Servo motor overheat alarm

I have a Kiwa Excel Center with a 3m control. It keeps throwing a servo motor overheat alarm. I did some testing 1 axis at a time and found it to be the Y axis. The servo motors are Fanuc 5m model A06B-0642-B011. They are dc motors with brushes. Anyone have any advice on where to start diagnosing this problem.
Thanks Dan

I would start by cleaning all the brush dust out of the motor.

I need to ask the obvious... Is the motor pretty hot when the alarm trips?

Not hot at all, the alarm comes up while feeding or rapid. I took out all the brushes and blew the brush area out with air. The armature still looked like copper with a very slight amount of tarnish.

There are a plethora of diagnostics that come to mind.
DISCLAIMER: I am not that familiar with your machine or the software/firmware setup.
If you can get to the ball screw on Y are you able to turn by hand, with machine power off?
Does the control allow you look at the real time logic state of different sensors i.e. limit switches, thermocouples etc.. A lot of times this will quickly allow you to narrow the scope of your search.
Is it an overheat I the true sense, which it sounds like it ain't. At least on the boat anchor I have deal with it is some times difficult to determine if the controller saying the servo pulling too much current during a given period of time and alarms out to prevent an overheat. Look at your schematic and test the component(s) that sense heat at the component level. If is okay work your way back, eliminating a wire problem. Ohm out the windings on the servo if they are all close in value move on. Meg the cable from the servo to driver.
Again not being familiar with your bit of kit makes it hard for me to trouble shoot from my sofa, but I had similar situation.
Very large HMC was telling me an axis was pulling way too much current. Servo ohmed ok, cable ohmed out okay as well. Day later after plenty of head scratching and consulting with greater minds that mine. I decided to repeat the meg on the servo to driver cable, low and behold the cable was leaking big time. So what was different between test 1 and 2 ? Rain yup it had rained a lot the night before test 2. Turns out our shop was built on land fill, the pit in which the machine sat just a titch above the water table. So when the 10 year+ cable which was run below the pit was now wet. Yha had to be their but what I am getting at is work from simple to complex in testing stuff, if someone else did a test repeat it and by all means use your test gear as an aide (Have meters, meggers etc. that are not crappy cheap 2 reasons: it won't blow up in your hand if you mess up and will give good data.). A lot of times part or all of problem is located between the driver's seat and the steering wheel.

camaro_dan67 said:

Not hot at all, the alarm comes up while feeding or rapid. .....

Click to expand...

The OH alarm is activated by a NC thermo switch built into the motor. When the motor gets too hot the thermo switch opens the circuit to generate the OH alarm.

Does the OH alarm occur in approximately the same axis position? If so, that is a pretty good indication of a bad connection of failing cable. The wires for the thermo switch are in the encoder cable.

If the OH alarm occurs very randomly or intermittently at any axis position it could also be a connection or cable problem but could also be a flaky thermo switch.

The alarm is very random at any position. The thermo switch idea sounds like its worth looking into to me. I got this alarm once or twice a few years ago when i was taking some heavy cuts but only the other day did it come back and get frequent. Now that you mentioned the thermo switch it reminds me of a circuit breaker in a home electrical box. If you trip that type of breaker to many times it seems to make them wear out or easier to trip. Can you tell me where the switch is in this type of motor.

I tested the 4 pins on the motor to ground. Pin D to the case has a resistance to the case no matter what position the rotor is in. The other 3 have no reading.

Conflicting info found so I deleted original content

Vancbiker said:

The OH alarm is activated by a NC thermo switch built into the motor. When the motor gets too hot the thermo switch opens the circuit to generate the OH alarm.

Click to expand...

Is the thermistor built into the motor on the old DC servos? If memory serves, in the newer Fanuc drives it's part of the encoder. Cheaper and easier to replace.

For the motors equipped with a thermo switch it was bonded to the case in between the magnets.

I learned over the weekend that the early Fanuc DC servo motors did not have a thermo switch while later on they did. That's why I deleted the content of post 9. I had posted up connector pin numbers that deal with the thermo switch but now need to figure out how to determine if a motor has it or not before posting info about jumpering it out for troubleshooting purposes.

My lathe with a 3T and DC servos has thermal switches, it was built in 83. If you take the cable connector loose from the motor the wires leading to the thermo switch are evident, they are the 2 smaller wires leading down into the motor. I did that to track down the problem on mine. Turns out it was only a bad connection, after removing and replacing the cable connector the alarm went away. Every time I have had a problem with that machine it has been a connection so I have loosened and reseated every connector and cable on it as a precaution.

Ed.

Thanks Van and Ed, I have the motor out and will look for a switch. I'm going to take it apart tonight to investigate one of the 4 motor pins having a resistance to the motor case.

I got a chance to open the motor tonight. The pin I thought was shorted is actually a machine provided ground to the motor case. The others were not shorted. The section with the brushes was very dirty with carbon deposits. I will clean that tomorrow. It did have some kind of sensor in the casting where the brushes mounted to. The sensor has 2 greenish wires coming out of it, those are spliced to white wires that run to the center of the encoder plug. Can I test that switch or do you guys know a model number for it. I'm thinking it might be easier to just replace it.

I do not know a Fanuc part number for it. I have a note showing the temp range at 80C-90C. It is an NC type thermo switch. It should have a circuit through it when cold and open when hot. If you suspect a flaky thermo sensor you can jumper the white wires together. If you can then run with no alarms then the thermo switch is bad. If you still get alarms then it is likely the encoder cable or connectors is bad.

I still haven't fixed this problem I checked the motors the cables cleaned the brushes and checked the motor thermal switch with no change. Could it be a motor that draws to much current or a bad velocity control unit? I'm about to start switching x and y motors then if that doesn't work I think I might have to switch velocity control units.

Could this be a voltage problem? At the input junction going in the machine between R and T I have 217 volts between R and S I have 217 volts and T and S ant T its 244.

That might be the answer. My Hitachi lists an input of 220V + or - 10% so I ended up with buck transformers to bring it down. Check your transformer or manual for proper input volts. 244V is just above 10%.

Ed.

Are you running on a roto-phase or similar? If this is line 3 phase power, you should not have that much variation.

The machine is running of of a rotary phase converter. Decided to switch the X and Y servo motors and when I took the x off I thought I'd clean the motor out. The inside of the motor was extremely carboned up and some of the carbon look like grease. The commutator was very dark colored. I cleaned all that up but before I swap motors I have to fix the encoder connector for the x. If after I swap motors the problem isn't fixed then I will look into input voltage.