Please explain watts/amps/volts

[Chenige]

How about this motor: http://www.hobbypartz.com/96m26-car-3650-2950.html

Watt: 900
No-Load Current: 2.1A
Load Current: 55A
No-Load RPM: 5W
Load RPM: 3.7W

First what does the 5W and 3.7W mean? What do they mean by load current. Seems like every motor uses different specification descriptions.

2nd, if I go with 3S I get: 900 watts/11.1V= 81 amps

Is this the max amps recommended for the motor? What is the maximum prop size that would keep me under the 81 amps at the ESC?

[lohring]

In my opinion amps are the real limit. It would take higher voltages than we are likely to run to jump any brushless motor's insulation. Since lipo voltages don't pull down much with load, it's easy to get really high current draws. Higher current carrying capacity is always a good thing.

Lohring Miller

[Make-a-Wake]

68 amps is what its "happiest" at, more just stresses it and heats it up. As sailr pointed out, the prop has huge input on the amp draw/wattage. In a boat app, the 3656 you posted should probably run a 38-42mm prop on 3s, if you throw a 50mm on it, amp draw soars as well as heat, and if continued to run the motor would fry.

Here is a similar size motor with the same kv and the wattage output rating is 865............much more realistic but still a bit low.

http://www.hobbyking.com/hobbyking/s..._Inrunner.html

[sailr]

Most motor manufacturers actually cheat. They take the highest voltage the motor can stand and the highest amps the motor can stand and multiply them to get the watts. In reality, the motor may never pull that many amps in an aircraft application and may exceed it in boat application. Watts are irrelevant in aircraft. THRUST is the most important factor. Watts can be changed in any application by changing the prop, battery, water conditions, (rough or smooth), proper trim of the boat/airplane, etc.

I pay no attention whatsoever to the meaningless WATTS published for a particular motor.

[sailr]

Most motor manufacturers actually cheat. They take the highest voltage the motor can stand and the highest amps the motor can stand and multiply them to get the watts. In reality, the motor may never pull that many amps in an aircraft application and may exceed it in boat application. Watts are irrelevant in aircraft. THRUST is the most important factor. Watts can be changed in any application by changing the prop, battery, water conditions, (rough or smooth), proper trim of the boat/airplane, etc.

[Chenige]

The wattage listed on those motors is very conservative, that motor can easily draw 80 amps in an aggressive setup. That would be over 1000 watts which is more realistic for a 3656 at 2700kv.

Why would they list conservative wattages? Would they not put the highest wattage down to make the motor more salable/easier to calculate needed ESC? I just do not get it.

Is 475 watts the maximum the company wants the motor to put out for durability?

[Make-a-Wake]

The wattage listed on those motors is very conservative, that motor can easily draw 80 amps in an aggressive setup. That would be over 1000 watts which is more realistic for a 3656 at 2700kv.

[Chenige]

OK so Watts = amps x volts

In the new turnigy motors I see they have all 3 specs and they calculate great.
Max amps = 120a
max volts = 14.8v
max watts = 1776 watts
http://www.hobbyking.com/hobbyking/s...dProduct=17699

1776 watts = 120a x 14.8V
I can understand that.

Another motor list these specs:
Max power = 475 watts
Max Load = 68 amps
Voltage range= 10 - 18volts
Required ESC = 80 Amps
http://www.hobbyking.com/hobbyking/s...idproduct=5146

So if I run 10 Volts: 475/10=47.5 amps
If I run 18 Volts: 475/18=26 amps

Nowhere do I get the 68 amps. What am I missing?