# Total Harmonic Distortion of Portable Generators



## Generator Newb (Feb 7, 2018)

Hello Everyone,

I just registered on this forum looking to obtain some insight on the total harmonic distortion (thd) metric for portable generators. I currently own a new Westinghouse WH7500E Generator which is 7500 continuous watts and 9k peak watts. However, it has come to my attention that it has a 9% thd. As such, I've become a bit paranoid that it will do damage to the electronics in my home and fail to adequately operate the variable speed blower on my furnace.

My panel is wired with a 30amp inlet and an interlock kit. 

My question is how a manufacturer can state a specified thd figure when it is pretty much all based on the rpms which are not electronically governed like standby or inverter generators.

I was considering buying a Briggs and Stratton 8k running/10k peak model because they state a thd of 3-6%.

Isnt thd largely dependent on different loads initiating and the rpm engine at that time? I don't understand how Briggs can claim a lower thd number than the Westinghouse model. Am I overthinking thing entire thing? 

Lastly, whether I keep the Westinghouse generator or move on to a Briggs, will the installation of an aftermarket natural gas kit have a material impact on the frequency or thd of these generators? I know there's an adjustment for the air/fuel ratio but I don't want to install it if it will negate the manufacturer's stated specs for thd. 

Thanks in advance for the information!


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## Generator Newb (Feb 7, 2018)

Any help would be greatly appreciated!


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## KRE (Nov 29, 2011)

First of all, no rope start generator will pass a <5% THD test, that is fact an most will not pass a <15% test. They tell you it will but the test they run is not to industry/utility standards. 
Fact there are gensets backing up wireless, hospitals, ect as well as parts of the gov that were bid to 5% THD or less an they will not pass a 15% test, that is pure fact!
Can you move the THD around with hertz, yes, but very little as THD is mostly influenced but the excitation system with closed loop systems being pure junk when your trying to control THD. A true PMG set is the only way to go, an no rope start gensets I know of are PMG types. 
You can clean up the sine-wave with caps an filters + isolation transformers, but why spend the time an money reinventing the wheel. Buy a quality gen set an be done with it. No matter the name on the box/crate if it's sold at a box store, I would not own it, period. 
Why trust you last line of defense to a cheap unit when you can buy quality an be done with it? Remember your going to pay for it one way or the other, you can pay for it up front on on the back side, but you will pay for it. The back side costing 2-5 times the front money. 
I've been at this for more that a week or two an can prove the horror storys, about what the MFG said vs what the cost of the gear they burn't up using faulty nto out right lies as info. If you have questions test it your self, you can rent test gear from many places, or for a quick an dirty method get a cheap 100Mz O-scope an look at the waves yourself. Make sure you test with an w/o load on both resistive an reactive loads, then change the power factors from leading to lagging an retest. 

Prime mover droop as very little to do with THD. Do not equate a 5% prime mover droop rating with THD, two different things altogether.
Berger power analyzers can be had for about $500.00-1K . Bergers are not the top of the line or the bottom, but they tell the truth everytime.


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## Generator Newb (Feb 7, 2018)

Thanks for the insight. Do you have a particular brand you recommend? So basically, without being electronically governed as with inverter generators or standby generators, any claim of less than 5% moot? 

I was considering going from my Westinghouse to a Briggs and Stratton as stated before in order to go from 9% thd to 3-6% but now you've called that all into question lol.


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## KRE (Nov 29, 2011)

Onan.
Electronic speed control has nothing to do with THD. THD is any distortion in the sine-wave caused by the excitation system, winding pitch angle, lamination/s an it's material make up, or compounded by the excitation system with elements of the load, aka none linear loading.


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## Generator Newb (Feb 7, 2018)

So if I don't want to cause any damage to the electronics in my home, does it make sense to only purchase an inverter generator or a standby generator?


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## KRE (Nov 29, 2011)

Generator Newb said:


> So if I don't want to cause any damage to the electronics in my home, does it make sense to only purchase an inverter generator or a standby generator?


Both have advantages an disadvantages. Neither are w/o issues when your dealing with rope start units. If you are that worried about THD I would just bite the bullet an buy a quality 10-15KW or larger unit an never look back.


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## Generator Newb (Feb 7, 2018)

Do you have any recommendations for a 7kwh+ unit with low thd? The portable Onan units I found on ebay are all less than that and obviously used.


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## KRE (Nov 29, 2011)

The used ONAN RV units are 4 times the quality of anything rope start no matter the maker or that makers reputation. I have installed many RV units on homes an every home owner has been more than pleased with them. Quite, stable under all loads, an safe for any application. Most are hook to automatic transfer switches, but some use a manual switch, while others use tie breakers. You can buy new ONAN RV units but they are not the cost of rope start stuff, but you get what your paying for in them. A good used unit maintained properly, will last 4x longer than the ones most hype as being the best rope start you can buy. Up front or the back side, it's your money.


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## bones1 (Nov 5, 2020)

How would you rate the large Winco generators in this regard? I believe they use Mecc alte brushless gear. I was looking for Onan 15K used for a long time but they are few and far between.


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## sportplumber (Nov 16, 2019)

I was disappointed to see the Onans are still 3600 rpm units. I think the older ones were 1800? Seems it is very hard to find an 1800 rpm unit these days. I always thought the lower rpm had advantages such as lower fuel consumption, sound and less likely to break since it is spinning slower.


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## tabora (Sep 6, 2018)

sportplumber said:


> I was disappointed to see the Onans are still 3600 rpm units. I think the older ones were 1800? Seems it is very hard to find an 1800 rpm unit these days. I always thought the lower rpm had advantages such as lower fuel consumption, sound and less likely to break since it is spinning slower.


Onan always made a mix of 1800 & 3600 RPM 60Hz units. My 1989 Onan is a 3600 RPM genset.

Cummins Onan still makes a number of 1800RPM (i.e. 4 Pole) generators. Use the product selection tool here to narrow their offerings down to your desired specs: Product Finder | Cummins Inc.

Here's an example of a 10K 1800RPM Diesel: Onan QD 10000 | Cummins Inc.
They also make 2880 RPM generators, for example this 7K gas unit: Onan QG 7000 | Cummins Inc.


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## bones1 (Nov 5, 2020)

There are a few 1800 rpm models on facebook marketplace


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## speedy2019 (Jan 29, 2019)

Someone said the main reason for electronics being damaged from generators is people dont look after them and they continue to run the gens when they are running out of gas, so they surge... And I honestly believe this, if people use and look after a generator properly, there shouldn't be a problem.


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