The frequency converters I have seen for electric motors generate a square wave voltage. To generate a sinus the duty cycle is varied to get sinus voltage in average and usually the current is close to sinus.
Then it come to quality I guess the large difference is in filters and coupling to control signal ground. There exist true sinus output but I think all of them are sold as true sinus for a higher price. Nicklas Karlsson On Mon, 16 Mar 2015 22:19:42 +0100 Bertho Stultiens <ber...@vagrearg.org> wrote: > On 03/16/2015 09:40 PM, Karlsson & Wang wrote: > > A shield is primarily intended to prevent electrostatic coupling from > > the outside world. So by grounding in the consuming end the shield > > will get the ground potential of the consumer and the signal cables > > will be shielded from different external electric fields. This should > > motivate why as you say the shield should be connected in this end > > only. If there are current there is also a potential difference. > > There are generally three cases to consider: > 1) a shield as a Faraday cage; this means that only one side of the > shield may be connected to go into the earth/ground star-point where you > maintain an absolute reference of zero (0V). The shield is used to dump > all (most) cable-internal EM radiation into a low-impedance star-point > via the shield. > > 2) a shield as in a coaxial conductor; here the signal is contained in > the cable and the shield is part of the conducting circuit. This works > on basis of very tightly controlled cable properties and is generally > limited to a specific frequency range per cable specs. > > 3) all other cases; the shield is connected at both sides and is > intentionally or unintentionally part of the conducting circuit. This > case will generally give you worse results in terms of EMI emissions and > protection. The shield will radiate and may act as an antenna to worsen > the situation. > > > > I consider the VFD to be a noise source since it have common mode > > voltage which will emit an electrical field. There is also a > > capacitance between the VFD cables and shield. Since Shield impedance > > on high frequency is far from zero the shield around the VFD cables > > will not be at GND potential. The most common method is to increase > > common mode inductance by a filter but I have also seen active > > filters which reduce the common mode voltage and multiple step > > voltage inverters. > > With respect to VFD systems; the amount of junk they produce depends on > the quality of the converter. > > The best version generates a relatively pure sinusoidal output and the > EMI it generates is very minimal. Such VFD can normally be connected > without problem with unshielded cabling. > > The lesser quality emulates a sinusoidal output, but has substantial > higher harmonics. These VFDs are not too shabby, but they can cause > interference. The best solution is to filter the output before putting > it on (long) cables. No shielding is required when the harmonics are > under control, but it generally does not hurt to use a Faraday cage type > shield. > > The cheap VFDs are poor substitutes and generate nearing square-wave > output. The amount of EMI from higher harmonics is high and is often > difficult to filter at the source. These VFDs should be used with both > output filters and shielded cabling. > > The remaining problem that may arise is EMI from the motor. The > remaining harmonics may radiate from the motor just as easily. That > cannot be solved with shielding of the cables. You must ensure proper > earthing of the motor as well and it should be enclosed in a proper > metal casing. > > It should also be noted that VFD frequency changes cause harmonics in > the output. If you turn on/off the hard way, then you can introduce some > transients that are very hard to control. The best way is to control the > up-/down-going frequency such that no abrupt changes can occur and > therefore no transients are allowed to be created due to too fast changes. > > -- > Greetings Bertho > > (disclaimers are disclaimed) > > ------------------------------------------------------------------------------ > Dive into the World of Parallel Programming The Go Parallel Website, sponsored > by Intel and developed in partnership with Slashdot Media, is your hub for all > things parallel software development, from weekly thought leadership blogs to > news, videos, case studies, tutorials and more. Take a look and join the > conversation now. http://goparallel.sourceforge.net/ > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users -- Karlsson & Wang <nicklas.karls...@karlssonwang.se> ------------------------------------------------------------------------------ Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
