Note 3 :
- Non-linear load, crest factor 3:1
Can anybody explain pls.
@Ludo the crest factor is a simple numeric indication of the distortion of the waveform. That distortion is the result of harmonic content in the waveform. The harmonic content is sometimes the result of a non linear load.
For example if you have a resistive load the current through the load will be a sine wave. Crest factor peak / RMS value. Crest factor will equal 1.414.
If however the load is a switch mode power supply, that switches at a point in the sine wave the crest factor will increase. That will distort the sine wave because of harmonic content caused by the load. The rating of the crest factor of 3 gives the highest crest factor that the inverter can deliver the rated power.
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I found this explanation:
As I understand, the peak power on the top of the sinewave times 3. Switch mode power supplies without pfc charge there high voltage capacitors at the top of the sine wave by just using a rectifier bridge which causes this effect.
A non linear load is one where the current is not directly proportional to the supply voltage. This can be due to a switch mode converter in the unit.
Crest factor is the ratio of the peak current to the mean current of the circuit.
Neither is directly related to wave form distortion - except if the non linear load is very fast acting - within the 50Hz cycle - In this case, the power factor would also be <1. Some non linear loads do have unity power factor (The phase difference between the current and voltage waveforms), but draw the same power as the supply voltage is increased say from 220 to 240V. This results in a reduction of supply current at 240V, and a negative resistance slope.
Other units incorporating a single diode to rectify the supply, would only take power on one half cycle, and would cause waveform distortion. These are typically used for very small loads, so would not have much effect on a stable source.
Crest factor is normally a measure of system diversity - many different loads connected to one source, as in a house. If everything is turned on at the same time, load current is very high. Usually this is not the case, and only a few loads are on at one time. This gives a much lower mean current, averaged over several hours.
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So youâre really telling me that with this waveform
https://shuttlelighting.com/dimensioning-a-upsinverter-for-led-crest-factor/
The mp has its highest output ?
Donât forget, he power is equivalent to the surface under the curve.
Nice theory.
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That (red) wave-form is the input Current waveform of an old style Bridge rectifier, with a large reservoir capacitor. This has a very poor power factor, and high harmonic content, and is no longer deemed to be an acceptable load by CE approval authorities. Crest factor does not apply to that type of waveform. Most good LED drivers will follow the blue waveform and have a PF close to unity, and little harmonic content.
Solid state inverters can usually deliver the full output current over a wide range of power factor, and usually donât care too much about the harmonic content of the load*. IF you have a load with the RED current waveform, then the peak current needs to be within the peak current capability of the inverter, taking into account all other loads.
- A load with high Harmonic content like the red waveform will mess up the Voltage waveform on the output of the inverter, causing a high voltage harmonic output. This can interfere with other loads on the system.
That is NOT the answer to my question.
This is the only way to achieve crest=3
Note 3 to what?
Donât just write, try reading
As Simply as possible then: It describes the inverterâs capacity to deal with a ânoisyâ current wave form, generated by a time varying load.
This is the question.
Sorry missed this
https://communityarchive.victronenergy.com/questions/67869/victron-multiplus-rating.html
OK, so I checked back to the original thread too.
> Oscillogram 1 shows the voltage (red) and load current (yellow) of typical 600VA âquasi-sineâ UPS with 10x Verbatim 8.5W GU10 LEDâs (#52310) without a dimmer â notice the 360V peak voltage produced by the UPS! The discontinuous voltage output of the UPS causes severe lamp instability when operated with a typical leading edge dimmer, shown in Oscillogram 2 and such UPS systems should thus be avoided when dimming LEDâs.
I donât think this guy knows much about modern âsinewaveâ inverters. These are built like class D audio amplifiers, and ARE capable of delivering current over the whole sine wave, not just in small peaks like the red trace. The Cheap Chinese inverters are provided with a âslowâ (<1Hz) voltage feedback, more sophisticated designs have feedback at >50Hz, so the actual output voltage can be corrected throughout the cycle. I should note here that the PEAK voltage of a true sine-wave of 240V rms is 340V, so the 360V peak of his inverter is not a surprise.
The intent of the inverter specification is to enable an engineer to determine the motor starting capability of the inverter. Direct on line starting of an electric motor will cause a very short current spike of 6 -8 times the full load running current. The 3x crest factor gives a peak output of 9kW (for a very short time) for a 3kW mean power output. This indicates that the inverter should be able to start a 1.5 - 2kW motor, and then run some other loads too.
Over a slightly longer time - 10s c/f <10mS, the Max output of the 3000W inverter is 5500W -at 25C, with temperature de-rating above this. Continuous - for several hours - the rating drops to 2400W.
For strange waveforms, yes the power is equal to the area under the current curve - at constant voltage. However, you need to be aware that IF the current waveform is not a sine wave, and / or is displaced in phase from the voltage waveform (This is Harmonic Distortion and Power Factor) then the simplest measurement is to multiply the current be the voltage and get a VA rating, not in kW.
Iâm sorry that this gets very technical, but hope it helps.
Your question:
The mp has its highest output ?
Under what conditions? The inverter will give its highest output power into a unity power factor, resistive load. It will be capable of higher peak powers, but for short time only, with the restriction that the mean power is <3000VA, and the crest factor of the load is <3.
I think you are totally off track here.
The inverter delivers the voltage, the load reacts with a current. Ofc the inverter can NOT deliver the current over the full sine wave, there is no loadâŚ
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