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Re: Back driving a CIM
Free speed drag torque for a CIM can be calculated based on data sheet parameters as follows:
Free power at 12V = Free current x 12V = 2.5A x 12V = 30 Watts
Effective resistance = 12V / Stall current = 12V/130A = 0.092 Ohm
Winding loss at free current = 0.092 Ohm x (2.5A)^2 = 0.57 Watt
Drag power at free speed = Free power - Winding loss = 29.4 Watt
Drag torque at free speed = 29.4 Watt / (5300 RPM x pi/30) = 0.053 Newton-meter
I can confirm from measurements in my motor lab that this theoretical figure is pretty close to what we really get, after the motor warms up a bit. Initially the drag (and the free current) are a bit higher because the grease and the brushes are a little stiff when cold.
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Back to the OP: yes it matters a lot if the motor is connected to a speed controller. The calculations above give drag at zero current; when the CIM is connected to a speed controller its induced voltage (back-EMF) will excite the controller and that excitation will cause current to flow, charging up the controller's capacitors and creating additional drag torque. Anyone who has pushed an FRC robot and seen the little LEDs come on knows what I mean.
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Richard Wallace
Mentor since 2011 for FRC 3620 Average Joes (St. Joseph, Michigan)
Mentor 2002-10 for FRC 931 Perpetual Chaos (St. Louis, Missouri)
 since 2003
I believe in intuition and inspiration. Imagination is more important than knowledge. For knowledge is limited, whereas imagination embraces the entire world, stimulating progress, giving birth to evolution. It is, strictly speaking, a real factor in scientific research.
(Cosmic Religion : With Other Opinions and Aphorisms (1931) by Albert Einstein, p. 97)
Last edited by Richard Wallace : 12-12-2014 at 16:48.
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