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[Guy Holt]

I've heard this suggestion before but I don't understand the physics behind it. Switch mode power supplies generally have a line-voltage rectifier and capacitor-input filter as their first stage. It seems to me that this topology would be relatively insensitive to input voltage waveform.

Since SMPSs depend on peak voltage to charge their smoothing capacitors, they do not operate well on a modified square wave. In his article "The “Hows” and “Whys” of Inverters and Inverter Generators," John De Armond, clearly explians why that is the case using as his example a simple three step modified square wave inverter generator: the Honda EX350.

        Output waveform of a Honda EX350 square wave inverter generator

The illustration above is an oscilloscope shot of the output of the Honda EX350. Notice the RMS voltage indication on the right side - 120 volts even though the peak voltage is only 142 volts. For a true sine wave, the peak voltage would be 120 x 1.414 = 169 volts. This difference in peak voltage is what makes or breaks the operation of modified square wave inverter generators with the SMPSs of electronic equipment where they work fine on construction sites.

Voltage and the current output waveforms of a Honda EX350 square wave inverter generator
powering 300W incandescent light

The photo above shows a scope shot of both the voltage and the current output of this generator driving a resistive load (a 300 watt incandescent light.) As you see, a modified square wave works well for a resistive load like an incandescent light. Things get a whole lot more interesting when one connects a SMPSs to the generator. As you can see in photo below the solid-state ballast of the fluorescent lamp slightly distorts the voltage waveform (creates a spike) and creates all kinds of current oscillation. This kind of harmonic activity can cause a noticeable audio buzz, equipment to malfunction, or shut off.

Voltage and the current output waveforms of a Honda EX350 square wave inverter generator powering fluorescent light

Another common problem with modified square wave generators is that they can starve electronic equipment of power. In his article, John De Armond illustrates this problem by comparing how a battery charger works on grid power verses how it works on a modified square wave power supply. 
To paraphrase him a battery charger typically consists of a transformer, a rectifier, and support electronics like charge control circuitry. On each half-cycle of the 60 hz line voltage, the voltage first increases and then decreases in the shape of a sine. The transformer secondary of the battery charger follows this voltage. Connected to the secondary is the rectifier that converts the AC to DC (rectified AC) for battery charging. Only when the instantaneous AC voltage exceeds the battery voltage plus the 0.7 voltage drop of the rectifier does current flow to charge the batteries. The illustration below shows this effect. The two lines at “1” and “2” mark on the voltage sine wave where the rectifier starts conducting and causing current to flow.

Problems arise when a charger of this type is connected to a modified square wave inverter. Recall from the first photo above that the peak voltage of a modified square wave does not rise as high as a sine wave (142 volts verses the 169 volts of a true sine wave.) The horizontal line in the illustration above shows about where the square wave would reach. In this particular case, the square wave would never reach a voltage sufficient to make the rectifier conduct and so the battery would never charge even though power is connected, the LED indicators light up, and a true RMS voltmeter would indicate about 120 volts. This is another fundamental problem with modified square wave inverters in production applications. Audio/video production equipment, computers, and battery chargers require a nearly pure (low distortion) sine wave input.

If there are a preponderance of gear that used SMPS without power factor correction, the harmonic currents drawn by SMPSs can so distort the voltage waveform to the point that, as in the case of the battery charger above, the peak voltage is never sufficient to charge their smoothing capacitors. Since this is getting off topic, see my white paper on the use of portable generators in motion picture production  available at  http://www.screenlightandgrip.com/html/emailnewsletter_generators.html for details.

Guy Holt, Gaffer
ScreenLight & Grip
www.screenlightandgrip.com

[Stephen Swaffer]

So, to risk going aginst the grain, having been around production welders fromLincoln in some exotic applications up until a couple of years ago my curiousity was raised.  Lincoln has the ability to custom design a weld pulse to minimize spatter on wire welding , etc.  It would seem they could easily do a very good sine wave if so desired.

The welder Mike mentioned is not your dad's welder/generator.  It derives its weld output from a 3 phase stator feeding a bank of 6 diodes (see print at end of owners manual.)  # phase being easier to get a smoother rectified output from.  This goes to a thyristor bank that is controlled by a microprocessor for weld output.  The 120 V/ 240 V aux power is derived from a completely separate stator winding.  Since a sine wave is produced by the physics of a magnetic field rotating within the stator winding, why would the output not be a very stable sine wave?  Arguably better than the POCOs since you don't have everyone elses dirty non-linear loads connected?

The only connection between the welder and the genny is mechanical and possibly incidental magnetic.  I suppose if you are using the welder for some special effects lighting you might get some rpm variation that might cause issues, but otherwise you'd be happy with shore power at plus or minus 10%, why not this genny?

I am asking-not arguing:)

[g'bye, Dick Rees]

Hey, guys.

How about addressing the OP's situation instead of turning yet another thread into a master class?  As it stands now, it appears he may have a choice between a "real" generator unit (the G20) or the welder.

Anyone got any opinions on his situation?  And he' not on a ship...

[Tim McCulloch]

Hey, guys.

How about addressing the OP's situation instead of turning yet another thread into a master class?  As it stands now, it appears he may have a choice between a "real" generator unit (the G20) or the welder.

Anyone got any opinions on his situation?  And he' not on a ship...

Correct.

The OP's take should be:  1- get me a "real" generator, the kind intended to provide temporary office power, not for welding or powering small electrical tools; 2- the municipal government better make damn sure they have this grounded and otherwise 100% up to Code, or they'll be in the same boat as Salina, Kansas, where a young girl was electrocuted by MUNICIPAL wiring, installed and maintained by the MUNICIPALITY.  How much liability does the city manager/city council/mayor wish to assume?; 3- in light of 1 & 2 above, the OP should run, not walk, away from this potential problem child.

Seriously.

The event is being held at the mall for considerations that go beyond commons sense (like use a facility built for this purpose) and it's unlikely the City Fathers will be persuaded to move it.  The best that can be hoped for is a real generator with a real ground rod.  The hole bored for this rod can be filled with epoxy (which is what the tent vendor will do with their guy anchor holes).

[g'bye, Dick Rees]

Correct.

The OP's take should be:  1- get me a "real" generator, the kind intended to provide temporary office power, not for welding or powering small electrical tools; 2- the municipal government better make damn sure they have this grounded and otherwise 100% up to Code, or they'll be in the same boat as Salina, Kansas, where a young girl was electrocuted by MUNICIPAL wiring, installed and maintained by the MUNICIPALITY.  How much liability does the city manager/city council/mayor wish to assume?; 3- in light of 1 & 2 above, the OP should run, not walk, away from this potential problem child.

Seriously.

The event is being held at the mall for considerations that go beyond commons sense (like use a facility built for this purpose) and it's unlikely the City Fathers will be persuaded to move it.  The best that can be hoped for is a real generator with a real ground rod.  The hole bored for this rod can be filled with epoxy (which is what the tent vendor will do with their guy anchor holes).

Yup.  It's amazing how towns will cheap out and give themselves a pass on safety.  I WILL be inspected on any of my regular outdoor gigs, but when the Mayor's office calls up and wants me to do something for them, it's always for little or nothing and codes be damned.  After all, it's for the Mayor, so we don't need to follow code or insure public safety...

[Ray Aberle]

The hole bored for this rod can be filled with epoxy (which is what the tent vendor will do with their guy anchor holes).

well shit, there's the answer right here! I mean seriously, the mall's saying it's OK for the tenting people to poke holes in their purty pavement but not the gennie provider??

Jason, there is  very little -- make that nothing I could add beyond what Tim and Dick have already said. You've got a responsibility to do everything in your power to ensure safety on the job site. If something went sour, I know I wouldn't want to be thinking "If ONLY..."

Is there an outlet on the nearby lighting poles that you could plug a grounding line into? (I'd never thought of that as a possibility before, but saw a major local company doing that to ground power at a festival...)

-Ray

[Tim McCulloch]

well shit, there's the answer right here! I mean seriously, the mall's saying it's OK for the tenting people to poke holes in their purty pavement but not the gennie provider??

Jason, there is  very little -- make that nothing I could add beyond what Tim and Dick have already said. You've got a responsibility to do everything in your power to ensure safety on the job site. If something went sour, I know I wouldn't want to be thinking "If ONLY..."

Is there an outlet on the nearby lighting poles that you could plug a grounding line into? (I'd never thought of that as a possibility before, but saw a major local company doing that to ground power at a festival...)

-Ray

To be fair, the mall isn't quoted as approving this... but I've been down the parking lot a time or 2 and know that when it comes to putting up food tents, bingo tents, etc in Kansas, one guys them to big-ass stakes.  None of that flouncy "ballast" that moves when a 3 year old leans on it...  The tent contractor we see most often does one of 2 things:  installs metal screw-in anchors into the asphalt or concrete and simply leaves the "socket" in the pavement, or drills out holes and fills them with epoxy when they removed the stakes.  The screw-in anchors go in with a sealant, too.  For places that use the same setup for annual or seasonal events this is a very cool way to deal with tenting.

[g'bye, Dick Rees]

And to be fair, OP states that the stage is open.   No top.  I haven't seen any mention of tents from him. I think the critical list boils down to getting the proper generator, getting 811 to check the island, setting a ground rod, covering all cable runs and adding GFCI's to the stage power side of things.

All of which should be do-able.  Just make sure the city does their part and pays fair money for the safety gear.

[Stephen Swaffer]

From the owners manual:

The RANGER 250 can be permanently installed as a standby power unit for 240 VAC, 3 wire, single phase, 35 amp service. Connections must be made by a licensed electrician who can determine how the 120/240 VAC power can be adapted to the particular installation and comply with all codes.

Seems to me the manufacturer considers this a "proper" generator??

[Craig Hauber]

From the owners manual:

The RANGER 250 can be permanently installed as a standby power unit for 240 VAC, 3 wire, single phase, 35 amp service. Connections must be made by a licensed electrician who can determine how the 120/240 VAC power can be adapted to the particular installation and comply with all codes.

Seems to me the manufacturer considers this a "proper" generator??

All the welders I've dealt with have a "utility/accessory" outlet of some kind -but it doesn't deliver the full rated power of the machine (-that's primarily being fed into the welding portion of it.)
So you're left with a big noisy motor intended for 100A or more just to run one 30A output?!

-you're better off with Honda with only that much output -less fuel, less noise, less weight!

Although I would probably take the welder over one of those parking-lot light tower trailers -another municipality favorite for stage power.

[ProSoundWeb Community]