A friend of mine has a Daewoo machine with a Mitsubishi MDS-A-SP-260 drive for the spindle. The drive works fine when lightly loaded, but when loaded more than about 35% for 20 minutes the drive shuts down on alarm number S01 003B. The book says this alarm is "intelligent power module" (IPM) overheat. When this happens the heat sink on the MDS drive is barely even above room temperature so it appears to be bogus. We have replaced all three power modules and the problem has not changed. Does anybody know how this temperature is sensed? Any idea what to check next? Thank you.

This alarm is a bit cryptic, IIRC it refers to the spindle motor temp sensor, check that the spindle fan, if it has one, is running and the filter is clear.
Check the temperature of the Sp. Motor.
Also check the spindle % load on the spindle status screen.
Al.

Thanks for the reply. Are you saying that this alarm is actually saying that the motor itself is too hot? I do know that the fan on the drive is functioning and the IPMs are not hot. The spindle load on the screen is saying approx. 35%.

Acording to the Spindle Manual, it is the Spindle motor sensor, this is usually buried in the winding. The sensor wires come out of the teminal box, they may be on a 9 pin plug, depending on the model.
In any case when the fault happens, and if you have to wait for a while to reset the alarm. Jumper the sensor out and see if you can reset, right-away.
If the motor is not hot, it may be because of a sensor problem.
Al.

The overheat protection circuit in the IPM may be malfunction. You may try to disable the alarm signal or just change a new IPM.