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Emc Test Failed, Any Help Is Appreciated.


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#1 JonAW

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Posted 03 December 2012 - 09:33 AM

Hi Guys,

 

We have recently installed a control panel on a site which has had an independent company come in to type test and certify the system.  Our electrical controls are not the only electrical controls on site and is not the only system to fail emc testing either, but we are under serious pressure to comply. 

 

The system is a straight forward waste recycle conveyor plant with various DOL motor's and 5 off VSD/VFD controlled motors. The drives used are WEG CFW08's which only have very basic filtering.  We only have to comply with the basic emc requirements for industrial applications and so we felt the drive chosen was adequate. We now know we have to add input filtering but we are unsure as to whether this will in turn completely rectify our problem. The installation as it stands is installed as follows:

 

All motor starters and drive equipment is installed in a single painted mild steel double door enclosure which is rated at IP55.  The doors are bonded with 6mm trirated cable directly to the backplate and chassis.  The drives are connected to the mains switch disconnector via a thermal/magnetic combined motor protection circuit breaker.  The cabling run from the output of the mpcb to the input of the vsd is standard trirated run in pvc trunking with no segregation from other circuits. The cabling from the output of the vsd is standard trirated run through a toroid, which is supplied with the drive, through the same pvc trunking as before to terminate at dinrail terminals.  The drive is earthed using trirated cable the same size as the output cable directly to the mains earth termination.  It is also directly earthed to the steel backplate via its mounting screws due to the frame of the vsd's being metallic.

 

The site install consists of each motor wired using swa cable.  The cable is terminated using a CW gland  to the gland plate in the panel.  The earth of the steel gland plate is used as the earth of the screen (swa) via the cw gland.  The cables, including the earth cable, are connected directly to the appropriate din rail terminals.     The cable at the motor end terminates in an inline pvc enclosed switch disconnector.  The SWA of the cable is linked using copper lugs and supplied banjos with the swa cw glands to keep the screen continuous.  It is then run from the output of the motor isolator using swa cable again to the motor terminal box.  The swa is again glanded using a cw gland directly to the terminal box. the motor cables are lugged to the approriate terminals and the internal earth wire connected to the earth terminal of the motor.  

 

Can anyone offer any advise as to how we can help reduce emc emissions?  We would like to be able to terminate the output cable to the motors directly to the vsd but due to the cable choice this is not practical or possible.

 

thanks

 

Jonny



#2 marke

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Posted 03 December 2012 - 10:06 AM

Hello JonAW

 

The important issue is the earth path from the motor frame to the VFD frame.

You must have a continuous low impedance High Frequency path from the frame of the motor back to the frame of the VFD.

To do this, you need a conductor with a large surface area for the whole length and an easy way to get that conductor is to use a quality screened EMC cable.

The screen should be bonded to the motor terminal box by an EMC gland that clamps the screen around the full circumference of the cable and is mounted in the motor terminal box with the paint removed where it is mounted. If the terminal box is plastic, use a clamp (saddle clamp or similar) to clamp the cable to the frame of the motor with the screen bard so that there is contact around the whole screen.

Do the same at the drive end. Many drives have built in clamps for clamping the cable and screen. If the drive does not include this, then the next best method is to use an exposed metal gear tray in the enclosure with the drive mounted on the gear tray and the cable clamped to the gear tray as close to the drive as possible.

If there is an isolator between the VFD and the motor, it is important to either keep the screen continuous through the isolator or to clamp the input and output screens to a common metal plate.

If you do not have a good return path between the motor and the VFD, any input filter will not be effective.

 

The return path to the frame of the VFD requires that the VFD includes coupling capacitors between the frame of the VFD and the DC bus. This is standard in Europe, but less common in USA and Asia. If the EMC filtering is not included, connect a 630VDC 100nF capacitor from the DC bus to the frame/chassis with as short a length of as fat a lead as possible.

Some drives have this filtering as an option with a switch or screw to select it.

 

see http://www.vfd-emc.co.nz for more information.

Best regards,

Mark






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