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Author Topic: 3-phase power demonstration  (Read 4368 times)

Mike Sokol

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3-phase power demonstration
« on: December 06, 2018, 07:32:33 pm »

I was just contacted by a company that wants me to train their crew and make a video for them on the differences between the main 3-phase transformer connections, and how to recognize them in the field. Yup, I need to get them up to speed on how to recognize and measure Delta, Wye and Wild-Leg Delta services, as well as 2-pole 120/240-volt service.

Of course it's easy enough for me to create a nice Powerpoint presentation, but I always say that nothing beats a real example. So I'm envisioning how to build a tiny 3-phase signal generator that will make 120/208 volts in WYE, Delta or High-Leg, in a place that doesn't have 3-phase power.

So the thought occurred to me that all I have to do is take any multi-track mixing program, generate a 10 minute track of a 60 Hz sine wave, duplicate it on tracks 2 and 3, then slip those tracks back 120 and 240 degrees out of phase. Then I just output the 3 audio tracks on 3 outputs of an I/O interface like my MOTU Audio Express. Next, I hook up three 120/240-volt primary filament transformers with the 6.3-volt side being driven by the TRS outputs on the MOTU. Finally, I can wire the 120/240-volt sides of the transformers into Delta, Wye or High-Leg Delta configurations with a few banana plugs. Of course, the MOTU can't drive any real amperage through the transformers. But I just need it to make 3-phase voltage in the 120 volt range that can be measured with a DMM.

I don't see why it won't work. Has anyone here tried something like this?     
« Last Edit: December 12, 2018, 07:29:06 am by Mike Sokol »
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Mike Sokol
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Tim McCulloch

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Re: 3-phase power demonstration
« Reply #1 on: December 06, 2018, 08:25:09 pm »

Ask yourself "WWNTD"  What Would Nicola Tesla Do?
 
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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

Riley Casey

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Re: 3-phase power demonstration
« Reply #2 on: December 06, 2018, 09:14:58 pm »

Nicola would claim wireless was better

Ask yourself "WWNTD"  What Would Nicola Tesla Do?

Mike Sokol

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Re: 3-phase power demonstration
« Reply #3 on: December 06, 2018, 09:22:14 pm »

Ask yourself "WWNTD"  What Would Nicola Tesla Do?
Well, Tesla always said he knew his inventions would work just envisioning them in his mind. He only had to do demonstrations to prove it to other people.

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Frank Koenig

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Re: 3-phase power demonstration
« Reply #4 on: December 07, 2018, 01:09:43 am »

Another approach would be to get your hands on a small, fractional-horsepower three-phase motor. These will work as a (not very good) single-phase motor with power applied to one of the three windings if you give them a spin to get started, which you could do with a starter cord and pully, a la lawnmower (good for dramatic effect). The other two windings will produce power suitably shifted in phase. You would still need a center-tapped transformer to simulate high-leg delta.

--Frank
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Marc Sibilia

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Re: 3-phase power demonstration
« Reply #5 on: December 07, 2018, 06:35:51 am »

Next, I hook up three 120/240-volt primary filament transformers with the 6.3-volt side being driven by the TRS outputs on the MOTU.
...
Of course, the MOTU can't drive any real amperage through the transformers.

I don't see why it won't work. Has anyone here tried something like this?     

I don't think you will get your 120/240V output without some amplification.  The outputs of the MOTU probably have some current limiting resistors for short circuit protection and they will drop most of the voltage with a low impedance load.  You could use a couple of cheap (under $25) Lepai 2020 amplifiers ( ~ 15 watts into 4 ohms) as buffers, as well as a form of protection for your nice interface.  They should work well for driving some 1A 6.3V transformers.

P.S. It will work. I just tried it with a 6.3 V 3A transformer and a Lepai, and it was pulling 0.35A on the 6.3V "primary" with no load on the 120V secondary.  It takes slightly more than 6.3V on the primary to get 120V on the secondary.  That's easy to adjust with the volume control.  The good thing about the Lepai is it will only do about 8 Vrms output, so the insulation on the transformer is relatively safe from stupid mistakes on input level.

Be careful and methodical (I don't think I need to tell you this, but it is for people following along).  With 240 V secondaries, you could see voltages of almost 1 kVrms with the worst case output wiring and input signal phasing.  This would likely comprimise one or more of the transformers' insulation, and may be more than the voltmeter's rated voltage.
« Last Edit: December 07, 2018, 07:07:27 am by Marc Sibilia »
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Mike Sokol

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Re: 3-phase power demonstration
« Reply #6 on: December 07, 2018, 08:12:59 am »

Another approach would be to get your hands on a small, fractional-horsepower three-phase motor. These will work as a (not very good) single-phase motor with power applied to one of the three windings if you give them a spin to get started, which you could do with a starter cord and pully, a la lawnmower (good for dramatic effect). The other two windings will produce power suitably shifted in phase. You would still need a center-tapped transformer to simulate high-leg delta.

--Frank

I'm pretty sure that all car alternators are 3-phase generators with a WYE winding configuration and a bunch of diodes to make DC. So all I would have to do is pull out the diodes and add a DC control voltage to the alternator rotor winding to adjust the output voltage on the stator windings. I could spin this with any kind of 120-volt motor, and add 12 volt to 120-volt filament transformers to create floating transformer secondaries just like POCO transformers in reverse. I can get a really small car alternator for $50 or so, an AC motor for another $20 and three filament transformers with 120/120 volt primary windings for maybe $20 each. I've got to take this as checked luggage on a plane so it could be mounted on a plate that would fit in a pelican case. The advantage of this type of rig is that it's more obvious to electrical power students than running Reaper on a Mac through an I/O interface. Plus it would be a good chance to demonstrate how 3-phase generators work. In addition, if the filament transformers were hefty enough it could easily generate a few amps of current per output for more interesting experiments such as what happens when the neutral opens up, etc...

The Mechanical Engineer inside of me wants to build this. 
« Last Edit: December 12, 2018, 07:30:51 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 #7 on: December 07, 2018, 08:20:34 am »

I don't think you will get your 120/240V output without some amplification.  The outputs of the MOTU probably have some current limiting resistors for short circuit protection and they will drop most of the voltage with a low impedance load.  You could use a couple of cheap (under $25) Lepai 2020 amplifiers ( ~ 15 watts into 4 ohms) as buffers, as well as a form of protection for your nice interface.  They should work well for driving some 1A 6.3V transformers.

P.S. It will work. I just tried it with a 6.3 V 3A transformer and a Lepai, and it was pulling 0.35A on the 6.3V "primary" with no load on the 120V secondary.  It takes slightly more than 6.3V on the primary to get 120V on the secondary.  That's easy to adjust with the volume control.  The good thing about the Lepai is it will only do about 8 Vrms output, so the insulation on the transformer is relatively safe from stupid mistakes on input level.

Be careful and methodical (I don't think I need to tell you this, but it is for people following along).  With 240 V secondaries, you could see voltages of almost 1 kVrms with the worst case output wiring and input signal phasing.  This would likely compromise one or more of the transformers' insulation, and may be more than the voltmeter's rated voltage.

The Lepai amps are a good tip. I was wondering about the actual impedance of the 6.3 volt transformers being so low that it would excessively load the output stage of the I/O interface. I'm sure there's current-limiting build-out resistors on the MOTU box which would probably drop the voltage swing too much since it's expecting a 600-ohm load at minimum. Looks like the amps are $22 each, so that's a cheap fix: https://www.amazon.com/Lepy-Amplifiers-Component-LP-2020A-Class-D/dp/B01FZKA28Y/ref=sr_1_fkmr0_1?ie=UTF8&qid=1544188691&sr=8-1-fkmr0&keywords=lepai+amplifier+quad

The Lepai amplifiers actually sound pretty decent. I bought one last Christmas for my son's television set to drive a pair of passive speakers, and it worked very well. Working backwards and considering basic efficiency losses, this should easily produce maybe 5 watts (500 mA) per leg on the 120/208 side of the filament transformers. 

And yes, I would fuse everything properly and consider insulation breakdown voltages. And even something as innocent looking as this can produce lethal voltages and currents. I know that you know that I know this, but you are correct that we need to remind all the possible readers about the hazards of playing with live electrical circuits. As I like to say, "Kids don't try this at home". 

The Electrical/Audio Engineer inside of me really wants to build this. 
« Last Edit: December 07, 2018, 08:33:49 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 #8 on: December 07, 2018, 09:42:31 am »

I think these transformers would work nicely. If I'm thinking about this correctly, I need the transformer "outputs" to be 120-volts for the 120/208 WYE configuration, and 240-volts with one of them center-tapped for the 120/240 Delta High-Leg configuration. The beauty of doing this with something like Reaper as a multi-track player is that I could let the class see the three sine waves on the computer screen and actually measure the voltage across the various transformer legs as I slip the track phases. That could be really educational, I think.

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Ron Hebbard

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Re: 3-phase power demonstration
« Reply #9 on: December 07, 2018, 09:43:02 am »

The Lepai amps are a good tip. I was wondering about the actual impedance of the 6.3 volt transformers being so low that it would excessively load the output stage of the I/O interface. I'm sure there's current-limiting build-out resistors on the MOTU box which would probably drop the voltage swing too much since it's expecting a 600-ohm load at minimum. Looks like the amps are $22 each, so that's a cheap fix: https://www.amazon.com/Lepy-Amplifiers-Component-LP-2020A-Class-D/dp/B01FZKA28Y/ref=sr_1_fkmr0_1?ie=UTF8&qid=1544188691&sr=8-1-fkmr0&keywords=lepai+amplifier+quad

The Lepai amplifiers actually sound pretty decent. I bought one last Christmas for my son's television set to drive a pair of passive speakers, and it worked very well. Working backwards and considering basic efficiency losses, this should easily produce maybe 5 watts (500 mA) per leg on the 120/208 side of the filament transformers. 

And yes, I would fuse everything properly and consider insulation breakdown voltages. And even something as innocent looking as this can produce lethal voltages and currents. I know that you know that I know this, but you are correct that we need to remind all the possible readers about the hazards of playing with live electrical circuits. As I like to say, "Kids don't try this at home". 

The Electrical/Audio Engineer inside of me really wants to build this.
Will you demonstrate "Scott T" interconnections as well? 
Toodleoo! 
Ron Hebbard
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