Power factor correction (cosφ)

Businesses with many electric motors — pumps, fans, screen motors — as is common in greenhouse horticulture, often deal with a low power factor (cosφ). This has direct financial consequences through the grid operator connection, alongside technical consequences for cable and transformer loading.

What power factor is and why motors are the culprit

The power factor (cosφ) expresses the ratio between the actually useful power delivered (kW) and the total apparent power supplied by the grid (kVA). Asynchronous motors — the most common motor type in pump and ventilation installations — have a cosφ that depends heavily on the load:

• No load: cosφ ≈ 0.2 — a largely inductive (reactive) load.
• Full load: cosφ ≈ 0.8-0.85.

A business with many motors that regularly run at partial load (such as ventilation motors that spend most of their time below full power) therefore builds up a structurally low average cosφ.

The grid operator's reactive-power surcharge

Dutch grid operators generally apply a threshold — often cited in the order of cosφ ≈ 0.85 to 0.9 — below which the transported reactive current is billed separately, in addition to the regular energy costs. Below that threshold, the invoiced reactive-power component increases as the power factor drops further.

Note: the exact threshold percentage and the tariff structure differ per grid operator (Liander, Enexis, Stedin) and per connection contract — for a specific business, always base this on the actual connection contract rather than on one universal figure.

Correction method: capacitor banks

The common correction method is installing a capacitor bank, sized in kVAr, which compensates the reactive-power component of the installation and pulls the cosφ toward the target value (usually close to 1). Besides the direct savings on the reactive-power surcharge, this also results in lower loading on cables and transformer — meaning the investment is often paid back in practice within 1 to 2 years for a heavily motor-loaded installation.

Points of attention: overcorrection and VFDs

• Overcorrection (an oversized capacitor bank, resulting in a leading rather than lagging power factor) is a real risk and can itself lead to a separate surcharge or instability — sizing must match the actual, variable load rather than the peak load alone.
• Variable frequency drives (VFDs), widely used to make ventilation and pump motors more energy-efficient, change the picture: they mainly introduce harmonic distortion (THD) rather than a pure inductive reactive-power problem. A standard capacitor bank is not automatically suitable for compensating harmonic distortion — that requires a separate assessment (for example detuned capacitor banks or active filters).

Common mistakes

1. Only considering power factor correction after an unexpectedly high energy bill — in an installation with many partial-load motors (typical for greenhouses), the reactive-power surcharge has often already been quietly present for a long time.
2. Sizing a capacitor bank on the peak load instead of the actual, variable load profile — this increases the risk of overcorrection during partial-load periods.
3. Applying a capacitor bank to an installation with many VFDs without assessing the harmonic distortion — a standard capacitor bank does not solve a THD problem and can even worsen it through resonance.
4. Assuming one fixed nationwide threshold percentage — this differs per grid operator and connection contract; check your own contract.

Related

• Distribution boards & selectivity between protective devices
• Greenhouses and Horticultural Enterprises
• Top 10 Mistakes in Greenhouse Wiring