Why is residential wiring known as single phase?

[K8MHZ]

What are we discussing then?

Single phase residential service voltages.

If they were 180 out of phase, Vna would = -Vnb, and the net sum would be zero. (120 + negative 120 = 0)

Since they are in phase, Vna does = Vnb and the net sum is 240. (120 + 120 = 240)

If you looked at all three voltages on a scope, you would only see two waves, as the two 120 volt waves would be superimposed upon each other. If you were looking at a system with a bad neutral, it would be easier to see. If you had 100 volts on one leg and 140 on the other you would then see all three waves and the peaks would all all line up (time wise) on the same side of the zero line at the same time, thus, all being in phase, and thus only a single phase exists.

 

[Joethemechanic]

LOL, I think we were set up! :slaphead:

Ok, here is one from one of my old text books

There were experimental traction motors built with either 6 or 8 phase motors (I can't remember which). And for some reason I think they were designed for light rail. And were in some kind of limited service in Europe.

From what I remember they were designed to prevent low speed cogging, when run from a low frequency VFD

Does anybody remember anything about this?

 

[iwire]

Ok, here is one from one of my old text books

There were experimental traction motors built with either 6 or 8 phase motors (I can't remember which). And for some reason I think they were designed for light rail. And were in some kind of limited service in Europe.

From what I remember they were designed to prevent low speed cogging, when run from a low frequency VFD

Does anybody remember anything about this?

An 'old book'?, it can't be that old if it speaks of VFDs.

We used to have a active member here that designed motors for specific applications, they were very powerful in a small package.

http://www.chorusmotors.gi/

 

[Joethemechanic]

Single phase residential service voltages.

If they were 180 out of phase, Vna would = -Vnb, and the net sum would be zero. (120 + negative 120 = 0)

Since they are in phase, Vna does = Vnb and the net sum is 240. (120 + 120 = 240)

If you looked at all three voltages on a scope, you would only see two waves, as the two 120 volt waves would be superimposed upon each other. If you were looking at a system with a bad neutral, it would be easier to see. If you had 100 volts on one leg and 140 on the other you would then see all three waves and the peaks would all all line up (time wise) on the same side of the zero line at the same time, thus, all being in phase, and thus only a single phase exists.

Hey, what about this crazy thing,,, the 120/208 that we call single phase but isn't?

So far I can only represent it with this unpronounceable symbol

 

[K8MHZ]

Hey, what about this crazy thing,,, the 120/208 that we call single phase but isn't?

So far I can only represent it with this unpronounceable symbol

120/208 is actually three phase. Who calls it single phase? We can utilize a single phase of that system, but it is still a three phase system. That's why you see the phase difference.

You bring up a good observation.

Since 120 plus 120 does not = 208, what gives?

(Hint, here is where the square root of 3 comes in)

 

[K8MHZ]

Remember that in 120/208 there are actually three 120 volt phases. If only one 208 (two hots) with a neutral is brought to a panel giving it only two 120 volt phases, it would physically resemble a single phase system. That is how 120/208 residential panels look. Checking the voltages will determine if you are dealing with a single or a multiphase system. In an industrial panel, all three phases will be present. That is easily identifiable as three phase, because you can see all of them.

Do you know where the square root of three comes in yet?

 

[Joethemechanic]

An 'old book'?, it can't be that old if it speaks of VFDs.

We used to have a active member here that designed motors for specific applications, they were very powerful in a small package.

http://www.chorusmotors.gi/

Well the 1980's are considered old now.

And back in the 1970's we built crude square wave "VFD like" devices in class.

 

[Joethemechanic]

Remember that in 120/208 there are actually three 120 volt phases. If only one 208 (two hots) with a neutral is brought to a panel giving it only two 120 volt phases, it would physically resemble a single phase system. That is how 120/208 residential panels look. Checking the voltages will determine if you are dealing with a single or a multiphase system. In an industrial panel, all three phases will be present. That is easily identifiable as three phase, because you can see all of them.

Do you know where the square root of three comes in yet?

yeah in the angle between the phases.

Because it is really 2 phases of a 3 phase system, but yet when we bring 2 phase wires and a N from a wye connected transformer we use it for single phase, and throughout the electrical trade, it is commonly referred to as "single phase 120/208"

 

[Joethemechanic]

BTW, now I'm digging back in 20 year old memories of trig class to remember how 120 degrees relates to the square root of 3.

I can tell you it is the ratio of the shorter side to the longer side of a triangle, but most of my trig is kinda rusty. Well except for "right triangle" formulas that I use all the time

Sine, cosine, tangent, it's got to be in here somewhere

 

[K8MHZ]

yeah in the angle between the phases.

Because it is really 2 phases of a 3 phase system, but yet when we bring 2 phase wires and a N from a wye connected transformer we use it for single phase, and throughout the electrical trade, it is commonly referred to as "single phase 120/208"

"A single phase of 120/208" is the correct terminology. "single phase 120/208" is technically incorrect. It is used to indicate that only two of the three 3 phase conductors, plus a neutral, are brought to a panel. Yes, without checking voltages, that panel would look like just a plain old single phase panel. Of course, the same folks that say '120/208 single phase' are usually aware that they are dealing with a three phase system, overall, and that's how we get the 208. I have never had anyone dispute that, unlike the dispute that often erupts over 120/240 having more than one phase.

 

[rbalex]

Jraef gave the best practical explanation in Post #27.

The problem understanding it is English ambiguity, a refusal to accept a common definition (as Jim pointed out) and semantics used simply to win points.

Periodic functions (sinusoids for example) are the same phase, thus ?single-phase? with respect to each other, where they have the same period and same origin (zero-crossing); Technically, amplitude and polarity are irrelevant with respect to phase. Multi-phase systems have the same period but different origins.

 

[kingpb]

BTW, now I'm digging back in 20 year old memories of trig class to remember how 120 degrees relates to the square root of 3.

I can tell you it is the ratio of the shorter side to the longer side of a triangle, but most of my trig is kinda rusty. Well except for "right triangle" formulas that I use all the time

Sine, cosine, tangent, it's got to be in here somewhere

Vector math, and

Vab = Van + Vnb = Van - Vbn

|Vab| = 2 * |Van|cos 30 (2 *cos 30) = 1.732 or sqrt 3

 

[K8MHZ]

BTW, now I'm digging back in 20 year old memories of trig class to remember how 120 degrees relates to the square root of 3.

I can tell you it is the ratio of the shorter side to the longer side of a triangle, but most of my trig is kinda rusty. Well except for "right triangle" formulas that I use all the time

Sine, cosine, tangent, it's got to be in here somewhere

You are over complicating it.

Let's take 120/208. Since there are three distinct 120 v to N voltages, and the line to line voltage is 208, it is easy to see that 120x3 does not = 208. It equals 360. 360/208 = 1.73. Square that number and you will get the number of phases, 3.

Now, let's apply the same formula to 120/240. Since there are two distinct 120 to N voltages, and the line to line voltage is 240, we see that 120x2 = 240. 240/240 = 1, and the square of that is 1, indicating a single phase.

I have never tried this with a true two phase system. I don't think it will work as since a 2 phase system isn't symmetric, there may be some other factors related to phase angle to take into consideration.

 

[Joethemechanic]

It's funny how I realized that 120/208 wasn't really single phase the first time. 1980-something my buddy and me were at a liqudation sale and he bought this 10 HP 3 phase radiac saw (abrasive wheel)

Well he only had 3 phase in the front of his building. There was a "single phase" sub-panel in the back, and of course he wanted the saw in the back.

Well anyway, this saw came from the auction with a single phase plug on the end of a piece of 8/3 so cord that some hack had installed on it.

Later on that week, he calls me and says "Johnny (his brother) plugged that saw in down the back, but when he takes a heavy cut, it blows the fuses. I'm like huh:?. I tell him that's impossible and that Johnny must be smoking crack because that saw can't run down there. It needs 3 phase. Pause on the phone, and then he says "Johnny says you're full of _______, Just come down here and look at it.


It took me a good two hours to figure out how the heck that 3 phase saw was starting on "single phase":ashamed1:

 

[Joethemechanic]

You are over complicating it.

Let's take 120/208. Since there are three distinct 120 v to N voltages, and the line to line voltage is 208, it is easy to see that 120x3 does not = 208. It equals 360. 360/208 = 1.73. Square that number and you will get the number of phases, 3.

Now, let's apply the same formula to 120/240. Since there are two distinct 120 to N voltages, and the line to line voltage is 240, we see that 120x2 = 240. 240/240 = 1, and the square of that is 1, indicating a single phase.

I have never tried this with a true two phase system. I don't think it will work as since a 2 phase system isn't symmetric, there may be some other factors related to phase angle to take into consideration.

Oh you didn't want the math/trig answer

BTW here is my 2 phase motor running on 120/240 delta (high leg) hack. I thought I invented it about 20 + years ago, but I recently found out my POCO has been doing it since the 50's to supply customers with 2 phase motors. I thought they only used the Scott-T, but the scott requires a special transformer.

 

[david luchini]

If they were 180 out of phase, Vna would = -Vnb, and the net sum would be zero. (120 + negative 120 = 0)

Since they are in phase, Vna does = Vnb and the net sum is 240. (120 + 120 = 240)

This is not correct per Kirchoff's Voltage Law. Per KVL, the sum of the potential differences around a closed loop is zero, or V1+V2+V3=0. With a load across A-B, V1+V2+V3=0 would become VL+Vbn+Vna=0 (or you could look at the loop in the opposite direction so that VL+Van+Vnb=0.)

So, VL=-Vna-Vbn, or VL=-Vna+Vnb.

If Vna equals Vnb, then the voltage across the load would be zero volts. (VL=-Vna+Vnb = -Vnb+Vnb = 0.)

If Vna equals -Vnb, then the voltage across the load would be 240 volts. (VL=-Vna+Vnb = Vnb+Vnb = 240.)

 

[K8MHZ]

Oh you didn't want the math/trig answer

BTW here is my 2 phase motor running on 120/240 delta (high leg) hack. I thought I invented it about 20 + years ago, but I recently found out my POCO has been doing it since the 50's to supply customers with 2 phase motors. I thought they only used the Scott-T, but the scott requires a special transformer.

Cool! !

 

[Joethemechanic]

Let's take 120/208. Since there are three distinct 120 v to N voltages, and the line to line voltage is 208, it is easy to see that 120x3 does not = 208. It equals 360. 360/208 = 1.73. Square that number and you will get the number of phases, 3.

Hmmmm, I always thought of it as the ratio between the long and short side of a triangle with a 120 degree angle

 

[K8MHZ]

This is not correct per Kirchoff's Voltage Law. Per KVL, the sum of the potential differences around a closed loop is zero, or V1+V2+V3=0. With a load across A-B, V1+V2+V3=0 would become VL+Vbn+Vna=0 (or you could look at the loop in the opposite direction so that VL+Van+Vnb=0.)

So, VL=-Vna-Vbn, or VL=-Vna+Vnb.

If Vna equals Vnb, then the voltage across the load would be zero volts. (VL=-Vna+Vnb = -Vnb+Vnb = 0.)

If Vna equals -Vnb, then the voltage across the load would be 240 volts. (VL=-Vna+Vnb = Vnb+Vnb = 240.)

We are talking about open voltages, not closed loops, and the relationship I am illustrating is not KVL.

Remember, we are summing voltages and getting a positive number as a result, not 0. The fact that the two 120 volt legs add up to 240 volts instead of 0 does not violate KVL.

 

[Joethemechanic]

Cool! !

being that those old 2 phase motors were designed as 220 volt machines, One set of windings would run a little high, and one set a little low since they are not interconnected.

In practice, I never had to even change an overload to make it work. There is only a slight current difference between the phases. Then there is the fact that machinery from the 40's and 50's had quite a bit of "design factor" built into it.