VFD confusion. Output filter, Line choke, du/dt filter ....

Hi again.

From several sources, I hear varying terms used for "devices" that are put on the output side of VFDs.
Can someone help me clarifying them all ?

The permissible lenth of unshielded or shielded cables also depend on what is on the output side of the VFD.

Here is what I have encountered, and what I think they are or do:

EMC Filter class A. Limits the EMC (doh).
Max cable lengths: 25 meter shielded.

EMC Filter class B. Limits the EMC. Better than A (doh).
Max cable lengths: 50 meter shielded.

LC Filter. Inductance (L) + Capacitance (C). A more powerful filter to allow longer cables and/or to reduce the dU/dt which ages the motor faster than with DOL start.
Siemens states that an LC filter increases the inverter load with 10-15% (!).
Max cable lengths: "Much longer than without an LC filter" ?.

Output Choke. Only Inductance. Reduces the dU/dt.
Max cable lengths: More than 100 meter shielded. More than 50 meter unshielded.

Jesper, the items you cite are classified by some European standards that I am not familiar with. Therefore, I can offer you some general help but nothing specific to the European specs.

There are two basic problems with drive output signals to the motor. The first involves the pulses that the drive generates. These consist of lots of high frequency components and, because the motor leads are essentially transmission lines, the further down the leads you go, the more ringing, overshoot, and out-of-control problems you get. The insulation sees this ringing, etc. as excess voltage so the insulation is stressed beyond its normal levels. Naturally, the longer the motor leads, the worse it gets.

The second problem has to do with these high frequency components radiating off the motor leads into the surrounding air, grounding system, and nearby electrical conductors especially those on sensitive equipment. This is the EMI/RFI you mention.

The solutions offered take a couple of different tracks also. First, you can limit the motor lead length. This is not always practical. Second, you can buy a better motor. Motors rated inverter duty and certainly those with an MG1 Part31 endorsement are better motors. Third, you can put "devices" into the motor leads, usually right at the drive output terminals, to strip out the high frequency components which cause the problems.

These "devices" are of various levels of effectiveness and, naturally, have different costs. Starting with the cheapest and least effective, there are reactors (simple series chokes in each phase lead), then dv/dt filters which combine caps resistors and chokes, then sine filters. The use of shielded motor lead cable is also becoming common. There are other proprietary devices being offered and my only suggestion is to read the specs carefully.

Hope that gives you a basic overview of the subject.

Have a read of the GAMBICA guide to power drive installations, follow the link to publications

http://www.gambica.org.uk/

Thankyou guys.

That gambica link was excellent.
I particular like table 2 in this publication (http://www.gambica.org.uk/pdfs/vsd3.zip), "Cost comparisons for preventive measures".

Here is an extract from ABs manual "Wiring and Grounding Guide (PWM) AC drives DRIVES-IN001E-EN-P

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"Wires with dielectric constants greater than 4 cause the voltage stress to shift to the air gap between the wires that are barely touching. This electric field may be high enough to ionize the air surrounding the wire insulation and cause a partial discharge mechanism (corona) to occur. The electric field distribution between wires increases the possibility for corona and greater ozone production. This ozone attacks the PVC insulation and produces carbon tracking, leading to the possibility of insulation breakdown. Based on field and internal testing, Rockwell Automation/Allen-Bradley has determined conductors manufactured with Poly-Vinyl Chloride (PVC) wire insulation are subject to a variety of manufacturing inconsistencies which can lead to premature insulation degradation when used with IGBT drives. Flame-retardant heat-resistant thermoplastic insulation is the type of insulation listed in the NEC code for the THHN wire designation. This type of insulation is commonly referred to as PVC. In addition to manufacturing inconsistencies, the physical properties of the cable can change due to environment, installation and operation, which can also lead to premature insulation degradation.​

The following is a summary of our findings:​

Due to inconsistencies in manufacturing processes or wire pulling, air voids can also occur in the THHN wire between the nylon jacket and PVC insulation. Because the dielectric constant of air is much lower than the dielectric constant of the insulating material, the transient reflected wave voltage might appear across these voids. If the corona inception voltage​

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(CIV) for the air void is reached, ozone is produced. Ozone attacks the PVC insulation leading to a breakdown in cable insulation. Asymmetrical construction of the insulation has also been observed for some manufacturers of PVC wire.​

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John, I think you are replying to a post in this thread:
http://www.plctalk.net/qanda/showthread.php?t=26892

Anyway, when I worked in high voltage power distribution as an apprentice, many years ago, I experienced the "corona" phenomen. It is like rot. When it first start it grows until your can HEAR it from inside an infected cable.