The Edison outlet I am going to plug my toaster into has a 110V AC voltage waveform. Let’s plug the toaster in. Toss in a slice of bread and start cooking. Mmmm toast!

Let’s stop time when the voltage phasor is such that the AC voltage waveform is at it’s positive peak (+110V wrt neutral).

Unplug the toaster and reverse the orientation of the plug.

Start time again.

Now the waveform in the Edison has not changed, but the voltage waveform seen by the toaster is inverted. I have NOT shifted the toaster’s voltage waveform in time (this would be the case if the phase had shifted by 180 degrees). I have changed the polarity of the voltage waveform in the toaster by switching which plug blade saw hot and which plug blade saw neutral.

Let’s build a single-phase distribution transformer (this will not be an exact replica just conceptual).

With a turns ratio (ratio of primary side windings to secondary side windings) of 131:2 we can take 7200V AC waveform at the primary and induce a 110V AC waveform at the secondary. This corresponds with a wye connection (line to neutral) to a 7200V/12470V distribution system. A 113:1 turns ration would be in line with a delta connection (line to line) to the same system.

Take a transformer core. Take a piece of insulated wire and wrap it 131 times around the core. Connect each end to a terminal and label the terminals H1 and H2. This is the high voltage side. Lets connect H1 to the "A" phase of the distribution system and H2 to the neutral wire of the distribution system.

Take two other pieces of insulated wire (much bigger wire – the secondary will be running much lower voltages and thus much higher currents) and wrap them (right next to each other, in the same direction) 2 times around the core. Label the starting ends A1 and B1. Label the other ends A2 and B2.

Label 3 secondary terminals X1, X2 and N. Connect the N terminal to the neutral wire of the distribution system and the wire leading to the neutral bus in the building. Connect the X1 and X2 terminals each to a hot (110V) leg leading to the building.

Connect A1 and B2 to terminal N. Connect B1 to X1. Connect A2 to X2.

Turn on the power.

(Current flowing in the primary windings creates a magnetic flux in the core. The magnetic flux in the core, linking the secondary windings, induces current in the secondary windings, resulting in voltage across the secondary windings. The “turns ratio” determines which side is higher or lower in voltage, and by how much.)

Stop time when the single secondary winding voltage phasor is such that A1 and B1 are at +110V with respect to A2 and B2 (individually and respectively). (The wires are in parallel, wound the same way, so the secondary winding AC voltage waveforms are identical.)

N (A1 and B2) is at 0V. X1 (B1) is +110V (wrt N). X2 (A2) is –110V (wrt N).

Switch A1 and A2 (so that A1 connects to X2 and A2 connects to N).

Now N (A2 and B2) is at 0V. X1 (B1) is +110V (wrt N). X2 (A1) is +110V (wrt N). (OOPS!)

Start time again.

The primary winding voltage phasor, and thus the primary AC voltage waveform, has not changed position. The secondary winding voltage phasor, and thus the AC voltage waveform present in the secondary windings is unchanged.

The voltage waveform seen at terminal X2 is inverted. I have NOT shifted the transformer secondary voltage waveform in time (this would be the case if the phase had shifted by 180 degrees). I have changed the polarity of the AC voltage waveform at terminal X2 by changing the orientation of the terminals to the A1-A2 winding.

For further investigation, please refer to Chuck McGregor’s Study Hall article
Polarity and Phase Explained. (You'll need Adobe Acrobat)

Also, this is very closely related to the periodic uproar about sound console manufacturers incorrectly labeling channel polarity switches with the symbol for phase. Search the archives for "polarity". You'll have plenty of bedtime reading.

Regards,

Doug


Reply posted by Chris Hindle on April 01, 2001

Hi Doug.

Didn't mean to ruffle any feathers, or bend anyone's nose out of joint. I am neither a licensed electrician nor a degree holder in engineering or electrical what-have-you. I frequently work in reception halls and church basements & community center back yards, so you can kind of guess where my energy needs lie. I am frequently dealing with the "No one opens the electric boxes here" mindset, and the simpler I keep the details of what I am up to, the better I can deal with these folks.

For convenience I talk of polarity with DC, and phase / legs with AC. If this is not 100 percent completely accurate, well, I am sorry. Nobody that I have dealt with to date gives a rats ass how a transformer is wired, and why there are 4 or 5 wires that I play with, when any plug they have ever seen has 3 or 2 plug things on them (me, I am very interested, and I even printed out your messages)

Don't get me wrong, technical accuracy is important, and certainly has it's place. Three hours before show time, with an antsy Maitre' D looking over my shoulder, I'm afraid some of the accuracy might have to go out the window, and bullshit a bit to get the damn AC up and running.

Chris.