ECN Forum Forums General Topics of the Trade General Discussion Area 75kva transformer

Tesla
I have no idea where you come up with those numbers but i am enforcing the code according to Ghosts numbers.

Tesla,

You are quite correct that the line-neutral current on the secondary of the transformer is 4X the line to line current on the corresponding leg of the primary delta.

However you are making the error that the 'line' current feeding the primary is equal to the current on each delta leg in the primary; this is not correct as the current on each supply leg must split through _two_ branches of the delta.

Given the example transformer, fully loaded (but ignoring losses).
75000/480/root(3)=
90.2A flows on each of the three supply conductors.

75000/480/3=
52.1A flows on each of the delta legs of the primary.

(75000/480/3) * 4=
208.3A flows on each of the wye legs of the secondary.

-Jon

I finally figure out 480 to 120 1 phase transformer the calculation offered by tesla might hold up. The transformer ratio is not 4 it is 2.3. Think of a 480/277 to 208/120 transformer the ratio is 2.3. I know this is a delta/wye configuration but the ratio is the same you take the primary voltage phase to phase and divide by the secondary line to line voltage. The only way to connect this transformer and get 120 volts is wye on the secondary. If you could connect the secondary at 120 volts Delta IE a 480 to 120 volt Delta / Delta then the secondary current is 75000/(120*root3)=360 amps but the current is 208 amps if it is a 120/208 transformer. The secondary OC is 208 amps or if protected on the primary at 100amps the seconday is 230 amps and a 225 amp panel is too small without secondary OC protection. If a 90 amp fuse holds then a secondary panel rated at 225 amps could be installed without secondary OC protection.

I know this is an old thread, but I see there is still some confusion as to the turns ratio of a delta-wye transformer (in this case, 480V x 208/120V).

According to mikesh, the turns ratio is 2.3:1 since 480V/208V = 2.3 (approximately).
While that seems like logical thinking, it is incorrect. You must remember that each ONE of the 3 coils of the delta-connected primary induces voltage onto only ONE of the 3 coils in the wye-connected secondary. (Look at a the schematic of a 3-phase transformer for reference.)

In a delta-connected winding, the line voltage is the same as the phase voltage (the voltage across a single coil). So, in this case, the voltage across each coil is 480V.

In a wye-connected winding, the line voltage is 1.73 times larger than the phase voltage. This is a result of two coils having voltages of 120V each, but being 120° out of phase from each other. In other words, 120V × 1.73 = 208V.

Since the voltage across each individual coil in the secondary is 120V (this is the phase voltage), you can clearly see that the turns ratio is 480V ÷ 120V = 4:1

What mikesh has calculated is the line voltage ratio (VL-Pri : VL-Sec). This is a valid calculation for the turns ratio of delta-delta and wye-wye transformers only (not wye-delta or delta-wye).

Just saw this thread, and have a quick comment;

Per the 75 KVA rating...

On the Primary (Delta) Side:

At Full Load, each of the (3) Windings will draw 52.08333- Amps at 480V, with a resulting 25 KVA per Winding.

Input Phase Terminations at Full Load will carry 90.208333- Amps, which is 52.08333- Amps x 1.732.

With a Balanced, Full Load output on the Secondary side, all (3) Input Phases (Phase "A", Phase "B" and Phase "C") will carry 90.208333- Amps.

Multiplying 90.2 Amps by the Three-Phase Voltage of 831 (480v x 1.732 = 831.36) equates in a total Apparent Power flowing in the (3) Primary Windings of:

74,956.2 VA
or simply 75.0 KVA

On the Secondary (Wye) Side:

At Full Load, each of the (3) Windings will carry 208 Amps.
The Voltage Ratio is 4:1, so each Secondary Winding is 120V.

With 208 Amps flowing per 120 V Winding, each Secondary Winding has an Apparent Power rating of 24,960 VA, or simply 25 KVA.

Connecting the (3) Windings together in a Star formation results in a Winding to Winding Voltage of 208 V (120 V x 1.732 = 207.84 V).

Multiplying 208 Amps by the Three-Phase Voltage of 360 (208 V x 1.732 = 360.256 V) equates in a total Apparent Power flowing in the (3) Secondary Windings of:

74,933.248 VA
or simply 75.0 KVA.

--Scott (EE)