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Author Topic: Generator power - Why square waves are bad, and other things to look out for.  (Read 10966 times)

Mark McFarlane

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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...
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Mark McFarlane
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Turn down what's too loud.

John Roberts {JR}

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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.   
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Steve M Smith

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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.
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Mark McFarlane

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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'.
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Mark McFarlane
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Turn down what's too loud.

Steve M Smith

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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.
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Tim McCulloch

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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.
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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.
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Mike Sokol

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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.   
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Mark McFarlane

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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.
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Mark McFarlane
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Turn down what's too loud.

Steve M Smith

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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.
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