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[Mike Sokol]

The 4-channel amp as well as the 12.6-volt transformers have arrived, but the 3-phase signal generator won't be here for another week at least (slow boat from China, literally). But I can use a 60-Hz sine wave on my phone to drive the amps, and my trusty O'scope to check out the waveform on the "primary" 115/230-volt side of the transformers. If that works as envisioned, then I'm going to mount everything on a backing plate that will fit in a Pelican case for transport.   

[Mike Sokol]

Here's the single-phase version of the demonstration rig. The little "400-watt" car amp makes around 8 volts RMS output without clipping, so I used the 6.3-volt secondary of the transformer. This easily makes 120/240-volts on the transformer primary which is actually my secondary.

I'm using a HOSA 1500mA 12-volt universal power supply to run this mess, but that could be a little light when I get all 4 channels up and running, so I'll upgrade it to a 5000mA 12-volt DC supply just so it doesn't die if an output of the amplifier gets shorted. But this is all very current limited so I shouldn't have any fireballs if something goes wrong.

Still waiting on my 3-phase signal generator from China, which hopefully will arrive this week. In the meantime I'm going to mount this down on a little board that will fit in a pelican case and wire it up all pretty with appropriate test points. Everything looks like it's going to work, so I'm a happy guy so far.

I've got to bring this demonstration to AES for show and tell.

[Mike Sokol]

Just ordered this 12-volt/30-amp regulated supply from Amazon for $19 with shipping. The 400-watt car amplifier has a 15-amp fuse, so this power supply should be able to supply sufficient current for worst case shorting scenarios without shutting down. It will be here on Wednesday.

[Mike Sokol]

So here's what I'm thinking about doing to demonstrate wiring of various 3-phase configurations. I was going to mount this on a piece of plywood, but my wife gave me an old white-board yesterday, so I started laying out how to mount the transformers on it. Then I drew out where I could mount 5-way binding posts for various connections and test points. After I did that with dry-erase markers it hit me. Why don't I just mount the transformers and 5-way binding posts on the whiteboard and put a hinge on the bottom so it can lay flat for transport or an overhead video camera, and it can flip up at an angle for direct viewing and testing by the class. I can literally sketch the various diagrams for standard Delta, Grounded-Leg Delta, High-Leg Delta, 3-leg WYI, 2-leg WYI and whatever else comes to mind (open leg Delta, perhaps?) right on the white-board, then hook it up using colored banana plugs. I can get 5-way binding posts in Black, Red, Blue, Green and Yellow (no white that I can find, but maybe yellow is OK), and get banana plug jumpers in these colors as well.

So what do you think? Is this a good way to bring a schematic on paper to life? As I've noted earlier, this system does make actual 120/208/240 volts AC at maybe 100mA current per leg, so students can use their own meters set to real scales for measurements.

As a side note, I'm installing a GFCI plug on the incoming 120-volt AC power cord feeding this entire rig, so that if someone accidentally gets between a hot test point and anything grounded the GFCI will trip before it electrocutes them. It would be a little embarrassing to kill someone in my seminar while trying to demonstrate electrical testing and safety.   

[Stephen Swaffer]

I like the white board idea-might steal it myself!

The GFCI is not a bad precaution-but you are already using transformers.  For the purpose of this demo, they probably need to be un-bonded so actually true isolation transformers.  Isolation transformers are a reasonable and common safety precaution often used on test benches-but if someone manages to get "bit" by one it will still present a balanced hot/neutral current to the GFCI.

[Mike Sokol]

I like the white board idea-might steal it myself!

The GFCI is not a bad precaution-but you are already using transformers.  For the purpose of this demo, they probably need to be un-bonded so actually true isolation transformers.  Isolation transformers are a reasonable and common safety precaution often used on test benches-but if someone manages to get "bit" by one it will still present a balanced hot/neutral current to the GFCI.

My plan is to create a bonded-ground system, just like the real thing. So the neutral of the transformer secondaries will be bonded to the incoming EGC from the wall receptacle. That's because I want to be able to demonstrate NCVT testing with this setup, and should be able to recreate a null/center position near the center of the transformers, that will cancel out a NCVT response. At least, that's what I've been told by a few old-timer electricians who don't trust NCVTs. Of course, I only trust them a little after I verify that they're actually working, and I would never use one as the ONLY way to verify that power was off in big panel or machine. But they're just one more layer of testing. 

[Dave Garoutte]

As you were describing it, I'm thinking, 'why not just use the white board?'.  five words later you read my mind.

[Mike Sokol]

As you were describing it, I'm thinking, 'why not just use the white board?'.  five words later you read my mind.

Hey, great minds think alike, or whatever. 

I think there's a teaching advantage to laying out a circuit that resembles its schematic counterpart. This transformer-triangle is how I tend to think about 3-phase power inside of my head, especially for high-leg or grounded-leg Delta service. And it works well enough for 120/208 WYE service as well. All easy to demonstrate schematically and literally with a few colored dry markers and jumper plugs on my white board.
 

[Mike Sokol]

The GFCI is not a bad precaution-but you are already using transformers.  For the purpose of this demo, they probably need to be un-bonded so actually true isolation transformers.  Isolation transformers are a reasonable and common safety precaution often used on test benches-but if someone manages to get "bit" by one it will still present a balanced hot/neutral current to the GFCI.

I would be really worried about an untrained consumer getting across two hot-legs of this this thing, but in this particular seminar I'm teaching AVL technicians what to look for when measuring power in a venue, which they already do all the time with their own meters. Plus I'll be sitting right in front of them watching like a hawk for any bad practices, like laying one hand on the transformers and the other on the meter probe. Come to think of it, I could choose not to ground the transformer shell allowing them to float. As long as they didn't leak too much internally that could offer an extra layer of safety. What do you all think?

[Stephen Swaffer]

I think I would bond the shells together so they stay at the same potential-a neon light connected between them and ground would be a super safe way just in case they became energized through a fault.  Should be safe enough as long as the whole contraption is GFCI protected.

I guess I assumed you would not always have a bonded neutral to work with-if you wire and demo a true delta.

[ProSoundWeb Community]