# vfd and other motor wires in same conduit



## sparky970 (Mar 19, 2008)

15hp, 480v motor fed from vfd and 1hp, 480v constant speed fan in the same conduit. Anyone see any issues with this? All wires are single conductor.


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## K2500 (Mar 21, 2009)

No. Are you having any issues with that?


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## Netree (Sep 3, 2011)

It will be okay and is very common. Conduit sized right.


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## sparky970 (Mar 19, 2008)

K2500 said:


> No. Are you having any issues with that?


No issues, it is a new install. Just didn't want to miss anything before it's too late. This isn't typically done at the industrial sites in our area.


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## Netree (Sep 3, 2011)

There are not thermals in the motor? Small price, but big insurance to tie thermals to the drive.


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## Big John (May 23, 2010)

Netree said:


> There are not thermals in the motor? Small price, but big insurance to tie thermals to the drive.


 Agreed, but no way would I run the RTD circuits back in the same conduit as the VFD, even if it wasn't a code violation it'd be asking for trouble.

-John


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## Netree (Sep 3, 2011)

Not thermocouples; thermal overtemp switches. One each phase in series, very common. You can tie to the drive with a relay and output inhibit input.


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## Big John (May 23, 2010)

Netree said:


> Not thermocouples; thermal overtemp switches....


 Same thing as internal thermal overloads? What size motors are we talking?

-John


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## John Valdes (May 17, 2007)

Netree said:


> There are not thermals in the motor? Small price, but big insurance to tie thermals to the drive.


I would use them too. All, VFD ready motors have them factory installed for a purpose. To be used.



Big John said:


> Agreed, but no way would I run the RTD circuits back in the same conduit as the VFD, even if it wasn't a code violation it'd be asking for trouble.
> 
> -John



Not all motors use RTD's or Thermocouples. Some use what we like to call (Klixons) in this trade. Klixon buttons are rated in temperature not resistance or millivolts like RTD's and thermocouples are. When the motor winding reaches a predetermined temperature, the klixon opens. Very similar to whats in a hair dryer. When it cools it closes back again.

I agree that thermocouples and RTD's should reside in the their own separate raceway. Klixons are not susceptible to noise or interference. But their supply is. It all depends on the way the klixon button is used. If it is used as part of the drive control power then I would seperate it. If it is used with control voltage independant of drive control source, then I say go for it. Put it in the same conduit or raceway.


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## piperunner (Aug 22, 2009)

Well if its a new install instructions with the vfd says you are not to install other control power or ac wiring with the output of the drive to load meaning the motor thats pretty common drive is all by itself no other wiring is to be with it and its to be as short as possible .




 What precautions must be taken when running the VFD’s power conductors and control wires? When running the power and control wires, the following guidelines should be followed: 

Install the AC input power wiring in its own rigid steel conduit.
Install the output motor leads in their own rigid steel conduit.
Install the control wiring in its own rigid steel conduit. Low voltage DC control wiring and 120 Vac control wiring should be run in separate conduits.
Make sure that all grounds are tightly connected, and properly earth grounded.
 NOTE: If multiple VFDs are mounted near each other, the input power wiring for each VFD can be run in the one conduit, and the control wiring for each VFD can be run in a second conduit, but each set of motor leads MUST be in a separate conduit. However, if one VFD is used to run multiple motors, the output wiring for all of the motors can be run in the same conduit. This out of the book


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## Netree (Sep 3, 2011)

John Valdes said:


> If it is used as part of the drive control power then I would seperate it. If it is used with control voltage independant of drive control source, then I say go for it. Put it in the same conduit or raceway.


Danke!

Yes, a small relay to isolate from the drive is important, but is little expense to replacing the motor if overheated. I think the 1hp motor is for cooling the 15hp motor?


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## Big John (May 23, 2010)

John, still not sure if we're talking about the same thing, but I've only ever seen internal OL's on compressors or small motors. You often run into "Klixons" on multi-horse motors and larger?

-John


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## Netree (Sep 3, 2011)

Big John, this helps explain, top of the page:
http://www.usmotors.com/Service/faq21.htm


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## John Valdes (May 17, 2007)

Big John said:


> John, still not sure if we're talking about the same thing, but I've only ever seen internal OL's on compressors or small motors. You often run into "Klixons" on multi-horse motors and larger?
> 
> -John


They are the same. When they heat up they open. When they cool down they close again. They might be different as far as manufacturer, but the principal is the same. 

On larger frame motors, the internal temperature switch (klixon) has leads that are brought out to the peckerhead (connection box). Here you can take these leads and do whatever you want with them. Some people just tie them together or separate them with wire nuts. (do not use them). In most all applications, these switch type buttons are used in the control scheme. When they open they act like a stop button would. 

On smaller frame motors and single phase motors (your examples) they are connected internally. That's why a simple hair dryer will restart on its own once it shuts off. It requires no reset. However. We almost never want to automatically restart a motor. So we allow the temperature switch to drop out the motor control circuit or stop the drive. Then it takes mechanical attention from the operator to restart the motor or equipment. Some small motors have a reset on the motor.

_Edit.....Hair dryers may need to be turned off then on again to restart the motor._

Some motors (special order) come with sensors like RTD's and Thermocouples. This is usually reserved for very large motors. Expensive motors. These sensors are in turn connected to temperature controllers that can be programmed to function as needed. These sensors can monitor winding, bearing and frame temperature and the technician can use these inputs to facilitate shut down or to trend the equipment. There are usually several/many sensors as compared to one or two switches (klixons).

Controls (drives) will accept most any type of input. You choose your input type in the programming of the parameters and tell the control (drive) what to do with the input.
Just remember that sensors produce a signal and switches open and close a circuit. That is the fundamental difference between the two common types of motor protection, not including overload.


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## JRaef (Mar 23, 2009)

Two threads going on here.
1) Multiple motor leads in the same conduit when one of them is from a VFD. *Really bad idea*. The reasons are: 
A) The VFD cables will be inducing voltages onto the non-VFD cables and vice versa, because the frequencies and synchonization will not be the same. If they were both non-VFD motor leads, no problem because the 3 phases all being from the same source will natually cancel each other out. Not so with VFDs. Even if you are running the VFD at 60Hz, that 60Hz is not in synch with the 60Hz of the other motor, so you will have mutual induced voltages on the cables that may damage the motor windings, the VFD, or both. Is it "done all the time"? Yes, and the VFD and motor suppliers LOVE that!

B) The VFD output cables have capacitance issues that create what are called "standing waves" in the circuit that can damage the motor windings even if you do not have another motor circuit in the conduit. But most inverter duty motors are now capable of withstanding this. However the non-VFD fed motor will not likely be inverter duty, so hese pulses could get induced onto its circuit as well and cause similar damge. It would not be as severe, but that just means it might take longer to show up.
These issues are why the VFD manuals will say, "Don't Do It!"​2) The "thermals" issue. 
A) VFD fed motors can get hot from the very fact that the power supply from the VFD creates harmonics IN THE MOTOR CIRCUIT, even if you have dealt with the harmonics on the line side. Couple that with the fact that in many cases the motor cooling will drop off with speed and it's a good idea to have a separate thermal monitoring system for a motor fed by a VFD. It is not mandatory here in the US however because the VFDs all now have to be UL listed as having Thermal Overload protection based on motor current. But it is still considered a "best practice" because of the non-current related thermal heating effects. That said, some VFDs take the extra step of tweaking their current based thermal protection algorithm in order to compensate for known heating effects at reduced speeds, so they may not recommend external thermal sensors in many cases. It really is a case-by-case issue. In most IEC design motors, the thermal sensors are standard issue and will be thermistors, so many EU based VFDs will come with a Thermistor Trip Input already built-in to the VFD. For NEMA design motors however, this is not the standard practice so you often will need to have ordered the motor with built-in thermal protection from the factory in order to get it. US Motors (the brand) as cited above, happens to do this as standard issue on their larger motors (and then makes you buy THEIR expensive monitor system), but even for them, not on the small motors. So because you need to think to remember to order them up front, you can basically decide on what type you want when you do that; RTDs, thermistors or "klixons".

B) Wiring for these sensors generally CAN be in the same conduit, but still, it's not a great idea. The circuits are usually just monitoring resistance (in the case of trhermistors or RTDs), so noise doesn't really affect that. But they go back to PC board inputs and unless someone has desgned really _really_ good isolation and shielding of those inputs, you can introduce all kinds of noise into the VFD microprocessor environment that way. Top end VFDs will do that, but lower cost VFDs, those often used by OEMs, not so much. If they last out the 1 year warranty, they don't have to worry about it (in theory). Shielded wiring can help, I also like to terminate them into separate signal isolators if I don't trust the VFD design.​


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## Netree (Sep 3, 2011)

:thumbsup: to you post.

I can say I have never had a drive go bad from this.


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## Wirenuting (Sep 12, 2010)

JRaef said:


> Two threads going on here.
> 1) Multiple motor leads in the same conduit when one of them is from a VFD. Really bad idea. The reasons are:
> A) The VFD cables will be inducing voltages onto the non-VFD cables and vice versa, because the frequencies and synchonization will not be the same. If they were both non-VFD motor leads, no problem because the 3 phases all being from the same source will natually cancel each other out. Not so with VFDs. Even if you are running the VFD at 60Hz, that 60Hz is not in synch with the 60Hz of the other motor, so you will have mutual induced voltages on the cables that may damage the motor windings, the VFD, or both. Is it "done all the time"? Yes, and the VFD and motor suppliers LOVE that!
> 
> ...


Great answer. 
Wish my boss could understand this. :-(


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## arifkunnath (Aug 17, 2011)

Good , informative


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