3 Phase Power Vs Single Phase Power

  1. I have to answer a question on the advantages a 3 phase power supply has for an x-ray machine and the production of x-rays as compared to a single phase power supply. Could you please tell me if I have this correct and if there is anything else I could add?

    - 3 phase is about 150% more efficient than single phase
    - Power in a 3 phase power supply never drops to zero but in single phase the power falls to zero three times. Therefore for a 3 phase power supply the power delivered is the same at all times.
    - Smaller conductors can be used in 3 phase power supplies and therefore it makes it 3 phase more effiecient at supplying power to large electrical machines like x-ray machines


    (I didn't know if I should post this thread in the Medical Science Forum or in this one....)
     
  2. jcsd
  3. what is this x-ray machine??!! some death ray gun that's as big as the Sears Tower?

    if the 3-phase is reasonably balanced (so that the return current through ground or the 4th "neutral" wire is zero or very low), 3-phase can carry more power per line (3 lines) than can single phase with 2 lines. (assume the same maximum current and line-to-line voltage difference.)

    there are electromechanical advantages to 3-phase AC motors. they're more efficient and need no "self-starter ring". the direction of rotation of a 3-phase motor is unambiguous.

    but for a piece of electronics, even reasonably high-powered electronics, i don't get it. i have no idea why an x-ray machine would work better supplied with 3-phase than single-phase since the power will need to be converted to DC in either case.
     
  4. NoTime

    NoTime 1,570
    Science Advisor
    Homework Helper

    Large X-ray machines are more likely to be industrial rather than medical.
    Living organisms do not fare well with large doses of X-rays.

    Anyway
    1) 150%? How so? Us EE types tend to think of efficiency in terms of how much of the input goes to producing the desired output.

    2) Yes, the phases overlap so a three phase to DC rectifier setup will never go to zero current.
    This could qualify as an advantage.
    However, you have this part "in single phase the power falls to zero three times" somewhat confused.
    Perhaps you could reword it.

    3) I'm not that familiar with 3 phase power and would have to work out an answer, but I think the total cross sectional area of the conductors will be about the same. Smaller yes, but more of them.

    X-ray machines require DC power.
    Three phase power requires more rectifiers, but allows another component can be smaller. On a cost basis this could be an advantage. What component can be smaller?
     
  5. stewartcs

    stewartcs 2,279
    Science Advisor

    I might re-word this part a bit. Efficiency is generally used to describe the ratio of the systems output to input so it may cause a little confusion with some.

    I think maybe your point was that when comparing the efficiency of 3 phase to single phase the ratio between the efficiencies is 1.5 in favor of the 3 phase.
     
  6. Oh ok :D Thanks for the help everyone
     
  7. The advantages of three phase power are many, but mainly:
    Three times the power available with the same three wires that would be used for a single phase service.
    As you stated, in a three phase service the power never drops to zero as it does with a single phase source.

    A 10KW load served by a 240V single phase source requires significantly larger conductors
    than would be required of a three phase 240V service. Savings due to the cost of smaller conductors and the ease of installing those smaller conductors (and possibly smaller diameter conduit) adds to the appeal of 3 phase power.
     
  8. The ripple frequency is higher (360Hz) and I believe those X-ray tubes produce X-rays only at the peak sof the ripple waveform. So you'll get 3-times as much X-rays.
     
  9. Oh wow thanks soooo much everyone!!!
     
  10. Astronuc

    Staff: Mentor

    The benefit is the reduced current in each wire for a given power level. The lines are rectified as was mentioned, and since the phases are shifted 120° from each other, one fills in the dips of the others.

    See - http://home.san.rr.com/nessengr/techdata/rms/rms.html#fullwave

    http://www.play-hookey.com/analog/full-wave_rectifier.html

    http://www.tpub.com/neets/book7/27b.htm
     
  11.  
  12. No, it IS single phase. Do a search on this forum and you will find this discussed in at least one other thread.
     
  13. stewartcs

    stewartcs 2,279
    Science Advisor

    In the USA it's actually split-phase power coming into the house. The transformer has a center tap that splits the primary's single phase input voltage into two voltages and a neutral to ground. The result is, if you measure the voltage between either of the voltages on the secondary to ground, you get 120 VAC. However, if you measure between the two voltages, you'll get 240 VAC.

    Hence, the split-phase terminology.
     
  14. true,true, if we measure between two voltages we get 240 volts, doesn't it mean between two voltages=two phases? , two phases gives 208 Volts (or 240 v as ussually some companies in Europe say to USA and/or Canadian customers when we buy some machines from them)

    example; you boght a machine from europe. specs say this machine operates on 240 v (208V in America) You are an installer and have to provide power supply of 208V. Would you run 1 neutral wire, 2 live wires, 1 ground wire?

    ***** i did it the way I just suggested.
     
  15. No. The only reason we have what you call 2 phases is because we center tapped the transformer. If we put multiple taps on the transformer would you call it 4-phase or 5-phase? Single phase (what you have been calling 2 phase) is never 208 volts in the U.S. You may have been in a situation where you THINK it is, but it is not. I'll explain more after the next quote.

    I don't see why you assume that it is 208 volts in the U.S. Normal 3-phase power in an office building or apartment for instance will be 208 volts with a wye configured transformer. The center of the wye is where one end of each transformer secondary winding are joined and grounded. There is 120 volts on each secondary winding so when measured from each 'leg' to ground you will measure 120 volts. However, when you measure between legs you will get 208 volts. Obviously this is not the case with single phase or what you have been calling 2-phase. This can be a problem for things that require 240 volts in a building that is serviced with 208 volt wye configured transformers. Hot water heaters and things of this nature don't really care so things like this can be run on 208.
    -
    Most buildings that house industrial equipment will be serviced with 240 volt delta configured transformers. What happens here is the transformer windings are connected all in series. They form a triangle on a schematic. Usually one of the secondary windings is center tapped and this tap becomes the neutral/ground. Now we can take 120 volt power off of each end of this transformer in reference to its center tap. This way we can run normal 120 volt items as well as 240 volt single phase items. The 3 'legs' in this system are typically labeled A, B, and C. A and B are the ones that come off of the center tapped transformer. The C leg is often referred to as the 'wild' leg or 'bastard' leg. It measures around 170 volts relative to the center tap (ground). However, when you measure between any of the 3 legs you will always measure 240 volts.

    BOTH systems 208 volts wye and 240 volts delta are commonly used in the U.S. Did you do a search like I asked?
     
    Last edited: Nov 5, 2007
  16. why would you need another neutral wire if you have a ground wire?
     
  17. NoTime

    NoTime 1,570
    Science Advisor
    Homework Helper

    Perhaps not for the European appliance if it works on 60hz.

    Some US appliances like a clothes dryer use the split phase.
    240v for the heater and 120v for the drum motor.
    So you need the 2 wires for 240 plus the neutral for the motor.
    The ground wire is for safety and must be the same size as the other wires. It should never carry current unless there is a fault.

    Other US appliances like a large window A.C. unit are straight up 240v, like the European appliance, so they only need 2 wires plus the safety ground wire.
     
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