Fanuc 6t controller/Blows fuses on the X Velocity controller

[MetallChips85]

Good Day, All,

I have taken on the task of repairing a CNC machine here at a shop i've been working for (Im a machinist who dabbles in electronics) and it is throwing a fuse on the Velocity contoller board, SDUX. Up to this point i have measured for continuity through the wiring harness and do not see any odd shorts to ground, the ground plane seems to be legit. The DC motor that drives the X axis has been removed and tested on the bench with a powersupply. I have Not verified that the Encoder within the DC Servo Motor is functioning correctly, i do not know of a good way to test it, to be honest. I have traced the power from the AC in , through a series of transfromers and have verified that the desired Voltage is reaching the DC Converter boards. I have Taken the Velocity Controller board out and measured all the SCR's, out of circuit and they seem to be functioning properly.

If anyone would like to get involved i would greatly appreciate the support.

Chadman

[rrinker]

 Wow, that brings back some memories. My degree is in electrical engineering but my first job post college was in a machine shop, in part working on maintaining the CNC machines. I think we had one or two tools with something as new as a 6t (this was in 1988). The more I see, the more I may remember, so I will definitely be paying attention to this thread and if I can think of anything I will definitely pass it on. The only fuse blowing incident I remember was not on the board but the spindle motor, caused by a rather incompetent operator. Fuse blew, we replaced it, told the operator to resume. As soon as the tool contacts the stock, he uses the feed rate override to crank it to maximum. Spindle stalls, fuse blows. Replace second fuse. Instruct operator not to do that. Same thing. Find his supervisor, explains what is going on, he shifts the guy to a different machine and brings someone else over, 4th fuse, and everything's fine.

 From an electronics standpoint - I would check the capacitors. Might have cooked one or more over time. On electrolytics, look for bulging, or actual signs of leakage of the electrolyte. Perhaps check the resistance on the power rails, it could be low but not low enough to trigger the continuity indication on your meter, which may be enough to case the total current draw to exceed the fuse rating.
 

[james_s]

Friends of mine own a machine shop and I've spent some time repairing various machines there. You mention SCRs, is this a very old servo driver? The modern stuff usually uses mosfets.

[MadTux]

Oldish DC-Servos used SCRs to control speed, my Maho 700P from 1980 had a bad servo driver too, thereby I know :-).
Usually 2 (x2 for 2 directions) SCRs per DC-Servo, one pair for each direction, one lead of the motor is connected to ground/center tap of transformer and the 2 SCRs are used to fire on negative and positive half wave for one direction.
Schematic of Indramat 3TRM2 of my Maho 700P (big picture):
https://www.mikrocontroller.net/attachment/332163/Indramat-3TRM2.png
Ux/Vx are outer taps of transformer, Ax goes to motor, other motor lead is connected to center of transformer.
 

As far as I remember, it used phase angle control on AC voltage to control the servo speed (current into DC-motor). 100% duty cycle for full speed and like 1% for slow feed. Tacho in the back of the servo motor gives feedback on how fast the servo turns and the servo controller adjusts the phase angle firing signal accordingly.

If you say, fuse blows and motor is fine, it might be that either a SCR is fried (shorted) or that 2 SCRs of the same motor fire at the same time (due to bad controller) and thereby the SCRs create a short. If that happened on my Maho 700P, fuses would blow too (380V/16A). SCRs survived OK though, in contrast to the darlington transistor modules on the spindle drive of my Deckel FP4NC ;-)

Perhaps check SCRs for continuity and/or try to disconnect the gate of the SCRs, so they can't fire. Be careful with your machine, since rogue servo drives can do significant damage, if you don't hit emergency stop fast enought 

[MadTux]

Heres a scan of the Maho electrical diagram with illustration. Note how the Servo motors are center tapped. For the servo to go into one direction, SCRs fire on positive half wave. Other direction, SCR fire on negative half wave. SCR fire on positive and negative half wave for some reason, short circuit and fuze blows  ;-(

Hope that helps!

[jmelson]

Wow, this is going to be a real mess.  The phase-controlled servo amps were quite complicated, and if anything went wrong, the trigger timing was messed up and it would blow fuses or worse.  There was an 18 V 3-phase transformer that distributed power and trigger timing to the SCR trigger circuitry.  On the oldest units, the servo amps were analog  so they got a velocity command from the CNC control, as well as a synthetic tachometer signal derived from the encoder.  Later models did all this control loop in the CNC control, and sent six trigger signals from the CNC control to the amp (but, I think those were transistor amps, not SCR).

I suspect it will be VERY hard to diagnose this problem, unless possibly some burned components are discovered.
There are a LOT of serviceable Fanuc servo amps floating around, but people usually want real money for them.

Jon

[duak]

If you don't have one already, here's a link to the maintenance manual for a 6T control: http://www2.i-logic.com/manuals/Fanuc-6T-Maintenance.pdf

I've never looked at an SCR servo.  Some questions off the top of my head:

Which fuse is blowing?
Is it throwing an alarm back to the control?
Does the motor and encoder turn smoothly?
Have you inspected the motor brushes and commutator?  If they are badly worn, the motor could be full of dust n' stuff too.
Have you checked the resistance between the motor terminals and the case both with and without the cable connected?
Does the motor connector look clean and dry? ie., not burnt or wet?
Does the X axis ballscrew turn smoothly?

Let us know what you find.

Best o' luck

[james_s]

I think I would start selectively disconnecting things in order to narrow down where the problem is.

I have no familiarity with this specific controller but I scrapped a few early 80s servo drives several years ago and I remember those had a card cage with a series of boards performing individual functions. With those it seems the X and Y axis units were also identical so if yours has a similar arrangement you could try swapping parts between X and Y.

[MetallChips85]

Tomorrow morning i will begin troubleshooting the machine, and there are many things that have been mentioned here that got me thinking.

This forum is great, and it's the company it keeps.

I installed an app that allows me to access the forum from my Droid much faster and will try to respond to the responses and i will post pictures, tomorrow as i get started and throughout the day for sure.

Sent from my TA-1052 using Tapatalk

[MetallChips85]

If you don't have one already, here's a link to the maintenance manual for a 6T control: http://www2.i-logic.com/manuals/Fanuc-6T-Maintenance.pdf

I've never looked at an SCR servo.  Some questions off the top of my head:

Which fuse is blowing?
Is it throwing an alarm back to the control?
Does the motor and encoder turn smoothly?
Have you inspected the motor brushes and commutator?  If they are badly worn, the motor could be full of dust n' stuff too.
Have you checked the resistance between the motor terminals and the case both with and without the cable connected?
Does the motor connector look clean and dry? ie., not burnt or wet?
Does the X axis ballscrew turn smoothly?

Let us know what you find.

Best o' luck

The brushes in the motor have been replaced. I haven't inspected the commutator.
It was mentioned that the motor could be full of carbon but that didn't seem to effect the DC bench test.
When i had the motor out, the i moved the X axis off the overtravel/limit switch, by hand, and was surprised by how "pornagraphically" smooth the action was.
I measured the resistance for the motor which was next to nothing and i used a megger,  there was no detectable leak to ground.

The motor connection inside of the motor is very moist looking but i could not measure continuity on the braided fibreglass insulation that was so oil impregnated.

The left most of the three 15amp 125v fuses is what pops. It doesn't pop if i disconnect the motor from the harness.

The motor turns smoothly, has less than 1 ohm,  actual .9 ohm of residence measuring the winding leads.




Sent from my TA-1052 using Tapatalk

[james_s]

Are those fuses on a 3 phase power input? If the same one blows every time that offers some clues. The first thing I would check is the rectifier if there is one, and then follow that phase to see what all it connects to.

[duak]

I looked thru the maintenance manual I referred to above and, if I've got the model number of the servo amp correct, I see on p.139 that fuse F4 is in parallel with a larger fuse F1.  I would bet that F1 is also open.  Fuses do "just go" but James suggests I'd check out all the SCRs connected to the fuses first. ie., if there is short or low resistance connection between the motor connections and the MCC (magnetic contactor)

The pages following show various scenarios and some tests. The problem with them is that it's an all or nothing situation in that most everything blows a fuse which is pretty series.  There's also a lot of energy flying around here making testing a bit hazardous.

Let us know how it goes.

[james_s]

A current limiter such as a 240V incandescent lamp (or a couple of identical 120V bulbs in series) or a water heater element in a bucket of water placed in series with the fuse could save some issues in the troubleshooting stage, reduce the wasting of fuses and the chance of collateral damage.

[duak]

Using light bulbs is a great idea.  I've never seen a 230 V incandescant bulb but two in series of the same wattage should work.

If the fuses blow only when the motor is connected, a 230 V 1-2 KW (??) water heater element in series with the motor should limit the current to a reasonable value but still allow the motor to turn with reduced torque.

[james_s]

240V incandescent bulbs are very common throughout Europe where 240V is the standard. They are less common in North America but I have seen 240V halogen lamps in high wattages, 1500W and larger which were common back when HID was expensive and power LEDs didn't exist. 277V bulbs are also available but not very common.

[MetallChips85]

Yes, that fuse rail is AC 3 phase, @ 208v. It is just before the Silicon Control Rectifiers.

[james_s]

So the SCRs run straight off the AC without a bridge rectifier to provide a DC rail? That should at least tell you which SCR may be triggering at the wrong time or getting stuck on.