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Homework Help: Electricity simple question

  1. Oct 28, 2014 #1
    1. The problem statement, all variables and given/known data
    why are there only 3 phase systems and 1 phase system, not other systems like 4 phase or more?
    why do electricity only use 50/60 Hz?

    2. Relevant equations

    3. The attempt at a solution
    for number 1 i assume if there are more phase, the price will be higher so it wont be used. But i need other reasons beside that.
    for number 2 i thought because at 50/60 Hz people can no longer differentiate if it is continuous or just blinking.
    is this right? or are there more reasons?
    thank you for answering
  2. jcsd
  3. Oct 28, 2014 #2


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    1. 3 phase just seems to be the easiest and most economical way. If you look down the big powerlines, you will always see pairs of 3 lines. Generators naturally make 3 phase power. The frequency 50/60 is simply set by the generator speed. 60 hz is 3600 RPM I believe. Due to the poles in a generator, the generator spins at a different speed, more likely 1800 RPM for example. You could use a 6 phase or a 9 phase or whatever, its just that 3 phase just makes the most sense monetarily and so forth.

    Perhaps you could say that the naked eye can't pick up 60 hz in lights or whatever, but I think It is set that speed more because of the speeds of the motors we use. Your AC condenser, blender, air compressor, house fan, etc....all turn at a nice speed because of this 60 hz. If the poles are set correctly, a generator spinning at 3600 RPM could nicely turn a motor at 3600 RPM. Thats nice harmony.....generator and motor are almost identical.

    3. Single phase simple pulls off of two of the line to line phases....or line to nuetral phase. Instead of three sin waves intertwined and shifted 120 degrees from eachother, the single phase sine wave is just that.....a SINGLE sin wave that repeats 60 times per second...or 60 hz.

    3 phase is best for distribution, commercial and industrial applications. It just makes sense due to the massive power needed.

    However, in residential homes, single phase works just fine. For example, say you need to turn a huge 200 HP motor in an INDUSTRIAL application that turns a huge water pump or something. This motor needs to last for 20 or 30 years. 3 phase would be best. Not only would single phase be akward, it would shake the bearings right out of that motor after a long time. 3 phase is much smoother. Picture three torque arms turning a motor....that's three phase....nice and balanced. Now picture one torque arm turning a motor. Not so smooth, definitelly not balanced.

    That being said, single phase is fine for turning you can opener, blender, refriderator condenser, mictrowave, dryer and even your smallish AC condenser in your home.

    USA uses 240/120 single phase system in homes. Places like UK and others do actually bring 3 phase into residential. 208/120 I believe.
    Last edited: Oct 28, 2014
  4. Oct 28, 2014 #3


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    The 50/60 Hz frequency setup is common for large grid use, but there are other frequencies which have been or are being used for special systems. 400 Hz systems are found powering certain radar systems, particularly in naval service, while there are some legacy systems which used low frequency (25 Hz) AC for special service, like some of the early pumping systems installed in New Orleans to keep it pumped out. These systems were used from about 1914 up to about 2005, but I believe they are now being converted to use more modern electrical equipment.

    This article gives a run down on the different frequencies which have been used:

  5. Oct 28, 2014 #4
    thanks psparky now I understand why they are using 3 phase but for the frequency i still get confused.
    So I can conclude that by history people were using low frequencies, but then they changed the frequency to 50 Hz or 60 Hz then all the equipment were adjusted to be used at 50 or 60 Hz, but is there any specific reason why there are using 50 or 60 Hz? if all the equipment all around the world were being operated at 100 Hz then i would assume that the generator would be 100 Hz too. Is this because at that time 50/60 Hz were the maximum they can produced? Then because of it all the equipment going at 50/60 Hz? But for some specific equipment, the generator would be generating more frequency?
    so more frequency is better then less?

    sorry for my bad English
    thank you for replying
  6. Oct 28, 2014 #5


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    50 or 60 hz seems to do the trick over time it would appear. Are there any devices in your house that don't spin fast enough for you???

    Here's the formula for motor speed (RPM) depending on frequency and number of poles in your motor:

    Ns= (120*f)/p

    Ns is the rpm of your motor (load)
    f=frequency of your electric supply, 50 or 60hz as you have suggested.
    p= number of poles.

    You can see that as you raise frequency, motor speed increases. Also, as you increase the poles in a motor, motor speed decreases. Or if a motor has less poles, the motor speed will increase.
  7. Oct 28, 2014 #6


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    That's just the point. There were a variety of different generating frequencies being used in the early days of AC power. Over time, it made economic sense to standardize on a certain frequency in order to reduce the number of different models of electrical equipment being made and sold (which can only use one frequency). In the US, 60 Hz was chosen, while in Europe and Asia, 50 Hz was made the standard. This article


    goes on at length about the different frequencies used.
  8. Oct 29, 2014 #7
    now i understand about the frequencies
    thank you
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