VFDs & PLC in same enclosure?

dogleg43

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Join Date
Dec 2005
Location
Indiana
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I’d like to hear your opinions about putting a MicroLogix-1400 and (3) 25HP 480vac VFDs in the same enclosure. Probably have a small control xfmr inside along with 4-20mA analog circuits.

I’m concerned about noise being a problem.

Thoughts, anyone?
 
As keithkyll said, use screened VFD cable for the motor leads and be sure to connect the screens as per the manufacturer manual for the drives.

Cable routing is important too. Cross power cables at 90 deg with signal and control where possible. Keep power cables in their own wiring duct away from control wiring as much as possible.

Ensure 4-20mA signals also use screened instrument cable. I like to clamp the screens on a screen bar below the analog card where space allows. But at the panel entry point also works fine most of the time.

Ensure all bonding / earthing is solid and use fine stranded flex for the earth especially for the drives to minimize skin effect.

For the size you're talking i think you'll be fine if you don't mess up any of the above too much.
 
Since day 1 12 years ago in one of our control boxes we have a transformer that supplies the high voltage a low voltage converter 4 inverters and 3 PLC's with out any problems and everything is ran all together.
 
From an arc-flash safety perspective, combination PLC/VFD panels simply shouldn't be built anymore to minimize PPE requirements for those who need to interact with the PLC and the low-voltage wiring associated with it.

Not impossible to separate these in each case, but putting a PLC into a VFD panel isn't doing anyone any favors.
 
From an arc-flash safety perspective, combination PLC/VFD panels simply shouldn't be built anymore to minimize PPE requirements for those who need to interact with the PLC and the low-voltage wiring associated with it.
Where is the difference here when it comes to any other control system that has a mix of AC power and low-voltage control ?
 
I agree with Paully's5.0.

control panels if possible need to be separate from vfd panels due to arc flash.
if they can't, build control panels separating the 428/240/120 controls.

At my current plant, no vfd's are located in plc panels due to arc flash concern.

regards,
james
 
"Arc flash concern" doesnt explain exactly what is your concern.
That there is power in the supply to a VFD is not different than what you have with a simple direct starter or a soft-starter.
But one thing that may be extra dangerous with VFDs, and that it is not obvious, is that they store energy for a relatively long time. That, and that some uneducated persons may look at it and think it is just a bigger PLC output, may lead to dangerous situations.
Is that what you mean ?
If so, indeed this must be taken care of (by signage and/or special covers), but to forbid VFDs in the same panel as the PLC seems excessive to me.
 
I put comm's outlets on the outside of my cabinets that contained PLC and 480 on any of the cabinets that were non ethernet, the ethernet ones are accessible from the maintenance office. This was the Arc Flash Requirement we got
 
Where is the difference here when it comes to any other control system that has a mix of AC power and low-voltage control ?

The arc-flash potential in a panel with 3 phase power will be much greater and more dangerous than a PLC panel mixed with single-phase AC and low voltage DC.

Here in the states, it's becoming more common for sites to require proper PPE for both arc flash and shock safety hazards. Designing control panels to mitigate the risk is much better practice than forcing the employees to wear arc-flash rated PPE and shock protection. Because let's face it, being in a control panel with the correct PPE is uncomfortable, and makes the job harder especially if you're wearing proper shock resistant gloves.

More and more companies here are requiring proper electrical permits to even open a panel with 120VAC. Delivering panels that isolate 24VDC, 120VAC, 480VAC enforce safety and help a technician to properly do their job w/o compromising on PPE or breaking policy. I'm sure everyone on this site is guilty of being in a VFD panel w/o arc-rated PPE/Face shield/gloves...etc.
 
Paully's5.0 said:
The arc-flash potential in a panel with 3 phase power will be much greater and more dangerous than a PLC panel mixed with single-phase AC and low voltage DC.
That sounds as your concern is that the short circuit current will be much greater with 3-phase AC power, than with single phase or DC.

Yes, indeed, it is normal to try and keep the AC power and control separate as far as is practically possible. But you dont have to split into separate control cabinets.
You "merely" have to bring down the short circuit current on the control side by some means. That is typically done by a control transformer for AC or a 3-phase-to-24DC for DC.

Over here you also are not allowed to open a panel if it has more than 48V and you dont have a full electricians education.
What you describe sounds to me to be an extra measure. I can see that it is certainly safer. But do companies really allow non-electricians to dabble with control panels, even if it is low-voltage ?
 
That sounds as your concern is that the short circuit current will be much greater with 3-phase AC power, than with single phase or DC.

Yes, indeed, it is normal to try and keep the AC power and control separate as far as is practically possible. But you dont have to split into separate control cabinets.
You "merely" have to bring down the short circuit current on the control side by some means. That is typically done by a control transformer for AC or a 3-phase-to-24DC for DC.

Over here you also are not allowed to open a panel if it has more than 48V and you dont have a full electricians education.
What you describe sounds to me to be an extra measure. I can see that it is certainly safer. But do companies really allow non-electricians to dabble with control panels, even if it is low-voltage ?

We have instrument guys (who are not supposed to touch anything over 110VAC ) work on the 4-20 ma stuff and electricians on the 575 VAC (or 480 VAC) stuff. It's easier for us in Canada to use two boxes. Using a barrier in the box is a pain.

The non-electrical people who mess with controls regularly are our HVAC guys. It is also the area where it is hardest to get suppliers to split 3 phase AC from controls and instrumentation.

If there is 3 phase coming into the box, you suit up for the arc flash rating. Period. If you want to limit the arc flash current, do so at the source. Current limiting fuses after the HMCP breaker or KDC breaker (depending on the load) is what we normally use. The required safety gear/level is listed on the outside of the panel.

The number of things that have been dropped across the terminals on our VFDs, the line reactors, and the near misses ... we have some guys that were able to come home to their families our of shear luck.

Electrical burns (burns of any kind) hurt for a long time. Recovery is a long time.

It's really tough to get people to suit up in heavy, hot gear when they 'just' need to check a signal. It's easier to remove the 3 phase from the box entirely. It just takes more space, more money, and more time!
 
That sounds as your concern is that the short circuit current will be much greater with 3-phase AC power, than with single phase or DC.

Yes, indeed, it is normal to try and keep the AC power and control separate as far as is practically possible. But you dont have to split into separate control cabinets.
You "merely" have to bring down the short circuit current on the control side by some means. That is typically done by a control transformer for AC or a 3-phase-to-24DC for DC.

Over here you also are not allowed to open a panel if it has more than 48V and you dont have a full electricians education.
What you describe sounds to me to be an extra measure. I can see that it is certainly safer. But do companies really allow non-electricians to dabble with control panels, even if it is low-voltage ?
The company I work for does
 

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