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Electrical ower service question

Discussion in 'Electronic Design' started by John E., Dec 15, 2005.

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  1. John E.

    John E. Guest

    Service to building is 150A, 115/220 single-phase. Proprietor wants to run a
    3-phase machine using rotary converter.

    How efficient are these converters?

    What is a safe maximum running load for a building with 150A service?
    Obviously the machine won't be pulling maximum rated current all the time. Is
    it safe to run 120A load? 140A load? Machine is mostly motor, running less
    than 2 hours at a time, typically (printing press).

    Building has only a half-dozen fluorescent lights, nothing else (no
    computers, etc.).

  2. Tim Wescott

    Tim Wescott Guest

    I don't know the answers to your questions.


    Many of the folks on rec.crafts.metalworking use 3-phase machinery in
    residential or light industrial buildings with no 3-phase power. So the
    interest level in rotary converters is high, there are a number of folks
    there with practical experience and even a few who seem to really know
    their stuff in this area.

    I'm going to cross-post this, even though there's already three groups
    on your list...
  3. Jon Elson

    Jon Elson Guest

    I don't think you'll get away with it. The big problem is the reactive
    which is a substantial part of the real current draw. If the machine
    draws 120 A
    of real current, and the reactive current is 60 A, the total current will be
    Sqrt(120^2 + 60^2) = 134 A. Also, if the 3-phase load is rated at 120 A
    per phase, the single-phase equivalent current is 207 A, without considering
    the reactive component. The 120 A at 240 V indicates a motor around
    35 Hp. The starting current will be in the neighborhood of 1000 A for
    several seconds, given the high source impedance for such a heavy load.

    Unless this press has a slow-start feature, you will blow the 150 A service
    entry breaker, and possibly the breaker on the pole transformer, when you
    start the press. Probably everybody's lights will dim so far that computers
    will crash, air conditioners, etc. will pop breakers, and so on for several
    blocks around.

  4. I am highly confused by the question. 120A of "mostly motor" drawing
    220V amounts to 35 horsepower. When I think of printing presses (like
    at a factory that was liquidating where I picked some stuff), it is
    hard to imagine a 35 horsepower motor on a printing press.

    I think that it would be good to clarify the needs of the owner, as
    everything hinges on it.

    A 5 HP easy starting motor (no large inertial loads or loaded
    compressors) can run on 120 amps. A 35 hard starting motor, probably
    not. Everything else is in between.

    So, the first thing that is in order is to find out the power
    requirements. That includes answering how much inductive load would
    there be, whether all motors start at once, how much reactive load and
    what degradation of 3rd leg quality his 3 phase equipment would

    Second, not all electric services are equal. Some would happily
    produce 120 amps and some woould not, as utilities sometimes install
    undersized pole transformers. Again, that should be found out before
    investing too much money.

    There are experts here who could help further when you clarify your
    power needs. I am not one of them, even though I built a couple of
    phase converters, a 10 HP one and a 17.5 HP one.

  5. CWatters

    CWatters Guest

    Check out the table ..
    It has figures for the single phase current need for a particular converter
    at max load. It suggests you need >160A if I've read it right. They give a
    phone number for advice.
  6. SQLit

    SQLit Guest

    Exceed the 150 amps and the overcurrent should open. Therotically you
    should be able to run 150 amps through a 150 amp service. Typically NO ONE
    DOES. The last 150 amp service I had was on a house, its total draw was
    about 70 amps.

    Pay real close attention to the other posters suggestions about reactive

    I used to partner with a steel welding company. They had a 200 amp 3 phase
    service on their shop building. Owner went out and bought a stud welder that
    they used infrequently. They made some type L copper fuses for their
    service and replaced the fuses every time they needed to use the stud
    welder. Nothing ever melted down. They were diligent about replacing the
    fuses. Eventually because of the demand the utility got wise and REQUIRED a
    service change.

    The service change is some thing that you should consider. No converter ever
    necessary. Also you had better look into the rate structure.
  7. Grant Erwin

    Grant Erwin Guest

    Yes, it can also run on 100 amps, 50 amps, or probably even 30 amps! 220V power
    at 30 amps is nearly 9 hp worth of watts. Sure it would draw more at startup,
    but it would run on a 30 amp circuit fine.

  8. Wayne Cook

    Wayne Cook Guest

    First question is what kind of drive does this press have? I must
    admit that I've only worked on one printing press but it's got a DC
    motor driven by a electronic controller for variable speed (as far as
    I can see this is pretty much a necessity for a printing press). Thus
    my question of what kind of drive.

    Actually that's a pretty small press to just have a 35hp motor. The
    press I work on from time to time has a 75hp DC motor on it and that's
    a smaller motor than was on the press originally. It's a press for
    printing newspapers and it's fairly small as far as news paper presses
    go (the owner talks about installing and running much larger presses).
    It's got 8 units (though only 7 are in service at the moment) and each
    unit is capable of printing both sides of the page. But 4 units are
    needed to print the color pages. Thus they have the capability of
    printing a max of 16 pages per section currently.

    Wayne Cook
    Shamrock, TX
  9. Guest

    Thats true if youve got the right type of overcurrent protective
    devices all along the line, but on a domestic supply this wont normally
    be the case, and they wont sit through the startup without tripping. If
    you can control the motor so as to startup gradually you should get
    round this, but whether that can be done depends on what the mechanical
    load is, whether its connected during startup, and what type of
    controller the motor has, if any.

  10. Grant Erwin

    Grant Erwin Guest

    What do you base this on? I have a 7.5hp motor I regularly start and it's wired
    to a normal 30A breaker in my panel. It's the idler motor on my phase converter.

  11. Eric R Snow

    Eric R Snow Guest

    I have 400 amps available to my shop though at this time am only using
    a 200 amp service. Just like any residential service. Which makes
    sense because I'm in a rural area with only single phase residential
    service offered. I make my own three phase like Grant does. Except my
    phase converter uses a 15 HP motor. It is wired through a 60 amp
    breaker. It has never been a problem.
  12. Rich Grise

    Rich Grise Guest

    The keyword here is "idler". It's a 7.5 hp motor, sure, but I seriously
    doubt that it needs anything like the startup torque would be if it had
    an actual 7.5 hp load.

  13. It all depends on what kind of load. For example, a blower is a nice
    easy starting load (develops resistance with speed), a compressor
    without unloaders is a hard load, etc.

  14. Grant Erwin

    Grant Erwin Guest

    Correct, Rich, but the OP did say "easy starting 5hp" ..

  15. Rich Grise

    Rich Grise Guest

    But, isn't the idler's job just to spin up the main unit, which then
    bootstraps itself?

  16. Not exactly. It is true that a 3 phase motor, once spun up, could
    continue to run, however, it would develop only 2/3 of horsepower (and
    suffer from unbalanced power that it was not designed for).

    With a phase converter, a loaded motor could develop 100% of its
    horsepower, provided that the capacity of phase converter is

  17. Guest

    A motor draws well above run current during start-up. How large this
    current is, and how quickly it diminishes depends on the motor type,
    how its controlled, and what its mechnical load is.

    A fuse or breaker will tolerate overcurrent of a limited amount and
    duration only before it trips. Again, what exactly it tolerates depends
    on the characteristics of the breaker/fuse.

    So whether your motor/breaker combination will run or not depends on a
    lot more than just the ratio of motor run power to breaker capacity.
    Some combos will run ok, some will never get off the ground, and some
    will sometimes start, sometimes not.

    Note also that breaker characteristics vary a lot between eg US and UK.
    Our domestic breakers are normally type B, whereas US breakers are
    closer to our type D, which are a lot more tolerant of overload than
    type B.

  18. DoN. Nichols

    DoN. Nichols Guest

    The typical US breaker tolerates quite a bit of *short-term*
    overload, but for long-term, the overload limit is a lot closer. I don't
    know how these compare with your Type-D and Type-B breakers.

    I believe that the short-term overload trip is magnetic, while
    the long-term overload trip is thermal.

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