Mitsubishi Inverter Troubles

[nineandtwothirds]

Hey all,
I posted recently about the Mitsubishi Freqrol Z300 Model FR-Z320-7.5K-UL inverter in my 1992 Fadal VMC 40.

It seems the brake resistor has been getting wayyyy too much current, enough to melt its aluminum housing (see pic 1) and create a fair amount of smoke.

After testing, all of the IGBT modules are good. Good diode readings and no shorts from gates to anywhere on all 3. Capacitors all seem good as well. Even the melted brake resistor is giving me a good resistance reading.

There is another micron 17.H ceramic or cement resistor that bridges the two sides of the contactor. Numbers read 40B 1.5K ohm (see pic 2). When checking this resistor with an ohmmeter in the 200 ohm range, I'm getting a reading of 1.9. That's well outside a 1% margin of error. But I don't know what the function of this resistor is and if it could be the source of my problems. There does seem to be some scorching near the terminals (pic 3).

Here are some machine symptoms that I have noticed over the past couple months that have been intermittent with slowly worsening regularity:

1. Upon startup of machine when warming up the spindle, I always get a spindle drive or control fault on the CNC readout. After ~30min of letting the machine sit, powered up, I can start the spindle no problem. It should be noted I don't have a parameter unit for this inverter, so I'm not getting codes from it. But every time I get the spindle drive or control fault, I have to reset the inverter as the alarm light is on.
2. Upon startup of the machine, I have been hearing more and more clicks, seemingly from the inverter and likely from the contactor, as it's the only moving part. When it started smoking, it was clicking like crazy. I have disassembled and cleaned, and tested. All looks good. But the contactor is what is controlling current to the brake resistor.
3. Every time the spindle starts up, during a program or just manually, I get a surge on the power gauge on my CNC display (pic 4) going up to almot 90% and then dropping down closer to 40% under no load and staying around there during cuts.
4. Roughly 1/20 times the spindle starts, usually during a program, it struggles to get going. The first time it happened, I ruined a tool. I have since programmed dwells after every spindle start in a program to prevent that from happening again.

To me, these ALL seem related to the brake resistor. But I'm open to opinions. I also really want to know what that micron resistor that's bridging the two sides of the contactor is doing.

All help and input will be appreciated.

Thanks,
Mat

 

[thermite]

There is another micron 17.H ceramic or cement resistor that bridges the two sides of the contactor. Numbers read 40B 1.5K ohm (see pic 2). When checking this resistor with an ohmmeter in the 200 ohm range, I'm getting a reading of 1.9. That's well outside a 1% margin of error. But I don't know what the function of this resistor is and if it could be the source of my problems. There does seem to be some scorching near the terminals (pic 3).

Having done THAT much detective work, why, then, have you not already replaced it, AND the damaged braking resistor? Then gone detectivating further if/as/when still No Joy?

That rig have "stem cells" that can GROW new components?

 

[Jraef]

My guess is that the resistor that melted is not the Braking Resistor, it is the Pre-Charge Resistor. The Pre-Charge Resistor is used whenever you initially power up a VFD to avoid having the capacitor charging current inrush damage the diode bridge or the caps themselves. All VFDs have some form of a Pre-Charge circuit, most use a resistor that is then shorted out with a little relay contact inside of the VFD after about 1 second.That is why you are not supposed to use a line side contactor ahead of a VFD, it can cause that resistor to overheat and burn out if the contactor is cycled on and off too rapidly.

The resistors on the contactor are probably the Braking Resistors, but I have no idea why there is a contactor, unless the Mitsi drive does not have a Braking Transistor and they are just automatically engaging the Dynamic Braking resistors whenever you give it a Stop Command whether you need them or not. That's not the way to do it, but I'm just speculating as to why there is a contactor with resistors mounted to it.

 

[Jraef]

Couldn't find a manual for an FR-Z320, but I have one for an FR-Z340, likely a successor. It appears as though there is an internal brake transistor and a standard internal braking resistor that appears to be the one you have burned out, mounted to the back of the heat sink. But there is an optional "BU" Braking Unit that you add if the internal resistor is inadequate to stop the load. I can't find any info on that, but it might be the thing with the contactor and larger resistor. In the manual though, it explicitly says that you MUST disconnect the standard resistor if you use the optional external one. So most likely someone didn't RTFM.

 

[thermite]

Couldn't find a manual for an FR-Z320, but I have one for an FR-Z340, likely a successor. It appears as though there is an internal brake transistor and a standard internal braking resistor that appears to be the one you have burned out, mounted to the back of the heat sink. But there is an optional "BU" Braking Unit that you add if the internal resistor is inadequate to stop the load. I can't find any info on that, but it might be the thing with the contactor and larger resistor. In the manual though, it explicitly says that you MUST disconnect the standard resistor if you use the optional external one. So most likely someone didn't RTFM.

Igor has one up online for the FR-X2XX. Code explanation, the diff is rated operating voltage, not age/revision-level.

http://manuals.chudov.com/Mitsubishi-Freqrol-FR-Z220-Manual.pdf

SAME NOTE appears in re disconnecting that onboard resistor when an external braking pack is added.

Looks as if you've nailed it. Again.




- Remove the remains of the onboard "fry-up".

- Replace the damaged external one. It may still have the expected Ohm's value, but it is "at least" impaired as to heat-transfer characteristics.

 

[JST]

A fried braking resistor may have additional causes....

If the voltage input is too high, AND the overvoltage trip point for the braking is set low, the braking resistor may be on too much of the time. It just responds to an overvoltage, and has no relation to actual braking other than that braking tends to raise the bus volts, so if the bus volts are high for any OTHER cause, such as a high input voltage, the "braking resistor system" will respond as if there were a braking situation. Since high input voltage is not really correctable with a resistor, the circuit may be on much more time than expected, and the resistor is not intended for that much on-time. Usually there is a duty cycle limit, but I do not know if it is active in all modes or where it may be set,

Your unit has a contactor for precharge, and a braking transistor. So clicking is indicating a possible problem with the precharge. The resistor may not be good, it may not be making contact until some warmup has occurred, since the block diagram indicates that the control power comes from the AC in, and not the DC bus.

Also, the issue with the external and internal resistors is actually NOT with the resistor. It is often with the IGBT. If the external module simply adds a resistor, then the two together may conduct too much current for the IGBT, and might damage it. The resistor has the same things happen to it regardless of whether there is another resistor or not.

If the external module is an entire system with IGBT and controls, then it may simply be that they do not want both systems on at once due to possible interactions.