Do 2 coils 90? apart supply a 2phase load ?

[dnem]

This is an offshoot from another thread on this board, ?Single phase / Two Phase Discussion?.
I posted this on Post #39

Reading this entire discussion up to this point has lead me to the conclusion that the word ?phase? has more than one meaning in our electrical industry and in our discussions and can not have an agreed upon usage until it is defined more specifically.

I?m seeing 3 different definitions being used:
1) phase coil, which can number 1, 2, or 3 (and theoretically higher but not found in reality)
2) phase load, which can number 1 or 3
3) phase conductor, which can number 1, 2, or 3

A single phase coil is connected to a single phase load.
3phase coils are connected to a 3phase load.
But 2phase coils are connected to a single phase load. . There is no 2phase load. . And that?s where most of the dispute is coming from.

So I made the statement, ?There is no 2phase load.?
But Engy made a statement in Post #40 that got me thinking.

You can only connect 1-phase loads to this animal, which is why I call it single phase.

It ain't two phase - doesn't have two phases 90degrees out of phase...
It ain't three phase - cause we only brought two of the three phases...

Maybe it's not single phase either

?It ain't two phase - doesn't have two phases 90degrees out of phase.?
I?ve never dealt with the 90? out of phase animal but going way back in my memory to the time when I was learning theory, I believe that I remember how this thing worked.

Rather than having a capacitor introduce a phase shift to provide starting rotation, I think I remember that this one has 2 transformer phase coils with voltages induced 90? apart. . One end of each transformer is brought together to provide a 3wire output. . The motor that would run off of this supply would also have 2 separate coils. . The phase difference between the motor coils would not only provide starting rotation but add torque during the entire motor running time.

I believe the common wire is grounded but there?s only one voltage. . Hot to hot is the same voltage as hot to ground. . Pick any 2 of the 3 leads and you get the same voltage as any other 2.

If I?m remembering all of this correctly than there is such an animal as a 2phase load. . This would be different from the 120/208Y 3wire wye supply with one of the 3 transformer coils missing. . It supplies dual voltage and the load uses either one voltage or the other, so every load connected is a single phase load. . Even appliances or other equipment that use both voltages, use them on independent functions within the appliance, like 208v heating element totally separate from the 120v controls. . So both loads are single phase loads.

But this rare 3wire 90? 2phase coils appears to supply the only example of an actual 2phase load that I can think of.

Can anybody either reinforce or correct my memory ?

David

 

[jcormack]

Two phase
From Wikipedia, the free encyclopedia


Two-phase electrical power was an early 20th century polyphase alternating current electric power distribution system. Two circuits, or "phases", were used, with voltages 90 electrical degrees apart in time. Usually circuits used four wires, two for each phase. Less frequently, three wires were used, with a common wire with a larger-diameter conductor. The generators at Niagara Falls installed in 1895 were the largest generators in the world at the time and were two-phase machines. Some early two-phase generators had two complete rotor and field assemblies, mechanically shifted by 90 mechanical degrees to provide two-phase power.

The advantage of two-phase electrical power was that it allowed for simple, self-starting electric motors. In the early days of electrical engineering, it was easier to analyze and design two-phase systems where the phases were completely separated, since this avoided the need for the effect of unbalanced loads. It was not until the invention of symmetrical components that three-phase power systems had a convenient mathematical tool for describing unbalanced load cases. The revolving magnetic field produced with a two-phase system allowed electric motors to provide torque from zero motor speed, which was not possible with a single-phase induction motor (without extra starting means). Induction motors designed for two-phase operation use the same winding configuration as capacitor start single-phase motors.

Three-phase electric power requires less conductor mass for the same voltage and overall amount of power. It has all but replaced two-phase power for commercial distribution of electrical energy, but two-phase circuits are still found in certain control systems.

Two-phase power can be derived from a three-phase source using two transformers in a Scott connection. One transformer primary is connected across two phases of the supply. The second transformer is connected to a center-tap of the first transformer, and is wound for 86.6% of the phase-to-phase voltage on the 3-phase system. The secondaries of the transformers will have two phases 90 degrees apart in time, and a balanced two-phase load will be evenly balanced over the three supply phases.

Three-wire, 120/240 volt single phase power used in the USA and Canada is sometimes incorrectly called "two-phase". The proper term is split phase or 3-wire single-phase.

 

[winnie]

Phase angle can be totally arbitrary.

Transformers used for high power rectifier systems are often arranged with multiple secondaries so that you have 6, 9, or more _different_ phase angles on the secondary side. The simplest system used is to have a three phase delta primary, and a dual wye/delta secondary, where you have 2 secondary coils on each phase, one connected as part of a wye set, the other connected as part of a delta set.

With a VSD that has suitable output stages, you can arbitrarily synthesize any phase angle difference you want. My current job involves research on electric motor systems where you _change_ the phase angles being supplied to the motor.

The 'historic' two phase system had a phase angle difference of 90 degrees between the phases. There were various arrangements, including a three wire system with a common conductor, 4 wire systems where the two phases were electrically isolated from each other, and 5 wire systems where the two phases shared a common center tap.

In a 3 wire 90 degree two phase system, you have 3 supply legs, A, B and Common.
If we define A to Common to be 1V with phase angle 0,
Then B to Common is 1V with phase angle 90,
and B to A is 1.414V with phase angle 135 (think 45 degrees off from the reference)

-Jon

 

[Smart $]

In a 3 wire 90 degree two phase system, you have 3 supply legs, A, B and Common.

If we define A to Common to be 1V with phase angle 0,
Then B to Common is 1V with phase angle 90,
and B to A is 1.414V with phase angle 135 (think 45 degrees off from the reference)

Not that it makes much difference, the A to B voltage angle would be -45? if A to Common is 0?, and B to Common is 90?, where 0? is the reference. A voltage angle of 135? would be the inverted B to A voltage angle.

To the best of my knowledge, A-B voltage was never used.

 

[dnem]

In a 3 wire 90 degree two phase system, you have 3 supply legs, A, B and Common.
If we define A to Common to be 1V with phase angle 0,
Then B to Common is 1V with phase angle 90,
and B to A is 1.414V with phase angle 135 (think 45 degrees off from the reference)

-Jon

I would have thought that the B to A would have been 1V.

With the 90? phase angle, when A is max voltage, B is zero. . And when B is max voltage, A is zero. . So wouldn't the voltage be the same A to common, B to common, or A to B ?

 

[dnem]

Two phase
From Wikipedia, the free encyclopedia

It was not until the invention of symmetrical components that three-phase power systems had a convenient mathematical tool for describing unbalanced load cases.

Can anyone explain that sentence ? . How would symetrical components affect the mathmatical equations used ?

 

[al hildenbrand]

Can anyone explain that sentence ? . How would symetrical components affect the mathmatical equations used ?

In a nutshell, the math is simplified.

The real time sine waves in a 120/240 3 wire delta total to 6. There are 3 voltages and 3 currents. When the currents are different in magnitude from each other and not equally separated in time, and a power factor is present that shifts the voltages in time from the currents, the math required to analyze the sine waves is daunting, to say the least. Lots of calculus and trigonometry.

Taking the sine waves and representing them with phasors helps somewhat, but there are 6 phasors at odd angles and unequal magnitudes to then apply trigonometry to.

Symmetrical components is a trick of representing the unbalanced 6 phasors with three different sets of phasors. The three sets are the positive sequence, negative sequence and zero sequence. The positive and negative sequence sets of phasors are each comprised of three phasors of equal magnitude and spaced exactly 120? apart, and the sets are understood to be rotating in opposite directions.

The zero sequence phasors are an interesting animal in that the three phasors in this set, also of equal magnitude, all point in the same direction.

The real time voltages will be represented by on set of symmetrical components (9 phasors) and the real time currents also will be represented by another set of symmetrical components.

Manipulating the symmetrical components is a simpler math than manipulating the unsymmetrical phasors.

In a way, its kind of like taking the log of a number to add it or subtract it from the log of another number, and then taking the anti-log of the result to get what one would have gotten by multiplication. One multiplies, using logs by simple addition, which is a simplified math.

 

[hockeyoligist2]

Way over my head!!!!!!!!

 

[tallgirl]

I would have thought that the B to A would have been 1V.

With the 90? phase angle, when A is max voltage, B is zero. . And when B is max voltage, A is zero. . So wouldn't the voltage be the same A to common, B to common, or A to B ?

Nope, because the maximum value of sin (A) - sin (B) occurs at A = 135* and B = 215* (and again 180* later),which is still 90* apart. Since sin(A) = 0.707 and sin(B) = -0.707, for a total of 1.414, the maximum voltage is 1.414 times the voltage of A or B to common.

 

[al hildenbrand]

Way over my head!!!!!!!!

Yeah, it's like that.

It sounds horrible. But when one gets through the math of balanced circuits, and then starts that first asymmetrical circuit, it's a real eye opener the way symmetrical components helps.

 

[iwire]

But when one gets through the math of balanced circuits, and then starts that first asymmetrical circuit, it's a real eye opener the way symmetrical components helps.

Nice try Al,:smile: but I still feel like that is beyond me.

 

[winnie]

I would have thought that the B to A would have been 1V.

With the 90? phase angle, when A is max voltage, B is zero. . And when B is max voltage, A is zero. . So wouldn't the voltage be the same A to common, B to common, or A to B ?

Yes, but figure out the values at the 45 degree points around the cycle; these will be the zeros and peaks of the difference voltage between leg A and leg B.

There is a very useful tool that can be used to solve the various voltage differences in polyphase systems. If you limit yourself to sinusoidal voltages, all of the same frequency, allow different voltages and phase angles, then you can _represent_ each of those sinusoidal voltages as a simple vector. The length of the vector represents the voltage, and the angle of the vector represents the phase angle. Then all of this math simply becomes measuring the distance and angle of vectors on paper.

In this 2 phase example, we _represent_ leg A as a length 1 vector from the the origin, with angle 0. Leg B is a length 1 vector from the origin, with angle 90. The voltage from A to B is simply represented by the vector from the tip of A to the tip of B.

Using this approach, you can calculate the line voltage for any phase angle, and for imbalanced supply legs.

There is a similar vector representation of sinusoidal current that you can use to figure out how currents from different parts of the circuit will add up on shared legs.

-Jon

 

[hardworkingstiff]

Nice try Al,:smile: but I still feel like that is beyond me.

Yea, me too, but it's enjoyable reading it and trying to understand. Thanks everyone who's sharing their knowledge.

 

[dnem]

In a nutshell, the math is simplified.....
..... One multiplies, using logs by simple addition, which is a simplified math.

If that's the simplified explanation, then I'm in trouble. . I didn't understand a word you said :-?

David

 

[dnem]

In this 2 phase example, we _represent_ leg A as a length 1 vector from the the origin, with angle 0. Leg B is a length 1 vector from the origin, with angle 90. The voltage from A to B is simply represented by the vector from the tip of A to the tip of B.

Jon,

Would this be the same concept as used for right angle triangles ?
The side opposite of the right angle is always longer than either of the 2 other sides. . In this case the 2 other sides are the same length.

Now if the phase angle between the 2 phases is 60?, would every voltage be the same. . A to common, B to common, A to B ?

Math isn't my worst subject but it's not my best either. . If what I said above is right, then I think I might understanding the basic concept.

David

 

[winnie]

Jon,

Would this be the same concept as used for right angle triangles ?
The side opposite of the right angle is always longer than either of the 2 other sides. . In this case the 2 other sides are the same length.

Now if the phase angle between the 2 phases is 60?, would every voltage be the same. . A to common, B to common, A to B ?

Yes, exactly.

If the included angle is 0?, then the two legs have the same phase angle, and the voltage between them is 0.

If the included angle is 60?, then the voltage between the legs is the same as the leg to common voltage. You essentially have an 'open delta'.

If the included angle is 120?, then the voltage between the legs is 1.732 * leg-common. This is the relationship between line-line and line-neutral voltage in a common three phase wye system.

If the included angle is 180?, then the line-line voltage is 2 * line-common voltage. This is the relationship found in a common center tap single phase system.

-Jon

 

[al hildenbrand]

If that's the simplified explanation, then I'm in trouble. . I didn't understand a word you said :-?

I apologize, David. Let me try one other approach.

Symmetrical Components, as a math concept, were introduced in a paper presented by Charles Fortesque in 1918.

That presentation was over 35 years after Nikola Tesla conceived the rotating magnetic field principal and motors that were mechanically simple, developed useful horsepower and were powered by AC. With the manufacturing and marketing of Westinghouse, in spite of Edison, 3? long distance power distribution became the standard.

As the installed and operating transmission line miles accumulated and generators became more numerous and expensive, the ways that the system would break was hard to analyze. Although fuses and circuit breakers helped to protect the generators, there were faults and overloads that simple overcurrent protection didn't address.

As understanding Fortesque's math of Symmetrical Components spread, the development of PoCo distribution system protective relays occurred. The relays, electro-mechanical machines, would respond to the electrical manifestations of symmetrical components, and the relay would operate the circuit breaker. From Wikepedia:

Physically, in a three phase winding a positive sequence set of currents produces a normal rotating field, a negative sequence set produces a field with the opposite rotation, and the zero sequence set produces a field that oscillates but does not rotate. Since these effects can be detected physically, the mathematical tool became the the basis for the design of protection relays, which used negative-sequence voltages and currents as a reliable indicator of fault conditions. Such relays may be used to trip circuit breakers or take other steps to protect electrical systems.

And again from Wikepedia:

Such relays were very elaborate, using arrays of induction disks, shaded-pole magnets, operating and restraint coils, solenoid-type operators, telephone-relay style contacts, and phase-shifting networks to allow the relay to respond to such conditions as over-current, over-voltage, reverse power flow, over- and under- frequency, and even distance relays that would trip for faults up to a certain distance away from a substation but not beyond that point. An important transmission line or generator unit would have had cubicles dedicated to protection, with a score of individual electromechanical devices.

The electro-mechanical relay of the 20th century is being replaced by microprocessor based "numerical relays", but on an as-needed basis.

 

[dnem]

Yes, exactly.

If the included angle is 0?, then the two legs have the same phase angle, and the voltage between them is 0.

If the included angle is 60?, then the voltage between the legs is the same as the leg to common voltage. You essentially have an 'open delta'.

If the included angle is 120?, then the voltage between the legs is 1.732 * leg-common. This is the relationship between line-line and line-neutral voltage in a common three phase wye system.

If the included angle is 180?, then the line-line voltage is 2 * line-common voltage. This is the relationship found in a common center tap single phase system.

-Jon

One last question.

I can see the 120? difference in the sine wave pattern with the ground a flat base line reference line for a wye system.
I can see 120? difference in the angles of the wye.
I can see the 120? difference in the sine wave pattern with the ground a flat base line reference line for a delta system.
So why an I looking at a 60? difference in the angles of the delta ?

 

[dnem]

Al,

Everything you said in that last post was understandable so I went from 0% to 100% understood. . I can see what type of issues they were dealing with in the early days of power distribution. . What I don't see is how these issues differed from one system to another.

The 90? 2phase would have also provided either forward or backwards rotation.

"the mathematical tool became the the basis for the design of protection relays, which used negative-sequence voltages and currents as a reliable indicator of fault conditions."
And that couldn't have been done with 90? 2phase ?

David

 

[winnie]

One last question.

I can see the 120? difference in the sine wave pattern with the ground a flat base line reference line for a wye system.
I can see 120? difference in the angles of the wye.
I can see the 120? difference in the sine wave pattern with the ground a flat base line reference line for a delta system.
So why an I looking at a 60? difference in the angles of the delta ?

Simply point of view.

The voltage from B to A is always the inverse of the voltage from A to B.

The inverse of a sine wave cannot be distinguished from that same sine wave shifted by 180 degrees.

60 + 120 = 180, so depending upon which side you reference each of your voltages from, you can see either 60 or 120 degrees of phase difference.

-Jon