What is the power factor rating of an inverter?

Understanding the Power Factor Rating of Inverters

What does the power factor signify in a photovoltaic or wind power inverter?

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Introduction

Inverters are primarily engineered to deliver energy at a power factor approaching unity, especially when they are operating at full capacity. Although exact requirements can differ, a commonly referenced standard dictates that the power factor should exceed 0.90 when generating power that is 50% or more of the inverter's maximum output.

Older inverter models, however, often struggle with low power factors, particularly at diminished power levels. The capacitors utilized at the inverter's output, meant to mitigate high-frequency switching interference, can adversely influence the power factor. When the inverter produces substantial power, these capacitive loads have a minimal impact. Conversely, during lower output scenarios, the capacitive influence can cause the power factor to plummet to figures around 0.5.

Contemporary inverter designs typically offer adjustable power factors ranging from approximately 0.80 to 0.85 or even higher.

Assessing the Power Factor

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Power factor measurements can be taken directly using WattNode meters equipped for this function. Alternatively, one may estimate the power factor, which might not be universally applicable across all inverters due to variations in control mechanisms and output circuitry. The following formulas ignore the actual power consumption of an inverter in under-load conditions, which can range from a mere fraction of a watt to several watts for larger units. Generally, the reactive power caused by filter capacitors spans from tens to hundreds of VARs, making the minor idle consumption relatively insignificant when approximating the power factor.

Basic Formulas

Real power (watts)

Reactive power (VARs)

Apparent power (VAs)

Power factor

Specific Inverter Power Factor Formulas

Nominal inverter voltage

Current due to inverter filter capacitors, measured with no generated power

Real power (watts) produced by the inverter

Reactive power (VARs) from filter capacitors

Apparent power (VAs)

Power factor

In practical scenarios, the power factor should generally exceed 0.9 unless the output is significantly low, usually under 20% of the maximum. Therefore, one might presume a near-unity power factor during active generation and a near-zero power factor otherwise. However, in instances where an oversized inverter operates within the 10-20% power range, measurements may indicate power factors dipping well below 0.9, potentially reaching 0.5.

Power Factor Ratings of Inverters

The power factor rating in inverters encompasses two aspects.

Firstly, it pertains to the minimum tolerable power factor for output loads indicated in kVA. Typically, high-frequency (HF) inverters experience restrictions on their output power factor due to their dual-stage design coupled with high-voltage (HV) DC filtering capacitors. Excessive inductive loading can lead to over-voltage peaks at the HV DC points, potentially damaging the filter capacitors or high-voltage IGBTs. A common limitation is a load power factor no less than 0.8.

Secondly, it refers to the power factor presented at the input AC while charging batteries from the AC grid or generator. Some HF inverter/chargers implement standard rectifiers from AC input to HV DC, which results in diminished power factor during the charging process. This concern grows when battery charging occurs through an inverter/charger connected to a generator.

Low-frequency inverters tend to manage poor power factor loads more effectively and maintain acceptable AC input power factor levels during battery charging.

In practice, there are two categories of poor power factor. One results from high short peaks of output current loading on the inverter, induced by devices like rectifier-filter AC to DC appliances, commonly seen in three-phase inverter-driven HVAC systems. This leads to elevated peak currents, causing efficiency losses and reduced real output capabilities for the inverter.

The second category stems from inductive AC motors, such as pumps and standard air conditioning compressors. Due to the inductive load properties, the voltage peaks occur before the current peaks in the AC sinewave. Consequently, there could be brief moments of current reversal within certain AC cycle periods. This reversal may create challenges for HF inverters.

In both scenarios, inferior power factors lead to decreased inverter efficiency, resulting in greater losses and excess internal heating during high load conditions.

Inverters, along with generators and transformers, are generally rated for VA loading (also referred to as apparent power) rather than actual watts (real power). When the load power factor is 1.0, VA equates to wattage.

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