VFD Input Cables and Disconnect Switches 2

[NickParker]

I have a feeder to VFD's in which the project specs for cables are as below,

"Three symmetrical conductors for protective grounding and with a collective copper tape shield".

Can I put these cables only from the VFD to the Motor?, while maintaining different specs of cables for input to the VFD. Does disconnect connect switches are necessary near the sight of the Motor when fed from VFD?

 

[jraef]

You do not need special cable on the input side of the VFD.

Whether or not you need a disconnect within sight of your motor is not related to whether you have a VFD or not. There are multiple rules pertaining to whether or not you need one, we don’t know all of your circumstances. But none of those rules have anything to do with how the motor is controlled.

But if you DO need a local disconnect downstream of the VFD, you should use auxiliary contacts in the disconnect switch wired back to the VFD Control terminals to disable the output before the main contactsof the disconnect open, otherwise the opening of a disconnect on the output side under load can damage the VFD transistors.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[itsmoked]

Hey Jeff,
How do you set the VFD to instantly suspend output before your disconnect separates? Or is that even possible?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[xnuke]

You use a disconnect switch with an early-break contact. That contact is placed in the run-enable discrete input circuit. The VFD controls will stop the output before the main contacts in the disconnect switch open.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[jraef]

Keith,
What xnuke said. MOST drives come with a Disable / Enable input, or one of the inputs can be assigned that way, or the drive has STO (Safe Torque Off) which will accomplish the same thing: it deliberately shunts the gating power for all of the transistors to ground so that they all immediately stop firing, ignoring all else coming from the mP like Decel profiles etc. It’s only the cheapes drives that dont come with that in one form or another.

All aux contacts for disconnect switches are Early-Break by the way. I didn’t know that for the longest time. The only thing you must not do is use a 4 pole disconnect and use the 4th pole as an aux contact, because it will open at the same time as the main contacts which defeats the purpose. That’s not something you would likely do on a 100A disconnect, but on those small rotary 16A disconnects, I’ve seen people do that and wonder why their drives blew up.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[waross]

I have seen plants with fused disconnects in all the MCC cells. They all featured the early break auxiliary contacts. The contacts were used to drop out the magnetic contactor so that the contactor rather than the disconnect switch broke the load current. There is nothing new about early break auxiliary contacts. Those MCCs were new over 50 years ago.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[NickParker]

Thanks Jraef,

This thing of using disconnect switches baffles me, because earlier in one project, in a HVAC Package unit, the Unit itself had a Incoming breaker (Isolator), but still the owner asked us to install the disconnect switch. If the disconnect switch is solely for maintenance purpose, the Incoming breaker could be switched off and locked using a padlock, but why going for a disconnect switch?

 

[waross]

Some safety codes require visual galvanic isolation.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

If the drives had an integral disconnect AND were within sight of the motors, they would have been allowable. But if they were around the corner, there are other more stringent rules as to when that is allowed as a LO/TO point. One of those rules is that it must be a facility “under engineering supervision with established procedures”, and generally many HVAC installations wouldn’t qualify.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden