More Transformer Loading

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Re: More Transformer Loading

Rattus and Al are correct based on my lab data.

Rattus, I also succumbed to the "transformer flipped" fact.... and the first go round on the lab hook-up, I missed this on the wiring connections and wasn't gettng hardly any secondary current! I had the windings backward and voltages were "sort of opposite" of each other... then it dawned on me. In a wye, the same ends are connected together at the centerpoint.

Now that I understand this, the whole thing is rather obvious. Good job guys, y'all taught me something AGAIN!

Okay:

Experiment done with smaller loads than in this thread.

3 single phase transformers connnected delta on primary, connected wye on secondary, but only loaded across two phases as in the diagram in the second post of this thread.

Secondary current = .2 amps

secondary current was in phase with the 208 volts from line to line

secondary current was 30 degrees out of phase with the 120 v winding voltage

primary currents were:

Line 1 = .2
Line 2 = .2
Line 3 = .4

Primary current in Line 1 was 30 degrees out of phase with voltage of primary winding C
 
Re: More Transformer Loading

Interesting, I never thought of the dot convention as being unconventional. . .
 
Re: More Transformer Loading

Originally posted by rattus:
... I will blame that on him though for not drawing conventional transformer symbols ...
Rattus
Click to expand...
Now see how we get caught up too much in the "model" and overlook the physics :D

Al's analysis was correct; as Crossman's empirical tests showed.

While I showed the 120V line to neutral secondary voltage, the voltage across the load was irrelevant for the analysis as long as we "guaranteed" there was 10A through it and the two active phases had currents in phase with each other. Voltage phase relationships were also irrelevant as long as we guaranteed the source would provide whatever primary currents necessary to maintain the secondary amps.

By the way the ?tip-to-tail? delta connection and ?dot? relationship with the wye, is the most common ?real world? arraignment in US three winding delta-wye domestic transformers. Therefore, the example was also ?practical.? It is one of the ways to field check ?ratio/polarity? of a power transformer with only a single-phase source. Driving a small current through two windings of the wye and measuring the ?shorted? delta currents can confirm the ratio and polarity of the windings in two tests.

threephase%20imbalance.gif


[ March 14, 2005, 12:47 PM: Message edited by: rbalex ]
 
Re: More Transformer Loading

rbalex, the dots are standard, I am saying I like my symbols to show primary and secondary in close proximity with some straight lines between. OK, I should have know better, and I did. Just sloppy thinking.
 
Re: More Transformer Loading

Crossman,

Glad you did the experiment, but it sounds like you didn't believe us. I am hurt, but it doesn't affect my appetite.

That's OK, I have often checked something in the lab. Helps me remember it better.

This little exercise ought to help some of us understand 3-phase a little better--thanks to rbalex.
 
Re: More Transformer Loading

Well, it isn't that I didn't beleeeeeve you. It's just that I used the standard thinking for currents in a delta... and didn't even see that the wye secondary had a reversed winding.

When I did the experiment and messed up the connection, I was wondering "what in the world, why doesn't this work?"

I redid the connections and DING DING DING! it suddenly became totally obvious what was going on with the directions of the current flow in the windings!

And I must admit it is totally cool to have been exposed to this revelation!
 
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