CIM Voltage and Motor Suggestions

[Cuog]

DC motors can run at as much as double their intended voltage "safely" however 150% is usually the maximum recomended so 18V should be ok, just make sure to stop using it if the motors start getting too hot to avoid damaging the motors.

[sanddrag]

You could try one and if it doesn't work you're out $28. However, how about just using more motors?

[Kevin Sevcik]

Well at max power, the CIM is gonna be drawing around 100A at 18V, so it's going to have to sink about 1200W of heat or so. Which is a good bit. I'm not saying it's going to catch fire, but you're probably going to want some sort of cooling on it. I figure a ballpark estimate would be a rise of atleast 1 deg C every second the motor is running at max power at 18V, probably more.

And don't forget you'll want a pair of Victor 885s to power those.

[Richard Wallace]

Check out the CIM data given by its manufacturer. In particular, read these notes:

1) DUTY CYCLE:INTERMITTENT, CCW 3 MINUTES ON 2 SECOND
CW 3 MINUTES ON 30 MINUTES @64.0 OZ-IN,
2) LIFE:1000 CYCLES MINIMUM

shown near the lower right corner under the heading "Special Features." Admittedly it's a little cryptic, so let me try to parse it. In plainer English it says that, if you use the motor to drive a 64 oz-in (4 lbf-in) load that requires a cycle of motion in which it first turns counterclockwise as fast as it can from a 12V supply for 3 minutes, then rests for two seconds, then turns clockwise as fast as it can from a 12V supply for 3 minutes, and finally rests for 30 minutes before starting the same cycle again, then the motor will last for at least 1000 of these cycles. The load cycle thus described is typical of a winch, which is the application for which the CIM was originally designed. So the total operating time for this load rating (12V, 27A, 4 lbf-in, +/-4320 RPM intermittent reversing duty) is 6000 minutes or 100 hours.

Back to your question: can you get more power out of the CIM by increasing the supply voltage. Kevin correctly pointed out that you can approximately double the peak power if you increase the voltage by a factor of sqrt(2). Now your question is: can the CIM handle that?

The question amounts to asking how to de-rate the CIM for operation at higher voltage and higher loading. Cuog recalls correctly that dc motors can typically be operated at up to 150% of rated loading; however, this increase does not come for free -- the penalty for overloading a motor is that it will wear out faster. You may not care about this if your robot only needs to last for a few operating hours.

Two overload stress factors will tend to shorten the CIM's expected life. One is running at higher speed, which will cause both the bearings and the brushes to wear faster. The other is running at higher currents, which will cause the armature coils to run hotter, thereby degrading the life of the insulation, the commutator, and the bearings, since these are all parts of the overheated rotating assembly.

I wish I could give you a rule of thumb for de-rating the CIM -- something like "subtract half the operating life for every 10 Celsius degrees beyond the rated insulation temperature." But I don't have one, and I don't know who does.

[Kevin Sevcik]

I'll also point out that if you power just these two motors from even a 20Ah 18V battery, you're likely to get about 3-4 minutes out of a fully charged battery. Assuming the battery doesn't end up in a puddle on the floor. A constant 200A draw is a huge load.