Eddy Current
Senior Member
Im having a hard time picturing how a stators magnetic field can rotate?
How does a stator's magnetic field rotate in an AC motor?
- Thread starter Eddy Current
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In the simplest form, you use a three phase winding oriented so that each phase winding produces a field offset by 120 degrees from the other two windings. That way as the field strength in each winding changes over time during the AC cycle, the direction of the resultant magnetic field (the vector sum of the field from each of the three windings) appears to rotate.
For single phase motor, you use a secondary winding (start or run winding) which is both rotated and fed through a series capacitor which offsets the current waveform from that in the main winding.
Eddy Current
Senior Member
When you say "over time" do you mean each 360 degree cycle?
K8MHZ
Senior Member
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- Electrician
It doesn't really 'rotate'.
Three phase is easier to understand. Oversimplified but, there are three (or more) electromagnets forming a 'circle' if you will. Each magnet is evenly spaced around the 'circle'. They are turned on and off so the most intense magnetism moves from one electromagnet to the other. That movement appears to be circular and will engage a rotor to move in a circular (spinning) motion.
Three phase is easier to understand. Oversimplified but, there are three (or more) electromagnets forming a 'circle' if you will. Each magnet is evenly spaced around the 'circle'. They are turned on and off so the most intense magnetism moves from one electromagnet to the other. That movement appears to be circular and will engage a rotor to move in a circular (spinning) motion.
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That may be easier for some to understand, and provides a usable explanation of why the rotor turns in response, but if you look at the vector which represents the total magnetic field at any moment, it actually does rotate. That vector may vary in amplitude somewhat and may not rotate at a perfectly uniform rate, depending on the geometry of the coils and pole pieces, but it does rotate.
"Eppur, si muove"
--Galileo Galilei
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Exactly.
zog
Senior Member
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This is a pretty good animation, watch the whole thing, it should all come together. Each color is a pole 120 degrees apart from one another.
http://www.youtube.com/watch?v=KFY84ZiwEi0
K8MHZ
Senior Member
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The way I see it, each phase has it's own magnetic field. Each phase's field increases and decreases, but it doesn't move to the next phase. This waxing and waning of separate magnetic fields causes a rotating attraction, but the fields don't really rotate.
The same effect can be done with lights in a circle. Switched on and off properly, it will look like the lights are rotating, but of course they are not.
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The problem with that analysis is that at any point there is exactly one value for the magnetic field, regardless of how many sources are generating it.
The separate fields from the different poles are not detectably isolated they way the light from a lamp is.
Here is a thought experiment to demonstrate my point: Make your lights in a circle spotlights pointed down with an overlap of coverage and instead of turning them on and off you dim and brighten them the way a three phase source would (like a chasing light sequence).
Now instead of looking at the individual lights, measure the brightest lit point on the floor below the ring of lights. It will actually move continuously around the circle. It may get dimmer and brighter, larger and smaller, speed up and slow down, but the center of that spot will move continuously.
In the actual case of a motor, the sources of the magnetic field are not moving, just as the lights are not moving. But the strength of the resulting field (overall brightness of light) is really moving. The fact that the magnetic field has a direction vector just makes it harder to draw pictures, but the effect is the same.
gar
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130822-1405 EDT
At any point in space the sum of individual magnetic fields at that point produces a single composite magnetic vector with both a direction and magnitude.
If you sum any number of sine waves of exactly the same frequency and with displaced phase angles between the waves, then the result is a single sine wave of exactly the same frequency and a phase angle relative to any one of the original source sine waves.
Now if you consider the physical structure as described by GoldDigger and plot the angle of the + peak point of the composite magnetic vector you will find its angle changes linearly with time and the peak remains at a constant value.
In the practical world there is some slight fluctuation in phase angle and amplitude as mentioned by GoldDigger. However, a 3 phase motor has almost constant torque. Possibly as good as a DC motor.
In a single phase motor there is only one +/- magnetic vector direction. Thus, a pulsating torque.
At a local ball bearing manufacturer, when they were still in town, someone had the bright idea to change the motors on bearing durability test machines from DC to AC, and they used single phase motors as the replacement. Large motors are not required. After this change their test results were producing much shorter lifetimes than expected caused by the non-uniform torque from the single phase motors.
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At any point in space the sum of individual magnetic fields at that point produces a single composite magnetic vector with both a direction and magnitude.
If you sum any number of sine waves of exactly the same frequency and with displaced phase angles between the waves, then the result is a single sine wave of exactly the same frequency and a phase angle relative to any one of the original source sine waves.
Now if you consider the physical structure as described by GoldDigger and plot the angle of the + peak point of the composite magnetic vector you will find its angle changes linearly with time and the peak remains at a constant value.
In the practical world there is some slight fluctuation in phase angle and amplitude as mentioned by GoldDigger. However, a 3 phase motor has almost constant torque. Possibly as good as a DC motor.
In a single phase motor there is only one +/- magnetic vector direction. Thus, a pulsating torque.
At a local ball bearing manufacturer, when they were still in town, someone had the bright idea to change the motors on bearing durability test machines from DC to AC, and they used single phase motors as the replacement. Large motors are not required. After this change their test results were producing much shorter lifetimes than expected caused by the non-uniform torque from the single phase motors.
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- Location
- Placerville, CA, USA
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- Retired PV System Designer
Except during starting of course. Something has to provide a second field direction to get the rotor turning in the first place.
In some cases (capacitor-run motor) that winding and its associated off-parallel magnetic field will be present all the time. But even that will not produce a uniform field or torque, so gar's main point remains unchanged.
K8MHZ
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- Electrician
I understand what you are getting at. I still envision three, but as you would measure (from a single viewpoint) you would 'see' one.
As for the bearing mfgr. Would they have been DBB? They used to have an office here in Muskegon and I think at one time we even made bearings here for them. I thought they had a cool logo.
ggunn
PE (Electrical), NABCEP certified
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Just having pulsating torque, by itself, will not impose any greater load on the bearings. Motor vibration as well as bearing load forces caused by the interaction between the motor shaft torque and the machine load connected to it, on the other hand....
Eddy Current
Senior Member
Eddy Current
Senior Member
So in Dc motors the magnetic field pulsates and AC motors the magnetic field bounces back and forth? For some reason i was just imagining a constant beam of magnetic field.
weressl
Esteemed Member
Magnetic field is induced by changing current in the stator.
The magnetic field FROM the stator induces changing current in the rotor.
The induced current in the rotor creates a magnetic field.
The rotor and stator magnetic fields are of the SAME polarity, therefore 'repulsing' each other and THAT creates the rotary motion.
(I went to the sister school of engineering named after the discoverer of the principle and creator of the first 3 phase motor,)
K8MHZ
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- Electrician
Tesla U in Serbia?
gar
Senior Member
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- Ann Arbor, Michigan
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- EE
130823-0823 EDT
GoldDigger:
My preference is to call all those starting techniques two phase. And both shaded pole and capacitor run are really two phase in magnetic field operation, but sourced from single phase.
K8MHZ:
The manufacturer was Hoover Ball Bearing, but at the time of this test owned by NSK. Now they have left town.
ggunn:
When you have 100 years of data and experience in durability testing you don't change the test procedure.
GoldDigger:
When you have a long history of results from a test machine, only change the motor, and this grossly changes the results, then that implies a difference relating to the motor.
Going back to a DC motor, and this eliminates the problem, then it is quite clear evidence of the motor being the source of the problem.
Then what is different? The motor pulsates. A high inertia load could be added to the single phase motor and this would essentially eliminate the pulsation to the test. This was not tried.
I was not involved in this problem. It was related to me by a bearing engineer.
Eddy Current:
In a DC motor there is a fixed in space constant magnetic field from a permanent magnet, or a wound field with a DC current. The rotor is a group of coils connected to a commutator that synchronously switches the coils as the rotor rotates.
A brushless DC motor is essentially an AC synchronous motor with added control circuitry. This has a constant magnetic field on the rotor, and a controllable rotating magnetic field from the stators.
You can make the rotating field move as slowly as you desire. For example just stop rotation.
Stepping motors are generally a 2 phase synchronous motor. Originally square current waveforms were used. But today it is more likely a modulated current is the excitation. This reduces resonance problems and provides micro-stepping. I believe that attraction of opposite poles is the usual description of how the rotor turns.
In a stepping or synchronous motor as the torque load is increased the phase shift of the rotor between its actual position and its theoretical position increases until the breakaway point, then sync is lost.
.
GoldDigger:
My preference is to call all those starting techniques two phase. And both shaded pole and capacitor run are really two phase in magnetic field operation, but sourced from single phase.
K8MHZ:
The manufacturer was Hoover Ball Bearing, but at the time of this test owned by NSK. Now they have left town.
ggunn:
When you have 100 years of data and experience in durability testing you don't change the test procedure.
GoldDigger:
When you have a long history of results from a test machine, only change the motor, and this grossly changes the results, then that implies a difference relating to the motor.
Going back to a DC motor, and this eliminates the problem, then it is quite clear evidence of the motor being the source of the problem.
Then what is different? The motor pulsates. A high inertia load could be added to the single phase motor and this would essentially eliminate the pulsation to the test. This was not tried.
I was not involved in this problem. It was related to me by a bearing engineer.
Eddy Current:
In a DC motor there is a fixed in space constant magnetic field from a permanent magnet, or a wound field with a DC current. The rotor is a group of coils connected to a commutator that synchronously switches the coils as the rotor rotates.
A brushless DC motor is essentially an AC synchronous motor with added control circuitry. This has a constant magnetic field on the rotor, and a controllable rotating magnetic field from the stators.
You can make the rotating field move as slowly as you desire. For example just stop rotation.
Stepping motors are generally a 2 phase synchronous motor. Originally square current waveforms were used. But today it is more likely a modulated current is the excitation. This reduces resonance problems and provides micro-stepping. I believe that attraction of opposite poles is the usual description of how the rotor turns.
In a stepping or synchronous motor as the torque load is increased the phase shift of the rotor between its actual position and its theoretical position increases until the breakaway point, then sync is lost.
.
weressl
Esteemed Member
Tesla finished his University Studies in Budapest, Hungary. Many people claim that his 'inventions' were stolen by his US financiers, namely Westinghouse and Edison, but maybe his comeuppance was due to the karma that he had worked together in Hungary with Kando, Balthy, Zipernowsky, Deri and he himself failed to identify them as at least co-inventors on many of his Patent Applications. IMO he was a shameless thief of ideas. Hey those guys were 5000mi away, who will know any better?!
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