Measuring power and cosφ with a three-phase power clamp meter

The guide on power-factor correction covers why a grid operator applies a reactive-power surcharge below a certain cosφ threshold, and the [guide on harmonics & THD](/guides/nen-1010/harmonischen-thd) covers the distortion that variable-frequency drives and LED drivers cause on the grid. This article covers the measurement itself: how a three-phase power clamp meter determines active, reactive and apparent power and cosφ in the field, and why the result can differ from what a simple current clamp alone would show.

What a power clamp meter measures

A three-phase power clamp meter combines three current clamps (one per phase) with voltage leads (line-neutral or line-line, depending on the measurement configuration) and calculates, per phase and combined:

• Apparent power (S, in VA/kVA) — the product of measured voltage and current, without accounting for phase displacement.
• Active power (P, in W/kW) — the power actually converted into useful work, smaller than S as soon as current and voltage are not in phase.
• Reactive power (Q, in var/kvar) — the power that oscillates back and forth between source and load without performing useful work, caused by inductive (motors, transformers) or capacitive (long cables, capacitor banks) loads.
• Power factor (PF = P/S) — the ratio between active and apparent power.

Displacement factor (cosφ) versus true power factor (PF)

With a purely sinusoidal current, cosφ and the power factor PF are equal to each other: cosφ is then simply the cosine of the phase displacement between voltage and current. As soon as the current is distorted — for example by variable-frequency drives, LED drivers or other non-linear loads, as covered in the harmonics-and-THD guide — the true power factor PF diverges from the displacement factor cosφ of the fundamental alone: PF also accounts for the extra apparent current caused by the higher harmonics, while cosφ does not. An installation with many VFDs can therefore show a cosφ that looks fine, while the true PF is noticeably lower due to harmonic distortion.

Practical relevance

When assessing whether a power-factor-correction capacitor bank is needed or correctly sized, the right quantity must be used: for an installation with a lot of non-linear load (VFDs, LED drivers), the displacement factor cosφ alone gives an incomplete picture, and the true power factor PF (or the separate readings of P, Q and S per phase) is the more reliable basis for sizing a correction — a capacitor bank sized purely on cosφ can, under high harmonic distortion, even cause resonance with the grid impedance instead of improving the power factor.

Common mistakes

1. Connecting current clamps the wrong way round (arrow against the direction of current flow) — this reverses the sign of the measured active and reactive power, causing a load to be incorrectly displayed as capacitive/exporting instead of inductive/consuming.
2. Connecting voltage leads to the wrong phase, or not checking the phase sequence — a mismatched phase assignment between a current clamp and a voltage lead gives a completely incorrect power calculation for that phase, even though current and voltage look normal individually.
3. Reading only cosφ and assuming it equals the true power factor on an installation with significant harmonic distortion — see the explanation above of the difference between the two quantities.
4. Measuring at a moment with atypical operating load (for example while production is idle) and extrapolating that snapshot to the full operating situation — power and cosφ vary with load, and a single snapshot does not represent the average or peak load.

Related

• Power-factor correction (cosφ)
• Harmonics & THD limit values
• AC/DC current clamp — Hall-effect measurement errors
• True RMS versus average-responding multimeter