VFD Failure Due to Distance 6

[PLCSlick]

Three months ago I commissioned A conveyor system and since we have gone through 3 VFDs on the same two motors. It is important to note that these motors are the furthest from the VFDs in the entire system (about 340 feet). Onsite maintenance has installed a load reactor to solve the problem and the problem still exist. It is a 2hp PowerFlex 40 and it is driving a 2hp motor. Rockwell support seems to think it is distance related but everything I have been reading says that long distance runs will cause voltage spike that can ruin a motor. We haven't had any problems with the motor but the drives are being destroyed.

 

[Skogsgurra]

Cable capacitance is quite high in a 340 ft cable. The charging/discharging of the cable may be a little too much for a 2 HP drive. Installing load reactors may seem to be a good idea, but the reactors may resonate at the carrier frequency or multiples thereof. So the drive may see a load that sometimes "sucks" more current than it was designed for.
You can sometimes hear the overcurrent in the reactors, a sharp kind of chirping sound. If you hear that, you need to do something.
Best thing is to put a current clamp on the motor leads and look for ringing. If you see ringing, you need to do something about it.

What can ve done? If you are lucky, you may find another carrier frequency where the problem doesn't exist. If you can't find such a frequency, you need to add damping resistors in parallel with the reactors. For a tiny drive like this, I think tha 10 or 22 ohms at 25 - 50 W is a good starting value.

Have had this problem at a large Dutch bank (cooling fans for the data center) where drives died at a rate of one or two per week. Damping resistors helped.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Marke]

You could add sinusoidal output filter to the VFD to reduce the dv/dt and this will reduce the effect of the cable length and capacitance. You could also oversize the drive one size to give you a little more current capacity.

Mark Empson
Advanced Motor Control Ltd

 

[sreid]

The free wheeling diodes in the IGBT co-paks should clamp any ringing churent. But some manufacturers use the co-paks with the smaller diode and they may not live.

 

[GroovyGuy]

Hi PLC,
I have installed hundreds of VFDs over my career, and have never experienced 3 drives failing in such short order. The load reactor is only there to protect the motor. A distance of 340ft should not be an issue with a properly sized dv/dt filter on the drive output.
What is the make/model of the 'output reactor' that was installed?
What part of the drive is failing?
I have never been a fan of Rockwell drives, there are much better units in the market today.
GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[PLCSlick]

Would simply replacing the drive with a larger drive solve my problem?

 

[PLCSlick]

I should add that after further inspection, I have found that the distance between the drive and the motor is closer to 460 feet. The electricians routed the conduit around existing structures to make installation easier and in doing so added much extra length to the wires.
When testing for "Ringing" what should I expect to see when measuring current using a clamp? Also just to make sure we are on the same page here, if I determine that I have ringing in the motor leads I should adjust the output frequency of the drive in hopes that the ringing will be reduced or eliminated?
My question remains can I just increase the VFD size to solve this problem or do I need to add dampening resistors or dv/dt filter?

 

[jraef]

It might, but without knowing the amount of cable capacitive charging current there is, it's a craps shoot as to how much bigger.

Take a look at this document, page 100.

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

This is a chart for the maximum TESTED distance from drive to motor on PowerFlex drives, this page is for the PF40s. This document is based mostly on the potential for motor insulation damage, but also includes cable capacitive charging current. An indirect way of telling the maximum distance with regard to charging current by itself is to look at the first set of columns, titled "No Solution" (meaning nothing on the output). On there it says the max distance for a PF40 to a standard motor is only 40ft, and for a motor designed for inverter duty, meaning insulation rated at 1488V max, is 275ft. That 275ft value is based mainly on the capacitive charging current capability of the drive. You have exceeded that distance. Now notice the colum to the right showing a reactor, the distance is 600ft. Unfortunately this chart doesn't address oversizing the drive as a solution, because that would only address the capacitive charging current capacity of the drive, it does nothing for the motor risk.

To that last point, also note the significantly shorter distances involved if NOT using an inverter rated motor. If yours is not, it's likely your motor is in its final moments as well.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[LionelHutz]

You would adjust the carrier frequency. The output frequency is typically understood to be the operating frequency or the frequency which sets the motor speed and changing that will most likely not help.

You are running the motor at varying speeds that require a VFD, right? If not, the simplest and most robust solution might be to get rid of the VFDs.

 

[itsmoked]

PLCSlick; You will likely see nothing of use with a clamp-on since all the issues are very fast and transient.

For a VFD to be 'simple' and energy efficient the drive outputs can only be fully ON or fully OFF. To get sinewave-like results the periods of time that the outputs are ON or OFF are varied simultaneously for all three phases in some fairly complex patterns. The ON/OFF periods are controlled by the carrier frequency which tend to be somewhere around 3 to 16kHz. The drive does math and figures out how many of these short ON/OFF periods are needed to result in the motor believing it is getting, say, 56Hz power.

For various reasons you can typically set the carrier frequency to different fixed values, often about 6 to 10 different frequencies. One of the common reasons you'd want to change it is for noise. At the lower frequencies motors usually make far more annoying audio 'whining' noise. However, at higher frequencies the effects of line charging, ringing, and drive output transistor heating all increase as do currents flowing about the system due to parasitic capacitance.

Part of the issue is just how fast the ON/OFF edges of the waveforms can change which has little to do with the carrier frequency because the ON/OFF is what it is for the output circuitry and drivers. However, the carrier frequency sets how often these transitions happen. How often they happen can drive various elements of the cable (inductance and capacitance) into resonances and into creating standing waves that can double the waveform voltages in some cases. That's what can trash the drive's output drivers and the motor's insulation.

The longer the cables the more capacitance. The more the capacitance the longer the actual drive output has to be ON for it to successfully charge up enough to get the power into the motor, also the more current that must pass thru the output components which heats them.

Turning the carrier freq down doesn't reduce the effects caused by the system and the ON/OFF edges but it certainly reduces the number of 'rings' and the number of times the drive has to try to charge and discharge the cable run and should have the drive output drivers running cooler. It may alter the tuned standing wave possibilities some also.

That's why we suggest moving the carrier freq down which is not the 'motor frequency'.

A bigger rated drive has less issues with it's output drivers expecting to drive larger currents thru bigger motors but instead driving more capacitive charging current. That's why that's on the table too.

A drive a month.. ugh. No chance you can move the drive to the conveyor? Use the existing cable to get the supply to the drive on the other end of the run?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

I have current clamps that are OK up to 5 MHz and one that does 200 MHz, so it is definitely possible to do the measurements.

A Rogowski coil may also work, but they either have a low BW (like the LEMFLEX and FLUKE - around 20 kHz) or ring heavily (the PEM for instance), but go to 10 MHz or above. The one that I use the most is the I-prober that goes to 5 Mz. No ringing and if current is too high, you can use it without ferrite concentrator and measure leakage field. That will show the character of the ringing. The 200 MHz is a Fischer, it has a rather high lower frequency. Like tens of kHz.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[jraef]

When you say the drives are "failing", do you know what that means? Have you asked for an analysis of them to see exactly what is failing? A-B will do that, but ONLY if you ask for it. To get it, you get a authorization to send a failed drive in and ALSO request an RCFA (Root Cause Failure Analysis). Someone will disassemble the failed drive and do a thorough forensic "CSI" like evaluation as to what killed it. That can be very helpful in a situation so that you are not guessing as to the cause by talking to people over the phone or in anonymous internet forums and throwing money at problems needlessly.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[waross]

You may consider asking ABB to put you in touch with a submersible pump expert.
Small motors and long cable lengths are often encountered in deep wells.
In my area, we often find water at 500 feet down. A pump from 1.5 HP to 3 HP may be 300 or 400 feet down the hole.
The problems with long cables to relatively small motors may be similar to your installation.

http://www.abb.com/cawp/seitp202/0ab6c25f1b7da067c1257707004c0668.aspx

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

 

[PLCSlick]

After physically visit site and measuring the run I found that it is longer than 700 feet. The route of the conduit was absolutely ridiculous and unnecessary. We have opted to move the drive to a closer panel and run new pipe and pull new wires and add a bit of extra IO. This will get us down to 200 feet and should solve the problem. I was hoping to avoid doing this because it is costly. I believe it is best at this point to just pay to do it right since it was done wrong initially. It wasn't a total loss since I was able to be educated by all of you. Thank you

 

[itsmoked]

Yep. Sounds like the best plan.

By "IO" that means you haven't forgotten that the drive probably needs several command wires run to it. If they have to run near noisy drive cables you might do better by having all the IO just be MODBUS or something equally robust and not some pansy analog signal.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[PLCSlick]

All three panels are controlled by one ControlLogix L71 processor with remote ControlNet racks in Panel-02 and Panel-03. The Drives are currently in Panel-03 and we are moving them to Panel-01. I just have to add an extra output card to the existing rack in Panel-01 and modify the PLC code accordingly. The control wires are routed in separate Panduit in the panel to avoid noise from the 480v motor wires.

 

[itsmoked]

Most excellent. Carry on!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[3DDave]

Can you buy onsite maintenance a tape measure for the holidays?