CNC Machines Crash On Power Sags

[Davebones]

Sometimes we have a disturbance on the power into the plant causing our CNC machines to crash . The type I'm talking about happen in a split second .Sometimes this causes our spindles to stop and sometimes distroys the tool and the part . My boss was wondering what we could do to keep the machines running and do a controlled shutdown .I don't know if surge protection would work . Don't see UPS as a solution as we are talking about a lot of machines and load . Any thoughts would be appreciated ...

 

[bob]

If this because your control relays can not ride through the instantaneous interruption.

 

[zbang]

I don't see how you can do a controlled shutdown without power, and that means some sort of UPS (maybe a flywheel type). You might be able to keep the computers and servos running but let the larger motors stop. This also means that the controller needs a power-lost input and the programming to retract or make-safe the tooling.

Is the cost of lost tools and parts over time more than the cost of the UPS? If it only happens once a year, replacement tooling might be cheaper.

 

[sii]

Do your machines turn off or just stop motion?

 

[gar]

110428-1758 EDT

Davebone:

Are you sure the problem is caused by a sag vs maybe a high voltage transient?

You say an instant. Is this 2 cycles or 60 to 180 cycles, 32 milliseconds vs 1 to 3 seconds? Assuming the spindles and servos derive power from a DC bus with substantial capacitance, then a short perturbation of even 3 seconds may not cause their failure.

Therefore I assume that the control part of the system, meaning the computer that controls the spindles and servos, is probably at fault. This might be transient susceptibility in some fashion, not likely the power supply. Again because there is substantial capacitance storage in the power supply.

Maybe MOVs and filtering of the AC to just the control power supply might help. But the noise could get thru in some other fashion.

It can be tough to solve your problem but you need more information. For example supply voltage waveforms resolved to maybe 0.1 millisecond from just before and just after the disturbance. Also what fault messages do the controls display? I am fairly familiar with HAAS machines. These machines trip out nicely on loss of phase with no damage. In the great northeast blackout we had some very large and moderately long voltage changes, both low and high, and mostly lost some fuses in the HAAS machines. No part or machine damage.

Are you drip feeding when these problems occur, or are you using an internally stored program?

.

 

[boboelectric]

I would call in the mfg. service rep.Maybe they would have a fix.

 

[sameguy]

Have you found why/what is the cause of the power probl. ?
Called the POCO ?
Is this on every CNC or just off X# feeder?

 

[broadgage]

I once found a CNC machine that appeared unduly sensitive to power sags.
A speciailst engineer found an internal fault in the machine.
As posted above, these machines use a DC bus internally, and there is intended to be capacitive storage to ride through short voltage drops of a second or so, in this case the capacitors had failed and replacing these components cured the problem.

 

[Davebones]

I do appreciate the information .I just wire in the power when we get new machines . I will show this to the guys that work on them so hopefully we can isolate this a little better . Again thanks for the input ...

 

[Davebones]

Will try to have some answers to some of your questions on Monday after talking with the guys that work on the CNC's .....

 

[gar]

110430-1101 EDT

Davebones:

It is real important to get a better handle on what kind of transient causes the problem.

The most important first question is the duration of the power perturbation, and is it simply a sag, or a rise, or a combination of both, and if there is a very large fast transient like thousands of volts.

.

 

[Electric-Light]

Do CNCs usually support separate power sources for drive-train and control systems?

My laser printer is like the drive train. The computer to send the signal is the control. If they're on the same circuit, the computer's UPS switches over to battery everytime the printer's fuser kicks in.

They're on a separate branch circuit and problem went away.

If the ENTIRE electrical system is plagued with sags, you can put all the controls behind a relatively small double conversion UPS. For "power on" signal, you could use a HVAC blower motor or something that has a centrifugal switch and power the motor from power-train power, then hookup the switch to control side. You'll just change the flywheel size/weight to control the "delay" before "power fail". The flywheel also provides natural protection against hunting since once it shuts off, it can't re-accelerate to speed instantaneously.

 

[gar]

110430-1516 EDT

Electric-Light:

Consider a typical 20 Hp HAAS machine today. It is a delta load, nominal 240 V with taps for adjusting up and down.

From memory I believe the taps are adjusted to provide about a 325 V DC bus. This DC bus supplies all the high power components, meaning spindle and servo drives.

On many of the machines there are some 3 phase motors, like a coolant pump, that are directly supplied from the 3 phase power.

Then there is the electronics that is powered by a conventional personal computer power supply. This is supplied from an internal control transformer that provides 120 V single phase. You do not want to power this from an external unrelated AC source. But it might make sense to insert a UPS at the computer power supply input. I am not sure this would help much other than to possibly be a filter.

There is under and over voltage monitoring of the DC bus that when triggered creates a fault signal that stops everything. This does not cause tool or machine failure. There is phase loss detection also. Many other fault conditions are sensed.

From a short transient problem the most likely cause is that the computer receives a bad command or produces internally a bad command that causes the machine to produce a motion it should not.

If the noise gets into the computer via the AC supply, then a filter on the AC to the computer power supply might help.

If drip feeding thru RS232 and noise enters this way, then it will usually cause a parity error which stops the program, and the machine. Spindle may still run but there is no motion. This could cause a problem on a tapping operation.

.

 

[Besoeker]

110430-1516 EDT

From memory I believe the taps are adjusted to provide about a 325 V DC bus. This DC bus supplies all the high power components, meaning spindle and servo drives.

On many of the machines there are some 3 phase motors, like a coolant pump, that are directly supplied from the 3 phase power.

Then there is the electronics that is powered by a conventional personal computer power supply. This is supplied from an internal control transformer that provides 120 V single phase. You do not want to power this from an external unrelated AC source. But it might make sense to insert a UPS at the computer power supply input. I am not sure this would help much other than to possibly be a filter.

Gar
We make machine tool drives. Like many (or most) variable frequency inverters, the control electronics is fed from the DC bus via a switch-mode power supply. As you noted, the DC bus is likely to have considerable capacitance so anything from the DC link onwards should be able to ride through a transient.*
The input converter may be four quadrant - I think that common on machine tool drives - and that might be a little less tolerant of supply borne transients.
A transient phase loss for example might trigger an uncontrolled shut down with undesirable consequences for the driven load.

*If, on the other hand, controls within the drive or any other parts of the drive controls system derive power from the AC input supply, then your idea of puting that on a UPS could well be worth a go.