How does a current transformer work?

Current transformer operating principle:

• Electromagnetic induction (Faraday's law) : Alternating current through primary winding generates time-varying magnetic field in iron core. Secondary winding (coil) wound around same core intercepts magnetic flux; induced EMF (voltage) drives secondary current through connected load (meter, relay).
• Primary winding : Usually 1–3 heavy-gauge copper turns (thick, low-resistance wire) connected in series with main power circuit. Current flows directly through primary; no energy loss (ideal resistance <1Ω).
• Iron core : Ferromagnetic toroid or C-shaped core (laminated steel or nickel-iron alloy). Core concentrates magnetic flux from primary winding, ensuring efficient coupling to secondary. Saturation design (maximum flux density) determines CT linearity across operating range.
• Secondary winding : Many turns (hundreds) of fine wire wound around core. Current flowing through secondary creates opposing magnetic field (Lenz's law effect). This opposition limits secondary current; secondary current = primary current ÷ turn ratio.
• Transformation equation : - Np (primary turns) = typically 1–2 for power CTs. - Ns (secondary turns) = dozens to hundreds depending on ratio. - Secondary current Is = (Np ÷ Ns) × Ip (primary current). - Example: 50/5A CT has Np=1, Ns=10 (ratio 1:10). Primary 50A → secondary 5A.
• Secondary impedance matching : Secondary burden (connected meter/relay impedance) must match CT design parameters for linear response. Low-impedance load preferred (short secondary leads, <10VA burden typical). High impedance causes core saturation (secondary voltage rises, secondary current falls; non-linear response).
• Frequency response : CTs optimized for 50/60 Hz AC. Works across 45–65 Hz range (±10% frequency tolerance). DC or very low frequency (<10 Hz) poorly transformed; core doesn't couple flux efficiently.
• Power transfer efficiency : Ideal CT transfers power with ~100% efficiency (primary power = secondary power accounting for transformation ratio). Real CTs: 95–99% efficient; small heat loss in core/windings.

Socomec CTs use laminated silicon-steel cores (low hysteresis loss) optimized per IEC 60044-1 metering standard. Secondary insulation rated 3 kV dielectric strength (safety isolation). Optim-Elec verifies CT specifications (accuracy class, burden, saturation point) per measurement application. Contact optim-elec.com.