TRANSFORMER at NO LOAD OPERATION | TRANSFORMATION RATION | TURN RATIO — HAND NOTES by Electrical Zindagi
Table of Contents
Introduction
Transformers are the silent workhorses of power systems — converting voltage levels efficiently without changing frequency. In this guide we focus on two practical ideas: what happens when a transformer runs at no-load, and how the turn ratio (or transformation ratio) determines the voltage relationship between windings. We'll keep things hands-on with short tricks, analogies, and a downloadable set of hand notes.
No-load Operation
No-load means the transformer's secondary is open (no connected load) but the primary is supplied with rated voltage. Even though no current is being delivered to a load, a small current — called the magnetizing current — flows to establish the magnetic flux in the core.
Think of the core like a bucket you need to keep filled: the magnetizing current is the tiny tap flow that compensates for leaks (core losses and magnetizing requirements). This current is small compared to load current but essential: without it the core couldn't establish flux and the transformer would not induce voltage on the secondary.
- Magnetizing current — small, reactive, required to create flux.
- Core (iron) losses — hysteresis & eddy current losses that occur even at no-load.
- Exciting branch — the equivalent circuit element modeling no-load behavior.
Turn Ratio & Transformation Ratio
The turn ratio (N1:N2) tells you how many turns the primary and secondary have. The transformation ratio (a) is usually written as a = V1 / V2 = N1 / N2 (for ideal transformers). In simple words: voltage scales with turns.
Example trick: If the primary has 1000 turns and the secondary 100 turns, the secondary voltage is 1/10th the primary. For 2300 V primary, secondary becomes 230 V. Keep this rule of thumb: voltage ratio = turns ratio.
Practical Example & Short Trick
Suppose a distribution transformer named "A" has a rated primary of 11 kV and secondary of 433 V. What is the turns ratio? Using a = V1/V2 = 11000/433 ≈ 25.4. So N1 is ≈25.4 times N2.
Quick trick: use round numbers for fast approximations — 11 kV ≈ 11000 and 433 ≈ 430, so a ≈ 25.6. For exam problems, round only after you compute — keep one extra digit during intermediate steps.
Also remember: in no-load the secondary will show the induced EMF equal to the ratio times the applied primary EMF (ignoring small drop due to magnetizing current). That means you can measure an unknown turn count by applying a known low-voltage and measuring induced secondary voltage — a common lab technique.
Common Mistakes & How to Avoid Them
- Mixing up turn ratio direction — Always write which side is primary and which side is secondary.
- Ignoring core losses at no-load — they matter for efficiency and temperature rise even when transformer is idle.
- Assuming the magnetizing current is zero — it isn't; protective devices and inrush currents must be considered.
Tip: draw the circuit and label the voltages and turns. Visual labels prevent sign errors in polarity or ratio calculations.
Download: Hand Notes (3D Button)
Click the big download button to open the handwritten notes (Google Drive). These notes contain concise formulas, exam tips and worked examples.
Download Notes (Google Drive)May You Like These Posts
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Tip: Visit Electrical Zindagi and pick the most relevant posts to link here for backlinks — replace table rows if needed.
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