Using a laboratory dynamometer set-up similar to one the shown in Team 1218's Behind the Design 2012
, my lab recently made some measurements of a CIM motor heating while loaded to its maximum power, running at 12 Volts from a regulated dc supply.
The first graph shows the mechanical power, RPM, voltage, current, and temperatures at several locations taken using thermocouples, two of which were installed inside the CIM. The brush guide temperature reached about 120 degrees Celsius in about 70 seconds, at which time the voltage was switched off. Notice that the internal temperatures, both brush guide and air, continue to rise after that -- this is caused by back-flow of heat from the armature windings. Notice also that after the internal temperatures begin to decrease, the motor case temperature continues to increase for several minutes.
The second graph shows mechanical power and temperature at several locations, on a longer time scale -- minutes rather than seconds. The embedded photo indicates locations of the thermocouples used to record brush guide and internal air temperatures. Notice that even after 50 minutes, the motor is still noticeably warmer than room ambient temperature. Cool down could be accelerated using a fan, but even so about 45 minutes are required to get all motor locations down to room ambient temperature.
Three observations that I think are important for FRC teams:
(1) maximum power available from the CIM motor is very sensitive to internal motor temperature (at 12V the power was 330 Watts when the brush guide was at 23 degrees Celsius, but decreased to 230 Watts when the motor brush guide was at 123 degrees Celsius),
(2) motor case temperature is NOT a good indicator of internal motor temperature. A motor that feels slightly hot to the touch on the outside can be hot enough to cook a turkey on the inside, and
(3) 40A circuit breakers, which are required by FRC Robot Rules to protect the wiring in CIM motor circuits, will almost surely open before a robot could reproduce the load conditions shown here -- CIM current at maximum power is 60 to 65 Ampere, depending on internal motor temperature.