Use the terms primary, secondary, step-up and step-down

4.5.6 The Transformer

Learning Objective

By the end of this lesson you should be able to use the terms primary, secondary, step‑up and step‑down correctly when describing a transformer.

Key Definitions

• Primary winding: The coil of a transformer that is connected to the input (source) voltage.
• Secondary winding: The coil that delivers the transformed (output) voltage to the load.
• Step‑up transformer: A transformer in which the secondary voltage is greater than the primary voltage (Ns > Np).
• Step‑down transformer: A transformer in which the secondary voltage is lower than the primary voltage (Ns < Np).

How a Transformer Works

A transformer consists of two (or more) windings wrapped around a common magnetic core. When an alternating current (AC) flows through the primary winding, it creates a time‑varying magnetic flux in the core. This changing flux links the secondary winding and induces an electromotive force (EMF) according to Faraday’s law.

The relationship between the number of turns in each winding and the voltages is given by:

\$\frac{V{\text{s}}}{V{\text{p}}} = \frac{N{\text{s}}}{N{\text{p}}}\$

where

• \$V_{\text{p}}\$ = primary voltage
• \$V_{\text{s}}\$ = secondary voltage
• \$N_{\text{p}}\$ = number of turns in the primary winding
• \$N_{\text{s}}\$ = number of turns in the secondary winding

Because power (ignoring losses) is conserved, the currents are related by:

\$\frac{I{\text{s}}}{I{\text{p}}} = \frac{N{\text{p}}}{N{\text{s}}}\$

or equivalently

\$V{\text{p}} I{\text{p}} \approx V{\text{s}} I{\text{s}}\$

Step‑Up vs. Step‑Down

Example Problem

A transformer has 500 turns on the primary winding and 1500 turns on the secondary winding. The primary is connected to a 230 V AC supply.

1. Identify whether the transformer is step‑up or step‑down.
2. Calculate the secondary voltage.
3. If the secondary supplies a lamp drawing 2 A, determine the primary current (ignore losses).

Solution

1. Since \$N{\text{s}} > N{\text{p}}\$ (1500 > 500), it is a step‑up transformer.
2. Using the turn‑ratio formula:

   \$\frac{V_{\text{s}}}{230\ \text{V}} = \frac{1500}{500} = 3\$

   \$V_{\text{s}} = 3 \times 230\ \text{V} = 690\ \text{V}\$

3. Power in the secondary: \$P{\text{s}} = V{\text{s}} I_{\text{s}} = 690\ \text{V} \times 2\ \text{A} = 1380\ \text{W}\$.
   

   Assuming ideal operation, \$P{\text{p}} = P{\text{s}}\$, so

   \$I{\text{p}} = \frac{P{\text{p}}}{V_{\text{p}}} = \frac{1380\ \text{W}}{230\ \text{V}} \approx 6.0\ \text{A}\$

Common Misconceptions

• “A transformer can change the frequency.” – In an ideal transformer the frequency of the output is the same as the input; only voltage and current are transformed.
• “More turns always mean higher voltage.” – The voltage change depends on the *ratio* of secondary to primary turns, not the absolute number of turns.
• “Power is created or destroyed in a transformer.” – In an ideal transformer power is conserved; real devices have small losses (core hysteresis, copper resistance).

Suggested Diagram

Summary

Transformers are essential devices for changing AC voltages. The primary winding receives the input voltage, the secondary delivers the transformed voltage, and the turn ratio determines whether the device is a step‑up or step‑down transformer. Understanding the relationships between turns, voltage, and current allows you to predict the behaviour of real‑world electrical systems.