# Poly-phase currents?

Discussion in 'Homework Help' started by Sonthaa, Mar 9, 2015.

1. ### Sonthaa Thread Starter New Member

Mar 9, 2015
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I'm in 8th grade and have no qualms about admitting that I have zero understanding of how electrical currents work. I understand that theres a rotating magnet but thats about it. I'm doing a paper on Nikola Tesla and the biography I'm reading says he invented a poly-phase current, "In the beginning, out of the obligations towards investors, he had to work in the area of carbon arc lights, but after that he started realizing his projects in the area of poly-phase alternating currents." My paper has absolutely nothing to do with said current however I would rather understand what I'm writing about than have a name with no explanation.

2. ### JoeJester AAC Fanatic!

Apr 26, 2005
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Last edited: Mar 9, 2015
3. ### amilton542 Active Member

Nov 13, 2010
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If you have zero understanding of polyphase current then I would start by making a comparison of the advantages as opposed towards a d.c. system which made Teslas's mission plausable.

4. ### WBahn Moderator

Mar 31, 2012
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First try understanding the basic ideas behind alternating current, AC, in which you have a single generator. Then start reading about systems in which you have multiple AC generators that are synchronized with each other to produce power in multiple lines that are used as a set. The most common of these is the widely used "three-phase" power.

Lots and lots of material out on the internet so go have a gander and then come back and we'll see if we can't fill in the gaps.

5. ### crutschow Expert

Mar 14, 2008
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I trust you don't mean it requires multiple generators to generate three-phase power as each generator generates three-phase power all by itself, whether connected to others or not.

6. ### wayneh Expert

Sep 9, 2010
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That's actually a key piece. A magnet moving past a conductor (such as the wiring in a generator) induces current to flow. The amount of current is proportional to the changing magnetic field strength. So for instance as a magnet spins past a coil, a positive current is induced as it approaches the coil and a current in the opposite direction is induced as it pulls away. One magnet gives us one phase of alternating current.

With more magnets, multiple coils COULD be arranged in series or parallel and there would still be just one phase. I won't dig into the details, but it's also possible to arrange the coils and magnets so that the coils are, for example, 120° apart. A full rotation causes 3 pulses. Why bother with the confusion? Well again it involves a lot of details but basically it's about efficiency. Moving power over 3 phases is smoother and more efficient than a single phase. Tesla recognized this, along with the superiority of AC versus the DC that Edison favored.

7. ### WBahn Moderator

Mar 31, 2012
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No, I'm just talking conceptually. It's also a matter of semantics in that a three-phase generator can be considered to be three generators integrated into a single frame.

8. ### Tesla23 Senior Member

May 10, 2009
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There was a big battle between Tesla and Edison over AC against DC. I'm sure you can find information on this.

AC over two wires may be called single phase. The voltage cycles up and down in a sine wave shape.

Tesla's inspiration in polyphase was to use more than two wires, and arrange for the voltage waveforms across other pairs of wires to go up and down at the same rate, but reach their maximum values at different times. If we call the time between peaks 360 degrees, then if the voltage on another phase has its peak one third of a cycle later, it has been delayed 120 degrees. Three phase systems typically have their waveforms at 0, 120 and 240 degrees, each delayed a third of a cycle relative to another.

Tesla's brilliance enabled him to see that polyphase systems would allow simpler and more efficient electric motors and generators. The reason for this is that using a polyphase system you can easily and efficiently create that rotating magnet you mention. If you only have one phase, you can make a magnetic field that goes up and down and reverses, but it always points in the same direction (or opposite). If you have three phases at 0, 120 and 240 degrees, you can generate a rotating field. The Wikipedia article may help visualise:
http://en.wikipedia.org/wiki/Rotating_magnetic_field

9. ### #12 Expert

Nov 30, 2010
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For me, the important part is that the 3 phases produce equal torque in a motor at all times instead of pulses that vibrate the chassis and depend on the inertia of the rotor to carry the motor between power pulses.

10. ### crutschow Expert

Mar 14, 2008
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Integral to that is that the total power output of a 3-phase generator (sum of the 3 phases) is a steady value, the same as a DC generator.

#12 likes this.
11. ### amilton542 Active Member

Nov 13, 2010
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Not necessarily. Tesla's primitive induction motors were actually two-phase.