[bassguitarist1985]

Less than 5% is the NEC spec and almost a perfect sine wave. 25% THD is the lowest a modified sine wave inverter can achieve. 48.3% THD is a square wave! Its not advisable for sensitive electronics above 5%. Any conventional generator that can be that high is trash for powering a house. Construction site tools...eh I'm sure the tools aren't that picky.

UPS with Line Interactive topology will switch to battery with any THD over 5%. A UPS with Double Conversion "Online" where the incoming power is only meant to charge the internal batteries while the loads are being supplied by the batteries all the time (Incoming AC to DC, then back to AC for the load) is solely dependent on the input charger. Chances are with the Active Power Factor Correction circuit (run by an IC) will not tolerate much beyond 5% THD. It is not often you have a double conversion UPS with a Passive PFC circuit (basically a large capacitor at the input) anymore because such caps are expensive and often have a limited lifespan in such an application.

I'd go with an inverter generator if you want clean under 5% THD.

 

[bassguitarist1985]

Your research is accurate, but don't discount high THD. Over time high THD will cause failure in your PSUs. Harmonic voltage/current is not real power going to your load, just circulating in the system not actually doing anything other than contributing heat. Many SMPS with Active PFC circuitry can tolerate high THD to an extent, but over time it can wear out the control circuit operating the boost converter after the input filter/rectification stage from the incoming AC line.

Make sure you have the right voltage and frequency for the electrical end, and verify the prime mover is properly tuned so that the speed regulation is as tight as it can be. THD cannot be mitigated on conventional generators unless they are already rated less than 5%.

 

[bassguitarist1985]

That is one of the most misunderstood things about generator THD...damage from THD is often cumulative with electronics. People tend to say "well, it is working just fine with my high THD gen, perhaps making some slight noises, but working fine".

Yeah I've encountered this way too often with my small engine customers. Again many SMPS devices greater than 100 watts will have Active PFC input circuitry, and must comply with IEC61000-3-2 standard. Still they are susceptible to high THD that over time will damage the components inside. However generators do not adhere to that standard unless specifically stated in their spec sheet, and sooner or later, your expensive (sensitive) devices will fail on dirty power.

 

[bassguitarist1985]

I have no idea how the UPS increases the volts to 240 from the generators round abouts 230v without using battery power, because it doesnt click over to battery mode and the UPS inverter can't increase the voltage surely?

For small corrections like that a "Line Interactive" UPS uses a multi tap transformer called an autoformer. With the use of various relays inside it senses the voltage it will switch to a different tap on the autoformer to either step up or step down the incoming voltage to keep within a given normal range. However if the voltage moves beyond what the autoformer is capable of adjusting to then it will switch to battery power.


In a double conversion UPS the onboard battery charger charges the batteries all the time, and the loads are run by the internal inverter. Such a UPS for simplicity/reliability will have a large inductor and capacitor at its input to the charger called an LC Filter to handle any high THD and then rectify it to DC. Simple circuit and it works, but it sacrifices voltage/frequency range

Take the Triplite SU2200XLCD for example. The specs say a "nominal 100V AC; 110V AC; 120V AC; 127V AC" and either 50/60hz. Nominal 120V means 108-132VAC and either 50/60hz.

On a Cyberpower 1500PFCLCD Line Interactive, constant voltage fluctuations and speed droops on a generator cause it to switch to battery power because the sensing IC circuit for choosing the different autoformer taps/switching to battery power is getting too much noise from the high THD it cannot perform its function so it defaults to battery power. Eventually this unit will fail on generator power either because the IC sensing circuit fails, relay fails from too many cycles, and/or overheating of components not designed to handle it.



I suppose we should discuss PFC circuitry a bit as depending on the design topology, it can either tolerate high THD very well or be completely crippled by it.

Passive PFC: In SMPS is simply a large capacitor/inductor. Its referred to as the Pi filter. Cap is wired across the line/neutral, and the inductor is in series with the hot. Called an LC filter. It is then rectified to DC to be used by the switchmode portion of the PSU. The main advantages are simplicity, reliability and ruggedness, insensitivity to noise and surges, no generation of high-frequency electromagnetic interface (EMI), and no high-frequency switching losses. The main disadvantage is a narrow frequency/voltage operating range, and in greater than 100 watt power applications, those LC components get large and heavy.

***IOTA Engineering makes their DLS chargers/PSUs with this type of topology. I use their 120VAC to 12VDC 90A chargers to replenish my inverter battery bank. They handle high THD of a generator no problem, and the main limitation is the input voltage range can only be between 108-132VAC and frequency between 47-63hz. They have 240VAC variants with the same range too. ***If it were an Active PFC, then you would see specs like 50/60hz 100-240VAC input.

Active PFC: This circuitry use active electronics circuits, which contain devices like MOSFETs, BJTs, and IGBTs. There are so many topologies I could make a novel on it. The two main types are boost (increasing the line voltage on the output) and Buck (decreasing the voltage) on the output. Basically they use an integrated circuit microcontroller to run the MOSFETS turning them on/off really fast in a circuit with an inductor. This precisely controls the current/voltage output.

There is still basic passive filtering before the DC to DC Buck/Boost Stage I discussed above, but it is not made to be as robust to correct power factor since that work is being on by the IC circuit. If enough harmonic distortion enters that PSU, it will heat up those passive filtering components adversely, and potentially cause that IC circuit to malfunction/fail. When that happens any loads connected to it when that IC circuit lets go, then you got cascading damage that costs BIG $$$$.


This should explain why some devices handle THD well and others over time fail. It is a factor in failures on portable generator power, but it is Surge that kills more devices than THD. Its important to look at the specs of devices, and again low THD generators are far less likley to cause all this crap we are discussing. Very informative technical knowledge, but crap to the commoner not familiar with the science.

Cheers!

 

[bassguitarist1985]

Onan generators especially from that era are some of the best built generators you can find. Sadly the stuff they push out these days is more crap than ever. 6% at almost 3/4 load is not bad at all. I can't imagine it rising up much further closer to full load.

Keep that dinosaur rolling, very few generators made these days comes close to that build quality.