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Generators & Electric Motors General Discussion

Total Harmonic Distortion of Portable Generators


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  #1  
Old 02-08-2018, 03:05:26 AM
GeneratorNewb GeneratorNewb is offline
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Default Total Harmonic Distortion of Portable Generators

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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Old 02-08-2018, 03:54:27 AM
Thaumaturge Thaumaturge is offline
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Default Re: Total Harmonic Distortion of Portable Generators

Most new electronics won't be affected by 9% THD. Almost all now use switching supplies that first rectify incoming line voltage and turn it into DC. This is then switched into a small transformer and then rectified again to the final lower voltage. Feedback circuit controls high side switching to regulate output.
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Old 02-08-2018, 08:01:25 AM
GeneratorNewb GeneratorNewb is offline
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Default Re: Total Harmonic Distortion of Portable Generators

Thanks for the response. Do newer furnaces typically have switching power supplies as well? I'll be installing an 80% 2 stage, variable speed York furnace in a few months.

In your opinion, does it make sense to switch to a generator with 3-6% thd instead? In a power out yin only concerned with running lights, garage door openers, well pump, some led lights, furnace, and maybe a TV with router/modem.
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Old 02-08-2018, 08:30:09 AM
armandh armandh is offline
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Default Re: Total Harmonic Distortion of Portable Generators

the generator specifications I looked up show bonded neutral
this is not to code for an interlocking breaker transfer.

furnace controls are a bit tricky they are usually built to run on the mains
and anything else is trial and error. ask your HVAC contractor

IMHO
the more traditional generator excitation, like the power company uses,
has lower noise than the cap regulated cheap ones. YMMV

as to the furnace, a lot depends on how the 2 stage firing is accomplished

electronic or 2 speed wound motors
how well the electronic speed controls are made
tell the contractor you want very traditional furnace controls if possible.
usually the 80% ones are OK, when you get to 90% things are way more complex.
again ask the HVAC contractor I ran a pair of york 80% with no problem
originally on a 5KW Coleman
but those furnace models are 25 years old.

Ive seen flame controls that wont work with the hot and neutral reversed.

worst case an isolation transformer for the furnace might kill the noise
I have never had a problem [yet] but I have not tested my new [Goodman brand] furnace on the generator!
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Last edited by armandh; 02-08-2018 at 09:04:06 AM.
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Old 02-08-2018, 09:33:15 AM
GeneratorNewb GeneratorNewb is offline
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Default Re: Total Harmonic Distortion of Portable Generators

In your opinion, how should I ground the generator with an interlock switch? In your mind, does it make sense to sell the 9% thd generator for thr Briggs generator that states 3-6%? Thanks!
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Old 02-08-2018, 10:05:38 AM
Jack Hottel Jack Hottel is offline
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Default Re: Total Harmonic Distortion of Portable Generators

You have encountered one of the paradoxes of using generators. OSHA requires that portable generators have their neutral bonded to generator frame for use on job sites. The NEC requires that in a residence the neutral to ground bond be connected in the service disconnect panel and that there be no others. Furnaces using flame rectification require a neutral to ground bond to operate. There are other requirements, see attachment.
Before I purchased either a generator or a furnace I would make sure that the respective contractors would guarantee that the units proposed would work together, and that operation would be demonstrated before they were paid. Good Luck!
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Old 02-08-2018, 10:12:48 AM
GeneratorNewb GeneratorNewb is offline
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Default Re: Total Harmonic Distortion of Portable Generators

I apologize, but some of this is simply over my head. I know the generator ground is bonded to the frame but will this not work with an interlock kit? I'm not sure if it is also grounded inside the panel. Thanks a lot!
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Old 02-08-2018, 03:14:18 PM
Power Power is online now
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Default Re: Total Harmonic Distortion of Portable Generators

Quote:
Originally Posted by GeneratorNewb View Post
In your opinion, how should I ground the generator with an interlock switch? In your mind, does it make sense to sell the 9% thd generator for thr Briggs generator that states 3-6%? Thanks!
In my opinion, you follow code, which requires an unbonded 4 wire system. Then you get it inspected, so if there are any issues in future, you are covered.

I am friends with a heating contractor. During every storm, he gets extremely busy with no heat calls. He usually finds:
1. the flame control system will not work with generator setup.
2. the control board has failed.

He is a licensed plumber, not an electrician, so does not diagnose electrical issues.

The boards on high efficiency systems usually have to come from manufacturer, they are not stocked locally. He says after a storm, often talking about weeks for board delivery. People cannot wait that long, so he replaces furnace. $$$$!

My suggestion would be to find out if there are any models local plumbing supply houses stock the electronic parts for, and buy one of those.

Try to get assurances that the unit you choose will work on a standby generator (unlikely any manufacturer will give any guarantee, and most void warranty if unit is powered from a portable generator)
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Old 02-08-2018, 03:42:08 PM
GeneratorNewb GeneratorNewb is offline
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Default Re: Total Harmonic Distortion of Portable Generators

Do you suspect that the control board failures are due to thd issues or something else?
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Old 02-08-2018, 04:07:01 PM
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Default Re: Total Harmonic Distortion of Portable Generators

It's a great topic regarding furnace control boards on these new furnaces.
I had a Bryant 80% er put in last year to avoid the cost and complexity of the higher efficiency models.

Chatting with the installer he responded that I did good. He went on that the 90+ units have a $1200 control board. And,,, if it goes out it usually takes out the same board in the central air unit and he dreads telling people that the bill is $2700. They're soo sensitive, a lightning hit blocks away can kill them.

I asked about generators and being a sharp guy he said they don't like the choppy waveform of the newer inverter based units. Said they'll usually just not start but he's seen some smoke a device or two on the boards. So in my case with my 70's style generator with no electronics I'll cross my fingers and flip the switch.

*As to my nifty new $7000 furnace and AC, it's noisy as hell and only required two visits in the last year. The blower motor cooked itself then a fuse on the board popped. What do I expect. An American assembled machine with every component made in China. My old unit was 37 years old and never sneezed.

Don't buy Bryant, Trane, Carrier and many others as they are all the same machine painted different colors and are all Chinese.
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