On 03/17/2015 12:33 AM, Karlsson & Wang wrote: > 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.
Most modern converters are class D amplifiers. One main difference is in the headroom they provide in the switch-frequency. The cheaper ones have about one order of magnitude headroom whereas the really good ones have up to two orders of magnitude headroom, frequency wise (switch frequency vs. output frequency). There is a trade-off between output accuracy and cost of switching and filtering. It all comes down to money ;-) Second, as you note, the current may be sinusoidal, but that does not mean that the voltage is sinusoidal. A motor is an inductive load, where the voltage and current are not in phase. This results in a problem for control loops where you have to choose between current based or voltage based regulation. Both have merits, but ultimately, the result of the output is in the quality of the filters in the VFD and the impedance matching between VFD, cabling and motor, which have to span a considerable frequency range. > 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. Yes. The filters are very important. But you have to look at the system as a whole to build a good one. Components must be matched properly for best results. Non-sinusoidal drivers may be adequate for many systems, but when the power goes up, the EMI pollution generally increases too. It is a question of keeping EMI under control. Eliminating it completely is utopia. -- 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 [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
