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Starting and Stopping a phase convertor

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  • Starting and Stopping a phase convertor

    I have a rotary phase convertor--it works fine. The question is, "Is it better to let it run all day as you are working or turn it on and off as you use your 3-phase power? Does it use much power when you are not using 3-phase? Is its power usage in proportion to the 3-phase motor you are running or is it the same no matter what if anything you are running?

  • #2

    Stepper, I think you asked one basic question in 4 ways- the basic question is "what's cheapest way to run a three phase converter."
    Like every other question in the real world its a compromise- and best answer is dependent on your preferences.
    "run all day as you are working or turn it on and off as you use it"?
    Run it when you need it. Starting takes time for the converter to build to speed then you put your load on it. So time wise let it run before you need it. Money wise- the starting current is short duration so the residential type meter misses most of it. It cost you next to nothing to start the rig.

    " Does it use much power when you are not using 3-phase?". To get to the basic law of conservation of energy, a good 3 phase motor will not consume much power when idling, The energy it does use is developing heat (the motor gets warm even it idle) because of copper losses, fanning the wind, twisting the bearings, vibration, noise etc. The cost is low. Of course you do have noise, bearing wear, etc balanced against having instantly available power.

    " Is its power usage in proportion to the 3-phase motor you are running or is it the same no matter what if anything you are running?"
    Best way to tell the answer to this one is to go to your electric meter, look at the little disk that spins as the load increases. There is a mark on the meter, with a stop watch count how long it takes to make several revolutions- RPM. Then turn on your converter, see how much faster it spins, then load the converter and count again, then load you equipment (in the case of a lathe take a mighty cut and let it keep cutting) and count the meter disk RPM (with a safety man watching the unattended lathe).
    Bet you a beer, You will find the unloaded converter is relatively free, turn on the lathe and it will draw more but its still almost free, then load the lathe, and you will see a big increase in power used. The reason I am hedging my reply is that a large converter, driving a small load will use most of its energy to keep the converter spinning and may not even show a change in anything when a small motor is connected to it. and it may not show much change when a large unloaded 3 phase motor is connected. But if the converter/load motor are properly sized the changes may not be very great until the load motor is loaded to maximum capability- and that seldom happens in most shops and plants.

    All the above is relative- if you consume a thousand dollars of electricity per month and have a large converter the added cost may be say 25 dollars a month which I consider free, if you bill is 25 dollars a month and it adds 1 dollar I consider that free.

    Best advice: Count the disk turns (RPM), in all possible conditions and decide for your self. One experiment beasts a thousand expert opinions. Course you must know what the experiment really means- you CAN NOT use a ammeter to measure the power you are paying for. go to the power companies "WATT METER" on YOUR wall.


    • #3
      Thank you for the in-depth answer. Sorry about asking the same question 3 ways--must have been the early morning hour. The answer seems to be except for the noise let it run when using it intermittantly.
      Thanks again


      • #4
        Pete: I used to work questioning people. One basic rule for getting to the truth of some thing was "ask the same question from three differant ways". If the answer is consistant, the person is tellingthe truth, even if he thihks he is lying. If the answers differ, the person is not telling the truth , even though he thinks he is.

        Your questions were astute and should beasked by any one who is intersted in efficiency or saving a buck (which, absent govt intervention would be the same).

        Thanks for a good question.
        Its hard to find a question that I know the answer to. or more properly- its difficult to pose an astute question to which I have an answer


        • #5
          So I looked at your profile. I guess that Seattle is as far from Florida as you can get at stay in the lower 48 states. I am 2.5 hours from Canada and the same from the Pacific ocean. So I guess you don't have worry about me dropping in with a question and a coffee cup.
          What are your preference for old cars? Our high school has one of the few Autobody classes in Washington. The current major project in a 28 Ford Tudor custom. My machine/Metal shop students will build the frame and related pieces. My Drafting/CAD students will do all the needed drawings and renderings. I try to get all my pre-engineering students some hands on experience with machines and fabrication tecniques.
          Anyway thanks again for the help.


          • #6
            Stepside "I try to get all my pre-engineering students some hands on experience with machines and fabrication tecniques." is music to my ears. Too many engineers these days do not have a real feel for hardware and when they do design work it sure shows. Keep up the good work.

            Neil Peters
            Neil Peters

            When on the hunt, a broken part is better than no part at all.


            • #7
              Pete: JUst across the sound from Seattle is a little town named Hansville. There resides my sister, her hubby and kids. Sis is a Great grandmother now. We get there once a year when things work out right. Wife Takes courses at whidbey Art center on Whidbey Island. Her Hubby is one of my best friends, an ex engineer and was head of the Trident Program back in 1984 for Lockheed COrp.

              I would move there for the fishing and hunting and especially the Oysters and Crabs. Visit fishermans market every chace I get- not as a tourist but as a lover of seafood. Last time I was there damn near made my self sick on clam necture down by the ferries.
              Have a motor home, run to Vancouver BC just to visit gas light (?) district. Took it on ferry to Victoria. You are one lucky person except it rains to much for the wife. And I enjoy mexican border too. Last trip we hit Tiajuana, mexicali- and every bordertown to del rio texas. So far as dropiing in goes, I have a RV dump station and electrical hook up and in the family Motorcoach association I am listed as a stop over place and mechanics finder. Strangers are welcome to visit 24 hours with jus a phone call. A few stay much longer. Free water, electricity, an level concrete and IF they be nice access to a shop (that aint happened yet ).I like the old sports cars and Mercedes. wife has two cars a 12 cylinder and a 6 Jaguar. The mercedes I loved most was a 1959 300 gas fuel injected limosine that I shipped in from the Philippines. Have helped get some strange stuff running. Some english some Italian. I enjoy the strange stuff. Drove an old Citreon (looked like a 32 ford gangster car) in VietNam where I drove from tip to tip of the country. Damn if I don't think I have had more fun than any one else I know!!>
              The month I quit working- I enrolled in a body shop course at Saint Augustine but I quit cause thay were teaching kids to do Bondo way too thick. Would sure like to learn to do body fender work (straightening, shrinking, smoothing metals, a little leading) and Painting. The kids there were a good bunch, but the school fixed cars for the local dealers, did a slap dash job and a quick paint job. Sad thing. instructor was mid fifties, corrupt and had a good deal. So I made like a good alligator and drug my tail out of there.Wish I had found a place that teaches as you say your school teaches. May head out that way next year and if so I will try to see several persons. Hell, I can go coast to coast, border to border and not be more than hundred or so miles from some one I would like to see AGAIN.

              Just gotta do the Rodeo in Pendelton again and the one at Calgary (never done that one). So many things so litle time!!!!!!
              PS> I enjoy tours of industrial, schools, and meuseum facilities.

              NAMPeters: I agree with you. When I wnet to college, (Auburn Alabama) they had courses in machine shop, foundry, welding plus more. I made a fine living as a "consulting" engineer based mainly on what I learned in a year of physics, those shops I named and hitting the books on my own.


              • #8

                Calgary Stampede - where men are men and the sheep are scared. Where people who never stepped in fresh prairie muffins wear cowboy hats and boots and line dance till they puke. Sure glad I live here in the deep end of the gene pool and not in Calgary...

                Only place to see a bull on top of the grill in at least 1" slices. Pass the Mesquite/Garlic sauce please!


                • #9

                  If you get out this way you need to visit Oregon. First PRIME in September is fun for a part day, lots of small engines and lots of hot air. Then there is the "Spruce Goose" in McMinnville a definite must see.
                  Our autobody facilities are small but we have some good toys. My machine shop built them an English Wheel---They used it to build cab corners for my 72 stepside chev.---Did such a nice job it was in the Seattle Roadster Show as part of a student exhibit.
                  Last year they buit a Porsche 914 with a 350 small block and custom tailights ect.--good enough they could paint it gloss black.
                  My philosophy is --Get the students hands dirty, let them try many things, give them things to do besides computer games and rap music.
                  My students come back from engineering school and say "thank you for letting us actually build something"
                  Have a safe 4th


                  • #10

                    Don't be picking on us city slickers----The country folk invented blue jeans, but the city girls figured out that they look best if they are worn a couple of sizes too small.

                    You sure are correct as to the bull riding on the grill. If done right steak sauce is not required--actually should be outlawed.

                    Is the "sheep thing" something that only some men in Canada do or is it like Montana where Virgin sheep are those who can outrun any male human over the age of six?



                    • #11

                      I have been told the sheep thing is universal. I have enough trouble with women to add sheep into the fray! I suspected that about Montana - it is a few hours from Calgary you know...

                      Funniest thing I ever heard was this guy in in High School was out baling hay. Mom, Dad, and Sis, all devoute church goers - bring lunch out to him one hot August day. They caught him in a tender moment with Bessy the cow while standing on a hay bale.