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So Someone Please Explain.....

Old 12-13-2020, 04:28 AM
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Retiredat38
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Default So Someone Please Explain.....

Why everyone seems to demand to know what ones voltage and ESC is when selecting a Brushless motor?

Yes the voltage is important and the ESC needs to match the motor specs. But it's the motor is that is selected to do a specific job. i.e. turn a given prop at a given rpm. And once a motor is selected, the other components can be selected based on the motors requirements.

I want to turn a 3/4" prop around 5000 rpm under load. This means the motor will free run several thousand rpm more than that. So am I looking for a 1000 KV motor and a battery in the 7 to 9 volt range?

But what about the physical size of the motor? I figured I'd want something smaller physically based in large part on the physical limitations of the model itself. Another reason to go with a lower KV, more torque.

The ESC should be capable of handling the Amperage the Motor will draw. But why should it be selected before the motor? Doesn't make sense. Sort of like buying new tires and then going out to find a car they'll fit on.

The battery too, why must it be selected prior to the motor? Again, it doesn't make sense.

The prop, pitch, diam and rpm determines the work to be performed. The motor should be selected to do that work. The ESC and battery should be selected to properly support that motor. Anything else is bass ackwards to my way of thinking.
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Old 12-13-2020, 07:25 AM
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Got RPM
 
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“Everybody” asks that question because they want to supply you with the best answer. The work a motor does (watts) is determined by the voltage and the amps used. Watts = Volts x Amps. All the motor does is convert the electrical energy into mechanical energy.

If you need one horsepower (746 watts) to move a boat at a certain speed, then you can use 24 volts and 24 amps, or 12 volts and 62 amps, or 6 volts and 124 amps. So knowing the voltage chosen and the ESC capabilities are most important when choosing the motor. Once you know the power requirements and the voltage you want (6 volt batteries are cheaper but require a better ESC) then you choose the ESC which can handle the voltage and the amperage, then the motor size and Kv (rpm/volt). It all makes perfect sense, once you understand it.


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Old 12-14-2020, 03:35 AM
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I understand what you're saying. But it still comes down to selecting the components needed to do the work and I still say you should start with a motor capable of doing the work (or Watts) involved. And the rest can be determined and matched to that. But while you provide the formula for determining watts when you have voltage and amps, you don't say how to determine the wattage required for a given prop at a given rpm in a particular boat. Instead you require all components in order to do the math and decide if they're right or not. As for the Wattage, it appears you're simply taking a SWAG. Based on experience maybe, but a SWAG none the less.

So how does one determine the raw Wattage (or HP) required for a certain application without predetermined voltage and amperage?
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Old 12-23-2020, 03:55 AM
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Well it would appear real help is lacking. So I'll look elsewhere.
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