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Old 04-03-2014, 08:42 AM   #16
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Need help electrifying a brewery


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Originally Posted by Daniel Holzman View Post
To dmx: the equation you posted for power is correct only for DC, or single phase AC service. For three phase service, which is what the OPS indicates they have, power = V * I * 1.73, where V is the phase to phase voltage (208V in this case), and I is the current draw. The 1.73 comes in because there are three separate phases that contribute to power.
Well, the 1.73 is really there because of the relationship between L-L voltage and L-N voltage. For the DIY to understand, the simple formula is:

P= V(L-N) * I * 3

However, if you only have the V(L-L), then you introduce the following substitution:

V(L-N) = V(L-L) / Sqrt(3)

Making that substitution, you get

P = V(L-L) * I * Sqrt(3)

Mark

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Old 04-03-2014, 11:25 AM   #17
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Quote:
Originally Posted by Daniel Holzman View Post
To dmx: the equation you posted for power is correct only for DC, or single phase AC service. For three phase service, which is what the OPS indicates they have, power = V * I * 1.73, where V is the phase to phase voltage (208V in this case), and I is the current draw. The 1.73 comes in because there are three separate phases that contribute to power.
I think it's easier to understand as three separate 120V circuits. The sqrt(3) factor only comes into play as the ratio of 208/120.
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Old 04-03-2014, 12:46 PM   #18
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The ratio 208 to 120 occurs in a WYE circuit occurs because the 208 is measured phase to phase, and the 120 is measured phase to neutral. Even in a delta 3 phase circuit with NO NEUTRAL the total power is V * I * 1.73 where V is the phase to phase voltage. The 1.73 factor in the three phase circuit is not related to the presence of a neutral. The 1.73 factor occurs because the three single phase circuits that make up the three phase circuit are 120 degrees out of phase with each other, so at any given instant you get different power on each of the three circuits, because the voltages and currents are out of phase with each other. If you do the complex algebra calculations, you find that the multiplier is the square root of 3, which is 1.73.

If you had three separate single phase circuits that are in phase with each other, such as you have on three breakers on the same leg of a single phase panel in your basement, you would simply add the power of each circuit together to get the total power. If the circuits were identical, you would get a total of 3 * V * I. You don't get that in a three phase circuit because the circuits are out of phase with each other. Has nothing to do with the neutral.
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Old 04-03-2014, 03:52 PM   #19
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Originally Posted by Daniel Holzman View Post
The ratio 208 to 120 occurs in a WYE circuit occurs because the 208 is measured phase to phase, and the 120 is measured phase to neutral. Even in a delta 3 phase circuit with NO NEUTRAL the total power is V * I * 1.73 where V is the phase to phase voltage. The 1.73 factor in the three phase circuit is not related to the presence of a neutral. The 1.73 factor occurs because the three single phase circuits that make up the three phase circuit are 120 degrees out of phase with each other, so at any given instant you get different power on each of the three circuits, because the voltages and currents are out of phase with each other. If you do the complex algebra calculations, you find that the multiplier is the square root of 3, which is 1.73.

If you had three separate single phase circuits that are in phase with each other, such as you have on three breakers on the same leg of a single phase panel in your basement, you would simply add the power of each circuit together to get the total power. If the circuits were identical, you would get a total of 3 * V * I. You don't get that in a three phase circuit because the circuits are out of phase with each other. Has nothing to do with the neutral.
Yeah, I get the physics and math. You don't get what we're saying. Regardless of whether a neutral is being used or not, you can model the total load on a three phase circuit by reducing it to three single-phase "hypothetical" circuits between each phase and neutral. Take the load on each phase, multiply by the phase-neutral voltage, and add all three up. It's mathematically identical and much easier for non-techies to understand. It does not have anything to do with whether a neutral is actually being used or not.
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Old 04-03-2014, 06:46 PM   #20
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I'm interested in details of the brewery setup... I work for a brewery and love it.

What kind of process are you going to use and on what volume scale? ie: will you be lautering, boiling, mashing, fermenting, filtering at the same time or are you going to be a batch-by-batch operation?
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Old 04-03-2014, 11:01 PM   #21
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Quote:
Originally Posted by mpoulton View Post
I think it's easier to understand as three separate 120V circuits. The sqrt(3) factor only comes into play as the ratio of 208/120.

I was trying to explain it in the simplest way (ie single phase)
to make it easier for the O/P to understand.
I am well aware of the 3 phase system.

But the O/P might not be.

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