Current Transformer vs Rogowski Coil: AC Current Sensing for Power Electronics

Detailed Explanation

DC-capable current sensing techniques — shunt resistors and Hall-effect ICs — are covered in detail in Hall-effect current sensor vs shunt resistor, which names current transformers only once, in passing, to contrast their AC-only limitation against Hall-effect sensing's DC capability. This page covers the two dominant AC-only, non-contact current sensing techniques directly: the current transformer and the Rogowski coil.

Current Transformer (CT)

A current transformer works exactly like a power transformer, with the current-carrying conductor itself acting as a one-turn primary winding passing through the CT's core, and a secondary winding (often hundreds or thousands of turns) wound around a laminated iron or ferrite core. The changing primary current induces a proportionally scaled current in the secondary winding, following the transformer's turns ratio — a 1:1000 CT with 5 A flowing in the primary conductor produces 5 mA in the secondary.

Key characteristics:

• Self-powered. No external excitation supply is needed — the CT derives its output entirely from the changing primary current.
• Requires a burden resistor. The secondary current is converted to a measurable voltage across a burden resistor sized specifically for the CT — see the FAQ above for why this sizing matters.
• Saturates at high current or low frequency. Exceeding the core's rated volt-time product (from excessive current, too high a burden resistance, or operation below the core's designed frequency range) drives the core into saturation, where the output becomes nonlinear and no longer accurately represents the primary current.
• Cannot measure DC. See the FAQ above — a CT depends entirely on Faraday's law induction from a changing flux.

Rogowski Coil

A Rogowski coil is a helical coil wound on a non-magnetic (air or non-ferrous) core, typically formed into a flexible loop that clips around the conductor without needing to break the circuit to install it — a genuine practical advantage over a solid-core CT in retrofit or test-and-measurement situations.

Key characteristics:

• No iron core, so no saturation. Because there's no ferromagnetic core to saturate, a Rogowski coil has an extremely wide dynamic range, from small currents up to many kiloamps, without the nonlinearity a CT exhibits near saturation.
• Output is proportional to di/dt, not current. This is the coil's defining trade-off — see the FAQ above for why an integrator circuit is required to recover a current-proportional signal, and what happens if it's skipped.
• Needs external power for the integrator. Unlike a self-powered CT, a Rogowski coil measurement system needs a powered integrator circuit (built into most commercial Rogowski current probes and transducers).
• Flexible, clip-on form factor. Many commercial Rogowski coils are designed as a flexible loop that wraps around a conductor or busbar without disconnecting it — valuable for test and measurement applications and for large or irregularly shaped conductors that don't fit a fixed CT window.

Choosing Between Them

Fixed, cost-sensitive, high-volume applications (energy meters, protection relays, motor drive overcurrent protection) generally favour current transformers for their simplicity and self-powered operation. Test and measurement equipment, power quality analysis, and applications needing very wide dynamic range or a flexible clip-on form factor favour Rogowski coils.

Design Considerations

• Verify the CT's rated burden before finalising the secondary-side circuit. Adding a downstream amplifier, ADC input network, or protection circuitry all add impedance in series with the CT's secondary — the total burden the CT sees must stay within its rated limit, not just the resistor you intentionally add. See the FAQ above for the saturation consequence of exceeding it.
• Check the CT's rated frequency range against the actual signal. A CT sized for mains-frequency (50/60 Hz) energy metering may not accurately measure higher-frequency content — switching-frequency ripple current in a power electronics application needs a CT (or Rogowski coil) explicitly rated for that bandwidth.
• Confirm the integrator's own bandwidth and drift for a Rogowski coil design. Since the current-proportional signal only exists after integration, the integrator circuit's own frequency response and DC offset drift directly become part of the overall measurement's accuracy — this is not a passive, "set and forget" stage the way a CT's burden resistor is.

Zeus Design's electronics engineering team designs current sensing circuits for motor drive protection, energy metering, and power electronics test applications as part of complete product electronics design.

Common Mistakes

• Assuming a CT or Rogowski coil can measure DC or low-frequency current. See the FAQ above — both techniques require a changing flux and produce no usable output for steady DC; use a Hall-effect sensor or shunt resistor instead for DC or low-frequency measurement (see Hall-effect current sensor vs shunt resistor).
• Operating a CT open-circuited on the secondary (no burden connected). With no burden resistor to limit it, an open-circuited CT secondary can develop a dangerously high voltage across the winding, since the transformer tries to maintain its ampere-turn balance with no path to do so safely — always keep a CT's secondary loaded, even temporarily during test or calibration.
• Skipping the integrator on a Rogowski coil and treating the raw output as a current reading. Covered in the FAQ above — the raw di/dt signal is not current, and using it as if it were produces a plausible-looking but incorrect waveform, particularly for non-sinusoidal currents.
• Selecting a CT or Rogowski coil rated for mains frequency in a switching power electronics application. The core material and coil design suited to accurate 50/60 Hz measurement may not have adequate bandwidth or accuracy for switching-frequency current content — check the rated frequency range against the actual application, not just the nominal current rating.