Single Phasing effect on rotation of 3 Phase Motor
- Thread starter mull982
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Mull,mull982 said:
Your diagram shows a sine wave starting at time 0. In that case, we can define Vab to have a phase angle of 0. Then, Vba will have a phase angle of 180 degrees.
In fixed phasor form we have,
Vab = 240V @ 0, phasor arrow points to the right
Vba = 240V @ 180, phasor arrow points to the left
Phasor arrows do not move.
Fixed phasors are used in steady state analyses.
In rotating phasor form we have,
vab(t) = 339V*sin(wt)
vba(t) = 339V*sin(wt + 180)
Phasor arrows point away from each other and rotate CCW--one rotation per cycle.
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For this to happen you need a third point, You can then describe any two points (i.e. the ends of the single coil) in relation to the reference point.mull982 said:
The relationship between the two ends of a non-center tapped single coil do not change.
You are free to choose a third point, say your ipod, and correctly describe the relationship between the two coil ends and it. But I feel this simply adds confusion to the situation, which is why I suggest describing single-phase and three-phase systems/circuit based on the number of Line-Line voltages that exist regardless of the presence or absence of a neutral.
rattus said:
Ok I'm starting to see the vectors Vab and Vba.
I guess whast started all of this and whats leading to my confusion, is the vectors diagram I keep drawing which I am getting from this article:
http://www.allaboutcircuits.com/vol_2/chpt_13/9.html
mull982 said:
Mull, the diagram you reference describes the magnetic field in the motor. Do not confuse this with the phasors which describe the voltages.
I would argue too that the magnetic field is properly describe by a "vector" which has magnitude and direction while voltages are properly described by "phasors" which have magnitudes and phase angles.
Also, the "magnitude" is always a positive number.
"Negative magnitude" is an oxymoron.
Now, if you center tap your transformer to provide a neutral, the equations become,
Van = 120V @ 0
Vbn = 120V @ 180
van(t) = 170V*sin(wt)
vbn(t) = 170V*sin(wt + 180)
I would argue also, that the use of the neutral as a reference is standard procedure in engineering work.
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rattus said:
But it can add confusion to circuits that do not contain a neutral, like a single non-center tapped motor or transfromer winding, which are standard circuits.
winnie
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Guys, please don't sidetrack Mull's question with a rehash of the argument about inverse versus phase shift in single phase systems. We've already filled hundreds of posts on that topic, and while I am quite willing to go another round or three on the topic, let's do that in another thread 
Mull, here is my stab at the 'two vectors' issue:
In the real single phase motor, there is a _single_ changing magnetic field. This magnetic field does not change direction, but continuously changes magnitude, following the AC current waveform in the coil.
Now imagine that you had _two_ rotating magnetic fields, both rotating at the same frequency, but in opposite directions, aligned so that their peaks match up with the orientation of the real magnetic field in the motor, and each with a magnitude half the peak of the real field.
Take the _sum_ of these two "fictional" rotating magnetic fields. This sum will be the real field in the motor. When these two equal magnitude vectors point in opposite directions, the total flux is zero; when they point in the same direction the total flux is maximum, and because they are equal and balanced, the total flux always points along the same axis.
I put fictional in quotes above, because while the _real_ magnetic field has a fixed direction and is simply altering in magnitude, and these component rotating fields seem like a mathematical trick, they are present in a very significant fashion.
If you had some sensor which responded to one of these "fictional" rotating fields more strongly than the other, then rather then seeing the non-rotating balanced sum, that sensor would really 'see' a rotating field.
A squirrel cage rotor spinning in this fluctuating magnetic field would be at different slip relative to the two rotating component fields. Because of the different slip, the two fields will put different torque on the rotor. The rotor is responding differently to the two rotating fields, and thus 'sees' a net rotating field. If the rotor is spinning near synchronous speed for one of these component rotating fields, then it is at very high slip for the other. The net result is that one field will totally dominate the other.
Now when the rotor is exactly stopped, it has the _same_ slip relative to the two fields, and thus does not see a net rotating field. This is why single phase motors are not 'self starting'.
-Jon
Mull, here is my stab at the 'two vectors' issue:
In the real single phase motor, there is a _single_ changing magnetic field. This magnetic field does not change direction, but continuously changes magnitude, following the AC current waveform in the coil.
Now imagine that you had _two_ rotating magnetic fields, both rotating at the same frequency, but in opposite directions, aligned so that their peaks match up with the orientation of the real magnetic field in the motor, and each with a magnitude half the peak of the real field.
Take the _sum_ of these two "fictional" rotating magnetic fields. This sum will be the real field in the motor. When these two equal magnitude vectors point in opposite directions, the total flux is zero; when they point in the same direction the total flux is maximum, and because they are equal and balanced, the total flux always points along the same axis.
I put fictional in quotes above, because while the _real_ magnetic field has a fixed direction and is simply altering in magnitude, and these component rotating fields seem like a mathematical trick, they are present in a very significant fashion.
If you had some sensor which responded to one of these "fictional" rotating fields more strongly than the other, then rather then seeing the non-rotating balanced sum, that sensor would really 'see' a rotating field.
A squirrel cage rotor spinning in this fluctuating magnetic field would be at different slip relative to the two rotating component fields. Because of the different slip, the two fields will put different torque on the rotor. The rotor is responding differently to the two rotating fields, and thus 'sees' a net rotating field. If the rotor is spinning near synchronous speed for one of these component rotating fields, then it is at very high slip for the other. The net result is that one field will totally dominate the other.
Now when the rotor is exactly stopped, it has the _same_ slip relative to the two fields, and thus does not see a net rotating field. This is why single phase motors are not 'self starting'.
-Jon
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mull982 said:
There are several different ways to build a single phase motor.
The most common uses only a single winding and plays games with the flux density of the magnetic circuit. This is the type of motor used in most portable tools and very low HP requirements like clocks and bathroom fans.
More powerful single phase motors have two windings. Typical one of these windings is used for starting and the other for running.
winnie said:Guys, please don't sidetrack Mull's question with a rehash of the argument about inverse versus phase shift in single phase systems. We've already filled hundreds of posts on that topic, and while I am quite willing to go another round or three on the topic, let's do that in another thread
Mull, here is my stab at the 'two vectors' issue:
In the real single phase motor, there is a _single_ changing magnetic field. This magnetic field does not change direction, but continuously changes magnitude, following the AC current waveform in the coil.
Now imagine that you had _two_ rotating magnetic fields, both rotating at the same frequency, but in opposite directions, aligned so that their peaks match up with the orientation of the real magnetic field in the motor, and each with a magnitude half the peak of the real field.
Take the _sum_ of these two "fictional" rotating magnetic fields. This sum will be the real field in the motor. When these two equal magnitude vectors point in opposite directions, the total flux is zero; when they point in the same direction the total flux is maximum, and because they are equal and balanced, the total flux always points along the same axis.
I put fictional in quotes above, because while the _real_ magnetic field has a fixed direction and is simply altering in magnitude, and these component rotating fields seem like a mathematical trick, they are present in a very significant fashion.
If you had some sensor which responded to one of these "fictional" rotating fields more strongly than the other, then rather then seeing the non-rotating balanced sum, that sensor would really 'see' a rotating field.
A squirrel cage rotor spinning in this fluctuating magnetic field would be at different slip relative to the two rotating component fields. Because of the different slip, the two fields will put different torque on the rotor. The rotor is responding differently to the two rotating fields, and thus 'sees' a net rotating field. If the rotor is spinning near synchronous speed for one of these component rotating fields, then it is at very high slip for the other. The net result is that one field will totally dominate the other.
Now when the rotor is exactly stopped, it has the _same_ slip relative to the two fields, and thus does not see a net rotating field. This is why single phase motors are not 'self starting'.
-Jon
Great explanation Winnie Thanks! I understand now that these vectors are actually magnetic field vectors whose net sum is either in one direction or the other depending on the magnitude of the waveform at any given time. (I'm assuming the direction will also follow the right-hand thumb rule relating the direction of the magnetic field to the direction of the current flowing through the coil)
Now that I understand this is the magnetic field vector I was looking at, does the same principle apply to a three phase motor that is single phased. I'm thinking that the only difference is that instead of one coil we will now have two coils in which the magnigude of the magnetic field is changing (at the same time) in each coil. Does the fact that these voltages were 120deg apart have any effect on this issue or since they are now single phases are they 180deg apart just like a regular single phase source?
The one last part thats confusing to me is when we talk about the number of windings. When we talk about one winding are we just talking about a single coil is a motor in which the rotor is above or below this coil? Would a single winding look like figure A or figure B in the attached sketch? If we use two coils to start a motor as mentioned and then eliminate one of the coils after start that one coil will still allow the rotor to rotate, and wont cause it to come to a stop and therfore try to move back and forth between the ends of the coil?
Ive almost Got It!
winnie
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Sorry about the delay in reply; I spent a bit of time trying to find a good image of a stator winding. What I want to show you is really '3-D', and what I'd love to do is take apart a motor and sketch the thing out by actually pointing to the parts.
When we say 'one coil' we are actually over-simplifying. Virtually no induction motor actually has a _single_ coil, even the simplest 2-pole single phase motor. Instead there are a set of coils of roughly the same orientation. These coils literally _surround_ the rotor.
Here is a good view of a stator: http://www.ewh.ieee.org/soc/es/Nov1997/09/A_DIS1.JPG A 'coil' follows a path _down_ one slot, around the 'end turn' _up_ another slot, around the 'end turn', and back to the first slot. The relative placement of the slots determine the sort of magnetic field structure that the 'winding' will produce.
I found this set of animations: http://www.ece.umn.edu/users/riaz/mrmovies/listmovie.html and clip 8 ("Magnetic field distribution due to single-phase excitation. " does a pretty good job of showing a single phase 2 pole field. The dots at the edge represent the current flow in the slots, as seen edgewise.
Just as an aside: Keep in mind when you look at this that the 'big vector' representing the entire field is a tremendous simplification; what really matters is the distribution of the 'little vectors' representing the flux crossing the gap between rotor and stator; if you stay stuck on the 'big vector' then you will get stuck when you go to '4-pole' motors. (for another time)
A three phase motor has three _sets_ of coils placed at different orientations, each 120 electrical degrees apart. (Electrical degrees means "as measured relative to the magnetic field.") Each of the sets of coils has individual coils which are not at exactly the same angle; but they are treated as a distributed set with a single net angle.
When you single phase a three phase motor, you still have sets of coils operating, just with a somewhat different distribution than in a 'proper' single phase machine.
Phase angle cannot be defined with only two supply terminals. Voltage is measured between two terminals, and phase angle is measured between _two_ voltages. When you single phase a motor, all of the remaining energized coils are energized at the _same_ time; there is no phase angle difference.
-Jon
When we say 'one coil' we are actually over-simplifying. Virtually no induction motor actually has a _single_ coil, even the simplest 2-pole single phase motor. Instead there are a set of coils of roughly the same orientation. These coils literally _surround_ the rotor.
Here is a good view of a stator: http://www.ewh.ieee.org/soc/es/Nov1997/09/A_DIS1.JPG A 'coil' follows a path _down_ one slot, around the 'end turn' _up_ another slot, around the 'end turn', and back to the first slot. The relative placement of the slots determine the sort of magnetic field structure that the 'winding' will produce.
I found this set of animations: http://www.ece.umn.edu/users/riaz/mrmovies/listmovie.html and clip 8 ("Magnetic field distribution due to single-phase excitation. " does a pretty good job of showing a single phase 2 pole field. The dots at the edge represent the current flow in the slots, as seen edgewise.
Just as an aside: Keep in mind when you look at this that the 'big vector' representing the entire field is a tremendous simplification; what really matters is the distribution of the 'little vectors' representing the flux crossing the gap between rotor and stator; if you stay stuck on the 'big vector' then you will get stuck when you go to '4-pole' motors. (for another time)
A three phase motor has three _sets_ of coils placed at different orientations, each 120 electrical degrees apart. (Electrical degrees means "as measured relative to the magnetic field.") Each of the sets of coils has individual coils which are not at exactly the same angle; but they are treated as a distributed set with a single net angle.
When you single phase a three phase motor, you still have sets of coils operating, just with a somewhat different distribution than in a 'proper' single phase machine.
Phase angle cannot be defined with only two supply terminals. Voltage is measured between two terminals, and phase angle is measured between _two_ voltages. When you single phase a motor, all of the remaining energized coils are energized at the _same_ time; there is no phase angle difference.
-Jon
winnie said:
Winnie
Thanks again for the explanation and visuals they really help. I completely understand now how the magnetic field vector is a sum of all the individual little magnetic field vectors.
The one thing when looking at these little vectors was their orientation. Im assuming the red dots is the current coming in and out of the windings or slots as you referred to but what are the white dots representing(Strongest field)? Why are the arrows on one have os the bottom windings pointing in towards the rotor and the arrows on the other half point away from the rotor? Is this because of the orientation of the slots? Looking at it if the slots are all in a line and the current is passing through them the same I would think that the magnetic field arrows would point in the same direction not some in towards the rotor and others away. I think once I see this i'll have it.
winnie
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- Electric motor research
I've no idea what the white dots represent.
The reason that the arrows point to the rotor on one side and away from the rotor on the other side is that one side of the rotor (stator) is a north pole, and the other side is a 'south' pole. Take a look at the video right at the start. On the 'bottom' conductors, current is going into the page; on the 'top' conductors current is coming out of the page. The magnetic field is going left to right, lined up perpendicular to the 'coil' set.
-Jon
The reason that the arrows point to the rotor on one side and away from the rotor on the other side is that one side of the rotor (stator) is a north pole, and the other side is a 'south' pole. Take a look at the video right at the start. On the 'bottom' conductors, current is going into the page; on the 'top' conductors current is coming out of the page. The magnetic field is going left to right, lined up perpendicular to the 'coil' set.
-Jon
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- Wisconsin
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- PE (Retired) - Power Systems
winnie said:
I looks like they are simply reference points for the "red arrow" alignment.
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