Quote:
Originally Posted by ToddF
Very true, and good catch. I just took their numbers without checking their math first. My bad.
Let's take a look another similar motor listed: XK3674-1650KV Brushless Inrunner.
Specs.
# of poles: 2
Max Amps: 66A
Max Volt: 14.8V
Max Watts: 975W
Rpm/V: 1650kv
Running the math, the quoted max power actually equals the max current x max voltage. Derating to the 12 V on a FIRST robot (nominally) gives 792 Watts.
This one costs $31.86, so we get 1,584 Watts of power for $90.12 total.
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Also reduce it to the maximum currents allowed on an FRC robot (it has to sit behind a 20, 30 or 40A breaker, remember!), and you're down to 480W.
I find it impossible to believe that this motor operates at 100% efficiency. There's no power loss
at all due to friction in the bearings, or in generated heat in the motor?
The motors we deal with in FRC have been tested to death by teams, and the posted numbers are typically peer reviewed and confirmed to represent the output power available from the motor. The math we've done here can easily be applied to
any of our motors to find the input power available to the motor, with identical results.
The key here is to look at the
efficiency of the motor - that's where you'll be able to see actual gains in output power to your mechanisms. A supplier like BaneBots provides their motor efficiency right on the website. I ran a quick search, and I can't find any posted efficiency numbers for either of the motors you mentioned.
Granted, brushless motors are typically more efficient than brushed, but unless we know exactly how much more efficient, we can't really do a comparison. Is going from 70% efficiency to 80% efficiency (for example) worth ditching our current suppliers and sponsors? Is it worth ditching the known performance and longevity of our motors for new motors we have no experience with? Remember the debacle with the RS-775 case short a couple of years ago? I wouldn't want to see us make a drastic switch in our motors only to encounter something like that again... or worse!