richardv2 said:
Twice?? Could you please explain that.
"Twice the speed you need" sounds like a rule-of-thumb-type thing that tries to account for the acceleration that you need (without actually considering acceleration). Due to the way the motor speed decreases as you increase torque load on it, a motor that can move your robot at the twice the speed you need will output more torque if it's running slower at the actual speed you need, somewhat accounting for the torque+acceleration factor without needing to think much about it. I don't use that rule though. I use the following rule-of-thumb though since it's lower-level and still lets me attempt to characterize the motor entirely.
Rule of thumb: a DC motor operates most efficiently at 1/7th to 1/8th of it's stall torque. Conversely, this translates to a DC motor operates most efficiently at about 6/7ths - 7/8ths of it's no-load speed.
Rule (not a rule of thumb): DC motors output the most power at half stall-torque or half no-load speed (which occur at the same point).
You know the problem about rules of thumbs though (ie. sometimes false and no optimization)...I've used the 1/7th rule though to try and figure out more operating characteristics about motors where only stall torque is given. It seems to hold fairly true since I use it on every motor I come by where all the data is given to check it's validity and they have all come very close to the 1/7-1/8th range so far.
richardv2 said:
Suppose for the wheels I have, 200 RPM = 1 meter/second.
Wouldn't a 400 RPM rated motor be operating mostly in the 25-50% range of its rated speed. . . .While a 200 RPM motor would be operating at the high end of its range, which seems more efficient. So I guess I don't understand why I'd intentionally run a motor at the low end of its range.
Your example is too vague as you don't specify what "high" and "low" end of the range are (is it torque? or is it speed?)
One reason to oversize a motor is to account for the extra torque you may need at peak torque conditions. Another is a cooler running, longer lasting motor. You sacrifice steady-state efficiency however when running an oversized motor (oversized relative to steady-state conditions to be specific, there's never a point in oversizing the motor relative to peak conditions). Sometimes you have to oversize the motor by a lot beyond steady state conditions because the peak-loads required are much much greater than steady-state, and a motor sized near steady-state would stall, overheat and burn out whenever the peak-load was encountered.
You might run an undersized motor because it is cheaper, and smaller, and depending on the way you use it, may take less current and extend battery efficiency(batteries provide more total energy if smaller currents are drawn from them). This can be the case in intermittemt applications where the motor has off-time to allow cooling.
A DC motor runs more efficiently when it is closer to no-load (full-speed) than stall-load (zero speed). However, below the torque/speed where you get maximum efficiency, (at very close to no-load), the efficiency drops very fast (much faster than if you went beyond the max efficiency point to a lower speed/higher torque).
Represenative graphs:
**broken link removed**
You probably know this, but your last post makes me have to say this out loud:
Don't size a motor stall's conditions to your steady-state operating conditions.