When instlling VFD's

Should they be installed as close to the motors as possible or does it matter on the distance the motor is. Some of our motors are like 200'-250' away from the Control box. Would it be better to run the line that feds the power to the VFD to cover half the distance or run the power feed line all the way to the motor and then install the VFD???????


I was just looking at NORD VFD's they have a Software to program frequency inverters does all the VFD's have software like that.

Greg7683 said:

Should they be installed as close to the motors as possible or does it matter on the distance the motor is. Some of our motors are like 200'-250' away from the Control box. Would it be better to run the line that feds the power to the VFD to cover half the distance or run the power feed line all the way to the motor and then install the VFD???????


I was just looking at NORD VFD's they have a Software to program frequency inverters does all the VFD's have software like that.

Click to expand...

200ft sounds like a pretty long run for a motor cable compared to what i'm used to, but I'm not sure what limits are. The systems I work with are usually pretty compact. It might depend some on how dynamic you need the system to be.

I don't think ALL VFDs have software, but I'd say many do. It depends on the complexity. If a VFD only has a few parameters, it is easy to set from buttons on the front. If a VFD has 1000 params, you need software, for sure.

Hmm well one of ours that has a 200' run from the VFD to the motor keeps overloading even with a new motor overload and relay I read that each VFD has different distances a motor can be from the VFD but it talks in meters some might be 30 meters some might be 50 meters depends on the VFD. So i am not sure on the distance has to be with the power feed to the VFD if it had to be a certain distance or not.Like in one of our boxes we have 4 VFD's with two motors around 200' away one at just over 100' away and another at 30' away and they all are linked together in a chain like with low voltage wires that come from the PLC I think for the photo eyes that stop the motors when certain ones are blocked.

The way it was explained to me is that at longer lengths you run into a situation where the high voltage spikes in the output waveform are amplified due to resonance. Similar to a guitar string which vibrates at a frequency related to its length.

"Galloping Gertie", the Tacoma Narrows bridge is an example of resonance gone wild.
https://www.youtube.com/watch?v=CsKKDLKYsVU

Steve Bailey said:

The way it was explained to me is that at longer lengths you run into a situation where the high voltage spikes in the output waveform are amplified due to resonance. Similar to a guitar string which vibrates at a frequency related to its length.

"Galloping Gertie", the Tacoma Narrows bridge is an example of resonance gone wild.
https://www.youtube.com/watch?v=CsKKDLKYsVU

Click to expand...

So it's not good for the motor to be so far from the drive? And dose that apply to the power feeding to the VFD?

Greg7683 said:

So it's not good for the motor to be so far from the drive? And dose that apply to the power feeding to the VFD?

Click to expand...

Look into load reactors. I believe one case where they're meant to be used is when the distance between the motor and the drive is very long.

Mylespetro is correct. A load reactor is recommended for long motor leads. The amplified voltage spikes will degrade the insulation in the motor windings which will result in reduced motor life. The drive will most likely detect the insulation breakdown and shut itself down on an overcurrent fault.

Greg7683 said:

Should they be installed as close to the motors as possible or does it matter on the distance the motor is. Some of our motors are like 200'-250' away from the Control box. Would it be better to run the line that feds the power to the VFD to cover half the distance or run the power feed line all the way to the motor and then install the VFD???????


I was just looking at NORD VFD's they have a Software to program frequency inverters does all the VFD's have software like that.

Click to expand...

As mentioned, over 200, maybe 250 feet, you add load reactors (5% or 3% of full load current, depending on who you talk to). This stretches out the pulses, rounds out the corners sort of.

Over 250 you can use a dv/dt filter up to around 1000 feet. This further stretches the voltage changes.

Over 1000 feet you are best to use a sine wave filter. Output of the sine wave filter only looks like a sine wave at rated current, frequency, and voltage.

There are youtube videos that explain reflected waves much better than I can. To summarize:
- VFD outputs voltage pulses that travel down the motor cable
- when the pulses hit the change of impedance at the motor terminals, they partially reflect
- this is a feedback loop .. so at specific frequencies it can build up to about 2X supply voltage .. Kind of like the microphone squeal when the microphone gets too close to the speaker... depending on the details of motor impedance and motor cable impedance.

IGBTs are regularly rated at 2X to 3X supply voltage for this reason (and others). The capacitors see some of this ripple as well .. in some cases. The capacitors I have seen are rated 20% - 30% more than system voltage.

Greg7683 said:

Should they be installed as close to the motors as possible or does it matter on the distance the motor is. Some of our motors are like 200'-250' away from the Control box. Would it be better to run the line that feds the power to the VFD to cover half the distance or run the power feed line all the way to the motor and then install the VFD???????

Click to expand...

When I first began to implement ACVS drives back in the early 1980s, motor lead length was not an issue of particular concern any more than with any other motor installation. Those early technology drives used 6 step thyristors to simulate a sine wave output. It was in the late '80s or early 90's when I first encountered VFDs that modulated a very high frequency carrier wave to create a better sine wave simulation. That is when reflected waves became an issue.

The first problem that I encountered was with a 20 HP motor located about 200' from the drive. The motor was damaged, but it was not hot and it didn't smell like it had overheated. When it was sent to a motor shop for rewinding, they told us that the varnish had been blown off of the windings. We installed a set of reactor coils between the drive output and the motor and that resolved the problem.

Anyway, long story short -- yes, it is best to design for short motor leads to a motor that will be driven by a VFD. That said, you don't want to locate a drive out in the field when it could instead be located in a climate controlled motor control center. There are work-arounds for the reflected wave problem, and load side reactors are the most common solution.

See attached file

@Greg: This document is related to a specific brand model vfd but can be applied to your case.

I'll check them out thanks

I installed some ab vfd's several years ago.

the rep said 230 volts didn't need line & load reactors, I still put load reactors on any way.

for 480 volts, for distances 50 ft or longer, you needed line and load reactors.
load reactors to counteract the motor feedback and line reactors to prevent spikes to the control panel.

that's what he said.
cannot answer in regards to other brands.
james

I'll try and chime in with a link tomorrow, if works not to hectic, but I've ran into similar problems.

The issue is with the "electronic motor protector" type devices. The overload (not the drive) needs to be close to the motor to eliminate false trips, due to the harmonics.

This, of course, is typically not feasible, but nonetheless, I've ran into a few times. On occasion, I've simply just replaced a few electronic overloads with the old fashioned overloads with "heaters" to get around it. I'm not sure if this is the problem you are dealing with, but if you are using the newest, latest, greatest, electronic motor disconnect/overload/disconnect combo, this may be your issue.

Why do you need a motor overload? The VFD cam provide thermal overload protection. Or this this an American regulations thing?

Saffa said:

Why do you need a motor overload? The VFD cam provide thermal overload protection. Or this this an American regulations thing?

Click to expand...

As long as the VFD states that it provides motor protection, an overload is not necessary.

Typically when more than one motor is being ran on a VFD is when you would install overloads.


Curious as to why nobody has mentioned the configurable PWM frequency on some drives. This setting can help with long motor leads. Just be sure to check the manual before changing it because some drives have more range than others. 2khz and 6khz is pretty standard with 2khz being the default.