I've been having a perodical problem with the Spindle on my Fanic 6M milling machine. It is a DC spindle. From time to time it will blow either 1 or 2 of the 3 fuses on the spindle drive. It also sometimes when going to start up the spindle, almost starts, then errors and stops. On the spindle drive the second led from the top comes on "over current". I seem to be able to start the spindle at a low rpm and it goes ok, but when there is a sudden change in speed, or I press the spindle stop button, it will error. It seems that sometimes if I just let it sit there and rotate at a low speed for awhile, maybe 1/2 an hour, and then come back, it seems to have solved the problem and it all runs ok again. Other days it just won't start, always almost starting and then an error. I have been talking to a Fanuc engineer on the phone and he says that it could be a faulty SCR. I removed the wire from each of the large terminals on each SCR and checked the resistance to the smaller terminal and they all seem to be around 15 ohms, some around 13 and some 18. The technicial also said I need to check between the large terminal and the base with a megohm meter, which I did at 250V DC but am not sure I am doing it correctly as I get no resistace value at all. I have been reading here a little about what Dan has mentioned, hopefully he will read this and respond. Or anyone else that may be able to help. The Fanuc technician also said that the AOS02, AOS03 and AOS04 IC's may be faulty. They are very expensive to replace so at the moment I want to make sure it's not something else first. Any ideas? I just went out and bought this megohm meter hoping I would find a problem but as yet havent. I read Dan's explination of how the system works with the smaller SCRs controlling the field current, maybe there is something wrong with them? Any help is greatly appreciated.
BigDamo