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Author Topic: Brushless motor questions again  (Read 2790 times)

craggle

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Brushless motor questions again
« on: February 16, 2016, 10:36:22 AM »

Okay.....

I need two motors for my 1:12th Severn lifeboat and think I know what I need but just wanted to see what others thought. The props are about 60mm diameter at a guess. Not sure on pitch though.
The motors will be run independently of each other with separate batteries and independent speed controllers.

I am thinking 600Kv brushless motors on 3s (11.1v) batteries giving a no load shaft speed of 6660rpm.
The alternative is a 400Kv motor on 4s batteries giving 5920rpm

I think I'm fairly close on my guesses there as the Speedline instructions suggest a 400Kv motor on around 12V and a local 1:12th Tamar build which is a similar size model has 460Kv motors on 3s (11.1v) batteries.

Is it more efficient to go for more volts and a lower Kv motor or a higher KV motor and less volts or does it really not make any difference?

Thanks, Craig.
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sparkey

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Re: Brushless motor questions again
« Reply #1 on: February 16, 2016, 11:17:29 AM »

A lower kv motor on higher volts will give you more torque,a higher kv motor on lower volts will give you about the same shaft speed but with less torque,the choice is up to you.......Ray. 
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microgyros

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Re: Brushless motor questions again
« Reply #2 on: February 16, 2016, 11:27:58 AM »

A lower kv motor on higher volts will give you more torque,a higher kv motor on lower volts will give you about the same shaft speed but with less torque,the choice is up to you.......Ray.
However, rewind more or fewer number of turns, keeping the same delta or star, onto the same core won't change the torque, just the kv (or voltage) proportionally.
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microgyros

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Re: Brushless motor questions again
« Reply #3 on: February 16, 2016, 12:16:41 PM »

...Is it more efficient to go for more volts and a lower Kv motor or a higher KV motor and less volts or does it really not make any difference?

Thanks, Craig.
I forgot you, Craig.
Smaller wire diameter, whether more turns (lower kv) or parallel winding, reduces eddy current loss. But that is marginal.

Another consideration is esc heat generated by the bec component. If the esc has a linear bec, its increased power dissipation on the higher voltage may make the difference between the need for additional cooling.

Why not ask Speedline?

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craggle

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Re: Brushless motor questions again
« Reply #4 on: February 16, 2016, 12:43:42 PM »

Thanks chaps

I'm leaning towards the 400Kv motor now running on a 4s (14.8V) battery for a bit more torque.

Speedline do say in the instructions a 400Kv motor on around 12V. As you can't get a lithium 12V battery I'm torn between 11.1v or 14.8v hence undecided which one to go for.
By BEC I assume you mean for the receiver supply? If so I was going to fit a lead gel battery either 6V or 12V to run things like the receiver, rudder servo, lights, bilge pump, radar motor etc from so I was going to disable the BEC from the speed controllers if they came supplied with it and use the lead gel battery for the radio supply.
Will a BEC still get hot if it's not in use?

Craig.
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microgyros

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Re: Brushless motor questions again
« Reply #5 on: February 16, 2016, 01:17:35 PM »

If the esc is capable of beyond 4s it possibly has a switching bec which doesn't burn off excess power. That should be in documentation.
Unused, there is no current draw, the bec component power loss is near to zero even if a conventional (linear) type. 
A charged 3s Lipo is above 12volt. That is why my suggestion to ask for clarification. You could send him a private message.
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inertia

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Re: Brushless motor questions again
« Reply #6 on: February 16, 2016, 02:25:08 PM »

Craig
If you buy 3S LiPo batteries and the model turns out not to be quick enough then you'll have to buy new batteries. If you fit 4S LiPo's then you'll have speed in reserve (you can always restrict full throttle via the transmitter, assuming you're using a proper radio and not a toy one). Neither the motor nor ESC will be bothered about the difference between the two voltages.
DM
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craggle

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Re: Brushless motor questions again
« Reply #7 on: February 16, 2016, 03:11:40 PM »

That makes sense too, Can always slow it down, harder to increase speed I guess.

Will be using a Futaba FF6 transmitter with a Corona 2.4Ghz conversion fitted. The transmitter does indeed have the limiting function along with mixing etc.

Think I'm nearly there now then. Thinking of Turnigy 5055 400Kv motors, Turnigy marine water cooled speed controllers (120A) and 5000mah 4s, 14.8v, 30C battery packs.

I know 120A seems excessive on the controller as the motors say 55A max but assumed it was safer to over-spec them than under and there seems to be a big gap between 60A rated speed controllers and 120A ones. Not much difference in the price though.
Guess I'll need to find a charger as well.

Craig.
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essex2visuvesi

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Re: Brushless motor questions again
« Reply #8 on: February 16, 2016, 03:42:45 PM »

Always wise to go a little higher spec. Chinese amps can be smaller than EU standard amps
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inertia

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Re: Brushless motor questions again
« Reply #9 on: February 16, 2016, 03:49:43 PM »

This is the charger I use. It's idiot-proof (which is perhaps as well...).
Beware cheaper imitations (esp the ones with a built-in power supply). http://www.ebay.co.uk/itm/IMAX-RC-X200-TOUCH-SCREEN-BALANCING-CHARGER-/191313882910?hash=item2c8b322f1e:g:W7sAAOSwajVUL9Cj
DM
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microgyros

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Re: Brushless motor questions again
« Reply #10 on: February 18, 2016, 06:54:46 PM »

I'm leaning towards the 400Kv motor now running on a 4s (14.8V) battery for a bit more torque.
I missed that torque business popping up again. I'm going to put a bullet in it this time.

Think of a change in the kv value as change of rated voltage. A 500kv motor on 4s would produce the same torque & rpm as a 1,000kv with 2s. The AMP x TURNS product, and the magnetic field, is the same. 

In this case, the 600kv on 3s will be the more lively if your 600kv & 400kv are two wind options of the same motor with same copper weight.

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canabus

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Re: Brushless motor questions again
« Reply #11 on: February 23, 2016, 03:24:07 AM »

HI
Have a look at the Heli motors on Hobbyking they go down to 149KV at about 2750 watts, that is serious power.
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martno1fan

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Re: Brushless motor questions again
« Reply #12 on: March 07, 2016, 06:45:24 AM »

Outrunners have more torque and as long as you have plenty of room inside the boat they wont need water cooling on the mounts.
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