Single Phase or Polyphase?

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mivey

Senior Member
Nope.

As described, you have the same voltage induced in every turn of the secondary. This is the same throughout the transformer (neglecting minor flux leakages).

But you can have different _currents_ flowing via any of the available paths on the secondary.

Consider a standard center tapped single phase transformer. Depending upon the loads connected to the two legs, you can have different magnitude and phase of the current flowing on the two halves of the winding.

-Jon
Exactly

You only appear to have currents flowing in two directions because you choose to use the neutral as your reference point. If you use an end point of a the winding as your reference the currents flow in the same direction.

The number of phases should not depend on if a neutral is or is not used.
There are points in time where the currents will not have the same sign, even with an end reference.
 

mivey

Senior Member
No, I do not mean currents that are in phase but measure 180 degrees out of phase because of the selection of reference point, I mean currents flowing at real different phase angles.

Consider a conventional 120/240V center tapped transformer. I connect a 120V 12A motor from leg A to neutral, and connect 12A of tungsten lighting from leg B to neutral. The current flowing on the transformer from terminal B to neutral will not be in phase with the current flowing from neutral to terminal A.
Spot on.
On this point I am pretty sure that I _disagree_ with Mivey and agree with you. However this is why I posted that there are numerous different uses of the term phase, and some of these usages are not quite consistent with each other.
Don't interpret what I have said to mean that any phases (voltages) go away. they may just not fit in the system bucket of interest. They will wind up in a different system bucket.
The transformer that you've described is clearly a _single_ phase transformer.

Even with a center tap I agree that it is a single phase transformer. This is the common usage of the term phase to describe transformers.
It is also called a single-phase source because it is serving single-phase loads. It can also serve two-phase loads and be called a two-phase source.
But I'm not going to get my knickers in a knot if someone says that I tap phase A and phase B from the terminals of this single phase transformer. It is quite common and well understood to call each separate ungrounded leg of a system a 'phase'.

Furthermore, I am quite comfortable saying that the two legs of this single phase transformer are 180 degrees out of phase. This is yet another usage of the term, one that is entirely dependent upon the selection of reference point.
A load that require simultaneous delivery of two voltages with a 180 degree displacement would be a two-phase load.
 

mivey

Senior Member
Yes, you have what appears to be current from B-> N and current from A->N.
But you can also see this as current flowing from N->B and from A->N.

But the current in the winding comes from the magnetic field created by the primary. The single magnetic field is moving in a 'fixed' direction relative to the secondary winding, therefore the 'different' currents in the winding must actually be in the same direction.

My point has been.
The number of phases should not change based simply on the presence or absence of a neutral.

I have little problem with people saying the currents in a 120/240V system appear to be 180? apart when viewed from the neutral. But that does not make it 2-phase. While it might simplify describing the operation of a 120/240 system it causes problems for similarly constructed ones that do not have or use a neutral, like the 240/480 and the 24/120 examples I have been using.
The primary current is caused by the flux drawn by the secondary load. The flux created by the secondary current causes an opposing flux in the primary which creates the primary current. The only net flux remaining in the core is the magnetizing flux that creates the voltages.
Jim, Larry, please read the specific example again....[/qoute]Well said.
Since this is a single phase transformer, then the _voltage_ induced in the two halves of the secondary has to be in phase (barring losses and leakage, but these are small order terms). But the _current_ flowing the two halves of the secondary does not need to be the same magnitude, and does not have to have the same phase.
Even if there is only one primary pressure wave, it can still produce the same systems that you would get from two pressure waves through two primaries and two series-connected secondaries. The convenience of consolidating the two primaries does not limit the two series-connected secondaries.
 

mivey

Senior Member
I agree. If one puts a pair of batteries in series, you couldn't get 3v if they weren't 'in phase', and likewise with 240v.

Moving the reference point is the only difference between a dual-voltage supply and a bi-polar one. 'Additive' requires 'in phase.'
You are not thinking in terms of voltage systems. With the negative end reference point you can have a single-voltage +1.5v supply system and a single-voltage +3v supply system. With the common reference point you can have a single-voltage +1.5v supply system, a single-voltage -1.5v supply system, or a dual-voltage +-1.5v supply system.

The load that needs a dual-voltage +- supply does not have independent + loads and - loads. It truly requires a dual-voltage supply.
...Regardless they are two different currents, at different phase angles but still flowing in the same relative direction as their voltages.
No, they don't. I can post a plot if it will help.
So, what would you call it if both of these loads were in parallel on a single 120v secondary?
One voltage, one angle = Single-phase
 

mivey

Senior Member
I would call this a single phase system. But as I stated before, I would call the center tapped transformer with the motor on one winding half and the resistor on the other half a single phase transformer.
It would be serving single-phase loads. It is still capable of serving two-phase loads.
If you have a center tapped transformer, and you have a motor in parallel with a resistive load on one half of the center tapped winding, and nothing on the other half, then you still have two _different_ currents flowing in the two winding halves.

Additionally, the two circuits (the one going to the motor and the one going to the resistive load) carry current with different phase angles.

This simply reinforces my point: we are using 'phase' in many different ways, and depending upon which _commonly accepted_ usage you are using, you will get different phase counts.
You will get different phase counts in different system phase-counting buckets. You have not changed the number of voltages in the universe as a whole. Those voltages un-counted in one bucket will fit in a different bucket.
This is a _single_ phase transformer.

It develops a _single_ voltage phase in its windings.

It can supply current with different phase angles. Unless you have extremely odd loads (say a capacitor on one leg and an inductor on the other leg) the different phase angles of these currents will be more or less aligned.

It can carry different currents in the different halves of the secondary winding, with different phase angles (again, more or less aligned, but still different).
The secondary side can be configured to provide the same function you would get from two separate transformers and can thus serve two-phase loads.
Remember that a 'network' service, which clearly has different phase angles available, and is derived from a set of transformer secondaries that are developing different voltage phase angles, is still _defined_ as a single phase service. IMHO this is simply a different use of the word phase.
It is used to supply single-phase loads but is a source for two-phase power in the same manner that an open-wye primary distribution system is also called 2-phase.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
The load that needs a dual-voltage +- supply does not have independent + loads and - loads. It truly requires a dual-voltage supply.
Agreed, but again, the +12/+24v dual-voltage supply differs from the +/- 12v dual-polarity supply only by the chassis/signal ground reference. A pair of 12v batteries in series would power either type of load.
 

mivey

Senior Member
Agreed, but again, the +12/+24v dual-voltage supply differs from the +/- 12v dual-polarity supply only by the chassis/signal ground reference. A pair of 12v batteries in series would power either type of load.
Yes. But the catch is how we define a system of voltages. In any system bucket, the voltages will have the same magnitude. That does not mean that you can't put the other voltages in a different bucket (system). If you use two buckets, you have two systems. If a particular bucket holds n different voltages, that one system has n phases. It is a system of voltages and the question becomes: how many voltages (voltages with differing angles) can we supply to put in the bucket? The bucket with the most voltages becomes the highest order that the source can supply. The source can supply more than one system (grouping of voltages).

For the DC analogy, since a system has only one voltage level:
In the +12/+24v case, we supply two systems containing one voltage each: a 12v system and a 24v system.

In the +/-12v case, we supply one system containing two voltages.

Add: For loads that don't need a dual supply, we could supply two separate 12v systems, each having one voltage.
 
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LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
For the DC analogy, since a system has only one voltage level:
In the +12/+24v case, we supply two systems containing one voltage each: a 12v system and a 24v system.
Okay, and I'll bet you could power a 12v load between the +12 and +24v terminals, as long as they have common negatives.
In the +/-12v case, we supply one system containing two voltages.
Here, too, and obviously, the +/- supply can handle 24v loads.

Either type of supply could power either type of load IF a ground reference is not important to the load, which is untrue of most electronics.

In England, they have an end-grounded 240v supply (one phase of a Y), and their loads will function fine here as long as the load doesn't expect a grounded conductor.

Likewise, our (non-frequency-dependent in both cases) 240v-only devices will function there, because there are no line-to-neutral loads within them.

Add: For loads that don't need a dual supply, we could supply two separate 12v systems, each having one voltage.
Correct, which correlates with my battery example. With two 12v batteries, they can be connected in myriad ways, but if one wanted to parallel them, they must be "in phase."

Likewise, in order for them to add to 24v, they must be connected in seires and what would be the DC equivalent of "in phase" in AC, as must be either of the DC supplies we've been discussing.

Whether you ground one end or the center, both types of DC supplies are effectively two 12v supplies in series. To floating (chassis-isolated) loads, either supply type will do.

In order for the two halves of a CT secondary, or two separate windings, to add to twice the voltage, they must be in phase. And, we agree we're talking voltage, not reactive current.
 

mivey

Senior Member
Okay, and I'll bet you could power a 12v load between the +12 and +24v terminals, as long as they have common negatives.
Yes. It can supply multiple system types.
Here, too, and obviously, the +/- supply can handle 24v loads.
Yes. It can supply multiple system types.
Correct, which correlates with my battery example. With two 12v batteries, they can be connected in myriad ways, but if one wanted to parallel them, they must be "in phase."

Likewise, in order for them to add to 24v, they must be connected in seires and what would be the DC equivalent of "in phase" in AC, as must be either of the DC supplies we've been discussing.

Whether you ground one end or the center, both types of DC supplies are effectively two 12v supplies in series. To floating (chassis-isolated) loads, either supply type will do.

In order for the two halves of a CT secondary, or two separate windings, to add to twice the voltage, they must be in phase. And, we agree we're talking voltage, not reactive current.
That is right. You have two batteries with the "+" on the tit. For analogy's sake: redundant phases.

In AC, there is no universal "tit" on the AC battery so we must arbitrarily pick a reference point. For the DC equivalent, we can have positive tits and negative tits. So we can also have a series connection of a "positive" battery and a "negative" battery. We no longer have redundant batteries (or phases).


Follow this progression:

We can have two independent AC supply voltages that are 180 degrees out of phase. They are two separate phases. If we use the four secondary wires to feed two unequal but inter-related loads in a box, we are feeding the box two separate phases. The box is a combined load that is an unbalanced load as a whole and is using a two-phase supply.

We can take the two AC supply voltages and feed two different transformers. We use the secondaries to feed the two-phase load. We notice because the primary windings are 180 degrees out of phase that we can combine them while keeping the same secondaries. To do this we only have to use one phase on the primary that is twice the individual voltages (or adjust the windings ratio). The secondary is still serving the two-phase load with the original four wires: we still have the original two-phase load.

We now notice that we can combine two of the secondary wires and have the same supply to the two-phase load. We still have the original two-phase load.

Just because the re-configured transformer is now single-phase on the primary, and is normally used to feed single-phase loads on the secondary, does not mean that the secondary is not able to be a two-phase supply for two-phase loads.
 

mivey

Senior Member
Just because the re-configured transformer is now single-phase on the primary, and is normally used to feed single-phase loads on the secondary, does not mean that the secondary is not able to be a two-phase supply for two-phase loads.
FWIW, the reconfigured transformer is a single-phase transformer because it is a single-phase load to its supply. It is a single-phase load but can be a two-phase source. The device load type and device source type do not have to be the same. For example, the open-wye open-delta bank is a two-phase load but can be a three-phase source.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
It can also serve two-phase loads and be called a two-phase source. [...] A load that require simultaneous delivery of two voltages with a 180 degree displacement would be a two-phase load.

Here you are using 'two-phase load' in a fashion that is not consistent with any of the common uses of the term. Certainly it would not be called a 'polyphase' load.

My understanding of polyphase loads is that their operation depends upon the specific phase angle difference of the voltages applied to their multiple circuits. I am specifically distinguishing this from a multi-voltage load which has multiple circuits at different voltages, but does not care about the relative phase angles as long as the voltage requirements are met.

By your description above, a common electric cloths dryer supplied with a 120/240V circuit is a 'polyphase load', and I strongly disagree. The motor and controls require 120V, the heating elements require 240V, but the relative phase angle between these two circuits is entirely irrelevant. The phase angle difference of supply leg A and supply leg B are simply a result of the common approach of supplying these two voltages.

Even if there is only one primary pressure wave, it can still produce the same systems that you would get from two pressure waves through two primaries and two series-connected secondaries. The convenience of consolidating the two primaries does not limit the two series-connected secondaries.

A single phase transformer is substantially limited relative to a polyphase transformer. Unless you add energy storage, the power supplied by such a transformer _must_ fall to zero twice per AC cycle. With two primary circuits at different phase angles, you can supply power on a continuous basis. A single phase transformer with two output phases (two circuits displaced by 180 degrees about the reference point) can no more supply continuous power than a single phase transformer with only one output phase (one output circuit containing the reference point.)

It (a network service) is used to supply single-phase loads but is a source for two-phase power in the same manner that an open-wye primary distribution system is also called 2-phase.

Agreed. A network service is in reality a supplied by a polyphase source that is called a single phase, only because it is used to supply loads that don't care about phase angle. If you wanted to set up appropriate transformers, you could convert this into a common polyphase source and supply common polyphase loads. You could not do this with a corresponding center tapped single phase service.

-Jon
 

mivey

Senior Member
It is more helpful if you point out specific passages which you feel are important to the discussion such as from page 90 of your reference:
.
I agree. A lot of us have libraries full of reference materials. The useful part is finding relevant passages in those reference materials as well as adding any relevant commentary.

That said, from H-E's reference:
Each winding has two ends designated as 1 and 2. The HV winding is indicated by capital letters and the LV winding by small letters. If more terminals are brought out from a winding by way of taps there are numbered in the increasing numbers in accordance to their distance from 1 (eg A1,A2,A3...). If the induced emf at an instant is from A1 to A2 on the HV winding it will rise from a1 to a2 on the LV winding.
Again, the reference point is arbitrary. A rise in the emf from X23 to X1 will produce a rise in the emf from x23 to x1. A rise in the emf from X23 to X4 will produce a rise in the emf from x23 to x4. A rise in the emf from X1 to X4 will produce a rise in the emf from x1 to x4. All of these are valid reference frames. By picking a particular reference, you have defined a particular system. You have not made other choices of reference frames invalid.

If you go back in time to the beginning of time (say the emf application), and say that the emf is a rise from A1 to A2 on the primary, you can also see a fall from the midpoint to A1 and a rise from the midpoint to A2. I can show you a series of two primaries that produces the exact same results using a rise from A1 to A2 on one primary and a rise from A2 to A1 on a second primary. Both will produce the exact same rise and fall of emfs on the secondary. The reference point is not a universal given.

Using a single emf across a primary coil gives the exact result on the secondary that you will get with two 1/2 magnitude waveforms across the primary. The secondary can still be a source for two waveforms with a 180 degree displacement. It is not just math, it is a physical reality.
 

mivey

Senior Member
My understanding of polyphase loads is that their operation depends upon the specific phase angle difference of the voltages applied to their multiple circuits. I am specifically distinguishing this from a multi-voltage load which has multiple circuits at different voltages, but does not care about the relative phase angles as long as the voltage requirements are met.
Me too. But the feed to a 120/240 group of loads does indeed care about the relative phase angle as the neutral would only carry the unbalanced load. Concern with the unbalanced loading can reveal the two-phase characteristic of the load.
By your description above, a common electric cloths dryer supplied with a 120/240V circuit is a 'polyphase load', and I strongly disagree. The motor and controls require 120V, the heating elements require 240V, but the relative phase angle between these two circuits is entirely irrelevant. The phase angle difference of supply leg A and supply leg B are simply a result of the common approach of supplying these two voltages.
That is not per my description, or at least not what I am trying to convey. Since the relative phase angle is not an issue, that would not be an example of a two-phase load. A two-phase load would need the 180 degree displacement. The input for a two-diode full-wave rectifier would need such a source.
A single phase transformer is substantially limited relative to a polyphase transformer. Unless you add energy storage, the power supplied by such a transformer _must_ fall to zero twice per AC cycle. With two primary circuits at different phase angles, you can supply power on a continuous basis. A single phase transformer with two output phases (two circuits displaced by 180 degrees about the reference point) can no more supply continuous power than a single phase transformer with only one output phase (one output circuit containing the reference point.)
There is no requirement for steady delivery of power or we wouldn't have the stipulation over a hundred years ago that a polyphase system would be a system with two or more voltages with phase angles differences of 360/n degrees for an n-phase system.
Agreed. A network service is in reality a supplied by a polyphase source that is called a single phase, only because it is used to supply loads that don't care about phase angle. If you wanted to set up appropriate transformers, you could convert this into a common polyphase source and supply common polyphase loads. You could not do this with a corresponding center tapped single phase service.
I agree that a center-tapped transformer has tapped its potential and can't reach the realm of supplying a 3-phase system.
 

mivey

Senior Member
The motor and controls require 120V, the heating elements require 240V...
BTW, I have never said that 120 volts and 240 volts together would make a two-phase system. I have repeatedly said that the voltages in a system's phase-counting bucket must have the same magnitude. The 120 volt load is served by a 120 volt single-phase system and the 240 volt load is served by a 240 volt single-phase system.
 

gar

Senior Member
100318-1304 EST

From dictionary.com

Word Origin & History

poly-

comb. form meaning "many, much," from Gk. poly-, combining form of polys "much" (plural polloi); cognate with L. plus, from PIE base *ple- (cf. Skt. purvi "much," prayah "mostly;" Avestan perena-, O.Pers. paru "much;" Gk. plethos "people, multitude, great number," pleres "full," polys "much, plenty," ploutos "wealth," plethein "be full;" Lith. pilus "full, abundant;" O.C.S. plunu; Goth. filu "much," O.N. fj?l-, O.E. fela, feola "much, many;" O.E. folgian; O.Ir. lan, Welsh llawn "full;" O.Ir. il, Welsh elu "much"), probably related to base *pele- "to spread."

phase

1.any of the major appearances or aspects in which a thing of varying modes or conditions manifests itself to the eye or mind.

.....

8.Physics. a particular stage or point of advancement in a cycle; the fractional part of the period through which the time has advanced, measured from some arbitrary origin often expressed as an angle (phase angle), the entire period being taken as 360?.


pol?y?phase

?adjective Electricity.
1.having more than one phase.
2.of or pertaining to a set of alternating currents that have the same frequency but different phases and that enter a specified region at more than two points.

--------------------------------------------------------------------------------

Origin:
1890?95; poly- + phase
.
 

Hameedulla-Ekhlas

Senior Member
Location
AFG
I agree. A lot of us have libraries full of reference materials. The useful part is finding relevant passages in those reference materials as well as adding any relevant commentary..


The only reason that I did not interfere was that long discussion has been done and still there is no agreement. Since the real questioned has already been asnwered many times and there is only dispute on single phase and polyphase system and connection method. According to me,

An ac generator designed to develop a single sinusoidal voltage for
each rotation of the shaft (rotor) is referred to as a single-phase ac generator.
If the number of coils on the rotor is increased in a specified
manner, the result is a polyphase ac generator, which develops more
than one ac phase voltage per rotation of the rotor.

The number of phase voltages that can be produced by a polyphase
generator is not limited to three. Any number of phases can be obtained
by spacing the windings for each phase at the proper angular position
around the stator. Some electrical systems operate more efficiently if
more than three phases are used. One such system involves the process
of rectification, which is used to convert an alternating output to one
having an average, or dc, value. The greater the number of phases, the
smoother the dc output of the system.

Please download this pdf file too from below link

http://www.4shared.com/file/159137998/df0566dc/Polyphase_Electric_Currents_AC.html?s=1

and see these pages

Page-43 : combination of polyphase current
Page-45 : Two phase system and continue to page-46
Page-176 : Chapter X: Polyphase transformer
Page-179: Phase transformation to Page-181
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Now that I am back home, I have looked up some of my old college resources. Granted these are only two 'ancient' references, but it is what I was taught.

From Schaums' outline Series "Theory and Problems of ELECTRIC CIRCUITS" by Joseph A. Edminister, McGraw-Hill Book Company, copyright 1965.
Here he is discussing 3-phase system voltages
The system voltage is the voltage between any pair of lines, A and B, B and C, or C and A. And in the four-wire system, the magnitude of the line to neutral voltage is 1/SQRT3 times line voltage.
In this case he is describing Sinusoidal Current and Voltage.
If a voltage and a current are both sinusoidal functions of time, a plot of both to the same time scale will show a displacement between them except for the case of pure resistance. This displacement is the phase angle and never exceeds 90? or Pi/2 radians. By agreement this phase angle is always described as "what the current i does with respect to the voltage v"...
Another textbook, 'Electromechanical Devices for Energy Conversion and Control Systems' by Vincent Del Toro copyright 1968.
Transformers under no load
But, as has already been observed in the preceding step, the emf induced in the secondary winding must be in phase with the corresponding voltage E1 of the primary winding.
Transformers under load
By Lenz's law there is an emf induced in the secondary winding which instantaneously makes terminal c positive with respect to terminal d. When switch S is closed , a current I2 flows instantaneously from c to d.

So my take-aways, and the basis of my arguments, have been:
The number of phases can be determined by the number of voltages between pairs of line conductors.
Current is always 'in the same direction' as voltage even though there is a phase angle between them.
The direction of the voltage in a transformer secondary is related to the direction of the voltage in the primary.

If you want to ignore the physics of a single winding transformer, and consider it a 'black box' with voltage leads then, of course, you may manipulate the voltage and current directions in any way you wish. But, you should state that is what you are doing.
 
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mivey

Senior Member
Now that I am back home, I have looked up some of my old college resources...If you want to ignore the physics of a single winding transformer, and consider it a 'black box' with voltage leads then, of course, you may manipulate the voltage and current directions in any way you wish. But, you should state that is what you are doing.
What you are ignoring is that you are either using a preferred reference point or that you are using a pre-defined reference point. Nothing you posted showed I was ignoring physics.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Post 79
mivey said:
Let's use the 240/480 volt transformer with the X1, X23, X4 terminals for example so we can put in some real numbers for some example systems:
….
X23 Reference: We can have two single-phase systems with one voltage each. One single-phase system is 240@0 and the other single-phase system is 240@180. This system can serve two independent loads from separate sides of the winding...

Aren’t you saying there are two voltages in different directions available from a single winding?

Also
mivey said:
Regardless of the direction you pick to be positive, there are times when the current in one side of the coil is positive and the current in the other side of the coil is negative. They are actually flowing in two different directions.
Aren’t you saying, here, that a single winding can have current flowing in different directions, regardless of the voltage direction?

Post 80
mivey said:
Pick any direction you want, but the currents will not always flow in the same direction…

This contradicts the source I quote that applied Lenz’s law.
 
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