Quote:
Originally Posted by 1683cadder
Very helpful, but please elaborate on this point.
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brushed DC motor torque curves are linear, such that peak torque is produced at stall and 0 torque is produced at free speed. Current is proportional to torque, so at peak torque the motor will consume peak current, and at 0 torque the motor will consume free current. In this example, I'm going to use ideal motor specs as listed on the mfg's website. All speeds are rpm, torque in N*m, current in a.
This example isn't as obvious with the CIM and MiniCIM, so I'm going to use the CIM and RS-550 from BaneBots. Here are the graphs of their torque curves. CIM is blue, RS550 is green.
http://www.chiefdelphi.com/forums/at...d=13711310 01
Now we have to decide what ratio to mix them at. If we mix them at the ideal ratio to match free speeds (it happens to be ~3.63:1 reduction on the RS550 to match CIM speed), we can simply solve for the torque at a given speed and add them to get the yellow line. If we don't like the yellow line, we can mix at whatever speed we want. Note that the line is still linear, so we can simply adjust for the slope in the gearing. For example, the blue line is 3:1 RS550:CIM, the pink line is 2.7:1 RS550:CIM, the white line is 4:1, and the red line is 5:1.
http://www.chiefdelphi.com/forums/at...d=13711310 15
The real reason to do this is for current. Current is related to torque, so we can graph current like we did for torque (vs speed), assuming constant voltage. I skipped ahead and mixed them according to free speed, and summed them. In this graph, pink is CIM current, sloped blue is speed-adjusted RS550 current, red is sum current, light blue is 120a, yellow is 80a, green is 60a, flat blue is 40a. Based on the flat current lines, the CIM is at ~60a when the RS550 is at ~40a.
http://www.chiefdelphi.com/forums/at...d=13711310 27
If we make this graph again with 2.7:1 mix ratio, we end up with (RS550 current changed to white, other colors the same). Now the two motors hit ~40a at the same (adjusted) speed. Once the output speed overruns the slowest motor it will become a generator and produce current, loading the motor overrunning it. That is why the sum current intersects RS5550 current at CIM free speed then goes below it, as the CIM graph continues under the X axis as a negative current.
http://www.chiefdelphi.com/forums/at...d=13711310 34
So now the RS-550 is running at a lower operating point, and will draw more current than it's ideally mixed current (it's share).
Why would you do this? If you know the motor can handle more than it's share, you can match such that you get more low-end torque for accel, and more evenly distribute current through the electrical system (40a breakers handling different motors see closer to same current), which can improve accel performance. However, the motor consuming more than it's share will produce more than it's share of heat also, so you have to verify that the motor can handle the additional heat (for a CIM + MiniCIM it's probably OK, worry about the RS550 mixed 2.7:1).
It's really all about where you want to optimize the current and heat, since you can adjust for torque in the gearing later. You could mix these two motors at 2.7:1 so the currents are more evenly distributed, or mix at 4:1 so the RS550 has to handle less heat.