open delta secondary and 3 phase motor

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wirenut1980 said:
Jim, this is what I am after. Why does the open phase have worse regulation than the other two? How does this happen?
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In a 3 pot bank, the voltage drop across any transformer phase is the across the associated coil. For example, the voltage drop across phase AB is the voltage drop across the coil between A and B. In an open delta connection the voltage drop across the open leg is dependent on the voltage drop across two coils in series, since the open leg has no one coil supporting it.

The two coils introduce more drop than the one.

Jim T
 
jtester said:
In a 3 pot bank, the voltage drop across any transformer phase is the across the associated coil. For example, the voltage drop across phase AB is the voltage drop across the coil between A and B. In an open delta connection the voltage drop across the open leg is dependent on the voltage drop across two coils in series, since the open leg has no one coil supporting it.

The two coils introduce more drop than the one.

Jim T
Click to expand...
Right, JT. Furthermore, the phase angles are skewed such that the separation is no longer 120 degrees.
 
al hildenbrand said:
The primary, open-wye connected is 120? from the PoCo.
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Indeed. The voltage across the open side of an open Delta should theoretically be the same as it would be with a closed Delta.

In addition, if you were to open a genuine Delta at one corner, there should theoretically be zero volts across the open.

Theoretically.
 
Yes, but:

Yes, but:

al hildenbrand said:
:confused:

What causes that?

The primary, open-wye connected is 120? from the PoCo.
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You mean open-delta? That is true for the unloaded case. Now consider a real open delta system, and apply a load on the open side only. The phase angle of this load current in conjunction with the xfrmr impedances creates voltage drops that tend to close the open delta phasor diagram. The phase separation is no longer 120 degrees. The open side of the phasor diagram is shown below:

--Vd->--------Va'c'---------->--Vd->
C......C'..............................A'........A

Va'c' denotes the voltage on the loaded open side. Vac denotes the unloaded voltage. Now draw in the rest of the diagram, and it is clear that the separation has changed.
 
rattus said:
You mean open-delta?
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No, I meant open-wye.

I'm interested in staying with the opening post's construct. A mile of PoCo primary 4160 V GndY / 2400 V multi-grounded neutral, but only two of the three hot conductors.

Connect the PoCo primary to two step down transformers with their high side connected in an open-wye.

The PoCo provides a solid 120? to the transformer high side.

Connect the transformer secondary in an open-delta 120/240 volt 4-wire 3? where the centertap is grounded.

For the first, simplest, case. . .connect only a 30 HP 240 volt irrigation pump motor (a balanced 3? load). . .no other loads, single or three phase.

Second case. . .add to the motor your choice of single phase loads to the 120/240 volt transformer, and / or, 240 volt loads on the 240 volt transformer and on the open side of the delta.

Following your notation in your post #27 above Vab is the 120/240 xfmr and Vbc is the 240 xfmr. Vac is the open side.

The phasor angle of Vab and Vbc are locked in by the PoCo side, seems to me.

How does the phasor angle of Vac vary?
 
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Rattus,

Thanks. . .This is something that has always confused me. So bear with me.

I've reposted your Word file above:
Rattus said:
Consider the phasor diagram below which represents the voltages at the secondary of an open delta transformer bank. The solid lines indicate the voltage magnitude and phase with no load. Now apply a resistive load to the open side only and ignore the leakage reactance. The load current is in phase with Vab, therefore the voltage drops, Vc’c and Vaa’, are in phase as well. The result is that the load sees Va’c’. Clearly, the separation angles have changed.

This happens because the transformer is not ideal and we are feeding the open side from the other two transformers.

For that matter, if the secondary voltages become unbalanced for any reason, the separation angles will change even on a closed delta.

If we apply a balanced load, the diagram will be a bit different, but the open side voltage will sag more than the others and we see the same effect.

Now, if we have a perfectly stiff source and an ideal transformer, we would see no sag and no change in separation.

............................Open Side
RattusUnbalancedPhasorDiagram.jpg
Click to expand...
P.S. I'm not sure why, but when I open the file, my copy of Word 2003 wants to log into IP address 70.183.29.208 which is owned by Comcast. . .
 
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al hildenbrand said:
P.S. I'm not sure why, but when I open the file, my copy of Word 2003 wants to log into IP address 70.183.29.208 which is owned by Comcast. . .
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