Lathe VFD causing over current in AC unit

[MJakob]

So, machine shop. There's a Carrier Infinity AC unit which has a 100% variable compressor in it with a lot of electronics. There's a couple CNC lathes in the shop. When the AC unit is trying to run at it's upper end of speed and one of the lathes completes a cycle and it's VFD brakes the spindle motor, it causes an over current fault on the AC compressor unit...

You can put an amp meter on the line going to the AC unit and it will be pulling about 8 amps at 100%, then over the course of a second or two while the lathe is stopping it will push up to 10-10.5 amps. The control in the AC unit logs an over current condition and kicks the compressor out for a few minutes before it restarts, at which time the next lathe that ends it's cycle kicks it back out again. Eventually it has enough of this and just locks out completely.

There is no way in the control to limit the compressor speed to say, 50%. It's going to do what it wants to do.

I've tried a buck transformer to drop the voltage from 245 down to about 222, but it didn't seem to make any difference at all. I can't really detect any voltage increase coming from the lathes, at least not with a DMM. I have seen where a braking VFD can push a higher frequency out onto the incoming line.

What can I put on the line going to this AC unit that would smooth out the power going to it? Power conditioner? Sine wave filter? Anybody have any recommendations?

 

[iceworm]

Harmonic filter?
https://www.mtecorp.com/pages_lang/wp-content/uploads/MAP-PSP-E.pdf

I have not use any of these, but one of my contemporaries has - she liked liked it.

Before that , seriously consider hiring someone to do power monitoring and see exactly what is causing the issue. Suggest to do that before buying a harmonic containment field generator

 

[GoldDigger]

Sounds like a poor excuse for a VFD to me.
If the line voltage rises suddenly, there will be at least a few cycles of high current through the rectifier to charge the DC bus to the new equilibrium voltage. Any slow start (resistor?) provisions to limit rectifier current would not be operating at this time. I would not expect it to last as long as a second or two though.
Next best step would be to lower the voltage drop (rise) between the CNC VFD and the service and maybe substitute a braking resistor for the regenerative braking on the CNC VFDs.
Putting the A/C on a separate home run back to the service would lead to less voltage variation seen by it.

 

[iceworm]

(edit to add) Slow poster - following GD's post

One other thought:
Can you rig a temporary power source for the A/C VFD and power it from a different transformer or a panel closer to the service. Lower impedance to the service will cut down of the voltage distortion. (Not exactly true)

 

[hbiss]

Sounds like a poor excuse for a VFD to me.

Did you ask their A/C service company to look at it and possibly contact Carrier? Seems to me that would be the first thing to do before calling an electrician.

I can't believe this is the only installation of that unit that has this problem. Most systems use VFDs these days and I have never heard of anything like this.

-Hal

 

[Besoeker3]

So, machine shop. There's a Carrier Infinity AC unit which has a 100% variable compressor in it with a lot of electronics. There's a couple CNC lathes in the shop. When the AC unit is trying to run at it's upper end of speed and one of the lathes completes a cycle and it's VFD brakes the spindle motor, it causes an over current fault on the AC compressor unit...

Can you reduce the deceleration rate on the lathe VFD?

 

[GoldDigger]

And just to be perfectly sure, have you independently confirmed (instead of just being told) that it is an overcurrent fault and not an overvoltage fault?

 

[Besoeker3]

And just to be perfectly sure, have you independently confirmed (instead of just being told) that it is an overcurrent fault and not an overvoltage fault?

Good point, sir.

 

[synchro]

A sufficiently large 3-phase induction motor on the feeder to the lathes may absorb some of the transient energy during braking. It would be best if there was an inertial load on this induction motor to help absorb/supply energy as needed to stabilize the line.

 

[ptonsparky]

A sufficiently large 3-phase induction motor on the feeder to the lathes may absorb some of the transient energy during braking. It would be best if there was an inertial load on this induction motor to help absorb/supply energy as needed to stabilize the line.

Translated to mean a Line Reactor?

 

[Besoeker3]

Translated to mean a Line Reactor?

I wouldn't take it to mean that. Just a load on the supply to absorb some of the stored energy of the lathe.

 

[Dennis Alwon]

This thread is out of my knowledge level however it may be helpful to know if this is a new occurrence and the system worked fine previously or is this a new install.

Has any work work on anything been done previous to the symptoms?

 

[Jraef]

What kind of drives are on the lathes? Because unless the lathe VFDs are Line Regenerative Active Front End drives, which is unlikely on a lathe, then the braking is being done via a Dynamic Braking Resistor and nothing having to do with braking would have ANY effect on the line side; everything is blocked by the diode bridge of the VFD, no energy would come from the motor past the VFD itself.

Or are you assuming the lathes have VFDs when in fact they are 4 quadrant (Line Regen) DC drives? Or maybe the VFD has nothing to do with the braking and they have electromechanical brakes on them? Bottom line, something is missing from your story, whether you realize it or not. My inclination is that the spike in current on the AC unit is the result of a voltage drop when something is trying to stop the lathe, or, if by chance they ARE Line Regen VFDs, then the Active Front End of the lathe VFD is causing a resonance that is affecting the AC unit VFD. If that’s the case, it’s going to be difficult to fix; it may require putting an isolation transformer ahead of the lathes. Double check your facts and we will try to help more.

 

[Besoeker3]

What kind of drives are on the lathes? Because unless the lathe VFDs are Line Regenerative Active Front End drives, which is unlikely on a lathe, then the braking is being done via a Dynamic Braking Resistor and nothing having to do with braking would have ANY effect on the line side; everything is blocked by the diode bridge of the VFD, no energy would come from the motor past the VFD itself.

Or are you assuming the lathes have VFDs when in fact they are 4 quadrant (Line Regen) DC drives? Or maybe the VFD has nothing to do with the braking and they have electromechanical brakes on them? Bottom line, something is missing from your story, whether you realize it or not. My inclination is that the spike in current on the AC unit is the result of a voltage drop when something is trying to stop the lathe, or, if by chance they ARE Line Regen VFDs, then the Active Front End of the lathe VFD is causing a resonance that is affecting the AC unit VFD. If that’s the case, it’s going to be difficult to fix; it may require putting an isolation transformer ahead of the lathes. Double check your facts and we will try to help more.

Don't greatly disagree.
Yes, more information is always helpful.
An observation. The problem occurs during lathe braking. The only obvious connection between the lathe and the AC is the supply, the line. So, it would seem probable that it is a line side disturbance. From that you might reasonably infer that there is regen braking on the lathe. Added to that, the AC seems to recover from the blip whatever the "blip" is.

Scope the line voltage during the event maybe?

 

[gadfly56]

Can you reduce the deceleration rate on the lathe VFD?

Slower machine = lower productivity.

 

[gadfly56]

Good point, sir.

He put an ammeter on the AC supply during the lathe coast-down. What else do you need?

 

[drktmplr12]

He put an ammeter on the AC supply during the lathe coast-down. What else do you need?

a waveform

 

[Besoeker3]

He put an ammeter on the AC supply during the lathe coast-down. What else do you need?

An oscilloscope trace of the of the line voltage during lathe braking.

 

[Besoeker3]

Slower machine = lower productivity.

We did a few machine tool drives for the aircraft industry. Decel was from 10,000 rpm to zero in less than one second to within +/- half a degree position accuracy. But does the OP need that sort of dynamic performance?

 

[gadfly56]

We did a few machine tool drives for the aircraft industry. Decel was from 10,000 rpm to zero in less than one second to within +/- half a degree position accuracy. But does the OP need that sort of dynamic performance?

OK, clearly we have different reference frames for "reduce the deceleration rate". In your experience it's clear that 3-5 seconds would constitute a significant reduction in deceleration. OP didn't mention the braking time, which might be important information.