Vsd-shielded Vs Unshielded Cable
Posted 15 January 2009 - 12:19 AM
I have been going through a couple of VSD Manuals and I've noticed the following.
For EMC Requirements, VSD manufacturer advises to use Shielded cables
But when using shielded cables, the max length of the cable is reduced when comparing with the unshielded cable.
I thought it is contradictory. Say, the motor is 100m from the VSD. If I use shielded cable, condition 1 is satisfied, but for condition 2, I can only use a maximum of 50m of shielded cable, whereas I can go upto 100m if its unshielded.
1. Shielded VSD cables help reducing radiated noises and reduce interference (by returning the noise back to VSD and not radiating) and helps in complying with EMC requirements .
2. At the same time, by using shielded cables, the spikes are reflected back to the VSD itself. But, if an unshielded cable is used, the noises don't return to the VSD, but are radiated. So, by choosing to use a shielded cable at the output of the VSD, we are degrading the performance of the VSD because of the reflected noise. Hence the length of the cable is reduced when shielded cables are used.
So, after a particular length is exceeded, to meet the EMC requirements, one has to use a shielded cable AND output chokes.
Is my understanding correct?
Posted 15 January 2009 - 06:32 PM
There are two issues here, the installation to comply with EMC standards, and the installation to protect the VFD.
Firstly, the VFD is affected by the amount of capacitance on the output. If the cable has a high capacitance, it will cause high transition currents at each IGBT switching and this will result in high switching power losses. There is a limit to the level of capacitance that is acceptable by the VFD. Shielded cables have a higher capacitance than unshielded cables and therefore the maximum length of shielded cable is less than unscreened cable.
For EMC requirements, Screened cable MUST be used. The use of unscreened cable will result in much higher emissions than are acceptable for EMC regulations.
If your environment requires EMC complience, then you must use screened cables except where you use additional filters to the manufacturers specification.
If the cable is too long, you need to either site the VFD closer to the motor, or follow the VFD manufacturers recommendations for compliance with longer cables.
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Posted 15 January 2009 - 06:43 PM
Thank you so much for taking time to reply. Now I'm quite clear abt these.
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