4.2 Transformers: Construction, Turns Ratio & Types
Key Takeaways
- A transformer transfers energy between windings purely through mutual induction across a shared magnetic core — with no direct electrical connection between primary and secondary
- The turns-ratio relationship Vp/Vs = Np/Ns = Is/Ip governs an ideal transformer: the side with more turns has higher voltage and lower current
- Step-up transformers raise voltage (and lower current) for efficient long-distance transmission; step-down transformers lower voltage (and raise current) for safe end-use utilization
- Core-type transformers have windings wrapped around the core's legs; shell-type transformers have the core wrapped around the windings
- A bank of three single-phase transformers costs more than one three-phase unit but allows individual replacement and open-delta operation if one unit fails
4.2 Transformers: Construction, Turns Ratio & Types
Working Principle: Mutual Induction
A transformer is a static (no moving parts) electrical machine that transfers electrical energy between two or more windings through mutual induction — a shared, alternating magnetic field linking a primary winding (the side connected to the power source) and a secondary winding (the side connected to the load). Both windings are wound on a common magnetic core, usually built from laminated silicon steel, which concentrates and guides the magnetic flux between them.
When AC voltage is applied to the primary winding, it drives an alternating current that produces an alternating magnetic flux in the core. By Faraday's law of electromagnetic induction, this changing flux induces an electromotive force (EMF) in every winding that links it — including the secondary winding, even though the two windings have no direct electrical (conductive) connection between them. Energy crosses from primary to secondary purely through the magnetic field. This is also why transformers work only on AC (or switched/pulsed DC): a steady DC current produces a constant flux with no rate of change, and Faraday's law requires a changing flux to induce a voltage.
The Turns-Ratio Relationship
For an ideal transformer (no losses, no leakage flux), the voltage induced in each winding is directly proportional to its number of turns. This gives the fundamental turns-ratio relationship every master electrician must be able to apply:
Vp / Vs = Np / Ns = Is / Ip
where Vp, Np, and Ip are the primary voltage, primary turns, and primary current, and Vs, Ns, and Is are the corresponding secondary quantities. Notice that the voltage ratio and turns ratio move together, while the current ratio is inverse to both — this follows from conservation of power in an ideal transformer, where input apparent power equals output apparent power: VpIp = VsIs.
The ratio Np/Ns (or equivalently Vp/Vs) is called the turns ratio, often written a = Np/Ns and expressed in the form a:1 (for example, a 10:1 step-down transformer).
Worked Example
A single-phase distribution transformer has a rated primary voltage of 2,400 V and a secondary voltage of 240 V. The primary winding has 1,000 turns, and at full load the secondary supplies 50 A to the connected load.
Step 1 — Find the turns ratio: a = Vp / Vs = 2,400 / 240 = 10 (a 10:1 step-down transformer)
Step 2 — Find the secondary turns: Ns = Np / a = 1,000 / 10 = 100 turns
Step 3 — Find the primary current: Since Is / Ip = Np / Ns = a, then Ip = Is / a = 50 / 10 = 5 A
| Quantity | Primary | Secondary |
|---|---|---|
| Voltage | 2,400 V | 240 V |
| Turns | 1,000 | 100 |
| Current | 5 A | 50 A |
Notice the pattern: the side with more turns and higher voltage carries less current, and the side with fewer turns and lower voltage carries more current. This inverse relationship is exactly why transmission utilities step voltage up before sending power over long distances — the same power can be delivered at much lower current, which sharply cuts I²R line losses (covered further in Section 4.3).
Step-Up vs. Step-Down Transformers
- A step-up transformer has more secondary turns than primary turns (Ns > Np), so it raises voltage and lowers current. Power plants use step-up transformers at the generating station to raise the alternator's output (commonly a few kilovolts) to high transmission voltages (tens to hundreds of kilovolts) before the power travels over transmission lines.
- A step-down transformer has fewer secondary turns than primary turns (Ns < Np), so it lowers voltage and raises current. Utilities use step-down transformers in stages — first at substations to bring transmission voltage down to distribution voltage, and finally at pole-mounted or pad-mounted distribution transformers to bring distribution voltage down to utilization voltage for homes and businesses (in the Philippines, typically 230 V single-phase or 230/400 V three-phase).
Core Types: Core-Type vs. Shell-Type
Transformer cores are built up from thin, insulated laminations (to limit eddy-current losses — see Section 4.3) in one of two basic geometries:
| Core Type | Construction | Typical Use |
|---|---|---|
| Core-type | The core forms a simple rectangular loop; primary and secondary windings are wound concentrically around the two vertical legs | Large power transformers, where easier winding insulation and cooling access matter |
| Shell-type | The core has three legs; the windings are wound on the center leg while the outer two legs return the flux, effectively surrounding the windings | Smaller distribution and instrument transformers, where the core offers better mechanical protection and confines stray flux |
Single-Phase vs. Three-Phase Transformer Banks
Three-phase power can be transformed in two ways: with a single three-phase transformer built on one core with three sets of windings, or with a bank of three separate single-phase transformers externally connected in a three-phase configuration (wye or delta — see Section 4.3). A three-phase unit is more compact, lighter, and less costly for new installations. A bank of three single-phase units costs more and takes more space, but offers a practical maintenance advantage utilities value: if one single-phase unit fails, it can be swapped out individually, and the remaining two can even continue supplying reduced three-phase power in an open-delta configuration while a replacement is sourced — something impossible with an integrated three-phase unit, which must be fully replaced or repaired as a whole.
A transformer has a primary winding of 1,200 turns and a secondary winding of 300 turns. If the primary voltage is 480 V, what is the secondary voltage (assuming an ideal transformer)?
Which statement best describes a shell-type transformer core?
Why do utilities use step-up transformers at generating stations before sending power over transmission lines?