Transformers: Ratio, Current and Loading

Electrical Engineering · CHAPTER 4 / 5

Level: introductory undergraduate · Study session: 15–20 minutes including practice.

Learning outcome: Apply the ideal transformer ratio and separate kVA from kW.

Before you begin: Complete earlier chapters in this discipline; revise algebra and units as needed.

1. Understand the idea

An alternating magnetic flux links primary and secondary windings. The ideal voltage ratio follows the turns ratio, while current changes inversely to conserve power. Real transformers have winding and core losses, finite regulation and thermal limits. Nameplate kVA is not a direct statement of delivered kW.

2. Write the model

V1/V2 = N1/N2; V1I1 ≈ V2I2; loading = S/Srated

Define the symbols from the worked case before substituting values. Keep units consistent, record the assumptions and round only after the calculation.

3. Follow a worked example

An ideal 11 kV/230 V single-phase transformer has ratio 47.83. At 23 kVA secondary load, secondary current is 100 A and ideal primary current is approximately 2.09 A.

4. Explore the model

Figure: calculated teaching cases; straight segments connect sampled points, not experimental observations. Read exact values below.

Illustrative scenario sweep — synthetic educational data
Load (kVA)	Secondary at 230 V (A)	Ideal primary at 11 kV (A)
11.5	50	1.045
23	100	2.091
46	200	4.182

Increase apparent load at fixed terminal voltages. Both winding currents increase proportionally.

5. Reproduce it yourself

Copy the input cases into Excel or a notebook and name each quantity and unit.
Recalculate each result from the stated equation or rule. For qualitative cases, explain the state or partition decision instead.
Change one input, predict the direction of change, then calculate again.
Write one sentence explaining where the model stops being valid.

6. Check your understanding

What is loading for 100 kVA on a 200 kVA transformer?

Reveal the answer and check your work

50%.

Compare your result, units and reasoning. If they differ, return to the equation and test the worked case first.

7. Common mistake

Three-phase winding connections and line/phase quantities require separate treatment.

Further study and academic note

Explore the discipline reference and further learning resources. This is an original IIMEEE introductory teaching note with synthetic examples; it is not a peer-reviewed research result or a professional design specification. Figures were constructed from the listed teaching cases. Resource links support further study and do not imply endorsement.