2
\$\begingroup\$

Consider 3 windings, a, b and c. L1 is connected to A, L2 to B and L3 to C. The 3 windings are then connected in delta. If we could freeze frame the voltage waveform where L1 and L2 are positive maximum and L3 is negative, what will the current direction through the windings look like? Particularly through the bridge which connects a1 to b2...

Will we at any point have the current from 2 different phases going towards each other? enter image description here

\$\endgroup\$
1
  • \$\begingroup\$ As far as I remember, the current in any one phase is the sum of the currents in the other two, make sure of the signs... \$\endgroup\$
    – Solar Mike
    Sep 7, 2019 at 17:09

1 Answer 1

8
\$\begingroup\$

enter image description here

Figure 1. Three-phase currents in a balanced system at various points in time. They always sum to zero.

If we could freeze frame the voltage waveform where L1 and L2 are positive maximum and L3 is negative, ...

That never happens. Have a look at Figure 1 points (1) and (2) and you will see that only one phase can be maximum at any time and at that time the other two are at half-maximum and in the opposite direction.

... what will the current direction through the windings look like?

They will look the same but with the minor complexity that the currents are out of phase with the phase-neutral voltages.

Will we at any point have the current from 2 different phases going towards each other?

Yes. At Figure 1 (2), 90°, the current in on the black phase is split in two leaving on the red and blue phases at half the input current. 60° earlier (1) blue and black are at 0.5 positive with red at peak negative.

The currents always sum to zero, even if the load is unbalanced. If you run the three wires through a clamp-on meter it will read zero. (This assumes that the is no neutral connection.)


From the comments:

At the point where phase 1 and 2 are at positive 0.5 and phase 3 is at -1, which direction will the current between a1 and b2 be facing, up or down?

enter image description here

Figure 2. In the situation described the current through the yellow winding will be zero.

Also the current from phase 2 on winding B will pass through the winding and head towards a1 from b2. Won't these currents clash?

If currents "clash" in the mathematical analysis then you add them taking care to observe the polarity or sign of the signal. That way the answer can be zero.

\$\endgroup\$
4
  • \$\begingroup\$ Just a clarification, For star connections, an imbalance in currents will be consumed by the neutral connection, if there is no neutral it will instead shift the center points voltage towards, which allows the currents to sum to 0. \$\endgroup\$
    – Reroute
    Sep 7, 2019 at 22:08
  • \$\begingroup\$ You can have imbalance in star without having neutral current. e.g. Current in a star connected load when a ph-ph fault occurs in the system. The unbalance shows up as negative sequence in symmetrical component analysis, not zero sequence (no neutral current). \$\endgroup\$ Sep 8, 2019 at 3:02
  • \$\begingroup\$ At the point where phase 1 and 2 are at positive 0.5 and phase 3 is at -1, which direction will the current between a1 and b2 be facing, up or down? I say this because current will split, some goes through winding A, some goes through the bridge. Also the current from phase 2 on winding B will pass through the winding and head towards a1 from b2. Won't these currents clash? \$\endgroup\$
    – Ph3ng
    Sep 8, 2019 at 5:01
  • \$\begingroup\$ See the update. \$\endgroup\$
    – Transistor
    Sep 8, 2019 at 8:18

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.