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Discussion Starter · #1 · (Edited)
Hi,

I'm l looking to buy a Firman generator. 2 model is interesting me.

T08072 TRI FUEL 10000/8000W PORTABLE GENERATOR
P08008 GAS PORTABLE GENERATOR 10000W

Both are 10000w/8000w, motor is 439cc on both, looking the same except for the LPN input. This is looking fine.

But one thing is bothering me, T08072 is 11%-14% THD and the P08008 is 25%.
Is it an error in the description or the T08072 really is better ?

Thank you for any advice
Yannick
 

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They are different. See...


 

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Oops, I searched for P08003 instead of P08008. The specs on the Firman website are here...

 

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Yes I saw the spec on their website. Looks like the generator part of T08072 is almost 2x better than P08008 for THD.
Okay. I don't see any reason to doubt the THD differences in their specs. Conventional (non-inverter) generators can be all over the place as far as THD goes. It really depends on the market they are shooting for. Construction class generators can have 20%+ THD and be just fine. A table saw just doesn't care about THD.

Conventional generators can be made with less than 5% THD with careful design and windings.
 

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Discussion Starter · #6 ·
Problem is I'll buy it for my house, including electronic device such as computers. In fact I already bought the p08008 but the box is still not open. I'm thinking of return it to take the 11-14% thd instead. Those computers are all connected throw a UPS but I dont know how those UPS will react with 25% THD
 

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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.
 

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I have a APC Smart UPS 1400 add a Smart ups 750. Like you said, if THD is too high and switch on battery anyway, this doesn't help me at all. Best way should do to try it to see what is really going. on.
First, see if your generator is putting out power that is within limits of what the UPS can deem "good". So that means 120V @ 60Hz nominal. Oftentimes, the UPS may not accept power because the frequency (Hz) is way off... a common occurrence with non-inverter generators with an improper speed governor setting.

If the generator output is within norms and the UPS still occasionally switch between line and battery backup, the input sensitivity can be changed on APC Smart-UPS series. All my APCs are set to medium. I've had no hesitation from it when running off generator power. APC says that if your load can tolerate generator power, you can lower the sensitivity to medium.

The Smart-UPS detects line voltage distortions such as spikes, notches, dips, and swells, as well as distortions caused by operation with inexpensive fuel-powered generators. By default, the UPS reacts to distortions by transferring to on-battery operation to protect the equipment that you are plugging into the UPS. Where power quality is poor, the UPS may frequently transfer to on-battery operation. Battery longevity and service life of the UPS may be conserved by reducing the sensitivity of the UPS, as long as your equipment can operate normally under the conditions detailed below. The sensitivity circuit monitors Total Harmonic Distortion(THD), Change in Voltage over Time(dv/dt), and Frequency (Hz) out of tolerance.

NOTE:
The sensitivity circuit looks at these three parameters (THD, dv/dt, Hz) and makes a quantitative analysis based on all three things using a proprietary algorithm.

There are not pre-defined limits for each parameter. (IE: There is no way to say "At medium sensitivity, the UPS will transfer at X percent THD.")
Source: Video: What are the different sensitivity settings of a Smart-UPS? - APC USA.
 

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I have a APC Smart UPS 1400 add a Smart ups 750. Like you said, if THD is too high and switch on battery anyway, this doesn't help me at all. Best way should do to try it to see what is really going. on.
Unless you buy a "generator friendly UPS" as I have and the UPS plays nicely with my conventional (non-inverter) generator. But I only have my computer and 3d printer plugged in the UPS and I leave the rest of the devices(tvs, lights, internet router/phone, microwave, kettle) plugged directly in to the generator. And so far everything has been ok, but I haven't needed to use the generator that much yet during powercuts.
 

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My UPS APC and Cyperpower care about voltage and frequency (60Hz =+- 2Hz) Get those right and what ever THD is there, they have not cared. The THD story is more than distortion from the power source but includes the trash reflected back onto the line from reactive loads.
 

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First, see if your generator is putting out power that is within limits of what the UPS can deem "good". So that means 120V @ 60Hz nominal. Oftentimes, the UPS may not accept power because the frequency (Hz) is way off... a common occurrence with non-inverter generators with an improper speed governor setting.
If I remember rightly Ive seen my generator go off by 2hz, from 50 to 52, but I'll keep a eye on that the next time I run my generator as I know the voltage jumps about by about 5volts either side when loaded
 

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If I remember rightly Ive seen my generator go off by 2hz, from 50 to 52, but I'll keep a eye on that the next time I run my generator as I know the voltage jumps about by about 5volts either way when loaded
Should be fine for 50Hz.

My generator unloaded, goes to 62-63Hz. So far, my UPSs seems to tolerate it. But that's only for a short time. Once I've got most of the load up and running, the gen droops down to around 60-61Hz... totally within spec.
 

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Discussion Starter · #15 ·
Ok more clear now. Wrong voltage and frequency is what ups doesn't like. THD is less important that I thought.

To adjust the frequency---> motor speed. Something like that:

And for voltage---->something like that:

In addition, for the THD, all pc's are switching power supply. Meaning they are first filtered and rectified to DC and raise to about 400v dc, and switching it in a transformer as square wave (already noisy). And re-lower down at acceptable voltage with higher current, again, filtered. Knowing this. I agree that THD isn't the worst problem for electronic.

So, turn off the main breaker, start the generator, be sure voltage and frequency is on spec. When all is looking good, turn on the generator breaker.
Thank you for your advice et your time very appreciated.

Yannick
 

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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%.
 

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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.
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". :rolleyes:
 

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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". :rolleyes:

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.
 

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Again, there are two problems that I believe we can pull out of an entire dirty power discussion and do something about ... THD and Surges.

Unfortunately, talking about THD only from the gen side is making it sound like all problems are cured by getting a low-THD gen. They aren't ... everything in your house is dumping THD back out on the line (smps devices are among the worst by their very design, UPS, etc). If you were to argue that THD is bad, then your electronics are getting eaten up by the THD that is already in your house wiring, and not from the gen (how often do you run your gen, after all?). My belief (IMHO) is that THD is the lesser evil.

Think of the house wiring as a pond, and everything in the house has dirtied up the water already, because almost every device is dumping THD into that pond. Your "clean power" gen is dumping clean water into the pond, which immediately turns dirty (clean + dirty = dirty). Your "dirty power" gen is dumping dirty water into the pond (dirty + dirty = dirty). In commercial/industrial sites, they can clean their entire lake of dirty water with specialized equipment, and end up w/ less dirty water. The grid requires this of them.

When you transfer back to the grid, clean water is getting dumped into your dirty pond by the grid, along with the occasional (and free) instant-death-dealing surge. Kiss that HVAC goodby, not slowly from THD, but instantly, from a single surge event.

What can you do about a THD problem? I'm no EE, but as best I can tell, little to nothing ... what commercial/industrial sites can do, with an army of EE's at their disposal, isn't what we can do at the home level. Can you stick a $200 UPS in front of something that is supposedly susceptible to high-THD? Sure, but now that same UPS is dumping more THD back out on the wiring.

The only clean pool of water I can find is the small puddle that is internal to your device that has an SMPS in front of it; this is a DC puddle. Behind it, that SMPS is dumping THD back out into the AC house pond. Our prevailing codes in the US say that this is a one-way street only ... clean up incoming power, but do nothing about outgoing power.

What can you do about the Surge problem? I'm no EE, but as best I can tell, you simply install an SPD device in your load center, and smaller SPD devices in front of susceptible devices. Have an HVAC that costs 10k? Stick an SPD in front of it. Same w/ little $$$ electronics. No THD being dumped back out on the line, from your SPD effort.

So, do you get a low-THD gen? Absolutely no problem, if you are willing to trade fuel choice, authorized dealer network only, complexity, low-watts, and other issues for the chance to dump clean water into your dirty pond water. Economics-wise, it's better for the economy when you buy a $5000 honda, than a $1000 westinghouse. Ignore those trade-offs (and issues, costs & headaches) behind the curtain ...

As I run my gen's for 1000 hrs / yr, out in rural nowhere, I too, am excited by "inverter technology" ... but I don't know where the marketplace will go with their designs. A "jet-engine complexity" Honda, and other similar designs, are the first iteration, and they don't fit rural applications easily. They are more like a JD tractor that only the dealer can repair, if you can get it there. I've seen only one open-frame gen w/ an inverter panel, but it had none of the features of a westinghouse non-inverter open-frame.

I took the $4000 in savings, bought $300 worth of SPD's, and called it good, as surges were infinitely more fixable than THD. 5 years later, it's still good. If my $300 TV does die a horrible death (and I can somehow trace it to THD, in some form of electronic autopsy), I still have $3700 left over to buy another $300 TV ...

YMMV, and watch out for those marketing folks ...
 

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While it's true that THD will increase with distance from the utility or generator interface (Point of Common Coupling, or PCC), because of voltage sag; and while it's also true that the practical effect of poor THD will vary depending upon the current demand of a load (smaller loads generally being affected less), I'm not sure the dirty pond metaphor is very apt. Plug in a load and the source of the electrons traversing the circuit powering that load come from the utility interface (or generator)... not a mishmash of electrons from throughout the house wiring.

If the dirty pond analogy were accurate, the entire grid would have very poor THD as all those homes and businesses polluted the bigger pond. Fortunately, it doesn't quite work that way. Utilities typically provide grid power with a THD of 5% or less.

People can do whatever they want, of course. But make no mistake... THD is a thing. Part of the problem is that generators have been around for many decades and most of our individual experiences with them is that they "just work." And they usually work just fine for a very long time. But most of the loads that we collectively powered with those generators for all those years were linear loads. And linear loads aren't generally affected by high THD.

Nowadays we have a proliferation of non-linear loads. And non-linear loads are very much affected by high THD.
 
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