277/120 XFMR with 100 volt output

[lbudden]

Have 3 3kva single phase transformers about 15 years old each, located outside in enclosures. Each is 277 volt primary, 120 volts secondary. Incoming voltage is 280v., but secondary comes out right at 100v (with no load except the XFMR). The primary 277v. circuit is landed on H1-H3 (as shown on nameplate). Secondary wires come off X1-X3 (also as shown for 115 volt). The secondary wires are: black on X1 and white on X3, and X3 is also grounded to the common ground bar. All 3 transformers give the same readings.
Why isn't the output closer to 120 volts?

 

[charlie b]

Re: 277/120 XFMR with 100 volt output

How are you getting three primary feeders that are each 280 volts? Is it connected as a WYE from a 480/277 volt system. The ratio of 115 (rated) to 100 (measured) is the same as the ratio of 277 (rated) to 240. So is the primary supply coming from a 240 volt system? Have you measured all three primary voltages?

 

[lbudden]

Re: 277/120 XFMR with 100 volt output

There are 3 separate transformers in 3 different locations. EACH transformer has 277 volt single phase feeding the primary, and the output/secondary is supposed to be 120 volts (single phase).

 

[lbudden]

Re: 277/120 XFMR with 100 volt output

Perhaps I confused you with the original post--the 280 volts I mentioned is what I get with my Fluke 87III when I measure voltage. The 277 volts I mention is the nameplate voltage listed on the XFMR.

 

[coulter]

Re: 277/120 XFMR with 100 volt output

I'd look at the taps. Maybe they are set clear down.

carl

 

[mc5w]

Re: 277/120 XFMR with 100 volt output

The primary taps are set at +5% or more which would reduce your secondary voltage.

 

[templdl]

Re: 277/120 XFMR with 100 volt output

You mentioned H1 and H3 which leads me to believe there are more connections for the primary. Is the primary a single 277v winding with taps, i.e. then what would the H2 and H4 represent? Usually One connection is made to H1 and the second to H2, H3, or H4.which I am to assume.
Again, since I don't have the transformer "wiring diagram" the only other configuration of a transformer (other than a CPT) would be if were dual voltage having 2 separate primary windings which are either place in series or parallel depending upon your primary voltage, one winging is H1-H2 and the second H3-H4 which I doubt that you have.
As coulter and mc5w said, it appears to be a "tap" problem where H1 is your first connection and the second is either H2, H3, or H4. Again, I don't have the schematic in front of me but I it leads ne to believe that you should reconnect the H3 to the H4 position which would increase the ration and thus the secondary voltage. But please double check the schematic first to verify.

 

[lbudden]

Re: 277/120 XFMR with 100 volt output

I don't have the transformer nameplate info at home with me, but I'll check the H2 and H4 taps tomorrow. I do remember that H4 was marked 380V, and H2 was something under 277 (maybe was 208?). It was like a multi-tap primary, and the secondary was either 95v. or 115, depending on whether you landed on X2 or X3 (with X1 being neutral). Thanks for all of your input, time and advise!!!!

 

[LarryFine]

Re: 277/120 XFMR with 100 volt output

budden, you may have misunderstood Charlie's question. How are the primary windings connected to the incoming power? Are they phase-to-neutral, or phase-to-phase?

If they're connected phase-to-phase (i.e., fed via a 2-pole breaker) from a 240-volt Delta, then exactly what you described will happen. (You mentioned measuring 280, though)

If the transformers have dual windings, as the H-1/H-3 and X-1/X-3 labels suggest, make sure all wires are properly connected, not just the ones exiting the transformer.