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Author Topic: Burning Man Generator  (Read 752 times)

Mike Sokol

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Burning Man Generator
« on: September 12, 2017, 06:56:13 am »

I just got this email from an RV technician who had hooked up generators at Burning Man. He doesn't seem to understand the basics of 3-phase power and how neutrals and bonding works which makes it very dangerous. I don't have time answer him in detail this morning, but at first blush it sounds like an open neutral somewhere, NOT a bonding issue. What are your thoughts?

==================================

Mike,

I just returned home from Burning Man with a fleet of rental RVs.  I am an RV technician and semi electrician.  I lay power grids at Burning Man, but always use the single phase setting on construction type generators, and never have problems.  I've been reading some of your stuff, because I ran into a situation up there... One of my 50 amp rvs, smoked an air conditioner up there on someone else's grid.

When I went to investigate, I read 208 volts across the 2 hot legs, yet 125 at each hot leg to neutral.  I didn't quite understand, at the time, that those voltages can be considered normal when on 3 phase, but I was extremely troubled by the fact that they had nothing hooked to the ground terminal on the generator.  I didn't open their home made breaker box to see if they were bonded, so I'm assuming that they were not. 

My question is ( I am looking for possible explanations) when I read voltages inside my air conditioner, at the fan motor leads, I read 157 to neutral, and 68 to neutral on the different fan speed legs, and I wondered if running un-bonded could cause variations inside the rv, since there were multiple rvs on different combinations of 3 phase legs, without a ground lead...could the neutral be affected and misbehave at different points in the rotation of the alternator?  What if one of the rvs on the other phase pair was bonded and mine was not, for example?

There were probably 8 or 10 rvs on this particular generator, and who knows if any of them were bonded elsewhere in their circuitry, or if plugs were miswired...One thing I have learned in the rv business is that you never know what other people have done.  Not being very familiar with running off 3 phase, I sought answers on the internet, and ran across your articles, primarily based around the portable generator bonding issue. 

I ran 3 generators and 18 units on single phase with no issues on the grid I set up, but it seemed like a lot of others go with 3 phase...not sure why.  The generator rating doesn't change...just the wiring to the panel.  Maybe they can just use smaller cabling that way. 

Thanks,
Allen Grota

David Buckley

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Re: Burning Man Generator
« Reply #1 on: September 12, 2017, 08:06:27 am »

Lordy...

When I went to investigate, I read 208 volts across the 2 hot legs, yet 125 at each hot leg to neutral. 

Seems about right for a four wire 3 phase 208V supply.

I didn't quite understand, at the time, that those voltages can be considered normal when on 3 phase

Uh-huh....

...but I was extremely troubled by the fact that they had nothing hooked to the ground terminal on the generator. 

Lack of a ground is not going to cause any functional problems, though it may be a safety issue. 

My question is ( I am looking for possible explanations) when I read voltages inside my air conditioner, at the fan motor leads, I read 157 to neutral, and 68 to neutral on the different fan speed legs

If this is inside the A/C unit, and the A/C unit is expecting a 220V-like supply (which two hots of a 208 3P is "close enough for rock and roll") then those voltages could be entirely normal, as there could be any wiring arrangement used internally to supply the fans.  Without reading the service manual, or doing a comparison with a known good unit, or a bunch of wire tracing and measurement, we wouldn't know.

and I wondered if running un-bonded could cause variations inside the rv, since there were multiple rvs on different combinations of 3 phase legs, without a ground lead...could the neutral be affected and misbehave at different points in the rotation of the alternator?  What if one of the rvs on the other phase pair was bonded and mine was not, for example?

So what could he have done?

My worst guess: treating the three phase supply wires as two hots and a neutral, so the neutral wasn't "floating", but it certainly wasn't a neutral!  So what the caravan saw was 208-0-208 rather than 110-0-110.  This, of course, is friggin dangerous because RV metalwork could be 1110V above ground from the "real" ground and neutral.

This is so likely to cause folks to get shocked that because shocks have not been reported I think it cant have been the case.

So floating neutral is the next obvious candidate.  But how could this have been accomplished?  Perhaps the distribution wiring was correct but the fact there was no ground connection is a clue that the generator neutral was not connected to anything?  Surely this would cause all the RVs to have smoked kit?

Thus I'm thinking that perhaps there was an inconsistency in the distribution, and just his RV was miswired, as otherwise they would all have had problems.

Or maybe his dead A/C was due to an entirely unrelated cause?

The more I ponder this, the more I think that, it seems by accident rather than design, it was all wired OK, and this A/C unit had decided that its time was done.  The other scenarios all seem less plausible.


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Mike Sokol

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Re: Burning Man Generator
« Reply #2 on: September 12, 2017, 08:27:53 am »

My question is ( I am looking for possible explanations) when I read voltages inside my air conditioner, at the fan motor leads, I read 157 to neutral, and 68 to neutral on the different fan speed legs, and I wondered if running un-bonded could cause variations inside the rv, since there were multiple rvs on different combinations of 3 phase legs, without a ground lead...could the neutral be affected and misbehave at different points in the rotation of the alternator?  What if one of the rvs on the other phase pair was bonded and mine was not, for example?

Two quick notes, and then I've got to get back to work.

#1)  Generaly, ALL RV air conditioners in the US run on 120-volts AC. The only exceptions might be tour-bus grade air conditioners, but that's not this situation.

#2)  If you add 157 volts to 68 volts (each leg to neutral measurement), that's comes to 225 volts. That's a classic open neutral scenario with the 120-120 volt legs not dividing down the middle.

I posted his email here because it's a good mental exercise in how this sort of failure can also occur in any sound system. I'm betting there's an open neutral somewhere in the distribution. EGC ground bonding should have nothing to do with this, but as noted it could create a hot-chassis condition that could be dangerous to anyone touching an RV and the ground at the same time.
« Last Edit: September 12, 2017, 08:29:59 am by Mike Sokol »
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David Buckley

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Re: Burning Man Generator
« Reply #3 on: September 12, 2017, 09:14:10 am »

Generaly, ALL RV air conditioners in the US run on 120-volts AC. The only exceptions might be tour-bus grade air conditioners, but that's not this situation.

Yes, if the supply to the A/C is supposedly 120V then having > 120V in there is vastly suspicious.

But there is no report of many the RVs having trouble, just this one, so a local wiring error is a good possibility.

Which leads us to this:

If you add 157 volts to 68 volts (each leg to neutral measurement), that's comes to 225 volts.

Lets go back to the original post:

Quote
when I read voltages inside my air conditioner, at the fan motor leads, I read 157 to neutral, and 68 to neutral on the different fan speed legs,

There is no mention of "leg" in the original post, only voltages from the fan motor to neutral.  There is only one leg in the A/C because you have now clarified it is a 120V device.  The two voltage readings from the fan to neutral came from the fan being set to different speeds.  Critically, the voltage was measured at the same point.  So although the voltages do add up as you suggest, they were not the same as the classic lost neutral which takes readings between two different points at the same time.  So this data does not indicate the classic lost neutral in the way that you suggest.  But the fact that one voltage is above 120V does.  A fine point, but as you note, a good mental exercise.

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Stephen Swaffer

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Re: Burning Man Generator
« Reply #4 on: September 12, 2017, 01:03:38 pm »

Mathematically, 125 V is high for 208 3 phase.  I would agree it seems like an open neutral.

The 68 volts to the various speed legs of the fan is a useless measurement, IMO.

3 generators and 18 units on a grid with a "semi-electrician" at this level of understanding?  I would suggest hands off anything metallic for the duration!
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Rob Spence

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Re: Burning Man Generator
« Reply #5 on: September 12, 2017, 03:08:32 pm »

To the OP

Were the RV connections 30a 120v connections or 50a 240v connections?


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Jonathan Johnson

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Re: Burning Man Generator
« Reply #6 on: September 12, 2017, 05:23:40 pm »

I would suggest hands off anything metallic for the duration!

Considering the event, many of the people probably had their hands on things made of glass... which might explain the irregularities in the "grid".
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Jeff Bankston

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Re: Burning Man Generator
« Reply #7 on: September 18, 2017, 11:25:57 pm »

I just bought a 15,000 BTU A/C with heater for my 30' enclosed cargo trailer and it's power requirement is 115 volts.
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Bob Vaughan

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Re: Burning Man Generator
« Reply #8 on: September 20, 2017, 12:46:56 am »



Quote
I ran 3 generators and 18 units on single phase with no issues on the grid I set up, but it seemed like a lot of others go with 3 phase...not sure why.  The generator rating doesn't change...just the wiring to the panel.  Maybe they can just use smaller cabling that way. 


Ah, but the generator rating does change, and fairly significantly..  you lose a bit over 25% of the capacity in single phase configuration..

For instance, a MQ Whisperwatt with a prime rating of 25kva (20kw) when set to three phase, is only rated for 14.4kw when switched to single phase.

The power factor also changes, from 0.8 in three phase configuration, to 1.0 in single phase configuration, so the generator is actually being more efficient in single phase mode, but at a lower overall capacity due to the different configuration of the windings.


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Stephen Swaffer

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Re: Burning Man Generator
« Reply #9 on: September 20, 2017, 01:00:42 pm »

Power factor is not an indicator of generator efficiency.  From a CAT  genny dealers design page (Cashman Equipment to give proper credit):

Q. “If an AC generator is rated 480 volts and 900 amperes at 0.8 power factor, why can’t the generator produce 900 amperes at 480 volts at 1.0 power factor?”

A. “It can, or cannot, depending on how the generator set has been set up and rated.” How’s that for an answer!

In reality, the generator is capable of producing 900 amperes at 480 volts, but the engine side of the unit is not sized to be able to deliver the horsepower (kW) to permit the generator to carry the load at the rated speed under all conditions.

The answer lies in the difference between apparent power and real power:
Power factor is defined as the difference, expressed as a percentage, between the voltage and current sine waves. It is critical to understand the generator does not produce power factor – the load does. Therefore, the generator set must be able to react to power factor in the load.

Leading or Lagging Power Factor?
• Power factor can be leading or lagging, or in some cases, at unity.
• A leading power factor can be caused by capacitor-intense loads, a lightly loaded synchronous motor or an induction motor that is being driven by its load.
• Lagging power factor is caused mainly by induction motors.
• Unity power factor can be found in loads dominated by electronic devices or resistance loads such as lights and heaters.
• Average industrial loads include many motors, so the recognized standard is 0.8 lagging power factor. Leading power factor is practically unattainable with today’s loads.

The key here is to keep in mind PF will affect the genset’s overall output capability.

Assuming the same current output, both following statements could result:
1. Any PF in excess of rated ( greater than 0.8 ), the genset output is limited by engine horsepower.
2. Any PF less than rated  ( 0.8 ) output is limited by generator amperage.
« Last Edit: September 20, 2017, 01:04:15 pm by Stephen Swaffer »
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Steve Swaffer
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