Mike holt stated on one of his classes that in order for any eletrical equipment to work, that on one end had to be voltage and on the other side it had to have no voltage. So when the voltage goes to a three phase motor where does it go? Does it go to ground? I understand that the 3 phases are 120 degrees out but where does the voltage go since all the windings are connected? I know that the transformer goes to ground but does the motor do the same thing.

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Originally Posted by ELECTRIC EEL

... for any eletrical equipment to work, that on one end had to be voltage and on the other side it had to have no voltage...

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Incorrect statement. Voltage itself does not do anything. A voltage DIFFERENCE between two points does!

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Originally Posted by ELECTRIC EEL

so when the voltage goes to a three phase motor where does it go?

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So a motor runs on a voltage difference.
With one phase that voltage is AC to one winding. The voltage is sinusidal, and - in the US - with a frequency of 60 cycles. With two wires the applied voltage is 180 degrees shifted, i.e. when one side is positive, the other side is negative. Note that the voltages at both points always cancel each other out. At the transformer one side is connected to earth (called neutral), the other side is called "hot".

Three phase is also 60 cycles per second, but the three windings of the motor receive now each voltages that are 120 degrees shifted in phase between each other. So if the 3 phase 220 Volt voltage on wire 1 is momentary nill Volt, than wire 2 is about + 180 volt, and wire 3 is about - 180 Volt.

The resulting voltage of these three voltages is again always nill, whatever each individual voltage is. Therefore there is no need to connect a forth wire to the common point at all.

And at the transformer the situation is as with one phase : the common point of all three phases is connected to earth (the neutral is used only for one phase connections). And each three phase wires is "hot".

Capice?

I'm confused with the explanation, but I'm not sure I can do any better without pictures.

I'm going to throw out a statement and let's assume you've heard of it before. 3 phase can be thought of as being a Y transmission arrangement or a Delta (A triangle) arrangement of the phases and transformers can be used to switch between the two modes.

We can take the simplest case. The 3 windings of a generator are connected in the shape of a triangle and the windings of a 3 phase motor also connected in the shape of a triangle.
So, there are three terminals to our generator and 3 to our motor windings.

Between any two terminals say that there is 240 volts between them. That just means that the RMS value is 240 V. The instantaneous voltage between the terminals would be quite different. Say x(t) = VmSIN(wt), y(t) = VmSIN(wt+120), z(t) = VmSIN(wt+240) where 120 and 240 are the phase difference in degrees. The units are mixed up a bit.

So as you can see, these are in effect sinusoidal sources applied to a corresponding motor winding causing rotation.

The corresponding source has its corresponding return. There is no common point.

This is a simplistic view.

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Originally Posted by KeepItSimpleStupid

I'm confused ... There is no common point.

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OK, let me try again : inside the transformer and inside the motor are common points, at which the voltage is always just zero. 3 Phase wires are always connected to 3 windings that at the other side of each winding are connected to an (internal) common point.

Just draw a circle and put through the center X and Y lines. X line = zero volt. Top Y is max positive (say + 240). Bottom Y is max negative (say - 240).

Mark anywhere on the circle 3 points that are each shifted 120 degrees, showing the effect of the different phasing involved.

Add all three (voltage) values of these three points (measured to the nill line X) together, and the total resulting value is always nil (as I showed in my example). i.e. the "return" of one phase out of three is the middle of the voltages on the other two phases.

So there is no need to connect the common points in transformer and motor. All you need is three phase wires.

Hope I was more clear this time!

:rolleyes:

Ahh:

Say x(t) = VmSIN(wt), y(t) = VmSIN(wt+120), z(t) = VmSIN(wt+240) where 120 and 240 are the phase difference in degrees. The units are mixed up a bit.

which means that at any given time:

VmSIN(wt)+ VmSIN(wt+120)+VmSIN(wt+240) = 0

So if Vm = the 0 to peak value we get at t = 0:

VmSIN(0) + VmSIN(120 deg) + VmSIN(240 deg) = 0
Vm*0 + Vm*0.866 + Vm * (-0.866) = 0

So, you see that the instantaneous sum of the voltages will always equal to zero.

Hence, no ground is needed. No return is needed.