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VFD EMC compliance

Electromagnetic compatibility covers a wide range of phenomena including emission and immunity to harmonics, flicker, and conducted and radiated interference. The material presented in this paper relates to the conducted and radiated interference aspects of EMC.

Technical limits for emissions and immunity to interference are specified in a number of local and international standards of which Australian Standard AS 61800 Adjustable speed electrical power drive systems, Part 3: EMC requirements and specific test methods is typical. Clause 6.6 Engineering practice provides a methodology for dealing with C4 category equipment such as VFD systems as well useful information on problems associated with applying the more usual kind of AC line filter employed in low voltage appliances to power systems with isolated or impedance grounded neutrals (IT-network).

Practical resolution of interference issues usually centres around conducted rather than radiated issues and especially paying close attention to the path of high frequency common mode currents around the installation. In many instances, the use of screened VFD cables will be of assistance.

In order to achieve the required electrical performance at high frequencies, it is essential that the screen of the VFD cable have a 360ยบ connection to both the gland plate of the metallic (typically switchboard) enclosure containing the VFD and the motor terminal box. The correct type of metal cable gland to suit the screened cable should be used. The protective earth (PE) conductor should be terminated in the usual way to meet the local wiring codes.

Isolation switches wired between the VFD and the motor should be in a metallic enclosure with the VFD power cable screen properly terminated on both sides. Failure to properly terminate the screened power cable (or alternative metal sheath) will result in a severe degradation of the screened VFD cables performance at high frequencies and increase the possibility of EMC problems. The screened motor cable should only contain the phase and earth (PE) conductors of one inverter and the associated motor. Do not include other conductors inside the screen.

Wiring materials
There is a wide variety of materials available as well as a degree of misunderstanding concerning the benefits of material without explicit EMC related specification. The following table seeks to summarize the situation.

Category Technical Data Comment
1 Screened cable material from reputable manufacturers Technical data will be available to allow assessment of the performance of the material against specific criteria The manufacturer's claimed data can generally be relied on, provided that the proper installation and termination practices are strictly adhered to.
2 Generic materials with well understood EMC properties. For example, screwed steel conduit and MIMS cable The technical performance of these materials is well understood by analysis from basic principles. Specific data has been reported in reputable engineering research journals. These materials generally offer very high performance, provided that the proper installation and termination practices are strictly adhered to.
3 Material without specific EMC performance data. Armored cables and flexible conduit systems fall into this category when there is no EMC performance data provided. Note that there are high performance, fully EMC specified examples of these materials available which would make them part of category 1. None. Assessment of the likely performance by visual inspection is difficult and unreliable. These materials represent a high risk category because the EMC performance is simply unknown. Apparently similar materials may have widely differing EMC performance. In general, there is no control of the EMC properties during design or manufacture because this is not the intended application.

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