1-phase split-phase circuit: current flow not arbitrary?

[akyip]

Hey guys,

I was doing a few examples involving the 1-phase split phase circuit... this is the circuit type that is common in residential homes.

Can someone confirm if the current directions I drew up for this 120/240 V circuit is correct? It seems that each current must flow in a specific direction, and that the current flow for each branch is not arbitrary. I was doing a practice problem where I tried the circuit analysis with Ib flowing right and IB, 120 flowing up. I got the neutral wrong because my neutral current value ended up higher than what the solution stated. So it seems that each current must flow in a specific direction for the circuit analysis to be correct.

Can someone confirm this? Thanks!

 

[jd5191]

Idk if this helps, but your Ia equation has a typo based on your drawing.

 

[akyip]

Idk if this helps, but your Ia equation has a typo based on your drawing.

Yes, you're right. That IB, 240 should technically be IA, 240 in my equation for total A phase current... But IB 240 = IA 240 with the way I drew it 

 

[Zach Stone P.E.]

The voltage polarity of the second coil is backwards, it should be identical to the first coil. It helps to think of it as one coil tapped in the center (just like a center tapped phase of a center tapped delta connection). Tapping the coil in the center just changes the ratio by a factor of two, it has no change on phase angles, like this:

VAB = 240V<0

VAB = VAN + VBN

             VAN = VNB = 120V<0

VAB = 120V<0 + 120V<0

VAB = 240V<0

From there, all currents from a source flow from negative to positive voltage, and all currents drawn by a load flow from positive to negative voltage polarity. 

 

[akyip]

The voltage polarity of the second coil is backwards, it should be identical to the first coil. It helps to think of it as one coil tapped in the center (just like a center tapped phase of a center tapped delta connection). Tapping the coil in the center just changes the ratio by a factor of two, it has no change on phase angles, like this:

VAB = 240V<0

VAB = VAN + VBN

             VAN = VNB = 120V<0

VAB = 120V<0 + 120V<0

VAB = 240V<0

From there, all currents from a source flow from negative to positive voltage, and all currents drawn by a load flow from positive to negative voltage polarity. 

Zach, you're right. Thanks for the catch!