Transformer Ratios

[ZcoreX29]

I've seen a lot of distribution transformers that are delta-wye. In fact, a three phase one could be 480V-208/120V. Meaning the primary is 480V L-L and the secondary is 208V L-L or 120V L-N.

I don't think Spin-up has an error here. It's 4600V L-L on the primary with a 10:1 step-down ratio giving you 460V L-L on the secondary. The L-N would be 460/1.73=265V.

Besides, when have you ever heard of a 795V motor... that should throw a red flag that something isn't right.

The 10:1 is supposed to be turns ratio, not the voltage ratio. With that being said, the LN voltage would 460V on the secondary or 795V LL, but I'm with you that this isn't a typical voltage for motor. If this were a question from school, I would've said 795V and put a note for the TA/teacher that the question is confusing. It's all about understanding the concept for the NCEES exam anyways.

Agreed... 10:1 is the turns ratio... so given the transformer equation Vp/Vs = N1/N2 ====> 4600/Vs = 10/1 Solve for Vs = 460V. That's the L-L voltage on the secondary.

I don't see the error on that Spinup question either. Kovz solution is correct.

The turns/winding ratio of the transformer is equal to the voltage ratio. The connection schemes of the transformer changes/remain the phase of the voltage.

On a delta-wye or wye-delta transformer, a 10:1 turns ratio does not equal Vp/Vs where Vp and Vs are line-line voltages on the primary and secondary respectively.

 

[eksor_PE]

I've seen a lot of distribution transformers that are delta-wye. In fact, a three phase one could be 480V-208/120V. Meaning the primary is 480V L-L and the secondary is 208V L-L or 120V L-N.

I don't think Spin-up has an error here. It's 4600V L-L on the primary with a 10:1 step-down ratio giving you 460V L-L on the secondary. The L-N would be 460/1.73=265V.

Besides, when have you ever heard of a 795V motor... that should throw a red flag that something isn't right.

The 10:1 is supposed to be turns ratio, not the voltage ratio. With that being said, the LN voltage would 460V on the secondary or 795V LL, but I'm with you that this isn't a typical voltage for motor. If this were a question from school, I would've said 795V and put a note for the TA/teacher that the question is confusing. It's all about understanding the concept for the NCEES exam anyways.

Agreed... 10:1 is the turns ratio... so given the transformer equation Vp/Vs = N1/N2 ====> 4600/Vs = 10/1 Solve for Vs = 460V. That's the L-L voltage on the secondary.

I don't see the error on that Spinup question either. Kovz solution is correct.

The turns/winding ratio of the transformer is equal to the voltage ratio. The connection schemes of the transformer changes/remain the phase of the voltage.

On a delta-wye or wye-delta transformer, a 10:1 turns ratio does not equal Vp/Vs where Vp and Vs are line-line voltages on the primary and secondary respectively.

Yes, they are line-to-line voltages, V_p and V_s, and that is why the line-to-neutral voltage will be V_ln = 266V.

 

[ZcoreX29]

I've seen a lot of distribution transformers that are delta-wye. In fact, a three phase one could be 480V-208/120V. Meaning the primary is 480V L-L and the secondary is 208V L-L or 120V L-N.

I don't think Spin-up has an error here. It's 4600V L-L on the primary with a 10:1 step-down ratio giving you 460V L-L on the secondary. The L-N would be 460/1.73=265V.

Besides, when have you ever heard of a 795V motor... that should throw a red flag that something isn't right.

The 10:1 is supposed to be turns ratio, not the voltage ratio. With that being said, the LN voltage would 460V on the secondary or 795V LL, but I'm with you that this isn't a typical voltage for motor. If this were a question from school, I would've said 795V and put a note for the TA/teacher that the question is confusing. It's all about understanding the concept for the NCEES exam anyways.

Agreed... 10:1 is the turns ratio... so given the transformer equation Vp/Vs = N1/N2 ====> 4600/Vs = 10/1 Solve for Vs = 460V. That's the L-L voltage on the secondary.

I don't see the error on that Spinup question either. Kovz solution is correct.

The turns/winding ratio of the transformer is equal to the voltage ratio. The connection schemes of the transformer changes/remain the phase of the voltage.

On a delta-wye or wye-delta transformer, a 10:1 turns ratio does not equal Vp/Vs where Vp and Vs are line-line voltages on the primary and secondary respectively.

Yes, they are line-to-line voltages, V_p and V_s, and that is why the line-to-neutral voltage will be V_ln = 266V.

The line to neutral voltage would be 460 V. Below is the closest I can find to a proof at the moment. On a delta-wye transformer, there's always a 1.73 difference in line to line voltages in addition to the turns ratio
http://www.drhenrylouie.com/uploads/17-Three_Phase_Transformers_part2.pdf

 

[eksor_PE]

Yes, they are line-to-line voltages, V_p and V_s, and that is why the line-to-neutral voltage will be V_ln = 266V.

The line to neutral voltage would be 460 V. Below is the closest I can find to a proof at the moment. On a delta-wye transformer, there's always a 1.73 difference in line to line voltages in addition to the turns ratio
http://www.drhenrylouie.com/uploads/17-Three_Phase_Transformers_part2.pdf

From this thread, there is a misconception between the turn ratio and the impedance of a transformer. The turn ratio of a transformer does not change regardless of its connection configuration such as delta-delta, delta-wye, or others. The turn ratio of a transformer will only change if the number of turns, physically, increase or decrease, V = 4.44*f*N*Phi.

The connection configuration of a transformer changes the value of the transformer’s impedance; hence, the current across each phase will change either by a multiple or a ratio of 1.732.

With all that said and other users implied, if the turn ratio of a transformer is given, the equation V_p/V_s = N_1/N_2 or I_1/I_2 = N_2/N_1 is used regardless of the transformer’s connection configuration.

For practical application, you can build a three-phase transformer from a three single-phase transformers bank with same characteristics. I attached an actual nameplate of a transformer, which is in delta-wye configuration. At full load, you can evaluate the turn ratio, currents, and voltages of a delta-wye configuration.

Like what others have said, this kind of question is a free pass question.