# three phase electricity

Discussion in 'Homework Help' started by andrewmm, Mar 6, 2016.

1. ### andrewmm Thread Starter Member

Feb 25, 2011
27
6
Not home works as such,

I'm retired, and this is for an uncle of mine,

Hes very up , especialy for his age, and keeps wondering / asksing us why his big wood working machines need three phase, and what the advantage is.

I try, but my background is lots of mathematics, trig functions, etc.
Hes a time served wood worker, finished school at 14 etc.

does any one have any links to a nice demo of how the currents in the three phase cancel out ,
was thinking about doing a spread sheet or something, was wondering if anyone knew of one on line
ta

2. ### shteii01 AAC Fanatic!

Feb 19, 2010
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andrewmm likes this.
3. ### andrewmm Thread Starter Member

Feb 25, 2011
27
6
thank you
that look sperfect

4. ### profbuxton Member

Feb 21, 2014
232
68
please note that the phases are actually 120 degrees apart ie: phase B lags 120 degrees behind phase A and phase C lags 120 degrees behind B. If they were all in phase you would not have a rotating magnetic field in your motor and it would go nowhere.

5. ### WBahn Moderator

Mar 31, 2012
17,446
4,698
Depending on what depth of understanding your uncle is looking for, the following might be sufficient for his curiosity:

There are two main advantages for using three phase over one single phase. First, the more phases you have the less current you need in any one phase and hence the losses in the line resistance is reduced. The more power is being delivered to a machine, the more efficiently you need the delivery to be because otherwise you waste more and more power.

Second, you get smoother power to the shaft -- which may or may not be important in woodworking. If you are using single phase (including split-phase), the power pulses to a peak and then drops to zero and then pulses to a peak. But in three phase as the power from one phase is peaking the power in another phase is rising while the power in the third phase is falling. The timing is such that the total power from all three phases remains constant at all times -- all that changes is how much of the torque is coming from each phase -- yielding a steady torque to the motor shaft at all times instead of the pulsing torque you get with single phase.