Wow. I can’t believe I missed that. Thanks.
We were doing some Physics (yes, finally there is a purpose to that class! ) and we calculated a speed of ~22 inches a second under a 5 pound load using two Tetrix motors idk if it is relevant to anyone but…
Hi Richard, for the results with the Tetrix NiMH battery, with voltage >14V, is there any way you could provide the info on RPM vs. torque (I’m curious how the stall torque increases with voltage–does it scale proportionately?) and current vs. torque? Then we can re-run our simulation for the various gearing and wheel size ranges we’re considering. We wanted to look at whole range of motor operation at ~14V, and not just the peak power point.
I realize you were unable to attach the raw data because of file size limitations, but if there is a way you could send us a more limited set of data that would be much appreciated. Thanks very much for your help!
*With Tetrix battery: (voltage > 14V)
Motor 1: 16.5 Watts peak output mechanical power at ~90 RPM, 178 RPM initial motor speed
Motor 2: 16.0 Watts peak output mechanical power at ~90 RPM, 173 RPM initial motor speed
Motor 3: 14.2 Watts peak output mechanical power at ~90 RPM, 172 RPM initial motor speed
Motor 4: 15.2 Watts peak output mechanical power at ~90 RPM, 176 RPM initial motor speed
NOTE: I wanted to include the raw data and plots of speed and power vs. torque; however, they cannot be attached to this post because the file size is too large (>101.8 kB).
Test Temperature: Ambient (22 degrees C)
Software: M-Test 5.0
Dynamometer: Magtrol HD 705-6N (Calibrated 01/12/2011 by NT) Dyno Controller: Magtrol 4629B (ML144)
Power Supply: Sorensen XG 150-11.2*
Yes, stall torque does scale proportionally with voltage, because increasing the supply voltage will increase the stall current.
Torque-per-ampere does not scale; it is the same constant (given by the magnetic flux linkage, which of course does not change) for all voltages.
When I get some time I will create a reduced version of the measured curves for speed vs. torque and power vs. torque, at each of the test voltages for each motor. We recorded too many data points to include the raw file.
With regard to climb rate modeling, there is a simple approach that skips the conversion to energy.
For your example of 22.5 W net output power (two motors @ 15W x 75% efficiency:
Converting units gives 22.5W * 0.7375 = 16.6 ft-lbf/s
Divide by the weight (in pounds) that you want to lift, and you get the theoretical top speed
16.6 /5 lb = 3.3 ft/sec,
or 2.8 seconds to climb 110 inches.
The trick, as you point out, is to get the right wheel radius and gear ratio to get to the peak power point for your weight. The estimate of efficiency is key, since it needs to include all sources of friction/torque. As I learned (again) this year, mis-estimating this can put you near stall, and smoke a motor.