Three-phase calculation confusion

[oldsparky52]

View attachment 22584

Still representation.

Note that 120 volt AC is RMS voltage or effective DC equivalent voltage and that actual peaks are about 170.

Look where V2 and V3 are at relative to same time V1 is at the + peak. They are both at same level on the - side.

Look where V2 and V3 are at relative to same time V1 is at zero crossing, they are same voltage but opposite polarity.

Look at any point along the time line and see where each phase is at that instant in time.

Using your drawing, when V1 is at it's peak (+120V to neutral) V2 is at -60V to neutral. So the voltage somewhere gets to 208V V1 to V2. It appears that it will be before V1 peaks (less than 90 degrees) since V1 to V2 voltage is approaching zero after the V1 peak.

Do you know where that 208V potential between V1 and V2 occurs? Somewhere less than 90 degrees and greater than 0 degrees.

It's great that all of you share these insights.

 

[Besoeker3]

View attachment 22584

Still representation.

Note that 120 volt AC is RMS voltage or effective DC equivalent voltage and that actual peaks are about 170.

Look where V2 and V3 are at relative to same time V1 is at the + peak. They are both at same level on the - side.

Look where V2 and V3 are at relative to same time V1 is at zero crossing, they are same voltage but opposite polarity.

Look at any point along the time line and see where each phase is at that instant in time.

Thank you.

 

[oldsparky52]

Do you know where that 208V potential between V1 and V2 occurs?

Would it be at the 0 crossing of V3?

 

[wwhitney]

If I have three 2-pole circuits drawing 59 amps seems like that means I have 108 amp on each phase.

That's not actually right. Assuming identical loads, when the A-B current combines with the B-C current on the B leg, the current waveforms are not in phase. So you end up with only 59 * sqrt(3) amps on the B-leg.

Does the 1.732 multiplier come into play here? I didn't think so.

Yes, as above.

Cheers, Wayne

 

[kwired]

Using your drawing, when V1 is at it's peak (+120V to neutral) V2 is at -60V to neutral. So the voltage somewhere gets to 208V V1 to V2. It appears that it will be before V1 peaks (less than 90 degrees) since V1 to V2 voltage is approaching zero after the V1 peak.

Do you know where that 208V potential between V1 and V2 occurs? Somewhere less than 90 degrees and greater than 0 degrees.

It's great that all of you share these insights.

Keep in mind 120 and 208 are RMS voltage. Peaks are approximately 170 and 295.

here I found better drawing that shows peak voltages.



RMS voltage will be represented with a straight line if the wave is a pure sine wave.

Phase to phase voltage is 1.732 times each phase voltage, not sure I know how to explain why.

 

[oldsparky52]

I try to conceptualize how things are working. With the OP's setup, I'm trying to visualize how the currents are flowing in the panel feeders.

 

[Coppersmith]

Air handler with MCA of 59 amps - sounds to me like it probably has electric heat strips in it. If that heat can't run at same time as condenser unit then you only have the air handler load to account for for a common supply.

I checked with the A/C contractor. It does have electric heat strips in it. It also is a heat pump. They can be on simultaneously. Therefore the heat strips amperage is in addition to the A/C amperage. Also there is a plaque on the A/H unit that lists all the possible heat strip units that can be installed and the MCA and MOP for each. The installer marks which is installed. The inspector expects me to heed the MCA and MOP listed. Those are the values I gave in post #1.

 

[oldsparky52]

Phase to phase voltage is 1.732 times each phase voltage, not sure I know how to explain why.

I just accepted the square root of 3, as in 3 phases expressed in a single-phase terminology (voltage is usually between 2 points, so single-phase reference). Don't know if that's right or wrong, but ... makes sense in my head.

 

[GoldDigger]

They peak 120deg apart.

Yes, and when they are both the same distance off peak at the same time the voltages are equal and add as scalars.

Sent from my XT1585 using Tapatalk

 

[david luchini]

Waveforms. Try it.

Kwired was kind enough to show the waveforms. It shows exactly what I said, so I'm still not sure what you are confused with.

 

[kwired]

I try to conceptualize how things are working. With the OP's setup, I'm trying to visualize how the currents are flowing in the panel feeders.

Well to simplify it lets talk about three 10 amp line to line single phase loads. Voltage doesn't really matter.

Connect them all between two supply leads - they are all in parallel and will draw 30 amps on the supply conductors. Most don't have trouble figuring that out.

Balance them across a three phase source and each supply line draws 1.732 times the individual load current. So supply conductors will each draw 17.32 amps for the same three loads. Not so easy to understand, but think of this - at the point where "A" splits off to two of the loads, you have current flowing three different directions kind of all at the same time, but to then complicate it more two of those directions are drawing 10 amps and the third direction is drawing 17.32 amps.

 

[oldsparky52]

... but to then complicate it more two of those directions are drawing 10 amps and the third direction is drawing 17.32 amps.

Right, believe it or not, I got that. The thing that I try to conceptualize is the dynamics of the current flow one moment to another. The 10 amps flowing in each of the loads (AC, so a simple back and forth) but the feeder having a 17.32 current draw feeds 20-amps on 2 separate loads, so that means the current from one load is feeding the other load without going out the feeder (to the tune of 2.68-amps).

Let's say in your example, one load is 10-amp and the other is 15-amp. Would it be proper to say that the common feeder load would be (10*1.732+5) 22.32-amps?

 

[kwired]

Right, believe it or not, I got that. The thing that I try to conceptualize is the dynamics of the current flow one moment to another. The 10 amps flowing in each of the loads (AC, so a simple back and forth) but the feeder having a 17.32 current draw feeds 20-amps on 2 separate loads, so that means the current from one load is feeding the other load without going out the feeder (to the tune of 2.68-amps).

Let's say in your example, one load is 10-amp and the other is 15-amp. Would it be proper to say that the common feeder load would be (10*1.732+5) 22.32-amps?

AFAIK yes, I don't see there is anywhere else for the unbalance to go.

 

[mahaney03]

This was the avatar of a once active member that is now deceased. Kind of shows you the voltage wave of each phase in relation to the others, slowed down of course, if you increased rotation to 3600 RPM the waves would be at 60Hz.

What! When did this happen? Was there a post I missed? He always provided such useful info. I hope he's preaching wisdom too in the next life.

 

[Besoeker3]

Using your drawing, when V1 is at it's peak (+120V to neutral)V

120V is an RMS value, not a peak.

 

[Strathead]

I don't know as much as I think I understand (or something like that), so I have a question for the more educated.

Where is (let's just pick A-phase) in the sine wave where it has the maximum load to the other 2 phases? It appears from just looking at a sine wave drawing that it might be when A-phase is at it's peak (either high or low). Is this correct? If it is incorrect, where in that sine wave would A-phase have it's maximum current draw (let's just use the op's scenario with all fans running)?

I think you are confused. A phase sine wave is A phase sine wave, it doesn't "relate" to the other phases in the way you seem to be trying to apply. it represents the current across A, so it is at is maximum current when it is peaking period.

 

[oldsparky52]

I think you are confused. A phase sine wave is A phase sine wave, it doesn't "relate" to the other phases in the way you seem to be trying to apply. it represents the current across A, so it is at is maximum current when it is peaking period.

I may be confused.

The "relate" part is most likely poorly worded.

In the drawing that kwired posted, when V1 is at 90 degrees it is +120V to neutral and V2&3 are both at -60V to neutral (a voltage of 180 between V1 and either V2 or V3). I was just trying to find out where V1 (at what degree) is when the voltage between V1 and V2 is 208V.

 

[Strathead]

Air handler with MCA of 59 amps - sounds to me like it probably has electric heat strips in it. If that heat can't run at same time as condenser unit then you only have the air handler load to account for for a common supply.

Caveat...

Unless the Condensing unit is actually a heat pump.

 

[Strathead]

I may be confused.

The "relate" part is most likely poorly worded.

In the drawing that kwired posted, when V1 is at 90 degrees it is +120V to neutral and V2&3 are both at -60V to neutral (a voltage of 180 between V1 and either V2 or V3). I was just trying to find out where V1 (at what degree) is when the voltage between V1 and V2 is 208V.

Yes, with respect you said load. Load is amperage. You didn't reference voltage in that question. The voltage sine wave and relationship is completely different than the amperage sine wave and relationship. One is constant the other is cumulative.

 

[oldsparky52]

Yes, with respect you said load. Load is amperage. You didn't reference voltage in that question. The voltage sine wave and relationship is completely different than the amperage sine wave and relationship. One is constant the other is cumulative.

Thank you, I will try to do better.