ECN Forum Forums Technical Discussion Electrical Theory and Applications Transformer Loading

I have a cutomer who thinks his transformer is overheating. Of course he has no concept how hot transformers really are. He has even stated that lightly loaded transformers run hotter than fully loaded devices.

This is a 6 KVA three phase, T connected transformer. The primary is connected to a 3 phase 3 wire 480V primary and the secondary is connected for 208Y/120V 3 phase 4 wire.

The transformer output is wired to 3 individual single pole fuse holders. X1 is fused at 10 amps and has a single phase loading of 7 amps. X2 and X3 are presently unloaded (no fuses installed).

I know that we will have poor effciency and regulation, but other than these what is wrong with this application?

JBD,

Sorry for the late reply to your transformer Q.

First off, the load per line [X1, X2, or X3] to common [X0] will have a maximum of 2 KVA on this 6 KVA Wye system. Right now, it looks like your less than 1 KVA with the 7 amp load on X1.

Got a few questions to throw your way,

• Is this 7 amp load Linear? - Did you check with a multi Hz / true RMS ammeter??.
• Was this transformer's connections altered in any way??
• When you mention the "TEE" connected transformer, is this a reference to an Open Delta TEE, or Scott connection??
• Is there any excessive current flowing through the grounding electrode conductor from the X0 termination?? - excess of 0.25 amps?

Circulating Harmonic currents might be causing an excessive heat in either the Secondary winding, or the Primary winding[s] - possibly both, depending on the source which is reflecting the Harmonic noise.

Other than this, incorrect polarities would be a cause for excessive heat. Another would be poor ventilation, or just a bad winding / insulation failure [short between winding segments, or to the core].

As you know, transformers will normally be hot - even at idle [due simply to losses consuming true power (wattage) and result in heat output]. When they are abnormally hot [maybe followed by letting some smoke out ..], then there is definitely a problem!

There should not be a real problem with this system, as it's a 4 wire Wye secondary. It should handle upto 2 KVA L-N without too much of a power quality issue, since each secondary is equal to a simple 2 wire circuit.
The primary coils will react to the change when a secondary coil, or coils, "drain" current - which will allow the primary to draw in more KVA to match the requirements on the secondary. The problem on a dual voltage system - such as this 208Y/120 4 wire Wye is the primary side will have protection set for "X" KVA @ "X" voltage. If using one coil [lower of the two voltages], it could easilly be loaded up higher than it's real maximum rating, but never trip the primary OCPD. Having fuses on the secondary as you described is good.

Any particular reason why the load wasn't distributed, instead of all on one line?

Good luck

Scott SET.

P.S. Thank you for the suggestions regarding schematics terms!!!

Scott,

Most small (under 15KVA) 3 phase transformers are constructed using two windings a main and a teaser with the neutral X0 point being located on the teaser winding, this is different than a typical 3 winding construction. (It is similar to the Scott-Tee which is used to generate a true 2 phase system from a 3 phase source.)

The purpose of the transformer was to provide 120VAC power to a parts cleaning machine. The existing circuit is for the control circuits and the open circuits are for future customer connections like portable tools or other ancillary equipment.

I did not check the harmonic content as I do not feel it important on a two wire circuit.

JBD,

Are you sure this also would apply to a 4 wire Wye transformer???
I could see it being done on a Delta 4 wire secondary where X0 is the center tap of one winding, plus the need for L-N loads is much lower than L-L and 3 phase loads.
Also could see a 3 wire Delta, even a Delta primary with a reduced size coil - but that shows how much I know

To me, this would be an inadequately constructed secondary coil, since the 4 wire Wye is 3 separate 2 wire coils connected at a common point [which becomes the common Neutral, or X0], instead of a center tapped winding. Single coil's rating would be exceeded very quickly.

Let me know your thoughts on this, and if I am out of the world here, please direct me towards something that explains the reasoning behind it [AKA: What 'da heck is going on! ]

Scott SET

Square D transformers that have 208Y/120 and 480Y277 secondaries which use wiring diagram #8 in publication 7400CT9601 available at http://www.squared.com/, are all Tee connected.

I don't know how to include pictures so here is the text version (also from Square D's Dry Type Transformer Study Course) of the secondary winding arrangement.

On your graph paper draw a coil which is vertical and 2.08 inches tall, label the top as X1, the bottom as X3, and put a dot at the midpoint.

Now draw a second coil 1.80 inches long from the midpoint of X1-X3 and going horizontal to the left to terminal X2.

If you measure from X1-X3, X1-X2, and X2-X3 in all cases you will have 2.08 inches which corresponds to 208V.

Now if you place a dot (X0) in the exact center of the equilateral triangle formed by X1-X2-X3, you would find it falls on the horizontal winding you drew. A measurement would show it to be 2/3 of the way between X2 and the midpoint, or 1.2 inches which equals 120 volts.

The disadvantage is the windings need to use larger conductors, but this is not a big deal with small KVA sizes.
The advantage is less core steel.

What I was rambling about in the last post was the physical winding size of the teaser winding being smaller than the Main winding. That's what was getting me conserned. Seems like an easy one to fry.

The Tee connection [will need to be equally done on primary and secondary] will be more symmetrical than a Vee setup.

From what I remember about Tee's [correct me if I am wrong ], if each coil [transformer] is 0.557 of the total load KVA, then the total rated capacity would be 15% more than required. This would be applied to L-L-L connected equipment.
I do not remember any information regarding KVA per loads on L-N connections, as the models and analysis items I dealt with were towards L-L and L-L-L connections.

It's definitely going to work as a Wye system, it's clearly connecting similar to Wye - just not the traditional 120 degree rotation of separate coils, but 90 degree rotation.
Other than this, it's just a Wye! [as yo have stated ].

One final Q: Does the teaser winding have an 86% tap point, where the X0 line would originate?? I haven't checked the site you linked yet, so it's all relying on "memory"... which should make everyone laugh here!

Scott SET

P.S. - I'll check out the schematic later on tonight, then maybe draw it up and post it to the new forum.

would any one like to chat about electricity, transformers, motors,
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I am a 15 year electrician and would like to learn more..I take my California State contracting exam on 9/13/01......... my icq and email should be visable to users....