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Author Topic: 3-phase power demonstration  (Read 6714 times)

Tom Bourke

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Re: 3-phase power demonstration
« Reply #50 on: December 15, 2018, 12:45:07 am »

Is there a difference between high leg delta and wild leg delta?

High leg delta is what we think it is. Wild leg delta is the same as high leg, except that only two transformers are used? Or is that the "corner ground" just mentioned?
Wild and high leg delta are the same as far as I know.  A delta connection of 3 240V coils with one being center tapped and that center tap bonded with ground.  The high leg is about 208V to neutral. Since the high leg is not direct referenced by a bond or single coil to ground it can vary widely, with respect to ground, depending on loads.  That is where the "wild" comes from.

The 2 transformer delta V configuration is used where the power company wants to save on transformers yet have the ability to upgrade.  From an end user stand point it should not matter unless you draw more that the POCO provisioned.

Corner grounded delta is where one of the 3 hot legs is also bonded to ground.  Mutually exclusive with high leg delta and Y configurations.  You can almost treat it like split/single phase, other that the voltages being off.  I have only personally measured it in an old building at a very old elevator control panel.  I have also heard about situations using it for non motor loads causing catastrophic inductive heating  in the conduit.

Buck/ boost transformers can take a 277 and drop it to 240 or take 208 and boost it to 240.  And then is it 240 or 220 the person was looking for?  110 or 120 or 125?  To hook my single phase 230 volt air compressor to 208 three phase I have a buck/boost transformer.

That's the real complexity of measuring electrical systems.  An industrial situation is very different from an AV situation.  Add on a situation with some one who knows just enough to be dangerous?

My point about go/nogo measurements is based on how you get the voltage your seeing.  When you factor in +-10%  variance and allowed drop , it can get very confusing. 
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Mike Sokol

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Re: 3-phase power demonstration
« Reply #51 on: December 15, 2018, 10:49:16 am »

That's the real complexity of measuring electrical systems.  An industrial situation is very different from an AV situation.  Add on a situation with some one who knows just enough to be dangerous?

My plan is to teach these AVL technicians how to recognize various transformer configurations and know what's safe and what to avoid. I've done enough shows at big hotel conference rooms where the facility guy shows me where to hook up power and says that "everyone" uses some really sketchy looking receptacle. I've learned not to trust anyone until I measure it myself.

This is especially true with new churches that have moved into old industrial warehouses, many of which have 3-phase, high-leg Delta panels. The other thing to be aware of is that even the ones with 4-wire WYE transformers often have undersized neutrals since much of the original industrial load was 3-phase motors. The undersized neutrals can easily be overloaded when you hook tungsten dimmer packs on a single phase, which can create odd-order triplen currents on the neutral.

I'm not sure how many residential electricians (even licensed ones) would recognize all these flavors of 3-phase power since they would probably never encounter them in a non-industrial situation. But 3-phase power can be dangerous if you don't know what you're doing. Much to teach about.   
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Tim McCulloch

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Re: 3-phase power demonstration
« Reply #52 on: December 15, 2018, 12:43:51 pm »

My plan is to teach these AVL technicians how to recognize various transformer configurations and know what's safe and what to avoid. I've done enough shows at big hotel conference rooms where the facility guy shows me where to hook up power and says that "everyone" uses some really sketchy looking receptacle. I've learned not to trust anyone until I measure it myself.

This is especially true with new churches that have moved into old industrial warehouses, many of which have 3-phase, high-leg Delta panels. The other thing to be aware of is that even the ones with 4-wire WYE transformers often have undersized neutrals since much of the original industrial load was 3-phase motors. The undersized neutrals can easily be overloaded when you hook tungsten dimmer packs on a single phase, which can create odd-order triplen currents on the neutral.

I'm not sure how many residential electricians (even licensed ones) would recognize all these flavors of 3-phase power since they would probably never encounter them in a non-industrial situation. But 3-phase power can be dangerous if you don't know what you're doing. Much to teach about.

I think it's very possible to try and teach too much to lowest tier hotel AV techs.  I think I'd start with a "go/no-go" module and then delve into theory after lunch for those still interested.

The very first post in this forum is about a hotel meeting room with an Edison power outlet that's labeled "microphone."  I don't expect hotels to do much better.  I agree that meter everything is best practice and one I use, but even at my level of interaction I don't care if a service is 480v delta or 208/277 high leg or 120/208/240 if I don't get the correct meter readings.  I can't change it - my only recourse is to not use it and get someone from the hotel or convention center to label it.

Again I'm all for knowledge (except for forums run by Social Knowledge, but that's another matter) but I think this needs to focus on safety of personnel, guests and equipment.  The *theory* behind 'unsafe for our use' services is nice but unnecessary for most AV techs.  All that is necessary is for techs to recognize over voltage and not use it.

This all said, I'm guessing there is a story or 2 behind this request.  Can you share?
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Mike Sokol

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Re: 3-phase power demonstration
« Reply #53 on: December 15, 2018, 06:20:21 pm »

This all said, I'm guessing there is a story or 2 behind this request.  Can you share?
I've not spoke at length to their head tech guy yet, so I don't have any intel as to what caused this line of inquiry. We're supposed to talk on Monday, so that's going to be my main line of questioning. What kind of power situations do they find themselves in, and was there some specific incident that went sideways? Right now I just have a bullet list of topics to cover, so I do need to know spin and depth of my training class. But as you said, many times you just need to give technicians a go/no-go type of SOP.
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Mike Sokol
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Mike Sokol

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Re: 3-phase power demonstration
« Reply #54 on: December 15, 2018, 06:30:12 pm »

Check this out. I found a 3-phase signal generator from China for $31.88 with shipping. Signal level appears to be 5 volt P-P which would drive any small power amp nicely. https://www.ebay.com/itm/1-00-200KHZ-3-Phase-Sine-Signal-Generator-0-to-360-Frequency-meter-Counter-/172793617133?_trksid=p2349526.m4383.l4275.c10#viTabs_0

Circling back to the top of the thread, adding a pair of the Lepai 2020 amplifiers at $20 each might be the best choice for a voltage-only 3-phase demonstration. And these little transformers could get the voltage swing up to 120/240-volts as well as Delta or WYE configurations. 

If this works (and I'm reasonably sure it will), then it's certainly cheap enough, small enough and simple enough for a tabletop demonstration.
« Last Edit: December 15, 2018, 07:02:44 pm by Mike Sokol »
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Stephen Swaffer

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Re: 3-phase power demonstration
« Reply #55 on: December 15, 2018, 10:39:23 pm »

I think it's very possible to try and teach too much to lowest tier hotel AV techs.  I think I'd start with a "go/no-go" module and then delve into theory after lunch for those still interested.

The very first post in this forum is about a hotel meeting room with an Edison power outlet that's labeled "microphone."  I don't expect hotels to do much better.  I agree that meter everything is best practice and one I use, but even at my level of interaction I don't care if a service is 480v delta or 208/277 high leg or 120/208/240 if I don't get the correct meter readings.  I can't change it - my only recourse is to not use it and get someone from the hotel or convention center to label it.

Again I'm all for knowledge (except for forums run by Social Knowledge, but that's another matter) but I think this needs to focus on safety of personnel, guests and equipment.  The *theory* behind 'unsafe for our use' services is nice but unnecessary for most AV techs. All that is necessary is for techs to recognize over voltage and not use it.

This all said, I'm guessing there is a story or 2 behind this request.  Can you share?

I would disagree to an extent.  I realize my background is industrial-but there is no technical reason not to install a 3 phase load only panel in a commercial facility to supply HVAC loads, etc.  The first 3 phase panel Mike showed was designed for such a use-no isolated neutral bus.  A panel like this will have the correct voltage readings-but on a 200 amp panel the difference between a properly sized grounding conductor and a properly sized grounded (aka neutral) conductor is from a #6 to a 3/0.  Similarly a 3 phase receptacle with a grounding conductor but no neutral would be likely to have too small a conductor to run single phase loads.

This "hack"-using a grounding conductor as a neutral is a code violation and a very real safety issue-and it is one of the most common ones I see (short of ignoring the importance of the grounding conductor/safety ground).

In my opinion, knowing how to check voltages is critical-as is determining if a 3 phase power source is a legitimate source of single phase power.  The "wye"-"delta" determination Mike originally mentioned-but the critical thing is if the 4th wire of the "wye" is available at the panel/receptacle-not necessarily if the transformer is delta connected.
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Tim McCulloch

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Re: 3-phase power demonstration
« Reply #56 on: December 16, 2018, 12:23:01 pm »

I would disagree to an extent.  I realize my background is industrial-but there is no technical reason not to install a 3 phase load only panel in a commercial facility to supply HVAC loads, etc.  The first 3 phase panel Mike showed was designed for such a use-no isolated neutral bus.  A panel like this will have the correct voltage readings-but on a 200 amp panel the difference between a properly sized grounding conductor and a properly sized grounded (aka neutral) conductor is from a #6 to a 3/0.  Similarly a 3 phase receptacle with a grounding conductor but no neutral would be likely to have too small a conductor to run single phase loads.

This "hack"-using a grounding conductor as a neutral is a code violation and a very real safety issue-and it is one of the most common ones I see (short of ignoring the importance of the grounding conductor/safety ground).

In my opinion, knowing how to check voltages is critical-as is determining if a 3 phase power source is a legitimate source of single phase power.  The "wye"-"delta" determination Mike originally mentioned-but the critical thing is if the 4th wire of the "wye" is available at the panel/receptacle-not necessarily if the transformer is delta connected.

AV technicians will not be dealing with any source of power that doesn't come from some kind of NEMA outlet (whether or not it's the right outlet for the service is another matter).  They won't be poking around in panels or wiring outlets.  It is not necessary for them to understand 'why' an outlet doesn't meter correctly, only that it does not.  In a hotel or convention center the responsibility to make it right is with the engineering or maintenance dept. or they will call this electrician from Iowa... ;)

Again, I think imparting solid, foundational knowledge is good thing in general.  What I'm doubting is that AV techs working mostly in the same venue, same meeting rooms and ball rooms will *need* to understand the theory nearly as much as knowing they're on the wrong service.

For technicians working in unfamiliar facilities, again knowing that they shouldn't use a particular outlet or service disconnect when it "meters funny" is more important than their being able to identify the nature of the errant service.

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Frank Koenig

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Re: 3-phase power demonstration
« Reply #57 on: December 16, 2018, 12:44:50 pm »

From having read all the above it sounds like a really good thing for these AV techs, and maybe for the rest of us as well, would be a nice, durable pocket chart that has illustrations of all the NEMA receptacles they're likely to encounter along with the expected meter readings. It would also call out receptacles that are unsuitable for Y-connection for lack of a neutral, as well as the now deprecated 3-wire home range and dryer receptacles.

The rule is if it doesn't meter as expected, don't use it, and enquire.

--Frank   
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Stephen Swaffer

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Re: 3-phase power demonstration
« Reply #58 on: December 16, 2018, 03:57:10 pm »

AV technicians will not be dealing with any source of power that doesn't come from some kind of NEMA outlet (whether or not it's the right outlet for the service is another matter).  They won't be poking around in panels or wiring outlets.  It is not necessary for them to understand 'why' an outlet doesn't meter correctly, only that it does not.  In a hotel or convention center the responsibility to make it right is with the engineering or maintenance dept. or they will call this electrician from Iowa... ;)



Mike's OP indicated they wanted him to teach how to recognize transformer connections, etc.  That leads me to assume  :o  that somehow they had an issue and whoever was involved was doing more than plugging a NEMA plug into a NEMA receptacle.

Frank's suggestion seems to be a good one.  There are plenty of reference charts out there-but one edited for AV techs or even better one for "these are acceptable receptacles for our feeders or gear" might be even better.
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Tim McCulloch

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Re: 3-phase power demonstration
« Reply #59 on: December 16, 2018, 04:23:18 pm »

Mike's OP indicated they wanted him to teach how to recognize transformer connections, etc.  That leads me to assume  :o  that somehow they had an issue and whoever was involved was doing more than plugging a NEMA plug into a NEMA receptacle.


That's why I'm waiting for the back story on this.  For hotel or convention center work NO AV tech is working inside a panel, ever.  At the hotels we work in the only services accessible to line staff are on some kind of outlets (including CamLocks, but then the switch has to be unlocked by a building engineer).

Hell, we can't even have ETCP-certified electricians tap tails into company switch lugs at the PAC, a city employee electrician has to do it.  Not because there was an issue (ever, in the 20 years I've worked in the venue) but because a non-electrician city worker was replacing a light switch and didn't test for power down.  No injury, no property damage, but the venue manager had a hissy fit and penalized the people trained and certified to do their jobs on stage.

Kansas isn't a bastion of progressive thinking (if they could eliminate workman's comp they would) so I don't think I'm in Nanny State.  I've worked in places with scary wiring, bogus "distro boards" and all kinds of non-compliant wiring but that wiring wasn't used with outside products (in house distros only, those kind of things).

My concern over trying to instill too much info is that the trainee's brains shut down and they absorb nothing, rather than taking away the parts about preventing electrical fires and not killing people through rejection of non-conforming outlets.
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"Practicing an art, no matter how well or badly, is a way to make your soul grow, for heaven's sake. Sing in the shower. Dance to the radio. Tell stories. Write a poem to a friend, even a lousy poem. Do it as well as you possible can. You will get an enormous reward. You will have created something."  - Kurt Vonnegut

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Re: 3-phase power demonstration
« Reply #59 on: December 16, 2018, 04:23:18 pm »


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