Post
by DrkAngel » Jul 23, 2011 9:12 am
It started out as a little bit laughable, but is so persistent that it has become annoying ...
Forum members that rate their motors by how many watts they can pump into them!
They don't seem to recognize that pumping in double the watts, might produce 50% more torque, but also produces 350% the damaging heat.
1000w = 750w torque + 250w heat = 75% efficient
2000w = 1125w torque + 875w heat = 56% efficient
The "watt" rating, on an electric motor, is a measure of a motors power output!
Not the input watts!!!
A "750w motor" is equal to 1 horse power. (750w = 1hp)
The motor might require 1000w input to produce 750w peak output, this would mean that the motor is 75% efficient!
Various motors attain between 75-90% "peak"* efficiency!
Controllers vary widely in efficiency, also!
"Peak"* efficiency is between 80-95%.
Batteries vary widely in efficiency, also!
eg. SLA batteries expended at 1C are 60% of rated, Li-ion at .5C are about 100%+.
* - "Peak" efficiency is attained somewhere near 75% of top (no load) speed.
At a dead stop, efficiency is 0%, but improves as "peak" is neared.
At 5mph 1000w input, might produce, 20% efficiency, or 200w motor output.
.......... 1500w input, might produce, 15% efficiency, or 250w motor output.
750w output might require
750w/75% motor efficiency = 1000w/80% controller efficiency = 1250/60% SLA efficiency = 2083 watts used!
Yes, I do realize that dynamometer testing is the only accurate way to measure motor output, and beyond the reach of most everyone, but please ... realize that input watts is not directly relateable to output watts!
Last edited by
DrkAngel on Mar 04, 2017 9:38 am, edited 3 times in total.