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Inverter power factor rating

ronnie263

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Feb 29, 2020
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Hello everybody

When an inverter is said to have a power factor of 0.8 what exactly does it mean. Is this power factor in reference to the load output it can handle or it's in reference to it as a load when connected to a grid supply.

Thank you.
 
There are two things relating to PF rating on inverters.

One is output kVA load minimum tolerable power factor for output loads. HF inverters are usually limited on their output power factor loading due to their two-stage design with HV DC filter capacitor. Too much inductive loading can creating a problem with peak over-voltage on HV DC voltage point that can damage its filter cap or high voltage IGBT's. Common limit is no worse than a load power factor of 0.8.

Second is power factor presented on input AC when battery charging from AC input grid or generator. Some HF inverter/chargers just use regular rectifiers from AC input to HV DC point which results in poor power factor during charging from AC input. This is of more concern when charging through inverter/charger from a generator.

Low frequency inverters are much better at handling poor power factor loads and provide good AC input power factor during AC input battery charging.

In practice there are two types of poor power factor. One is due to high short peaks of output current loading on inverter caused by simple rectifier-filter AC to DC power supply appliances, like you might find on an VFD drive three phase inverter air conditioner. This just causes high peak currents resulting in more inverter efficiency loss and therefore less real output power capability.

Other type of poor power factor is due to inductive AC motors, like pumps and conventional air conditioner compressors. Because of the inductive component of load, the AC sinewave voltage peak occurs before the AC sinewave load current peak. There can actually be a short period of load current reversal during a portion of AC cycle. A HF inverter can have an issue with this short reverse current push.

In both power factor cases, the inverter efficiency will degrade causing greater inverter losses and internal heating at high loads.

Inverter, generators, and transformers are really rated for VA loading (called apparent power) not actually watts (real power) of load. When load power factor is 1.0, VA equals watts.
 
I noticed there are Victron Inverter/chargers with a power rating of 5000va and 4000w. Does this mean to say 4000w is the maximum at at a power factor of 0.8? Does it also mean that when running a load with a power factor at unity 5000w becomes permissable?
 
I noticed there are Victron Inverter/chargers with a power rating of 5000va and 4000w. Does this mean to say 4000w is the maximum at at a power factor of 0.8? Does it also mean that when running a load with a power factor at unity 5000w becomes permissable?
Yes, but in Victron case it is a matter of how they rate their transformer use case. Victron is a low frequency inverter. It can take a much poorer power factor for a short AC motor startup surge period.

During the approximately 0.4 second induction AC motor startup, the motor load is highly inductive which is a double whammy for HF inverters. High surge current and highly inductive load during motor startup surge period. Power factor during AC motor startup is commonly below 0.5.

Low frequency inverters are inherently bi-directional. For inductive motor load the small portion of AC cycle with negative current flow will be pushed back to batteries. For a HF inverter it just pushes to the HV DC filter capacitors and causes the battery to HV DC converter to effectively shutdown during the short AC cycle period of reverse AC current flow. This may cause instability in battery to HV DC converter regulator feedback control loop causing oscillations that can damage battery to HV DC converter.
 
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