480-208Volt vs 480-240V Transformer

[FaradayFF]

Greetings,
For a dry-type, 3-phase transformer, can a transformer go from 480/208V to 480/240V? My understanding is not, because the ratio of the number of turns between the high side winding and the low side winding is fixed. The taps could be used to increase/decrease the voltage level on the unit, which would increase/decrease the voltage on the primary/secondary proportionally, but the ratio of primary to secondary stays constant.
Is my understanding correct?

Thanks,
EE

 

[Hv&Lv]

Greetings,
For a dry-type, 3-phase transformer, can a transformer go from 480/208V to 480/240V? My understanding is not, because the ratio of the number of turns between the high side winding and the low side winding is fixed. The taps could be used to increase/decrease the voltage level on the unit, which would increase/decrease the voltage on the primary/secondary proportionally, but the ratio of primary to secondary stays constant.
Is my understanding correct?

Thanks,
EE

Question...
What does the change in taps do?
If you change the taps to 5% over, what’s the TTR now?

 

[petersonra]

Greetings,
For a dry-type, 3-phase transformer, can a transformer go from 480/208V to 480/240V? My understanding is not, because the ratio of the number of turns between the high side winding and the low side winding is fixed. The taps could be used to increase/decrease the voltage level on the unit, which would increase/decrease the voltage on the primary/secondary proportionally, but the ratio of primary to secondary stays constant.
Is my understanding correct?

Thanks,
EE

The taps work by changing the ratio slightly.

 

[LarryFine]

If you change primary taps, you're adjusting the volts/turn ratio.

If you change secondary taps, you're adjusting the turns ratio.

 

[LarryFine]

For a dry-type, 3-phase transformer, can a transformer go from 480/208V to 480/240V?

Are you asking whether you can get 240v from a 208v secondary?

If so, no. Buck-boost units could be used to get you there, though.

 

[Carultch]

Question...
What does the change in taps do?
If you change the taps to 5% over, what’s the TTR now?

I haven't seen a transformer with taps that go between a 240V nominal setting to a 208V grid, or vice versa. That's a -13% or a +15% tap change that needs to happen.

The addition taps engage more windings. The subtraction taps disengage windings. Disengaged windings are parts of the transformer coil that become a dead-end circuit with one end not connected, thus they do not carry current or participate in forming the magnetic coupling. The ratio of windings changes, which means the voltage ratio changes. The KVA rating is the same, for all tap settings.

Consider a 480V:240V transformer, with 40 windings engaged on the 480V side when set to the nominal tap setting. It correspondingly would have 20 windings on the 240V side. Suppose this transformer has the following taps on the 480V side: (A)-5%, (B)-2.5%, (C) Nominal, (D) +2.5%, (E) +5%.

Given the following tap settings, here's the number of windings engaged on the 480V side:
(A) 38 windings engaged
(B) 39 windings engaged
(C) 40 windings engaged
(D) 41 windings engaged
(E) 42 windings engaged

 

[Hv&Lv]

I haven't seen a transformer with taps that go between a 240V nominal setting to a 208V grid, or vice versa. That's a -13% or a +15% tap change that needs to happen.

The addition taps engage more windings. The subtraction taps disengage windings. Disengaged windings are parts of the transformer coil that become a dead-end circuit with one end not connected, thus they do not carry current or participate in forming the magnetic coupling. The ratio of windings changes, which means the voltage ratio changes. The KVA rating is the same, for all tap settings.

Consider a 480V:240V transformer, with 40 windings engaged on the 480V side when set to the nominal tap setting. It correspondingly would have 20 windings on the 240V side. Suppose this transformer has the following taps on the 480V side: (A)-5%, (B)-2.5%, (C) Nominal, (D) +2.5%, (E) +5%.

Given the following tap settings, here's the number of windings engaged on the 480V side:
(A) 38 windings engaged
(B) 39 windings engaged
(C) 40 windings engaged
(D) 41 windings engaged
(E) 42 windings engaged

Thank you. I know how the taps work though...
I was asking it like that to get him to think about it...

 

[GoldDigger]

Greetings,
For a dry-type, 3-phase transformer, can a transformer go from 480/208V to 480/240V? My understanding is not, because the ratio of the number of turns between the high side winding and the low side winding is fixed. The taps could be used to increase/decrease the voltage level on the unit, which would increase/decrease the voltage on the primary/secondary proportionally, but the ratio of primary to secondary stays constant.
Is my understanding correct?

Thanks,
EE

Given the vector math relating the L-L voltage to the L-N voltage, the only way that three phase 480/240 makes sense is as a high leg delta with one of the 480V windings center tapped and the center tap grounded., If that is what you are asking about, it just requires the right secondary winding configuration. Just like 240/120 four wire high leg.