Paralleling multiple idlers
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  1. #1
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    Can anyone detail the parallelling of multiple idlers. I've looked at the possible topologies of such a system (at least as I seem to see it) and I am getting a real headache.

    In a multiple parallel system, all but one motor should be self-starting since there is already 3-phase available on the bus. However, trying to figure out how to connect it has me really confused.

    At first glance it seems that all that is necessary would be to connect the 3 phase lines (A,B,C) from the motor to the A,B, and C lines of the bus (with a disconnect between the motor and bus to isolate each additional idler. When that switch is closed, the motor would start just like any 3-phase motor should when supplied with proper 3-phase current. But then, after it's started, would it then feed the bus and increase to total system capacity?

    This seems just TOO simple and in my attempts to overcomplicate it I seem to get into 'front' feeding and 'back' feeding and current coming and going through the same wire.

    The more I learn about this subject the less I know. I wish I had taken a course in alternating current theory back in those dim days past when I was a physics major. Can anyone suggest a good (EASY...I'm a long time past calculus) text that would be a practical introduction to the subject.

    I sure would appreciate it if someone would give me a practical discription of how such a parallel system would be configured. (Would love to see a diagram).

    John
    [email protected]

  2. #2
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    John,

    You are correct about the paralleling, and the fact that only one motor (usually the biggest) need be self-starting. The beauty of rotary phase converters is that they self-synchronize.

    No, it is not TOO simple! What you are describing is commonly called a 'dual rotary' system, although you can have a triple rotary, or as many idlers as you want, to increase capacity. If you use the design which is posted on an earlier thread,

    http://www.practicalmachinist.com/phaseconverter.pdf

    all you need to do is use a branch breaker in the panel to supply the extra idler motor, just as you would do for a lathe. The only difference is that the idler (which WILL be self-starting as you say) will need to be fitted with run capacitors for best performance and efficiency. You can start this extra idler simply by flipping the breaker on. The single phase 'supply' current is supplied by two wires from the bus to the motor, while the third 'generated' leg is supplied by the idler and returns by the third wire from the idler motor to the panel bus. The common bus in the panel integrates ALL idlers and loads in your entire shop into one system.

    So far, if you follow what I've said, and you don't try to make it more complicated than the simple elegance that it is, you should have NO HEADACHES!

    If you WANT a real headache, that comes when you try to achieve positive sequence phase rotation for your ENTIRE SHOP. This can drive a very intelligent person nuts! Even a seasoned EE. We're potentially talking MIGRAINES here. What I'm talking about is wiring everything in your shop so that, if you connect up a new load motor whose leads are labeled T1, T2, and T3, or U, V, and W, the motor will rotate CLOCKWISE if you connect up the shop three phase supply leads A, B, and C in that order to the motor, with B being the high leg.

    If you know your EE theory and realize that phasor rotation is always counterclockwise while phase leg names are always clockwise, and you can apply this, you're home safe. But it can get VERY confusing in practice in a real shop. So confusing, in fact, that most people I know, including some electricians, just forget about phase sequence entirely, and control the shaft rotation direction at each motor by plugging it in, observing rotation, and switching any two of the three phase legs at the motor if the rotation is incorrect.

    This is definitely NOT optimal, as many machines can be damaged if the motor is run backwards.

    In too many shops I've seen, especially those powered by a home built converter, the receptacles are wired completely willy-nilly, except for ground. This means some receptacles will have clockwise rotation while others will have counterclockwise rotation.

    What every shop should strive for is to have all receptacles properly sequenced so that, for a female receptacle, you start with the GROUND prong hole and go CLOCKWISE with the prong holes being A, B (high leg), C, in that precise order. On Hubbell plugs, the A, B, and C are replaced by X, Y, and Z. For male plugs, the direction of reading is counterclockwise as you face the four prongs.

    All wiring in the shop should follow a common color code so that each color designates a specific phase or ground. I like the so-called 'BRB' schema, which stands for 'black-red-blue'. The A phase is always black, the B or high leg phase is always red (the 'high leg' delta is sometimes called the 'red leg' delta), and the C phase is always blue (or white insulation marked with blue tape at the ends). Ground is always green. These four colors are usually the ones used in building wire and rubber cordage.

    There are inexpensive phase rotation testers which can be very helpful in properly sequencing the phases.

    Hopefully preventing a migraine.

    [This message has been edited by bnelson (edited 05-27-2003).]

  3. #3
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    "I like the so-called 'BRB' schema, which stands for 'black-red-blue'. The A phase is always black, the B or high leg phase is always red (the 'high leg' delta is sometimes called the 'red leg' delta), and the C phase is always blue"

    The "high leg", formerly (and informally) called the "red leg", is now required to be "indicated" by orange throughout the premises, and panelboards are now required to be organized as Phases A, B and C, left to right.

    By convention, the B phase is that phase which has the highest line-to-ground potential, 208 volts in a 120/240 three-phase installation, where the center tap of the A-C phases is groundED.

    One reason for the orange indication is single-phase loads are not permitted on the B phase.

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    I read this doing a search for multiple idlers and I am confused still. Does this mean if I want to run a additional idler I just hook it up to a 3 pole breaker coming from the main idler and add running caps as needed? I really think I am missing something. Does the extra current just backfeed?

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    If you follow the integrated panel design refered to above, the three pole breaker for the extra idler goes into the panel, and the manufactured leg does indeed backfeed onto the panel bus. That way, any breaker in the panel sees the total power output from all the idlers.

    What you don't want to do is power the second idler directly off of the first idler, so that the breaker serving the first idler is having to supply power to both idlers. That's the function of the panel bus. The only thing the breaker for the second idler does is give overload protection and serve as an on-off switch for the second idler.

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    As far as checking the phases should I leave the manufactured leg disconnected from the circuit until after I get the voltage readings?

  7. #7
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    No. You may leave the manufactured leg connected, and measure the leg to leg voltages. If you've got a clamp ammeter, measure the three leg currents too.

  8. #8
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    I know this is an old thread, I've been reading like crazy, but something I haven't been able to determine is related to the design below:
    Quote Originally Posted by bnelson View Post
    ...

    all you need to do is use a branch breaker in the panel to supply the extra idler motor, just as you would do for a lathe. The only difference is that the idler (which WILL be self-starting as you say) will need to be fitted with run capacitors for best performance and efficiency. You can start this extra idler simply by flipping the breaker on. The single phase 'supply' current is supplied by two wires from the bus to the motor, while the third 'generated' leg is supplied by the idler and returns by the third wire from the idler motor to the panel bus. The common bus in the panel integrates ALL idlers and loads in your entire shop into one system.
    Quote Originally Posted by bnelson View Post
    If you follow the integrated panel design refered to above, the three pole breaker for the extra idler goes into the panel, and the manufactured leg does indeed backfeed onto the panel bus. That way, any breaker in the panel sees the total power output from all the idlers.
    Does this design (common bus) allow you to create a larger converter which can handle more amps than the original converter?

    Lets say I have a phase converter rated for 15-3 phase amps where L1, L2 (inputs) come from a single phase breaker and are connected to common buses in a 3 phase panel. The generated leg, L3, from the phase converter also goes to a bus (i.e the design referenced above). If I add an idler which can also handle 15-3 phase amps by connecting all lines to the common bus, can I connect a 30 amps worth of load to the common bus without exceeding the amperage of any of the phase converter/idlers (assuming the original single phase break supports the total amperage)?

    I appreciate any comments.

    Thank you,
    Don


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