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QT-15 bizarre 24V circuit grounding problem.

oliver7

Plastic
Joined
Jan 10, 2017
Location
Hutto, Texas
:ack2:
Our QT-15 lathe with a T-32-2 controller has lost it's ground on all 24Volt ground terminals.
When I measure voltage on the plus and minus of the power supply I get correct voltage, but when I measure positive to machine ground I get nothing,
and when I measure negative to machine ground I get 24Volts.
It literally appears that the electron flow is going backwards through the 24VDC power supply.
The Multimeter is showing negative 24V when my black lead is connected to machine ground and the red lead is connected to negative of the power supply.

I verified with all our other Mazaks in various condition (controller on/off. Machine on/off etc..) and the entire 24VDC system has constantly continuity to all grounds with a 0.2 Ohm.
The resistance of through the positive side of the 24V circuit is on all of my machines around 300 Ohms.

The resistance from the negative wire into my 24V circuit to ground on the faulty machine is 39 MOhm when connected with the positive clamp to the neg. wire and 16MOhm when I reverse it.

Some component of my 24V system is not getting ground and it pulls the power though the power supply "backwards".

I have never seen anything like it.

Anybody out there that has seen anything like this?

I replaced the 24V supply with three other units and had all systems isolated from each other, but cannot get ground into my G24 labeled terminals.
I am just about to exchange entire components like front panel of control, PCBs of junctions and mechanical relay board PCB.

I rejuvenated the entire ground bar at the bottom of the e-cabinet, I also had the 24V power supply isolated from the machine's 100VAC supply and ran it with outside 120VAC.

But when I take my Ohm meter to the bare wires plus and minus of the 24 volt circuit, my resistance is through the roof.
If anybody has a good tip to isolate the component that pretty much blocks my ground side of the circuit that would help a lot.

Now, additional information that might help, the transformer had cooked one leg coil so my spindle servo overloaded on two phase and blew a component .
I had the spindle control rebuild and use an identical transformer from an AJV right now for trouble shooting.

All in and output voltages are correct.

Also, when I run a separate wire from the ground bar to the negative of the 24V power supply, some machine ready and machine fault circuits are closing and the hydraulic power unit comes on.
But my control gives me a persistent NC alarm and a 128 Emergency Stop error, which I have extensively trouble shot through its circuits already.

I have re-initiated the control, I have had the NC send out to Mitsubishi (no fault found) and have triple checked all my spindle control parameters.

But after I was done with all of that, I could not ignore my 24VDC problem of not having ground connections on my G24 terminals.


Thank you for any input that focuses on my established main issue of not having connectivity on my 24V ground side.

Oliver
 
If the 24 volts read correctly at the power supply it must be OK.
Due to your "the transformer had cooked one leg coil so my spindle servo overloaded on two phase and blew a component" condition, I would think that the connection of ground and 24- has been disturbed. In this case you will see measurements that don't make sense.
The fact that "I run a separate wire from the ground bar to the negative of the 24V power supply, some machine ready and machine fault circuits are closing and the hydraulic power unit comes on" fits my scenario, there must be other sections of the 24v- circuit that have been removed from their ground connection.
 
ifixcnc,

that is what my conclusion was as well, I simply cannot locate the section that doesn't receive ground.
The issue I have with the theory that the leg cooking might have anything to do with it is that the 24 volts section are pretty much strictly separated from the 100VAC and the 230VAC and also the 180VAC that I have coming from that transformer.

I traced pretty much all connections from the PCBs to the front panel card, to that power supply in the front panel and back.
I am suspecting the mechanical relay board though and have seen one on eBay for $195. I might jump on that.

I do appreciate the brain storming though. I might have tunnel vision already, since I have been dealing with this now for months.

I did take the front panel board out, separated the key board, cleaned all connectors with electrical cleaner and put it all together.
Now one thing I have found was that someone had spliced power off the main shunt breaker into a switch and then routed the SOOW cord through a panel hole to a skimmer that doesn't exist anymore.
One hot wire was rubbed bare and that was the reason why sometimes the 150A breaker would trip when re-setting the breaker.
When I saw that, I was so happy because I thought it was the reason why ground gets sucked or compensated somehow by having power leak into the frame or something like that.
I am actually pretty good at trouble shooting missing or wrong signals etc. and electricity in general, but this is kicking my behind.

My last thought right now is to simply replace large sections, components. But there must be a better way to find the issue.

I traced the ground lines in the PCB on the relay today like an idiot, but I just cannot find anything clearly wrong with it.

Oliver
 
wippin boy,

no, this machine has only one 24V power supply, but there is a 24V out on my NC power supply that is not used, I was thinking about trying that, but have to clear with Mitsubishi how much load it can take.

Oliver
 
I would unplug anything that has the 24 volts, one at a time and see if it will power up. Fine tooth comb of prints may shed some light on the 24v distribution.
 
I would unplug anything that has the 24 volts, one at a time and see if it will power up. Fine tooth comb of prints may shed some light on the 24v distribution.

ifixcnc,

I have had everything isolated individually, literally disconnected the entire central nervous system, studied the electrical diagram traced all and every emergency, machine fault and machine ready circuit.
Lifted the wires out of the wire ducts.
Measured resistance through all components, moved components like the chuck, the axis (manually), checked switch states, ohmed all the switches, tried to force bits.... nothing has cleared the 128 emergency stop error.

I had the on off switches off, measured resistance etc, since on the T-32 which we have on our Slant Turn 50 as well, a 128 emergency stop message is cleared by pushing the control on switch.

I now learned that I could install the EIA/ISO option and that way gain ladder access. I haven't gotten a price from Mazak for that option yet.

I have two questions for you:
One:
Do you know if there is a way to force bits on this controller?
I can do it on my M-Plus and M-32 but when I try to do it on the T-32 it only flickers up real quick but returns to the original settings.

Two:
In my electrical diagram, there are some semiconductor components (transistor and diode in some) that involve 24V grounding symbols as well. This tells me that it has to do with the termination of the 24V circuit after the load, meaning grounding for all those circuits.
Have you ever dealt with those symbols?

In the attached pictures you can see one. Sp.Drive_OHS_OLR_EMG2.jpg

I cannot resize the pictures of the e-diagram right now. This forum only accepts 19kb file size...
I'll try to attach them later.

Oliver
 
Sorry for the delay, I have been up to my azz in alligators lately. I have not spent a lot of time on Mazak's. Call Dennis Black at 859 342 1700 he is an factory tech that has bailed me out a time or 3.
 
Thank you for the contact ifixcnc, I appreciate it.

Take a putter with you in the water to beat the alligators on their eyes like the golfer did last week... ;)

Oliver
 
Follow up:

Ken Terry from Terry Maintenance helped me to finally put an end to the 128 Emergency Stop error on this QT-15 lathe.

He did acknowledge that the missing ground in the 24VDC system is quite baffling, but the actual reason for the persistent 128 Emergency Stop message came from both axes drives.

To recap real quick, when I got to the machine, the 128 message was up and could not be cleared, I discovered that the spindle drive was off and found that one of the three 230VAC legs was missing, coming from the main transformer.
It obviously had smoked something inside the drive due to two phase condition.

I had the spindle drive repaired and verified everything I could possibly think of, including trying to isolate the axes drives, but I couldn't, since I didn't know how to make the NC not look for the drives.
Well, Ken did and both of the rives showed a 55 error now and the 128 Emergency Stop was gone.

Conclusion: The two phase condition damaged all three drives, but only the spindle drive showed signs of being damaged.
Both of the axes drives showed A6 on start up and E7 when the machine was up. All normal, no hint for me to think that the drives took a hit.
But, the 24VDC system having lost ground inside a section that needs to terminate to ground interfered with my trouble shooting flow, since I was constantly pulled back into thinking that that is the reason for the 128 error.

Well, the verdict is still out, I need to swap drives from another QT-15 we just bought and see how the 24V system is behaving.
I have routed ground externally to the negative lead of the circuit.

I'll report my final findings.

Oliver
 








 
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