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« on: February 04, 2007, 05:08:19 PM »
Wire insulation breakdown voltages have nothing to do with anything. If you exceed the insulation breakdown voltage you have really screwed up the design and the system will fail.
There are no 1000V spikes inherent in a VFD. If they were generated by the switching devices (e.g., opening an inductive [motor] load, the electronics must eliminate them or the switching devices would generally fail. This is usually accomplished by a reverse diode across the switching device to dump the resulting energy spike.
VFD motors are designed for variable speed. Motors that are not will probably not suffer from imbalance. There may be other electrical issues, but they are balanced. There may be resonances, but balanced is balanced.
Properly designed systems do not put noise back on the power line. A VFD drive generally first converts line voltage to a DC supply using the same type of switching power supplies mentioned by Bob NH. This power supply effectively decouples the VFD signal from the line. A DC supply meeting current standards also corrects power factor to the line.
Yes a square wave generates all harmonics of the fundamental frequency. No it does not get back to the line in a properly designed system. In a poorly designed system it mostly annoys things like X10 remote controls which operate at very low frequencies. The harmonics may also annoy an AM radio with little line isolation or via radiation for short ranges. As noted, so will a PC or flourescent light. I have some lights I bought from HD and they ruin my FM radio reception. They have lousy line isolation and the tubes radiate. Local radiation is short range and since the wavelength of the frequencies involved in an VFD are very long the equipment is not a very efficient antenna; hence minimal ambient radiation. A switching power supply (which also generates square waves) can have switching frequencies into the megahertz range and are much more likely to be a problem.
I am also not sure why a VFD drive would provide less overall efficiency when matched to the load characteristics than any other possible solution. Certainly not pumping at max capacity and throwing away some large fraction of the energy applied via a bypass back to the source. That is not why VFD drives are very popular industrial solution as a energy saving device. A VFD pump coupled with the appropriate sensor should provide a very effective pumping solution without wasted energy.
I do not believe that the concept of "carrier frequency" applies to a VFD controller.
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