter is probably the best bet, second link.

any motor with power handling greater than the MFA and similar Kv will give you the performance you want

http://www.hobbycity.com/hobbyking/store/uh_viewItem.asp?idProduct=8139&Product_Name=Turnigy2836_Brushless_Outrunner_1000kv

http://www.hobbycity.com/hobbyking/store/uh_viewItem.asp?idProduct=3888&Product_Name=TR_35-48-B_900kv_Brushless_Outrunner_Eq:_AXi_2826

Regards
nick

Thanks, I like the sound of that 36mm motor. How about an innrunner, aren't they supposed to have bettercontrol at lower RPM's/ easier to mount cool etc..? How about this one
http://www.hobbycity.com/hobbyking/store/uh_viewItem.asp?idProduct=9823&Product_Name=Turnigy_540L_V-Spec_Inrunner_w/_Impeller_1025kv



Pair of these will be enough for that boat I'd have thought-

http://www.hobbycity.com/hobbyking/store/uh_viewItem.asp?idProduct=5112&Product_Name=Brushless_Outrunner_TowerPro2409-18_1000kv_/_180W



Those things are tiny!

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Re: MFA 850 - Brushless replacement.
« Reply #4 on: July 19, 2010, 02:20:10 PM »

Quality of speed control is more dependant on the quality of the magnet spacing on the motor and the controller used. The latest controllers tend to be better especially at start up, to the extent that there is little difference between these and brushed motors.

Outrunners are better IMO than inrunners for marine use unless you want to wind hideous amounts of RPM.

Those motors maybe small, but they are rated at 180 watts and two of them equals almost half a horse- just how fast are you planning on making this fishing boat go?!!

Andy

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Re: MFA 850 - Brushless replacement.
« Reply #5 on: July 19, 2010, 03:11:11 PM »

As a side note, I've been experimenting with brushless cans for the fix-a-upper Aquabat that Ron kindly brought along to the Dive-in for me.

I'm decided to splash out and go with a $6 motor- http://hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=5354

I tried this with the original 50mm twin bladed prop in a bucket- well I was impressed, I could just about hang onto it with three lipos and my watt meter showed it was pulling about 10A. This is well within the motors spec.

That equals about 110 watts or so.

Should be a bit more lively than stock (not really trying to compete with a Subdog on with this one as the hull shape is quite draggy), as the original mild 500 sized motor was only drawing about 25 watts, which is about right for a 6 volt 4AH gel cell.

I'm also going to try the twin leveller split rear hydrovane approach that Norbert uses on his Dolphins, as torque roll is likely to be a bitch with a 50mm prop cranking about 10k on a sub that displaces about 2kg.

debssnal

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Re: MFA 850 - Brushless replacement.
« Reply #6 on: July 19, 2010, 09:27:18 PM »

Hi,
    this is the motor you need x2
http://www.giantcod.co.uk/xyh4250-800kv-outrunner-p-405282.html
you will also need x2
http://cgi.ebay.co.uk/Seaking-60A-Brushless-Electric-Speed-Controller-426U-/160431674732?cmd=ViewItem&pt=Radio_Control_Parts_Accessories&hash=item255a79156c
dont forget you are lucky to get a mfa850 to turn at more than 8000rpm under load.
hope this is of some help to you
kind regards
Alan
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Re: MFA 850 - Brushless replacement.
« Reply #7 on: July 19, 2010, 09:40:06 PM »

Those motors are over half a horsepower each, in a boat a tad over three feet. You're kidding aren't you?

Mankster

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Re: MFA 850 - Brushless replacement.
« Reply #8 on: July 19, 2010, 09:54:14 PM »

Thanks guys, so if i am running the same props at the same voltage on 2 different motors with the same KV, they should both pull the same/similar amps?? And the bigger more powerful motor should rev a little higher closer to its no load KV/ run cooler,  because its not being bogged down as much realative to it max power???

nick_75au

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Re: MFA 850 - Brushless replacement.
« Reply #9 on: July 20, 2010, 09:42:58 AM »



I'm decided to splash out and go with a $6 motor- http://hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=5354

I tried this with the original 50mm twin bladed prop in a bucket- well I was impressed, I could just about hang onto it with three lipos and my watt meter showed it was pulling about 10A. This is well within the motors spec.

That equals about 110 watts or so.

Should be a bit more lively than stock (not really trying to compete with a Subdog on with this one as the hull shape is quite draggy), as the original mild 500 sized motor was only drawing about 25 watts, which is about right for a 6 volt 4AH gel cell.


That really surprises me, though what is the pitch?, my 1250 Kv motor draws 50 amps on a carbon racing prop on 3 cell though the pitch of that is closer to 3 inch

Nick
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Re: MFA 850 - Brushless replacement.
« Reply #10 on: July 20, 2010, 10:31:28 AM »

Not sure exactly- would have to measure it. It's a fairly bog standard plastic two blader with a fairly coarse pitch, I'd say about 1.3 that of the diameter is my best guesstimate.



Regarding the KV and current, nope, it depends on the diameter of the stator and can, as this directly affects how much torque the motor can deliver and this is independent of KV, although the two do both affect the torque of the motor. e.g in brushed motor terms a speed 720 running at 4500RPM will give considerably more torque than a speed 500 because the armature is a larger diameter.

nick_75au

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Re: MFA 850 - Brushless replacement.
« Reply #11 on: July 20, 2010, 11:36:53 AM »

That looks like a much finer pitch than the prop I've got, which explains the difference in current. That prop was too much for a 36 mm outrunner %%

Nick
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Re: MFA 850 - Brushless replacement.
« Reply #12 on: July 20, 2010, 06:01:37 PM »

Fair enough. Perhaps you have a prop more aimed at i.c. use. Prop shop general purpose props are about 1.4 times the diameter in pitch, but their fast electric range are between 1.2-1.4.

Brushless motors can handle the higher pitched props, whether the hull can handle the power is quite another issue isn't it.

I wouldn't want to go any coarser than this for the sub it's on, which is only about 20" long.

Brushless motors produce gob loads of torque compared to brushed equivalents and I think a lot of people are over egging the custard when swapping them out.

debssnal

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Re: MFA 850 - Brushless replacement.
« Reply #13 on: July 22, 2010, 09:11:37 PM »

Those motors are over half a horsepower each, in a boat a tad over three feet. You're kidding aren't you?
no i am not kidding.i have a set for my Revenge.the motors are just under 1hp each.
regards
Alan
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Re: MFA 850 - Brushless replacement.
« Reply #14 on: July 23, 2010, 12:03:51 PM »

Sounds extreme to me.

Mankster

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Re: MFA 850 - Brushless replacement.
« Reply #15 on: July 23, 2010, 01:51:26 PM »

I have gone for the 36mm Turnigy 900Kv ones. I am sure its more than the boat need but it better to have too much than too little! Next step is to get some high capacity Lipos  to extend running time, or joing a couple of packs in parallel.
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