14.1 Transformer Construction & Operation

14.1 Transformer Construction & Operation

Quick Answer: A transformer couples AC circuits by mutual induction. AC in the primary produces changing core flux that induces EMF in the secondary. Cores are laminated silicon steel (core-type or shell-type). Polarity marks identify terminals that go positive together so windings can be phased correctly.

CAAS SAR-66 Module 3 topic 3.15 Transformers follows R, L, and C circuits (3.14) and precedes filters (3.16). Aircraft buses are typically 115 V AC / 400 Hz; many loads need other AC voltages (instrument lighting, radar supplies, specialty AC). Transformers provide those levels without sliding contacts or continuous mechanical conversion. This section builds construction and operating vocabulary before losses, turns-ratio maths, and three-phase/autotransformer variants.

What a Transformer Is

A transformer is a static electromagnetic device with:

1. A primary winding — connected to the AC source.
2. One or more secondary windings — connected to the load(s).
3. A ferromagnetic core — low-reluctance path guiding nearly all flux from primary to secondary.

There is no conductive connection between isolated primary and secondary in a conventional two-winding transformer (contrast autotransformers in Section 14.3). Energy transfers magnetically.

Aircraft context: Airborne units are usually dry, compact, and sized for 400 Hz. Higher frequency means less core mass for a given flux-density limit than a 50/60 Hz utility transformer of similar power — a direct weight benefit.

Operating Principle — Mutual Induction

Faraday and Lenz

When primary current is alternating, core flux Φ is alternating. Faraday’s law for a coil of N turns:

e = −N (dΦ / dt)

The secondary sees essentially the same mutual flux (ideal assumption), so:

es = −Ns (dΦ / dt) and ep ≈ −Np (dΦ / dt)

Hence for sinusoidal excitation the RMS magnitudes satisfy:

Vp / Vs ≈ Np / Ns

(Detailed turns-ratio and current relationships are Section 14.2.)

Lenz’s law sets the polarity of the induced EMF so that secondary current (when a load is connected) produces flux opposing the change that created it. That opposition appears at the primary as an increased primary current — the mechanism of power transfer from source to load.

Why Steady DC Does Not Transform

Steady DC primary current produces constant flux (after magnetisation settles). Then dΦ/dt = 0, so secondary induced EMF is zero. A DC step transient can induce a pulse, but continuous transformation of power requires changing flux — hence AC only for normal transformer action. Applying DC to a primary also risks high magnetising current and core saturation because winding resistance alone limits current once inductance’s opposing effect dies away.

Worked idea — 400 Hz vs 50 Hz. For the same peak flux density B and same core area A, Φ_peak = B A. Induced EMF magnitude scales with f × N × Φ_peak. At 400 Hz versus 50 Hz (8× frequency), the same EMF needs roughly 1/8 the core volume product (N A) if B is held similar — the engineering reason aircraft magnetics can be small and light.

Core Construction

Material

Cores use silicon steel (typically a few percent silicon) to raise electrical resistivity (helps eddy-current suppression — Section 14.2) while keeping high permeability and a narrow hysteresis loop. Soft magnetic behaviour (low coercivity) reduces hysteresis heating each AC cycle.

Laminations

The core is not a solid block. It is stacked thin laminations (often ~0.3–0.5 mm class for power magnetics), each coated with insulating varnish or oxide. Laminations lie parallel to the flux path so magnetic reluctance stays low, while eddy-current loops are broken across the stack thickness. At 400 Hz, eddy and hysteresis design is especially important because both core-loss mechanisms rise with frequency.

Core-Type vs Shell-Type

Shell-type construction keeps primary and secondary close and nested, reducing flux leakage (flux that fails to link both windings). Module 3 expects you to recognise both names and that shell construction improves coupling.

Windings and Insulation

Windings are insulated copper (or sometimes aluminium on large ground units). Cross-section follows current: the low-voltage, high-current winding uses thicker conductors. Layers and bobbins set creepage distance for the voltage class. Multiple secondaries can share one primary (e.g., several instrument supplies).

Step-down / step-up naming refers to voltage: Ns < Np → step-down; Ns > Np → step-up. Current moves the opposite way for roughly constant VA (Section 14.2).

Polarity Markings

Polarity answers: which secondary terminal is instantaneously positive when a chosen primary terminal is positive?

Dot convention

Dots (or equivalent marks) on one primary and one secondary terminal mean those terminals have the same instantaneous polarity. If both dotted ends are taken as “starts,” voltages are in phase in the additive sense used on diagrams.

Additive and subtractive connection

When windings are series-connected for higher voltage:

• Additive (boost): connect so instantaneous voltages add — join unmarked (or opposite-polarity) ends appropriately per the marking standard so the series voltage is |Vp| + |Vs| (for similarly rated senses).
• Subtractive (buck): reverse relative sense so voltages oppose — series voltage is the difference.

Wrong polarity on paralleled secondaries causes circulating current and damage. Wrong boost/buck on series taps yields unexpected voltage.

Alphanumeric marks (H and X)

Many power transformers use H1, H2… on the high-voltage winding and X1, X2… on the low-voltage winding. A common subtractive standard places H1 and X1 so that when H1 is instantaneously positive, X1 is also positive (same instantaneous polarity). Always follow the nameplate/diagram for the unit in hand — Module 3 cares that you know markings exist to show in-phase terminals, not that you memorise every national marking table.

Worked polarity check (conceptual). Primary H1–H2 driven from 115 V AC. Secondary X1–X2. With H1 and X1 same-polarity marked, a voltmeter from H1 to X1 (with an appropriate series connection test per training procedure) reads near the difference of the two winding voltages when windings are connected subtractively for the test, versus near the sum for additive — the classic polarity-test idea used in training shops.

Ideal Transformer Snapshot

For teaching, the ideal transformer assumes:

• Zero winding resistance
• Zero core loss
• Infinite permeability (negligible magnetising current)
• Perfect flux linkage (no leakage)

Then Vp/Vs = Np/Ns = Is/Ip and Pin = Pout. Real units deviate (Section 14.2), but ideal laws are the exam calculation baseline.

Magnetising Current (Qualitative)

Even with secondary open, a small no-load / magnetising current flows in the primary to establish core flux and supply core losses. It is largely reactive (lags voltage ≈ 90° in the simple model) plus a small in-phase component for core loss. Do not confuse no-load current with load current — load current appears when the secondary delivers power.

Summary for Syllabus 3.15 Construction

Master primary/secondary roles, why laminations exist at a construction level, core vs shell, and polarity marks. Section 14.2 then quantifies turns ratio, losses, load vs no-load, and efficiency.