Conductors for VFDs

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charlie b

charlie b

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Anybody know of either a ?good design practice? or an actual NEC requirement related to the conductors between a Variable Frequency Drive (VFD) and the motor it serves? Specifically, do you need to use a conductor of a higher rated insulation system (e.g., a 1000V insulation, instead of the 600V used upstream of the VFD)?

This came up in a design discussion with a client who thinks the high level of harmonics generated by the VFD would damage the insulation, if the insulation system did not have a higher rating. He did not think this applied to the conductors from the branch panel to the VFD.

Can anyone confirm or refute the existence of such a requirement?
 
charlie b said:
Specifically, do you need to use a conductor of a higher rated insulation system (e.g., a 1000V insulation, instead of the 600V used upstream of the VFD)?
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I have never seen any manufacturer's literature calling for 1000V motor lead insulation.

The problem with the "higher" voltage output of the drives comes at the 180? turns of the motor windings. So motors may need better insulation but not their leads.
 
The specifications section of most VFD manuals I've come across, call out just copper AWGnn, for field wiring for UVW. By all means, the drive you are considering probably has such a section to be consulted.

The wiring manufacturer sites show specialty cables, which could be specified. I think this would be more for a warm and fuzzy feeling than anything else. There may be shielding, oil, etc., needs which would warrant it.
 
In many installations, we have had to replace standard cabling with "drive rated cables".

The R.F. interference from the drive cables was coupling into other sensitive circuitry.:)
 
We have never had any problems with drive interference using standard wire, but our installations are all in rigid steel conduit.
Don
 
I have been using VFD's for many years in an industrial environment (typically at 480V 3 phase) using MTW or THHN wire and have not experienced any problems. There are several white papers which explain IGBT reflective wave which can cause motor insulation motor breakdown if the motor is not inverter duty rated (1600V peak). The voltage spikes created by IGBT's increase in amplitude as the cable length increases so make sure you consult the manufacture's data regarding the maximum distance from the drive to the motor. If you need more information do a Google search on the phrase "IGBT reflective wave" and you will find all sorts of information.
 
The Allen-Bradley Powerflex manual (which is the only VFD I'm really familiar with) recommends shielded cable for most runs, although they say that regular cables or wiring is acceptable if they're not run near sensitive circuits. They don't specify anything over 600V. I believe they're required for longer runs (>300'). I had wiring for two VFDs installed in a single steel conduit without using shielded cable and while running one, there was enough voltage induced on the other's cable that it would turn on the display of the second even though there was no power applied. Now I specify VFD cables for all my VFD apps. A quick look at the Belden spec, one that I've used shows that it is rated @ 1000V. Think most specific VFD cables might be, too.
 
I have always used THHN or similar for VFD to motor connections. I never run any other conductors in the same raceway. I always use a line/load reactor on the output of the control as mentioned by the previous poster (Transcoil), unless the motor is very close to the drive.. Shielded cable is not needed and could increase the harmonic issue. The line/load reactor must be installed as close to the VFD as possible. There is little concern regarding the input branch circuit.
A 480 volt 3 phase VFD will produce voltage spiking as high as 1500-1600 VAC. That is where the line/load reactor comes into play, and the use of inverter duty motors. It will "for lack of a better word" tame this spiking to some degree. An oscillosope could give you an accurate measurement of reduced spiking.
This high voltage spiking is the result of the high bus voltage and the transistors (IGBT) internal to the VFD. At 480 VAC the bus voltage is around 800 VDC. As the drive operates it is converting DC to AC then switched at a frequency that is transfered down the conductors to the motor. That is why I mentioned and recommend line/load reactors.
The inverter duty motors are special in that they incorporate higher voltage coated magnet wire, increased speed capability and in some cases insulated bearings.
VFD manufacturers sell all kinds of peripheral equipment, and cables are one of them. They make more money on this equipment than they do on drives and motors.
 
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In general we have not used line or load reactors and all of the conduit runs are in excess of 100' between the drive and the motor. We also have not had any problems using standard motors (not inverter rated). I know that all of the white papers say that damage or failure of standard motors is common, but we have not seen that problem. There are about 200 motors on drives in the plant ranging from 1/2 hp to 500 hp. Most of the Ac drives over 100 hp have line and load reactors, but none of the smaller ones do. All motor leads are THWN in rigid conduit. If the motor leads are #4 or smaller the #14s for the 120 volt motor control circuit will be in the same raceway. With larger motor leads the control wires are in a seperate raceway only to avoid the damage that often occurs when pulling large and small wires in the same raceway.
Don
 
The problem we had with a customer was getting them to set up gear ratios so the non vfd rated motors would run at full rpm or close to it most of the time. They were trying to run them continuously with hi torque loads at low motor rpm. Cooling was a killer. THWN wires to motors have not been an issue.
 
Don....You are running control wiring in the same conduit as the motor conductors? Personally I would not do that. But if you are not having issues thats great.
If you have any signaling wiring, I hope you don't have them in the same conduit.

ptonsparky.....You can use use forced blowers on slow operating motors. Blowers with a seperate circuit that runs at full speed while the motor runs at low speeds. They mount to the motor in place of the fan and fan cover.
Gearing is a great tool for motor speed regulation. The motor can run at slower speeds with little effect on motor performance.
I prefer gear driven heavy loads in contrast to direct drive.
 
John,
All of our motor control (start/stop control) for both across the line starters and drives is 120 volt AC and is installed within the same conduit as the motor leads as long as the motor leads are #4 or smaller. We do not have any issues with that installation. The drives require an additional card to convert them from low voltage control to 120 volt control. The speed controls and other single circuits are in other raceways. The only issue we have had with the drives is that often the solid state outputs in the DCS won't stop the dirve...too much leakage current. We either install a ice cube relay or put a running pilot light in parallel to eliminate this problem.
Don
 
My experience is to use an output reactor from Transcoil or MTE sized for the drive if the leads are longer than 75 feet to limit the capacitive coupling on the leads. Anything else other than an inverter rated motor may be overkill. Also the higher the carrier frequency the more prone to high frequency spikes the drives may exhibit on long lead applications.
 
Don,
I am familiar with the expansion cards you speak of. The ones I know about are controled by the VFD. The VFD supplies 24 VDC to the relays on the card, then you can connect your 120 VAC control circuits to the relay contacts. Very handy when the existing control circuits are 120.

Boater Bill,
You are right on the money. I use the 100' rule myself. Or what the
manufacturer recommends.
Are you using reators on the line side? They do an excellent job to help eliminate nuisance tripping. I have had some manufacturers encourage line reactors to comply with the warranty. It's a good practice. I use them regularly. I always recommend them to customers and discuss the load reactor if the (distance rule) exists.
 
I never recommend/specify or deal with a VFd that does not have a 3% input line reactor. Life is too short to deal with the problems that can/will happen without one.
 
boater bill said:
I never recommend/specify or deal with a VFd that does not have a 3% input line reactor. Life is too short to deal with the problems that can/will happen without one.
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Just out of curiousity, what problems do you think you will have if you do not have an input line reactor?
 
Harmonics on the line is a big problem. Nusance tripping on the input breaker of the VFD is one that has cost me a lot of sleepless nights also.

The IGBT's of the VFD switching at 2-10KHz tend to put harmonic noise back on the power lines and other devices will start to trip or behave erratically unless an input reactor is installed.
 
Hi Charlie,

One thing you should consider is what will happen if the VFD rectifier stopped working. The entire system will be impacted by such a large resistance factor that in most cases all of the other motors in the system will overload resulting in permanent damage. Unfortunately, I know of a few cases where this happened.

Myself and another member of the Dolphins Software development team contacted a number of VFD manufactures about the extent of the resistive impact. What we found was that simply upsizing the conductor sizing ampacity an additional 50% would solve the problem. The resulting conductor size is then able to cope with the added resistance and eliminate any other inductive motor damage.
 
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