# VFD Disconnect



## splatz (May 23, 2015)

I think this depends on the drive, and many drives won't like a disconnect on the load side. If you can't just use a lockable disconnect on the line side to satisfy the requirement, I believe there are disconnects that have aux contacts that break before the power breaks. The aux contacts go back to the drive inputs to shut down the drive before the disconnect opens the power to the motor.


----------



## darren79 (Dec 20, 2011)

That is what our thinking was as well. VFD I have installed have been in the same room room as the controlled load so have always installed a disconnect on the line side.

I'm not that familiar with VFD but what is the reason they don't like having a disconnect on the load side.


----------



## splatz (May 23, 2015)

darren79 said:


> That is what our thinking was as well. VFD I have installed have been in the same room room as the controlled load so have always installed a disconnect on the line side.
> 
> I'm not that familiar with VFD but what is the reason they don't like having a disconnect on the load side.


I really don't know, and I assume it would depend on the drive. I could see how it would confuse the drive's program if you pulled the rug out from under it that way, although if that's all that happens I'd think a reset of the drive would put it back to normal. I don't know if it could cause any damage to the drive.


----------



## MotoGP1199 (Aug 11, 2014)

Easiest option is to just LOTO the drive or the supply to the drive. Then a disconnect is not needed at the location of the load. If you need a disconnect between the drive and the load(required by facility) you need a disconnect with the optional fast acting aux contact(early break) that opens before the power wires. This will need to be tied to the drive and programed to have the drive free spin(coast) when it is opened. If not you can damaged the IGBT switches on the inverter portion of the drive. When you open the contacts it draws an arc that is like a capacitive circuit and can overload the IGBT's of the drive. The higher chance of damaging the drive instantly happens when you close the disconnect after the drive while the drive is running. The inrush current of the motor starting while the drive is already running can overload the drive and damage it quickly.


----------



## joe-nwt (Mar 28, 2019)

delete


----------



## gpop (May 14, 2018)

We could start a little argument about the value of vfd cable especially as you are only going 100' but if you have the money why not. 

Opening the disconnect can cause damage to the drive over time so like others have pointed out you should install a the aux switch in the disconnect and wire that to the enable circuit on the drive. In real life you might have to open the disconnect hundreds of times to get a drive to blow. (it shortens the life of the drive rather than blows it up the first time you do it)
The real danger and annoyance is closing a disconnect on a running drive. New drives are cheap and are sized to the limits of the motor. They need to ramp the motor up to speed to keep the amps in check. Slam a disconnect up on a running drive and you will get a instant overload. Not only does that risk damaging the drive but its a guaranteed walk to the cabinet to reset it. 

A disconnect also gives you a exposed terminal between a drive and a motor. Beware that some volt meters can not measure voltage at this point. I suggest you get a fluke T5 as the drive may be stopped but the wires can still have high voltage DC on them. (A meter set to AC will not read DC, the T5 is ac/dc). The Vfd output to the motor is also pulse width modified DC not AC so some meters will read random. (vfds not only control hertz which is speed they also control voltage based on hertz)


----------



## paulengr (Oct 8, 2017)

If you insist on a secondary disconnect run two extra small control wires to it and install an aux contact in it (early break type). Set up and use it to break an enable on the drive and use it to coast to stop. That way by time the disconnect is open the drive already powered down.


----------



## Wirenuting (Sep 12, 2010)

paulengr said:


> If you insist on a secondary disconnect run two extra small control wires to it and install an aux contact in it (early break type). Set up and use it to break an enable on the drive and use it to coast to stop. That way by time the disconnect is open the drive already powered down.



Make sure you connect it to the proper drive input and that the drive is programmed for it.
JRaf posted about this awhile ago and gave great info. Different manufacturers call that type of stop different names


----------



## SWDweller (Dec 9, 2020)

darren79 said:


> Have a job coming up that requires the VFD be installed a 100' from the fan.
> 
> We will be running VFD cable and there will be a disconnect at the fan. Is there a special disconnect required when running VFD wire?


VFD cable: learned something today, last drives I did there was not such cable.
Depending on the size of the drive 100' May be to long, the specs of the drive should tell you and if you need any filtering.

Some manufactures offer kits for remote mounting the Estop. Which should solve your issues as long as the Estop is lockable. As for the control wires, you will have to run conduit for those. 
I w


----------



## Malywr (Jan 23, 2018)

SWDweller said:


> VFD cable: learned something today, last drives I did there was not such cable.
> Depending on the size of the drive 100' May be to long, the specs of the drive should tell you and if you need any filtering.
> 
> Some manufactures offer kits for remote mounting the Estop. Which should solve your issues as long as the Estop is lockable. As for the control wires, you will have to run conduit for those.
> I w


I think OP wanted disconnect by fen for safety service of that fan and most likely will never be open or closed under load

But you never know for sure. 
At my place we have disconnect on ever blower but is never used under load but it is me other people might use it differently. 


Sent from my iPhone using Tapatalk


----------



## gpop (May 14, 2018)

Malywr said:


> I think OP wanted disconnect by fen for safety service of that fan and most likely will never be open or closed under load
> 
> But you never know for sure.
> At my place we have disconnect on ever blower but is never used under load but it is me other people might use it differently.


Last place over 150 vfd's and everyone of them had a disconnect between the drive and the motor for lock out tag out. People will open and close them on a running drive which is frustrating as that normally means walking to the mcc's to reset them. Touch wood i haven't seen one blow due to this but its probably shortened the life cycle of the drive.


----------



## paulengr (Oct 8, 2017)

Controls on VFD's are 24 VDC, which means that they are not subject to induced AC voltages...you can run the controls in with the power conductors. The question you have to ask yourself is this: is milliamps of current a "current carrying conductor"? Most would say no...so other than physical space, it can run in the same conduit/tray if you use general purpose wiring.

As to the purported advantages of VFD cable picking on one particular vendor that promotes VFD cable heavily even when they make utter fools of themselves:


https://literature.rockwellautomation.com/idc/groups/literature/documents/in/drives-in001_-en-p.pdf



What is interesting about this version of the document (older ones were a little different is this):
First off they define "unshielded" cable as unshielded tray cable or loose cables run in nonmetallic conduit. EVERYTHING ELSE is called "shielded" whether it is actually shielded, or armored, or just run in metallic conduit. Then paging through all the tables, they ALL have a header giving length limits of "shielded or unshielded". NO extra length is given for using "VFD cable". In the past they used to tell you that you could extend length from say 80 feet to 100 feet before you had to add some kind of filter (load reactor, dv/dt filter, DC choke, etc.). This latest version doesn't mention any of this.


----------



## BAKC (10 mo ago)

darren79 said:


> Have a job coming up that requires the VFD be installed a 100' from the fan.
> 
> We will be running VFD cable and there will be a disconnect at the fan. Is there a special disconnect required when running VFD wire?


If you don’t use shielded cable you will have common mode issues. For VFDs you prefer a UL508 load switch and not a UL98 service disconnect. You will also need the auxiliary to decel your drive to stop before opening your disconnect. You will have to pass through your grounds in the disconnect but not bonded to your protective earth. You will also have to pass the shield through the disconnect, don’t cut them off and don’t use high inductive reactance pig tails. You need surface area to carry the shield. There are solutions for this if you search for VFD intermediate terminations.


----------



## paulengr (Oct 8, 2017)

BAKC said:


> If you don’t use shielded cable you will have common mode issues.


False. It increases the common mode current substantially which is capacitively coupled to and returned by the VFD cable but does ZERO for the motor. In a recent case the customer had 45 A peak currents on a 900 HP fan. At ABs recommendation they put in VFD cable. This increased the peak common mode current at the drive to 90 A, and still 45 A at the motor and still 6 weeks bearing life. The VFD cable is a very good capacitor but that’s it. The basic problem is this. Except neutral point clamped drives, ALL VFDs create substantial common mode voltages. If you have a 6 pulse drive you can output +DC Bus or -DC Bus on each phase. They cannot sum to zero so by definition you will have common mode currents. The only true solution other than NPC drives is to implement common mode filtering to reduce the peak currents below around 0.5-0.8 A/mm2 of bearing contact.

ALL motors have bearing currents. It’s been an issue since motors were invented. It just came back with a vengeance with the advent of IGBT drives:



> For VFDs you prefer a UL508 load switch and not a UL98 service disconnect.


UL 98 disconnects with horsepower ratings equal or exceed UL 508 in every way across the board. You are confused.



https://barr-thorp.com/wp-content/uploads/2011/06/UL-508-Application-Guide.pdf



[quote{You will also need the auxiliary to decel your drive to stop before opening your disconnect.[/quote]

That won’t happen in the milliseconds it takes to open. And there is no interlock blocking it. The correct answer as pointed out earlier is to put the drive into coast to stop (shut off transistors) as fast as possible. Use early break aux contacts at the disconnect. That being said I’ve seen countless 3 contactor bypassed VFDs without the early break function so just fine.



> You will have to pass through your grounds in the disconnect but not bonded to your protective earth. You will also have to pass the shield through the disconnect, don’t cut them off and don’t use high inductive reactance pig tails. You need surface area to carry the shield. There are solutions for this if you search for VFD intermediate terminations.


HF WILL capacitively and inductively couple to the bonded surface. So pretty much I disagree with isolating grounds. There is no substantive reason for doing so. It adds another drain which can only benefit your overpriced cable anyways. As far as your concerns about shield surface area apparently you know little about cable construction. Foil shields are purposely MUCH higher impedance than the phase conductors, at least 10 times higher. If they aren’t they capacitively couple very high currents leading to overheating and huge power losses through the cable itself. A shield does NOT carry substantial current. The exception is concentrically wound neutrals which is not what you are talking about. Those DO have the capability of carrying full current but have very high losses as a result.


----------

