VFDs and EMC 8

[Marke]

It is very important that VFDs are installed correctly in order to minimize interference to other equipment.
There are a number of basic rules that should be applied to the installation procedure to minimize problems and these are usually covered in the instruction manual for the VFD.

I have recently come across a number of VFD installations that are causing severe interference with flow meters and other equipment to the extent that some equipment has been damaged by the induced noise levels. These have not been installed in accordance with the manufacturers instructions.

I am looking for a simple test and/or equipment that can be used by an installer to see if the VFD installation is satisfactory. This does not need to be a full compliance test, but just a reasonable indicator that the installation is not "BAD", and that it is safe to install sensitive equipment near by.

Has anyone got any experience or suggestions?

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[Marke]

Hi dpc

The motor should be earthed back to the source of supply. and when ain inverter is used, the inverter is the source of supply. Make the earth low impedance as possible back to the source of supply (inverter output) and you will minimise the amount if HF energy that gets dispersed throughout the rest of the network.
The invertoer is earthed back to the mains earth (it's source of supply) so the motor is still connected to the main earth, but it is tha path that the earth current have to flow.
The problem is the High frequency energy must flow back to the iverter where it is generated. If you earth the motor back to the main earth, then the HF energy must flow via the amin earth and that spreads the distribution of the energy through other circuits and increases the level of interference.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[Skogsgurra]

The "swinging choke" is one of ABB:s marketing things. It is nothing but a reactor that has been sized so it takes care of steep edges and is allowed to saturate in-between. It helps reducing harmonics in the mains current, but does nothing for PWM and HF EMI.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Valvecrazy]

Thanks Mark and Skogsgurra
You have answered a lot of questions for me. From what you are saying, I have pretty much been doing the best I possible can. It is comforting to know that everyone else is having the same problems and it is not just me. I have been able to ground and filter out some of the noise and bearing currents but, many times I still can't get the radio controls and other things in that area to go back to working.

Really the only thing I work on is pumps that move fairly clean and cool water. I only work with submersibles, turbines, and centrifugal pumps from 1/2 HP to about 250 HP. In this particular niche, I have found that if I choose the pump carefully, and use a throttling or pump control valve, that valve control will give the same energy efficiency as VFD. If you know how to choose the right pump, it is simply not true that VFD's save energy and Valves burn energy.

I use across the line starters when possible, or if soft start is needed I usually go with the auto transformer reduced voltage all mechanical soft starters. Then by controlling the pump with a valve, I have eliminated ALL the noise, vibration, bearing currents, nuisance trips, environment control problems, and other complications.

I make a lot of customers happy when their cows start producing more milk, the udder diseases go away, and their radios go back to working. I replaced a diaphragm in a valve for a Dairy just yesterday. Somehow they had pumped a rock into the valve and the diaphragm had torn. The guy said this is the first time he had even been in the well house in over 8 years, when the valve was originally installed.

I know there are many things like dynamic braking and machine tools that really must have a VFD for control but, with the size and kind of pumps I normally deal with, I had rather use a valve about 98% of the time. As Mark said, I also think it is very important which drive you use and how it is installed. However, I think it is even more important to understand pumps, and know when an application is better served by a valve than a drive. I guess you can see why they call me Valvecrazy.

Sorry to have gotten a little off the original subject. My thanks to everyone for the excellent information.

 

[gepman]

I haven't done any measuring of EMC but I have had a lot of experience designing, installing, and starting up motors and control systems with hundreds of VFD's. In the industries that I have worked the standard is rigid steel conduit. When this is used for VFD motor leads I have had very little trouble even when the VFD is installed in the same enclosure as the PLC (although I have generally avoided this since PWM drives have become common). Whenever PVC or other type of non-metallic conduit is used I almost always have a problem with any kind of instrument that isn't a 4-20ma signal and sometimes I have problems with a 4-20ma transmitter. Special shielded VFD cable leads help (such as Olflex or Belden) but not as much as steel conduit. A lot of the European (and more of the American) equipment that I install does not have rigid steel conduit anymore.

There are studies (and calculation software) on EMI and the shielding effects of steel conduit versus other types of conduits at

http://www.steelconduit.org/index3.htm

Click on <resources/downloads> at the left side of the page and then <GEMI Analysis Research> at the top of the page.

 

[HCBFlash]

my situation is very similar to that of gepman, except that I have had to deal with more open cabling and PVC lately. My understanding (or assumption) is that by using shielded cable and terminating the shield at the motor, the metallic motor housing completes a metallic / conductive shield of the entire RF radiant circuit. It encapsulates the antenna in a grounded metal enclosure.

So, if the motor is mounted to a conductive (steel) framework, how will that framework act? Won't it be a carrier / emitter of some of the RF? If so, would a solid low impedance earth (ground) bond near the motor effectively eliminate it, or will the grounding conductor too become part of the emitter circuit, and wouldn't this additional "remote" ground tend to increase circulating currents from other points in the system particularly during any events of ground faults?

Any insights? What are your thoughts marke?




flash

Me wrong? I'm just fine-tuning my sarcasm!

 

[Marke]

Hello HCBFlash

Yes the steelwork provide alternative paths for the noise to be conducted and radiated from.
The current will take the lowest impedance path, so provided that the path between the motor and the inverter is very low impedance (to the high frequency component), then the spill over into the steelwork will be dramatically reduced.

Nothing is ever ideal, we just have to make the best with what we are given.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[HCBFlash]

marke,

thx of course for your insights.

Since I niether have access to the type of instrument you're wanting to make / obtain, how have I got this, at least in theory:

To summarize, in the case of a rigid steel conduit (a.k.a. GRC, IMC, Sch40, heavy-wall conduit), or even an EMT installation, the cable should still be OAS, shield terminated to both ends, and bonded to the conduit at both ends also. The preferable load end termination would be by copper conductor to the motor's chassis grounding terminal, where the shield terminates, to the conduit by use of a "bonding bushing". As long as the conduit is installed with tight threaded or compression style couplings, and use of Kopr-kote, or other conductive, corrosion inhibitor, and bonding bushings, it, in combination with the cables OAS, should very greatly attenuate any EMI, as opposed to radiate it. HOWEVER, if a break in the conductive pathway of the raceway occurs, a situation similar to a floating shield in an instrumentation or microphone installation will result, and additional antenna surface will result.

I'm perhaps pounding on this a bit hard, but this described installation method (and one I favor)is rapidly becoming a thing of the past, for a variety of reasons including perceived cost issues as well as young engineering talent that has no experience with it. The steel conduit website is seen by many as simply an industry marketing organization, and not to be taken too seriously. I have a lot of confidence in the comments of both yourself and Gunnar, as being technically sound and not slanted.


flash