Transformer Basics

A transformer is a static electrical device that uses Faraday’s Law of Electromagnetic Induction to transfer AC energy from one circuit to another without changing the frequency. In low-voltage applications from 110 V to 1000 V, transformers are used mainly for step-down duty (for example 480 V down to 110 V / 220 V for industrial and commercial equipment) or for isolation.

• Operating principle: when alternating current flows in the primary winding, it creates an alternating flux in the core, which in turn induces an electromotive force in the secondary winding.

• Voltage ratio: $V_p / V_s = N_p / N_s = a$, where $a$ is the turns ratio.

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2. Transformer Construction

Low-voltage transformers are usually of dry-type construction, consisting of a core and windings.

• Core-form: the windings surround the core. The construction is simple and insulation is easier to apply.

• Shell-form: the core surrounds the windings, giving better mechanical strength and suiting high-current applications.

(The figure above shows the difference in magnetic circuit layout between core-form and shell-form transformers.)

3. Transformer Loading

Under load, secondary current produces a counter magnetomotive force, so primary current rises to maintain the magnetic balance.

• Equivalent circuit: when analysing loading, the secondary impedance is normally referred to the primary side.

• Voltage drop: because the windings have resistance and leakage reactance, output voltage generally falls as load current increases.

4. Multiple Winding Transformers

Transformers with three or more windings. They are common where several voltage levels are needed at once (for example 110 V and 220 V together), or where a control circuit requires electrical isolation.

5. The Autotransformer

In an autotransformer the primary and secondary share part of the same winding.

• Advantages: when the voltage ratio is close to 1 (for example 220 V to 110 V), it is smaller, more efficient and less expensive than an equivalent isolating transformer.

• Disadvantages: there is no galvanic isolation, so a fault on the primary side can be transferred directly to the secondary side.

(Autotransformer construction, showing the arrangement of the common winding and the series winding.)

6. The Current Transformer (CT)

Used to measure large currents. A CT scales a high current down by a fixed ratio to a standard 5 A or 1 A signal for instrumentation.

• Warning: never open-circuit the secondary of a CT. Doing so produces extremely high induced voltages that can damage equipment and endanger personnel.

7. Three Phase Transformers

Industrial systems (380 V, 440 V, 480 V and similar) are mostly three-phase.

• Connections:

  • Wye (Y): provides a neutral point, allowing three-phase four-wire distribution.

  • Delta (Δ): suited to power transmission and circulates third-harmonic currents rather than passing them on.

• Common arrangement: the $\Delta-Y$ connection is widely used in industrial step-down systems because it provides a stable neutral point for earthing.

8. Transformer Per-unit Value Introduction

A per-unit (pu) value is a physical quantity (V, A, $\Omega$, W) divided by a chosen base value.

• Definition: $\text{per-unit value} = \frac{\text{actual value}}{\text{base value}}$

• Benefit: the per-unit impedance of a transformer is the same whether it is viewed from the primary or the secondary side, which greatly simplifies calculations in systems with several voltage levels.

9. Transformer Voltage Regulation

Voltage regulation describes how stable the secondary output voltage stays between no load and full load.

• Formula: $VR\% = \frac{|V_{NL}| – |V_{FL}|}{|V_{FL}|} \times 100\%$

• Effect of power factor (PF):

  • Lagging PF (inductive load): $VR$ is positive — the output voltage falls.

  • Leading PF (capacitive load): $VR$ can be negative — the output voltage may actually rise.

(The figure above shows the phasor relationships of the voltage drop at lagging, leading and unity power factor.)

10. Losses in Real Transformers and International Efficiency Standards

10.1 Transformer Losses

1. No-load loss (core loss): made up of hysteresis loss and eddy current loss in the core. It is independent of load and is present whenever the transformer is energised.

2. Load loss (copper loss): the $I^2R$ loss in the winding resistance, rising with the square of the load current.

3. Stray loss: eddy current loss produced by leakage flux in metallic structural parts.

10.2 International Efficiency Standards and Minimum Energy Performance Standards (MEPS)

Dry-type transformers below 1000 V are covered by strict regulations in most markets:

• European Union (IEC 60076 & Ecodesign Tier 2):

  • Tier 2 has applied since July 2021. Compared with Tier 1 it requires no-load and load losses to fall by roughly a further 10%, and introduces the Peak Efficiency Index (PEI).

• United States (DOE 2016 — 10 CFR 431):

  • Low-voltage dry-type transformers (LVDT) must reach a defined efficiency at 35% load. A 15 kVA three-phase unit, for example, must reach at least 97.89%.

• Canada (CSA C802.2):

  • CSA C802.2-12 is closely aligned with the US DOE 2016 standard and applies mainly to 60 Hz dry-type transformers.

• Australia / New Zealand (AS/NZS 60076 / MEPS):

  • Based on AS 2374.1.2, requiring a minimum efficiency at 50% load. The threshold values are normally classified by the highest voltage for equipment (Um) of the winding.

• India (BIS — IS 1180):

  • IS 1180-1:2014 defines efficiency Levels 1, 2 and 3, with Level 3 the most efficient. Many types now require the BIS certification mark for the Indian market.

• Brazil (ABNT NBR):

  • NBR 10295 specifies efficiency and test requirements for dry-type transformers, optimised for 60 Hz systems.

10.3 The Economics of Choosing a High-efficiency Transformer

A transformer that meets Tier 2 or DOE 2016 costs more to buy, but because transformers usually run 24 hours a day, the lower no-load loss typically pays for the difference in electricity savings within three to five years — and the total cost of ownership (TCO) over the life of the unit is lower.