ProSoundWeb Community

Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => AC Power and Grounding => Topic started by: Mark McFarlane on November 17, 2013, 12:50:19 pm

Title: Generator power - Why square waves are bad, and other things to look out for.
Post by: Mark McFarlane on November 17, 2013, 12:50:19 pm
I have a gig coming up in a few weeks where the venue will provide generator power.  It's actually a huge self-contained generator in a mini-building on skids. Looks like they needed a crane to get it off the truck.  I saw the unit last night but it was dark and couldn't see what was really inside, but it looked very serious/professional...., more akin to a power plant on drilling rig than a job site generator.  I should have gone home and got my scope...

Anyway,... I've seen several posts that said 'avoid construction-grade generators because they generate square-wave power rather than sine wave power like one would get off the grid.

What specifically is 'bad' about running our audio systems on square wave power?

Out of fear/ignorance I've asked the venue today if they can provide 60 amps off the grid using a local light tower as the source, which is how I have operated in the past (the event is on a cricket field).  The local power is extremely stable. I don't really want to decline the gig, it's a grand in my pocket for a days work, but I also don't want to risk damaging the 50-70K of gear I'll be bringing: digital console, active speakers, lots of fiber connecting things...
Title: Re: Generator power - Why square waves are bad, and other things to look out for.
Post by: John Roberts {JR} on November 17, 2013, 01:08:52 pm
Nothing as long as there are enough volts and enough amps.

One possible issue with actual square wave power is that many products pull power from the top of the sine-wave and average reading voltmeters will read a 200v p-p sinewave as only 70V average. The 200V p-p square wave will read 100V average. So be careful with how you meter it.

JR

PS: Generators generally make sine waves too, while the sine wave may be easily clipped or flat topped if generator runs out of poop.   
Title: Re: Generator power - Why square waves are bad, and other things to look out for.
Post by: Steve M Smith on November 17, 2013, 01:22:23 pm
There is no such thing as a generator which generates square waves.  It's not possible.  When you rotate a coil of wire through a magnetic field, it can only produce a sine wave.


Steve.
Title: Re: Generator power - Why square waves are bad, and other things to look out for.
Post by: Mark McFarlane on November 17, 2013, 01:46:52 pm
There is no such thing as a generator which generates square waves.  It's not possible.  When you rotate a coil of wire through a magnetic field, it can only produce a sine wave.


Steve.

Perhaps I misunderstood why many people have posted 'don't use construction-grade generators'.
Title: Re: Generator power - Why square waves are bad, and other things to look out for.
Post by: Steve M Smith on November 17, 2013, 01:58:59 pm
Perhaps many people have had bad experiences using generators which were too small for the task which just happened to be construction grade - whatever that means. 

It probably means that the voltage regulation is not as good as a better generator as it's not so important for electric motors in construction power tools.

Any generator, if overloaded, will produce a waveform which is not sinusoidal but I would be very surprised if it could produce an actual square wave. More likely a sine wave with the top clipped a bit like overdriving an amplifier. 


Steve.
Title: Re: Generator power - Why square waves are bad, and other things to look out for.
Post by: Tim McCulloch on November 17, 2013, 02:44:44 pm
Perhaps I misunderstood why many people have posted 'don't use construction-grade generators'.

Acoustic operating level (engine noise), line frequency stability and voltage regulation.  State-side, the nice white boxes on the trailers look the same, but the genset regulation and control systems are different.  Crystal controlled frequency vs. mechanical governor...

If your loading is low enough that the genset engine doesn't have to respond to dynamic changes, you'll be fine.  With smaller construction grade gensets and loading that is a larger percentage of capacity, neither voltage nor frequency will be stable.  Some newer equipment will not care, but other devices can lock up or shut down.

My best *guess*, from thousands of miles away and a vague description based on what you could see... you'll probably be okay.  There is likely a name and contact info on the side of the container.  You might contact them if a local technician is not available to assist you.
Title: Re: Generator power - Why square waves are bad, and other things to look out for.
Post by: g'bye, Dick Rees on November 17, 2013, 02:52:23 pm
I have a gig coming up in a few weeks where the venue will provide generator power.  It's actually a huge self-contained generator in a mini-building on skids. Looks like they needed a crane to get it off the truck.  I saw the unit last night but it was dark and couldn't see what was really inside, but it looked very serious/professional...., more akin to a power plant on drilling rig than a job site generator.  I should have gone home and got my scope...

Anyway,... I've seen several posts that said 'avoid construction-grade generators because they generate square-wave power rather than sine wave power like one would get off the grid.

What specifically is 'bad' about running our audio systems on square wave power?

Out of fear/ignorance I've asked the venue today if they can provide 60 amps off the grid using a local light tower as the source, which is how I have operated in the past (the event is on a cricket field).  The local power is extremely stable. I don't really want to decline the gig, it's a grand in my pocket for a days work, but I also don't want to risk damaging the 50-70K of gear I'll be bringing: digital console, active speakers, lots of fiber connecting things...

http://www.tripplite.com/en/products/model.cfm?txtModelID=3053

For your critical gear.
Title: Re: Generator power - Why square waves are bad, and other things to look out for.
Post by: Mike Sokol on November 17, 2013, 03:06:41 pm
There is no such thing as a generator which generates square waves.  It's not possible.  When you rotate a coil of wire through a magnetic field, it can only produce a sine wave.
Steve.
Correct, all generators make sine ways naturally. Only a battery powered inverter makes square waves, with the better ones making modified square waves and the really expensive ones making true sine waves. Even a Honda inverter-generator (suitcase type) makes a modified square wave output, but it looks very much like a sine wave on a scope and should never cause a problem. A cheap square wave inverter will bleed a lot of buzzing noise into your audio gear and could cause interference with RF gear at times just due to all the harmonics running around.

One thing you may find in a construction grade generator is a lack of close RPM control, especially under load. So the frequency could vary up and down a bit, maybe 58 to 62 Hz depending on load. This should not be a problem for power amps or regular stage gear, but if an artist has a Hammond B-3 organ, that could cause the organ tuning to go up and down a lot in the middle of a set. That's because old-school Hammond organs run off of a synchronous clock-motor and need the line power at exactly 60 Hz to play in tune. The large Volvo generators we use for high-end concerts have some sort of crystal locked governor that holds the engine speed to the RPM required for exactly 60 Hz frequency.

Do make sure that the neutral and ground bus on the generator are bonded together and connected to an 8-ft ground rod driven next to the generator. And always check for underground wiring or plumbing before driving any ground rods. I would make the ground rod the responsibility of the company putting in the generator rather than messing with it yourself.   
Title: Re: Generator power - Why square waves are bad, and other things to look out for.
Post by: Mark McFarlane on November 17, 2013, 03:30:02 pm
Thanks everyone for the help. It sounds like I'll be alright.  The generator looks large enough to run several homes (you could probably park a Suburban inside) so there shouldn't be a load problem for my gear and some lights. I'll drive by tomorrow and if it's still onsite do some note/photo taking.  It was amazingly quiet for the size.

I don't think I have access to buy something like the Tripplite in time, but I have a 5amp voltage stabilizer I used to use in my darkroom decades ago that I could run my 01V96 on, although I'd need to modify it to add an earth ground.

Unfortunately, no B-3s at this gig.
Title: Re: Generator power - Why square waves are bad, and other things to look out for.
Post by: Steve M Smith on November 17, 2013, 04:22:30 pm
I have a 5amp voltage stabilizer I used to use in my darkroom decades ago that I could run my 01V96 on, although I'd need to modify it to add an earth ground.

I suspect that it has a switch mode power supply which is immune from voltage, frequency and waveform variations so probably wouldn't be much of an advantage.

I still have a darkroom but have never had the need to stabilise the supply for my enlarger.


Steve.
Title: Re: Generator power - Why square waves are bad, and other things to look out for.
Post by: Scott Wagner on November 17, 2013, 04:58:21 pm
Based upon your description of the genset, it's not a "construction" genny.  You'll be fine.  Bring your metering equipment to verify (as you should always do).
Title: Re: Generator power - Why square waves are bad, and other things to look out for.
Post by: Mike Sokol on November 17, 2013, 06:39:57 pm
Unfortunately, no B3s at this gig.

Too bad... I've been teaching sound mixing seminars at a bunch of Southern Baptist Churches lately, and many of them have a Hammond B3 and a pair of 122 Leslie speakers. I'm thrilled to hear one with a great player.

I myself am a B3 player and still have my 1954 model B2 which I converted to a B3 using Hammond factory parts back in the 70's. And it's still on its original tubes from 1954. Talk about solid engineering..

Back OT...
Title: Re: Generator power - Why square waves are bad, and other things to look out for.
Post by: Tom Bourke on November 17, 2013, 09:37:40 pm
Even a Honda inverter-generator (suitcase type) makes a modified square wave output, but it looks very much like a sine wave on a scope and should never cause a problem.
My understanding was the Honda i and EU series were VERY clean sine wave.  I have even heard  ham radio operators claiming they were better than what comes out of your wall!
Title: Re: Generator power - Why square waves are bad, and other things to look out for.
Post by: g'bye, Dick Rees on November 17, 2013, 10:28:18 pm
My understanding was the Honda i and EU series were VERY clean sine wave.  I have even heard  ham radio operators claiming they were better than what comes out of your wall!

Here are a couple shots of a 'scope trace on my Honda EU2000i, the first at idle, the second under a static load (1KW work light).

Title: Re: Generator power - Why square waves are bad, and other things to look out for.
Post by: Mike Sokol on November 18, 2013, 01:44:55 am
My understanding was the Honda i and EU series were VERY clean sine wave.  I have even heard  ham radio operators claiming they were better than what comes out of your wall!
I've read a lot of forums where they claim the Honda generators make a "pure" sine wave. But in the interest of accuracy, I think that while for all practical purposes they make a pretty darn good sine wave, they still have a few percent of THD. As seen on the scope they "look" like a "pure" sine wave, but it would be interesting to run a FFT on the waveform under various loads. I really don't think they're "cleaner" than power company AC from synchronized generators (excepting for transient junk, of course).

I've seen a few scope traces of "lesser" inverter-generators from cheap import manufacturers, and they really look like stepped sine wave outputs. Not pretty and with tons of harmonics. Still better than the cheap square wave inverters, but not as good as a Honda output which appears to be the gold standard of consumer inverter-generators.
Title: Re: Generator power - Why square waves are bad, and other things to look out for.
Post by: g'bye, Dick Rees on November 18, 2013, 09:04:10 am
I've read a lot of forums where they claim the Honda generators make a "pure" sine wave. But in the interest of accuracy, I think that while for all practical purposes they make a pretty darn good sine wave, they still have a few percent of THD. As seen on the scope they "look" like a "pure" sine wave, but it would be interesting to run a FFT on the waveform under various loads. I really don't think they're "cleaner" than power company AC from synchronized generators (excepting for transient junk, of course).

I've seen a few scope traces of "lesser" inverter-generators from cheap import manufacturers, and they really look like stepped sine wave outputs. Not pretty and with tons of harmonics. Still better than the cheap square wave inverters, but not as good as a Honda output which appears to be the gold standard of consumer inverter-generators.

I'll look for my notes.  We did take some THD readings and IIRC, they were comparable or cleaner than the house power from the grid.
Title: Re: Generator power - Why square waves are bad, and other things to look out for.
Post by: John Roberts {JR} on November 18, 2013, 10:34:18 am
I'll look for my notes.  We did take some THD readings and IIRC, they were comparable or cleaner than the house power from the grid.

As long as the waveform is not so distorted that the top is flattened and delivering inadequate voltage when rectified, the purity is not a significant concern.

JR

Title: Re: Generator power - Why square waves are bad, and other things to look out for.
Post by: g'bye, Dick Rees on November 18, 2013, 11:23:33 am
As long as the waveform is not so distorted that the top is flattened and delivering inadequate voltage when rectified, the purity is not a significant concern.

JR

A spouse mandated "house-cleaning" has apparently displaced my notepad from the tests.  IIRC, THD was more remarkable for being low than anything else.  I believe I was given the right to gloat about the power from the Honda.

I will not throw out a number until I can find the notes or see if the others present recall the range of THD.  All I remember is that it produced grins all around.
Title: Re: Generator power - Why square waves are bad, and other things to look out for.
Post by: John Roberts {JR} on November 18, 2013, 12:43:06 pm
A transformer is a bandpass device so extreme purity of the waveform is not a concern. If I was selling Honda generators I might hype that, but don't be confused into thinking this matters in the margin, only when distortion is extreme. 

JR

PS: I am  surely repeating myself by now so see ya later...  8)
Title: Re: Generator power - Why square waves are bad, and other things to look out for.
Post by: g'bye, Dick Rees on November 18, 2013, 01:04:03 pm
A transformer is a bandpass device so extreme purity of the waveform is not a concern. If I was selling Honda generators I might hype that, but don't be confused into thinking this matters in the margin, only when distortion is extreme. 

JR

PS: I am  surely repeating myself by now so see ya later...  8)

OK.  Just for s***s and giggles, here's a shot of the trace of an APC UPS, $165 from Office Max.  Two different resolutions...
Title: Re: Generator power - Why square waves are bad, and other things to look out for.
Post by: Mac Kerr on November 18, 2013, 01:33:44 pm
OK.  Just for s***s and giggles, here's a shot of the trace of an APC UPS, $165 from Office Max.  Two different resolutions...

Now the question is, does that matter any more than the stepping of a digitally sampled audio signal that is being converted to analog?

Mac
Title: Re: Generator power - Why square waves are bad, and other things to look out for.
Post by: g'bye, Dick Rees on November 18, 2013, 01:37:20 pm
Now the question is, does that matter any more than the stepping of a digitally sampled audio signal that is being converted to analog?

Mac

I was told not.  But as I said, it was posted for giggles.
Title: Re: Generator power - Why square waves are bad, and other things to look out for.
Post by: Tim McCulloch on November 18, 2013, 01:46:00 pm
I was told not.  But as I said, it was posted for giggles.

I worked with Giggles.  She was in the lighting dept at Baltimore Fishmarket (drinking in Disneyland) before it went under.
Title: Re: Generator power - Why square waves are bad, and other things to look out for.
Post by: John Roberts {JR} on November 18, 2013, 02:23:17 pm
OK.  Just for s***s and giggles, here's a shot of the trace of an APC UPS, $165 from Office Max.  Two different resolutions...

And what in the world does this have to do with generators?

JR
Title: Re: Generator power - Why square waves are bad, and other things to look out for.
Post by: Mike Sokol on November 18, 2013, 07:36:26 pm
Now the question is, does that matter any more than the stepping of a digitally sampled audio signal that is being converted to analog?

Mac

From the times I've been forced to run sound gear from a modified sine wave inverter (a parade float running from a cheap inverter in a pickup truck), I've noted there's a lot of extra buzzing going on just from all those harmonics in the waveform. In a D/A converter there's going to be some sort of low-pass filter to get rid of the ultrasonic frequencies (at least there should be). And most of the high frequency harmonics of the stepped sine output would be absorbed by the gear's power transformer. But those harmonics are like really bad triac lighting dimmers, and get into every guitar and high-impedance audio input.

I would agree that the Inverter generators from Honda are simply amazing with their clean AC output, and it's that same inverter function that allows them to run in eco-mode which throttles back the engine RPM to converse fuel when the electrical load is low. As noted earlier in this thread, a standard generator has to run at a constant RPM (1800 or 3600) to keep the frequency at 60 Hz. But the Honda's are smart enough that you can parallel two of them together and their outputs will phase-lock to each other at 60 Hz. Talk about cool stuff...
Title: Re: Generator power - Why square waves are bad, and other things to look out for.
Post by: g'bye, Dick Rees on November 18, 2013, 07:42:34 pm
And what in the world does this have to do with generators?

JR

Nothing.  It has everything to do with the interest in the "purity" of the wave forms.  I presented the pic simply as a visual representation of what some inverters do.

Title: Re: Generator power - Why square waves are bad, and other things to look out for.
Post by: John Roberts {JR} on November 18, 2013, 07:59:05 pm
Nothing.  It has everything to do with the interest in the "purity" of the wave forms.  I presented the pic simply as a visual representation of what some inverters do.
The natural output of a motor/generator is sinusoidal. Any distortion or reduced purity is likely related to load and source impedance, typically resulting in flattened waveform top/bottoms.

Inverters are a completely different technology converting DC to AC often with step wise waveform approximations. This extraneous information is IMO not germane to using a generator.

I will repeat my original advice that a distorted (I.E. flat topped) generator voltage waveform may not measure similarly to a clean sine wave. Care should be taken regarding measurements especially if used to make voltage adjustments. 

JR