Power factor vs Crest factor, non-linear load etc. - Page 1
Forum section: electronics / projects

author: Freesurfer
author_rank: Regular Contributor
timestamp: November 02, 2020, 05:25:51 pm
content: I'm reading a discussion about charging large battery banks with a high current charger, from an AC generator.The discussion goes on about how you cannot draw the full rated power from a generator to charge DC batteries and I am wondering if they are right or not.I am assuming a generator with a sine output (not modified sine). The specs of the charger in question states a power factor of 1. A power factor of 1 is equivalent to a resistive load (not inductive or capacitive). So at first glance it might seem like the charger should appear as a simple resistive load like a heater, and therefore have no issues. But I am wondering if power factor alone tells the whole truth or if there is more to it.There's another term called "crest factor" which I think very often gets mixed up with PF. A simple resistor will have a crest factor of 1.41 : 1 (where 1.41 is the peak current, compared to RMS current of the load)Some loads will only draw power for parts of the sinewave, and have a high current spike near the peaks of the sinewave and thus have a greater crest factor. For example if you are running a LED strip from simple rectified AC, there will be a portion of the wave where the voltage isn't high enough to run current through the LEDs, and once the LEDs do turn on there is a high spike in current at the peak of the wave. The resulting current wave isn't linear with the voltage waveform. So it may have a crest factor of maybe 5:1 (5 times higher peak, than RMS current of the load). Yet, this is still a resistive load as there is no current lagging behind, and no power ever discharged back into the power supply like you have with L / C loads.I have always thought of power factor (e.g. voltage-current phase shifted L / C loads) and crest factor (non-linear current wave) as 2 separate issues. But whenever this issue comes up in discussions about power supplies, people always refer to this phenomenon as "power factor", and any design to mitigate the high spikes is called "power factor correction".So I am a bit confused. Will a load with a poor crest factor, always also have a poor power factor? Even if there is no phase shifted current?

author: bdunham7
author_rank: Super Contributor
timestamp: November 02, 2020, 05:41:38 pm
content: The 'crest factor' in current results in what is commonly called a harmonic power factor. This is different from the typical phase-shift concept of power factor due to inductive or capacitive loads, but has similar consequences. So yes, if you are driving a load such as a typical linear power supply that has a bridge rectifier and a filter capacitor, you will get current in the center of the sine wave where the voltage is high enough and no current otherwise. However, you said the specs of your charger include that it has a power factor of 1. This indicates that it is power factor corrected, which means it has a PFC boost circuit that enables it to draw current arbitrarily, and that circuit is set up to draw current in direct correlation with the applied voltage, resulting in a current that mirrors the voltage just like a resistor. What is the charger you are using?QuoteYes.

author: Freesurfer
author_rank: Regular Contributor
timestamp: November 02, 2020, 06:13:53 pm
content: My particular one is a Victron Multiplus II 48/3000/35-32. It is an inverter+charger combo unit capable of delivering 35A charging current at 48V. I have been running it flawlessly with my Honda EU20i generator at its rated power. I had to configure the charger with a max AC input current of 7A with its software to make it stay within the honda's specs. And it works without issues and I can even overload the honda slightly by bumping it to 8A-9A if I want.So I was surprised to see that others were having problems and even victron (the manufacturer of the multiplus and other large chargers with PF=1) stated this in a manual.However reading over it again I think I may have misread it, and the extra headroom may be mostly be for the time it takes for the charger to drop the charging current when a load is switched on.

author: The Electrician
author_rank: Frequent Contributor
timestamp: November 02, 2020, 06:52:06 pm
content: Have a look at this:https://en.wikipedia.org/wiki/Power_factor#Distortion_power_factor

author: bdunham7
author_rank: Super Contributor
timestamp: November 02, 2020, 06:59:39 pm
content: Quote from: Freesurfer on November 02, 2020, 06:13:53 pmThat unit looks pretty robust and that type of device will have a fairly efficient and PFC-corrected SMPS for the charger. The issues others have could be transient loads or inrush currents, but I suspect at least some of those issues have to do with the actual performance of the generators vs their specs. Your EU20i at 230V would be 7A rated and 9A max, right? So your Honda performs to spec. Other generators may not play nice near their maximum outputs, making it necessary to derate them to avoid some type of regulation issue.