Not given in the exam, recall it.
What it is for
Transformer turns ratio is used in Electromagnetism (SPM Physics Form 5). Keep the SI units in every line of working. Electromagnetism
Symbols and SI units
| Symbol | Meaning | SI unit |
|---|---|---|
| Vp | primary voltage | V |
| Vs | secondary voltage | V |
| Np | number of primary turns (no unit) | , |
| Ns | number of secondary turns (no unit) | , |
Rearrangements
Vs = Vp × Ns / NpNs = Np × Vs / VpVp = Vs × Np / Ns
Worked example
Common trap
Understanding Transformer turns ratio
This relationship states Vp / Vs = Np / Ns. Here Vp is the primary (input) voltage and Vs the secondary (output) voltage, both in volts (V). Np and Ns are the number of turns on the primary and secondary coils; being pure counts, they have no unit.
The voltages relate to the turns because the changing magnetic flux links each turn equally, so the voltage induced in a coil is proportional to how many turns it has. More secondary turns give a higher output (step-up); fewer give a lower output (step-down).
To rearrange, cross-multiply: Vs = Vp × Ns / Np finds the output voltage, Ns = Np × Vs / Vp finds the secondary turns, and Vp = Vs × Np / Ns finds the input voltage.
A worked example, step by step
A transformer has a primary voltage Vp = 120 V with Np = 200 turns, and its secondary output is Vs = 600 V. Find the number of secondary turns Ns.
Rearrange for Ns:
Ns = Np × Vs / Vp = 200 × (600 V / 120 V) = 200 × 5 = 1000 turns.
Because Vs is greater than Vp, this is a step-up transformer, and the secondary must have more turns than the primary, which the answer confirms (1000 > 200).
Keep the voltage ratio the right way up: it is 600 / 120 = 5, not 120 / 600. The secondary coil has 1000 turns.
Common mistakes and how this is tested
The commonest error is inverting the ratio, mixing up which side is primary. Line up Vp with Np on the same side.
Another mistake is treating this as a power equation; turns ratio deals with voltage only, while current is handled by VpIp = VsIs. A third is adding units to Np or Ns, which are just counts.
Under calculate, substitute the numbers with units and quote the turns as a whole number. Under define, state that the ratio of voltages equals the ratio of turns.
Under explain, you may need to say why a step-up transformer needs more secondary turns, or why transformers work with alternating current but not steady direct current.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English), Sijil Pelajaran Malaysia: Format Pentaksiran mulai 2021, Fizik (4531) (Bahagian Pembangunan Kurikulum (BPK), KPM)