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VFD vs phase convertors for your project

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  • VFD vs phase convertors for your project

    I posted a reply to a phase convertor question in here, Sorry.. but as a electrician of 28 years I sometimes forget that everyone is not been as saturated with the knowledge base I have.
    Most phase convertors work with inductors and capacitors to simulate the missing phase, but the two outside legs usually go directly to the motor. Residential power consists of single phase, being ONE transformer with one transmission line power source, this picks one phase off the line giving a phase angle 180 degrees apart.
    Three phase picks three power legs off at 120 degrees apart. Graphing this you can determine it takes 360 degrees apart to make a completetion and return back to point of origin (0). In the 80's I had to install Westinghouse Vectrol softstart devices to reduce starting demand line loads for 200 horsepower motors. In this application you can adjust both phase angle and voltage. Tinkering around with a machine I had to fix, I had a unknown vibration I could not get out of the machine. Thinking it was mechanical, I flew in experts. Tinkering for days at a time we tried adjusting weights, shifting cogged belts trying to dampen the harmonics. Finally I hit the ideal to put a scope on it, sure enough it was running out of phase (not true 120 apart) and was generating vibration in the motor. I was so embarrassed.
    Now a true VFD does convert the input power to true DC and store it in capacitors, sometimes a single phase input and outputs a 3 phase output *the way I wired my bridgeport. The trigger board fires power transistors varying voltage and current to achieve a speed output as selected via keypad or speedpot, the neat thing is sometimes you can output up to 90 hertz and overspeed a motor if your gearing is not correct. "THE WARNING" is that the capacitors store a enormous amount of energy and can kill you long after you shut the power off, Just like television sets flyback transformer kills numerous technicians every year while being worked on and unplugged at the time. I once had a air-conditioning electrician transfer to my crew, he was used to shorting out the ac capacitors inside a ac motor with a screwdriver, he had no experience with Dc capacitors and sure enough he blowed about two inches off the screwdriver, he needed another screwdriver and another pair of glasses. HE was lucky. MOST inverters (vfd) have a led showing power on the dc bus. Mine lives for minutes after power is removed. My invertor was also cheaper at 230 dollars than the phase convertor when new.(I traded for)
    Utilizing the phase convertor, MY bridgeport pneumatic speed adj head was noisy, being the picky SOB I am I put a VFD drive on it. It was running and working just fine, just noisy. The best I could figure out was the two lines that connect direct to the motor were phased improperly being 180 out instead of 120. The documentation that came with the convertor, said a motor runnning in parallel (mass inertia dampening?) would dampen the signals and make it smooth and capable of full nameplate horsepower, I did not try it tho since I wanted the speed adjustment also with the vfd. Most people know if you run a motor with power on it, it will generate electricity? Dynamic braking (db stopping the motor rapidly) is what we call it in actual use and shunt the generated power to inductors or to the dc bus on other drives *most efficient.
    Tesla designed the three phase power transmission lines when people were still transporting things via horse and buggy, I read his autobiography and personally think he was too damn smart to be human.. ha ha..
    I hope this served to correct my previous omission.
    Yes I enjoy and pull on the knowledge base present here, I am learning machining and have picked up a lot here. Thanks for posting even the most obvious items, since we all must pick up and learn the things we need.
    Since I can't run all my equipment at once, I have thought of running a female twist lock plug from my inverter, and using the same one on several machines around my shop. Just watch out for the lil red eye power indicator. ha ha.. see ya ..