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Author Topic: 3-phase power demonstration  (Read 7138 times)

Mike Sokol

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Re: 3-phase power demonstration
« Reply #40 on: December 12, 2018, 07:54:56 am »

Once I commit to a 75-watt class amplifier, then 24-volt HVAC transformers with a 120/240-volt primary are really common. This one is 40VA which should match up very nicely with the amplifier. Cost is only $18 ea with shipping, so super-cheap. I'll need four of them since I'm going to hook up the 4th amplifier channel for who-knows-what. Perhaps it could drive a second service panel to demonstrate 1P 120/240-volt power.
« Last Edit: December 12, 2018, 08:43:52 am by Mike Sokol »
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Mike Sokol
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Mike Sokol

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Re: 3-phase power demonstration
« Reply #41 on: December 13, 2018, 05:14:54 pm »

This could be a start. I found a tiny 3-phase "alternator" that's used to demonstrate wind generator theory. I think it will output around 24 volts at 1,800 RPM, but who knows. I'm going to spin it with a little 1,800 RPM shaded pole fan motor. Yes, I know a shaded pole motor is not synchronous (like a Hammond B3 tone generator) and will slip a bit under load. But even if it ends up at 59 Hz that's good enough for a start. In any event I ordered one since at $8 with shipping it's cheap enough to throw away, and will a good test of my training protocol.
« Last Edit: December 13, 2018, 10:32:33 pm by Mike Sokol »
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Mike Sokol
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Stephen Swaffer

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Re: 3-phase power demonstration
« Reply #42 on: December 13, 2018, 07:53:50 pm »

So, motors have a lot in common with transformers.  An older semi-obsolete motor starting method is Wye-Delta start.  You supply 3 phase, and a set of contactors supplies power to the motor in a Wye configuration, then a timer switches contactors and motor switches to a delta run.  I'm thinking that hooking transformers up where the motor is and manually controlling the switching should allow you to hook up either a wye or delta supply going "backwards" through the relay.  I don't have a schematic handy but will check one out tomorrow.
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Frank Koenig

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Re: 3-phase power demonstration
« Reply #43 on: December 14, 2018, 12:46:33 am »

This could be a start. I found a tiny 3-phase "alternator" that's used to demonstrate wind generator theory. I think it will output around 24 volts at 1,800 RPM, but who knows. I'm going to spin it with a little 1,800 RPM shaded pole fan motor. Yes, I know a shaded pole motor is not synchronous (like a Hammond B3 tone generator) and will slip a bit under load. But even if it ends up at 59 Hz that's good enough for a start. In any event I ordered one since at $8 with shipping it's cheap enough to throw away, and will a good test of my training protocol.

It will make 60 Hz at 1800 RPM only if it has four poles per phase.  This one looks like it a total of 9 windings, so I'm not sure. I guess all I'm saying is you might need some other speed than 1800 RPM.
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Mike Sokol

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Re: 3-phase power demonstration
« Reply #44 on: December 14, 2018, 06:27:50 am »

It will make 60 Hz at 1800 RPM only if it has four poles per phase.  This one looks like it a total of 9 windings, so I'm not sure. I guess all I'm saying is you might need some other speed than 1800 RPM.

Yeah, I was trying to count poles and figure out the geometry. The specifications say it has 3 output wires, so it's probably connected up delta. Of course I could use a little variable speed DC motor to spin it at the correct speed, or a small pulley set to change the speed. But it will be easy to figure out once I can spin it. Now all I need is a 4-trace oscilloscope for these experiments. 
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Mike Sokol
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Mike Sokol

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Re: 3-phase power demonstration
« Reply #45 on: December 14, 2018, 07:12:57 am »

I'm now thinking about what I actucally need to demonstrate live in class. While it would be good to show a slide of how delta is converted to wye, and I could probably put test points on this gadget to show primary 3-wire delta voltages, I don't think anyone other than a POCO lineman would ever encounter 3-wire delta. So I think I need a 3-phase load center that I can switch between Wye 208/120 and High-Leg, 4-wire Delta 240/120. Then I could bring a small single phase load center to demonstrate single phase 240/120-volt wiring. For that 1p panel I could use a small 120/240-volt CT transformer fed from my B&K variable AC power supply to demonstrate low and high voltages.

One of the reasons I want some reasonable available load current is I want to demonstrate what you'll see when an undersized neutral is in a system. I first encountered this phenomenon at the Old Mill Inn some 40+ years ago when I noticed that when we fired up 50KW worth of PAR lighting on one of the panel legs, the voltage would drop down to maybe 110 volts, while the voltage on the other unloaded leg in the panel would rise to 130 volts or so. The only reasonable explanation was the neural couldn't support the current unbalance without a significant voltage drop.

Of course I had my lighting tech turn banks of our PAR lights on and off while I measured voltages in the open electrical panel. Not something I recommend to unqualified personnel, but I already had my Master Electrician license at the time and was taking classes for an EE degree, so it was a interesting live experiment.   
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Tom Bourke

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Re: 3-phase power demonstration
« Reply #46 on: December 14, 2018, 10:09:13 am »

I'm now thinking about what I actucally need to demonstrate live in class. While it would be good to show a slide of how delta is converted to wye, and I could probably put test points on this gadget to show primary 3-wire delta voltages, I don't think anyone other than a POCO lineman would ever encounter 3-wire delta. So I think I need a 3-phase load center that I can switch between Wye 208/120 and High-Leg, 4-wire Delta 240/120. Then I could bring a small single phase load center to demonstrate single phase 240/120-volt wiring. For that 1p panel I could use a small 120/240-volt CT transformer fed from my B&K variable AC power supply to demonstrate low and high voltages.
Having tried to teach complex concepts to simple AV techs, I feel your pain.  In my case most just wanted a recipe to make it work.  For power I would not make the test rig they deal with to complex.  A selection of the outlets they will have to deal with and what they should read.  Some instruction on the NEMA numbers and how to read a chart of connectors.

Having said that, here in Vegas we have mostly modern 120/208 Y or maybe 277/480.  Most venues have a maintenance department.  Mostly just need to make sure the connectors are wired right and the screws tight.  Other parts of the country and world don't have it so nice.  I have seen some strange in some of the old converted factory and mills in older towns.  Corner grounded Delta any one?  How about buck/boost transformers.  They don't need to understand how it works, just that it's not good for them to plug into.
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Mike Sokol

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Re: 3-phase power demonstration
« Reply #47 on: December 14, 2018, 05:34:26 pm »

Corner grounded Delta any one?

I've never seen one of these in the wild, so I'm considering whether or not to include a corner ground Delta in this seminar. At least I could put up a ppt slide showing how it will measure, but not spend any time on a live demonstration.

I was planning to include handouts with all the possible NEMA connectors as well as CEE-Form and Camlok color coding for various countries. Lots to do.   
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Jonathan Johnson

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Re: 3-phase power demonstration
« Reply #48 on: December 14, 2018, 11:21:46 pm »

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?
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Stephen Swaffer

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Re: 3-phase power demonstration
« Reply #49 on: December 14, 2018, 11:58:06 pm »

I would tend to agree that ow the transformers are wired is less important than how to measure-and really the big thing is are they dealing with 4 wire 3 phase, or just 3 wire (not counting equipment ground).

In the facility I work in, there are 3 phase/4 wire panels-ones that I can run 277 volt lights from, and there are a number of 3 phase/3 wire panels that can only be used for 3 phase loads-or maybe single phase line-line or 480 V.  They are all supplied from the same transformer.  You can feed a 3 phase delta primary transformer and get a 3 phase/4 wire 208/120 panel, if needed.

It would not surprise me to see a large subpanel feeding only 3 phase HVAC loads in a commercial building that did not have a neutral installed in the panel.  But, in the interests of having a grounded system, that panel will most often be fed with a wye connected transformer (at least in any newer install in most of North

Of course, identifying a wild leg or corner grounded would be important-but those are just voltage measurements.
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Steve Swaffer

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Re: 3-phase power demonstration
« Reply #49 on: December 14, 2018, 11:58:06 pm »


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