Understanding The Squirrel Cage Induction Motor.

[CasualKilla]

So I understand how the rotating magnetic field is created in the stator, so lets assume we have a uniform intensity rotating magnetic field and work from there.

 

Lets start from a stationary position. The flux lines from the stator magnetic field cut the squirrel cage bars, inducing current and thus inducing a north and south pole in the rotor. This induced 'magnet' attempts to align with the rotating stator field, but as it approaches the stator magnetic field rotation speed (synchronous speed) flux line cut the bars less often, thus less current and a weaker magnetic field induced by the rotor. This is why the rotor speed never quite reaches the synchronous speed since there is very little torque induced near synchronous speed.

 

Now the question: does the squirrel cage have a fixed N and S relative to itself? The SC seems symmetrical and does not have a defined position where you would expect a N and S pole to form, and thus I wonder why the magnetic field N and S induced by the rotor does not just get re-induced at a different position in order to stay aligned with the stator field instead of the rotor physically rotating. Hope this question makes sense.

 

I tried to lay out my understanding of the induction motor so that if there is a error in my fundamental understanding, that can be addressed first.

 

I am doing a thesis nest year, and I want to understand induction motors fully before I begin more technical research.

[studiot]

 

So I understand how the rotating magnetic field is created in the stator, so lets assume we have a uniform intensity rotating magnetic field and work from there.

 

Sorry it's so scruffy, but assuming your stator field is rotating anticlockwise and the instantaneous mag field is as shown;

 

 

The induced current in the cage wires (shown in section as small circles around the rotor periphery) are given by the right hand rule,

crosses into the paper and dots coming out of the paper.

 

The current in a rotor conductor generates a circular field round it, like any current carrying conductor, which reinforces the statorfield on the right and weakens it on the left of the conductor.

 

This is equivalent to a force turning the rotor in the same direction as the stator field rotates.

This jiggle in the field is shown in diagram 2.

[CasualKilla]

Hey studiot, nice to see you again. I'm slightly disturbed by how perfect those circles are, I hope those were not hand drawn, whahah . So you suggest dropping the induced poles in the rotor try to align with the stator field explanation thing, cus that where most of my confusion is coming from, and it seems like an oversimplified explanation, even if it is accurate? It seems youtube videos on induction motors always want you to imagine the rotor as an induced magnet trying in vein to align itself with an ever changing field. Does the rotor even have a pole and what is the relevance of the N and S poles of the stater field?

 

Gonna take a leap and assume the effect in figure 2 occurs at all the bars/wires, but the effect it greatest at near the poles of the stator field?

 

Edit: Yes the effect would be greatest at the stator poles, since the wires are cutting more flux lines there, but what does this mean for the overal magnetic field of the rotor. How are the poles formed (in the rotor)? This is something that has always confused me.

 

If I may add another question, Why does the strong magnetic field to the right and the weak field to the left create a net force of the conductor? I thought the force was produced by ie. the current and the magnetic field intensity. So the induced current in the conductor causes a force on the conductor in accordance to the RHR.

 

Edit: wait I think i got it XD. so the conductor is in a sense "in" the magnet field on the right and the "in" weaker one on the left simultaneously. The strong B on the right cause a net force to the left and the much weaker field on the left causes a force toward the right, this is where the "net force" comes from i assume? (figure 2)

Edited December 12, 2014 by CasualKilla

[Danijel Gorupec]

As studiot said, it is more productive if you drop the S-and-N image, and consider current-carrying wires in a magnetic field instead.

 

[Tip: I am not sure if you are aware, but instead of considering rotating rotor in a rotating magnetic field, you can always consider a stationary rotor in a rotating magnetic field - where the field rotation speed equals to the motor slip. For me, it is easier to think about induction motor if only the magnetic field rotates, while the rotor is fixed, or vice versa.]

 

When, as you said, the stator pole crosses a rotor bar wire, the voltage (EMF) generated in that bar wire will be at maximum. This however does not mean that the current will be at maximum too - it is because the rotor cage also has its inductance and the current will lag behind the induced EMF.

 

Consider now a very small slip case (in my image this equals to a very slow rotating magnetic field and a stationary rotor)... Because the low frequency voltage that is induced in motor cage, the lag between rotor bar wire current and EMF will be very small. This means that the rotor bar wire current is practically in phase with the stator magnetic pole that is crossing it. This also means that there will be force acting to that bar wire that will be directed tangentially to the rotor and that the motor torque will be generated (studiot depicted that)!.... Unfortunatly, that torque will be small because small current is induced in rotor due to small EMF induced (due low stator field frequency rotation). In the extreme case of zero slip the current and the torque will drop to zero.

 

[Note that the rotor resistance has the crucial effect - without it the rotor will be able to generate considerable torque even at near-zero slip... Thus, by regulating cage resistance you can regulate how hard or soft will the torque curve be in dependance on motor slip.]

 

Consider now the opposite case where the slip is very large (that is, the magnetic field rotates very quickly around a stationary rotor]. Now, due to rotor cage inductance, the rotor bar wire current lags about 90 degrees behind the stator pole that crosses it. The EMF generated in the bar wire is very very large, but still it can generate only a limited current inside the cage (because the cage has its inductance) - in fact, you can increase slip to infinity but the current generated in the motor cage will never increase over some value as the inductance limits it... As a result, at very large slip, the torque will again drop toward zero! The reason is as follows: Bar wire current lags 90 degrees behind magnetic field that crosses it and thus the direction of the generated force is now radial (not tangential) to the rotor - and because the current does not grow to infinity as the slip grows to infinity, even the existing tangential component of the force will soon diminish.

 

[Did you notice that in the high-slip case the force felt by the cage is substantial, but radial... This is the basis for inductrack magnetic levitation.]

 

Conclusion: The torque generated by motor tends to zero at both, the very small slip (due to small currents generated in rotor) and the very large slip (due to phase lag between stator magnetic field and rotor currents). The torque has its maximum at some finite slip value.

 

Edit: Your OP.... the physical rotation speed of the rotor is not the same as the rotor magnetic field rotation (that is, to currents inside its bar wires). The rotor magnetic field rotation speed is always the same as the stator magnetic field rotation speed, but will lag to it depending on the slip.

 

Edit2: There is a simplified calculation about a single wire loop in a rotating magnetic field here.

Edited December 12, 2014 by Danijel Gorupec

[studiot]

 

If I may add another question, Why does the strong magnetic field to the right and the weak field to the left create a net force of the conductor? I thought the force was produced by ie. the current and the magnetic field intensity. So the induced current in the conductor causes a force on the conductor in accordance to the RHR.

 

Edit: wait I think i got it XD. so the conductor is in a sense "in" the magnet field on the right and the "in" weaker one on the left simultaneously. The strong B on the right cause a net force to the left and the much weaker field on the left causes a force toward the right, this is where the "net force" comes from i assume? (figure 2)

 

 

 

Yes Amperes Law.

[CasualKilla]

 

 

When, as you said, the stator pole crosses a rotor bar wire, the voltage (EMF) generated in that bar wire will be at maximum. This however does not mean that the current will be at maximum too - it is because the rotor cage also has its inductance and the current will lag behind the induced EMF.

 

Ok, it is starting to come together now. Just one thing, So the emf is induced in the bars, but where is the ground?

 

http://imgur.com/tJxmGmX

 

This is the a diagram of a single rotor loop. the cross-hatched area I assume is at constant EMF, so I'm stuggling to understand where the current is flowing. The reason i assume the EMF is constant is that the field is the same for any cross section (figure A) and every point in the bar cross section is moving at the same speed. This is my mind should induce a uniform EMF, so there is no EMF (or Voltage difference) for the current to flow.

 

I am probably just showing my ignorance here, so forgive me.

[Danijel Gorupec]

@CasualKilla

 

When one says 'the EMF is induced in a bar' it means that there exist voltage difference between one end of the bar and the other end of the bar. (You can put the ground anywhere. For example, one end of the bar can be at 101V and the other end of the bar at 99V in reference to some ground - but this is not important in this case. The only thing important is that there is 2V voltage difference across the bar. This voltage difference will generate the same current invariably of the ground reference.)

 

...

 

Hmm... yes if you are saying 'EMF is constant across the bar length', although I would not say it this way as it sounds strange to me... Instead I would say the same thing the following way: If you divide the bar into many pieces of the same length, then the EMF generated across every such piece will be the same (btw, the sum of EMF's of every pice will be the EMF generated for the whole bar)...

 

I think that you might have wrong idea what the EMF is... EMF is not related to position where the bar is placed in magnetic field: if you place a steady bar anywhere into a steady magnetic field, the EMF generated across the bar will be zero. Instead, the EMF is generated inside the bar when the bar 'cuts' through magnetic field lines. [There are electrons inside the bar metal, if you move the bar so that it cuts through magnetic field lines, these electrons will start feeling a force and will start pushing to one or other end of the bar - therefore the voltage difference between two bar ends will be created.].. The EMF is a value that tells how much electrons inside a bar are pushed into one or other direction as the bar cuts through magnetic field lines.

 

The current flows through the loop you depicted (because the upper bar of the loop has EMF in one direction, while the lower bar of the loop has EMF in the other direction - therefore those two EMF's sum up). The current depends on the overal EMF and the loop resistance.

 

(I have difficulties with English, so maybe you will need to ask additional questions about what I wrote)

[CasualKilla]

@CasualKilla

 

When one says 'the EMF is induced in a bar' it means that there exist voltage difference between one end of the bar and the other end of the bar. (You can put the ground anywhere. For example, one end of the bar can be at 101V and the other end of the bar at 99V in reference to some ground - but this is not important in this case. The only thing important is that there is 2V voltage difference across the bar. This voltage difference will generate the same current invariably of the ground reference.)

 

...

 

Hmm... yes if you are saying 'EMF is constant across the bar length', although I would not say it this way as it sounds strange to me... Instead I would say the same thing the following way: If you divide the bar into many pieces of the same length, then the EMF generated across every such piece will be the same (btw, the sum of EMF's of every pice will be the EMF generated for the whole bar)...

 

I think that you might have wrong idea what the EMF is... EMF is not related to position where the bar is placed in magnetic field: if you place a steady bar anywhere into a steady magnetic field, the EMF generated across the bar will be zero. Instead, the EMF is generated inside the bar when the bar 'cuts' through magnetic field lines. [There are electrons inside the bar metal, if you move the bar so that it cuts through magnetic field lines, these electrons will start feeling a force and will start pushing to one or other end of the bar - therefore the voltage difference between two bar ends will be created.].. The EMF is a value that tells how much electrons inside a bar are pushed into one or other direction as the bar cuts through magnetic field lines.

 

The current flows through the loop you depicted (because the upper bar of the loop has EMF in one direction, while the lower bar of the loop has EMF in the other direction - therefore those two EMF's sum up). The current depends on the overal EMF and the loop resistance.

 

(I have difficulties with English, so maybe you will need to ask additional questions about what I wrote)

Thank you for that explanation, I think you finally made it click for me, especially the part when you said "the sum of EMF's of every piece will be the EMF generated for the whole bar" .Therefore I think the statement "The EMF is constant accross the shaded regions" is incorrect, it would be more accurate to say "the rate of EMF generation is constant" (I think you probably misunderstood my original statement).

 

So as you said each infinitesimally small element produces a small voltage and that adds up to give the voltage over the shaded regions. So the bar behaves like a impedance connected to an ideal voltage source of V=2*EMF where I will define EMF as the voltage across one shaded section of the loop. Therefore the current simply flows round and round the loop and it also follows that the voltage across half of the shaded section is only V=1/2*EMF etc.

 

http://imgur.com/4mNtVyl

 

I have added a diagram to explain what i am saying, I think this is exactly what you said, just presented in a different way. Can you confirm if this understand is correct?

Edited December 13, 2014 by CasualKilla

[Danijel Gorupec]

Yes, I believe you got it now. I found no error in what you wrote.

 

I have added a diagram to explain what i am saying, I think this is exactly what you said, just presented in a different way. Can you confirm if this understand is correct?

There is however one possible problem with the diagram you made... You depicted voltmeters and noted the voltage readings on them. This would be true only if you cut (disconnect) the wire loop at one place (say, at one side) so that the current does not follow through it. If you allow that current flows, then you also must account for wire resistance and the voltage drop on this resistance - so if the current is allowed to flow, the voltage readings on voltmeters may not be what you noted on you picture.

 

Also, and I think you already understand this, the wire loop has its inductance, so as the EMF frequency increases the actual current will lag behind the EMF. At 'infinite' frequency the current will lag 90 degrees.

[CasualKilla]

Yes, I believe you got it now. I found no error in what you wrote.

 

There is however one possible problem with the diagram you made... You depicted voltmeters and noted the voltage readings on them. This would be true only if you cut (disconnect) the wire loop at one place (say, at one side) so that the current does not follow through it. If you allow that current flows, then you also must account for wire resistance and the voltage drop on this resistance - so if the current is allowed to flow, the voltage readings on voltmeters may not be what you noted on you picture.

Ah yes ofcourse, that's a very important point, thank you for pointing that out.

[CasualKilla]

Thank you to Danijel and studiot for helping me so far, you guys have been amazing.

 

I have a new question I need to ask though. What exactly are eddy currents, as far as i can tell they get induced by the same mechanism as the current inside the bar conductors. Looking at diagrams they are depicted as spirals, which is also confusing. This Brings me to the iron core, which consists of parrallel laminated disks. Now I know the lamination with stop the current flowing through the core, and the reason for using an iron core is because it creates a high flux density around the rotor conductors. I can also see that you don't really want current flowing inside the core since the flux linkage will be poor (far from stator) and it will add additional losses to the motor, thus lowering efficiency. I would like to know more about these eddy current, why and how they are created, how they flow, and how the laminated iron core helps with them.

 

My 2nd textbook only mentions eddy currents and mentions that the laminated core design help alleviate them or at-least the problems they cause.

[Danijel Gorupec]

You answered most of your questions... indeed, eddy currents are induced the same way as the current inside bar conductors - the iron is conductive (although not nearly as much as copper or aluminum) and some circular currents will be generated inside if iron is immersed into a changing magnetic field. Those currents will dissipate energy (heat) over the iron resistance... If instead of one large iron piece, you put together many small electrically isolated pieces of iron, the dissipated energy will be smaller despite the fact that the overall volume of iron is the same

 

[The above can be shown by some easy math: consider one square wire loop 2x2 inches in a changeable magnetic field - the wire resistance is proportional to the wire length, and the EMF generated inside the wire is proportional to the loop surface area. The power dissipated by the loop is EMF^2/R... What if you replace this 2x2 loop with 4 loops of 1x1 inches? A single 1x1 wire loop has 2 times less length (resistance), but has 4 times less surface area (EMF) - this means that the power dissipated inside one 1x1 inch loop is 8 times smaller than in one 2x2 loop. Even if you combine power dissipation from all 4 small loops, the dissipation is still only half of the large loop.]

 

Both stator and rotor of an induction motor are laminated. You might wonder why rotor needs lamination as it already has short-circuited loops around it. I must admit that I don't have the right answer. I know that we prefer motors with 'harder' torque curve. A motor that does not have a copper/aluminum cage, but only a solid-piece-iron rotor, would be ridiculously 'soft' (its speed would vary very much with load variation) - therefore we, in practice, must have the cage. It is possible that in combination with the cage a non-laminated rotor would decrease efficiency, but I never tried that math... Can anyone else help us on this?

 

[Observe: it seems to me that in first approximation the increased cage resistance does not decrease induction motor efficiency - in fact, motors were once speed-controlled by varying its rotor 'winding' resistance. However, in a more detailed model of a motor, I guess, increased cage resistance would somewhat decrease motor efficiency and thus a solid-piece-iron rotor would be somewhat less efficient.]

[davidivad]

"The iron core serves to carry the magnetic field through the rotor conductors. Because the magnetic field in the rotor is alternating with time, the core uses construction similar to a transformer core to reduce core energy losses. It is made of thin lamination, separated by varnish insulation, to reduce eddy currents circulating in the core. The material is a low carbon but high silicon iron with several times the resistivity of pure iron, further reducing eddy-current loss, and low coercivity to reduce hysteresis loss."

http://en.wikipedia.org/wiki/Squirrel-cage_rotor

 

It also mentions the use of more than one cage for tailored motor characteristics and what i call a shunt for equipment that needs altered starting torque.

Edited December 16, 2014 by davidivad

[studiot]

 

You might wonder why rotor needs lamination as it already has short-circuited loops around it

 

Do you mean these? (extracted from the Wiki article in davidivad's post)

 

"connected at both ends by shorting rings forming a cage-like shape"

 

These are simply there to electrically connect all the induction bars together in parallel.

 

 

what i call a shunt for equipment that needs altered starting torque.

 

 

Do you mean a shaded pole motor?

 

http://en.wikipedia.org/wiki/Shaded_pole_motor

 

Pole shading is one of many starting methods employed, but we haven't got to these yet.

 

The thing to remember with induction motors is that a three (or more) phase motor will start on its own and many textbook explanations only treat 3 phase.

 

Most domestic supplies are single phase and require an auxiliary starting mechanism or the rotor would simply turn to assume the position of minimum energy and stick there.

 

Starting methods include additional windings, additional components (capacitors and/or inductors) to generate a phase shifted alternating signal or phase from the main supply so that there is a 'side push' as well as the main one.

Shaded poles are deliberately partially shorted to create an imbalance in the rotor response to the magnetic drive which again starts the motor.

[davidivad]

 

Do you mean these? (extracted from the Wiki article in davidivad's post)

 

"connected at both ends by shorting rings forming a cage-like shape"

 

These are simply there to electrically connect all the induction bars together in parallel.

 

 

Do you mean a shaded pole motor?

 

http://en.wikipedia.org/wiki/Shaded_pole_motor

 

Pole shading is one of many starting methods employed, but we haven't got to these yet.

 

The thing to remember with induction motors is that a three (or more) phase motor will start on its own and many textbook explanations only treat 3 phase.

 

Most domestic supplies are single phase and require an auxiliary starting mechanism or the rotor would simply turn to assume the position of minimum energy and stick there.

 

Starting methods include additional windings, additional components (capacitors and/or inductors) to generate a phase shifted alternating signal or phase from the main supply so that there is a 'side push' as well as the main one.

Shaded poles are deliberately partially shorted to create an imbalance in the rotor response to the magnetic drive which again starts the motor.

i mentioned it because i knew it was important in many of the motors i have serviced in the past.

the wording of the article seemed to dance around the subject a bit. i did not want to discredit my reference so i mentioned it externally.

almost every squirrel cage motor i repaired required a shunt of some kind.

your statement about phases is on the money.

[Danijel Gorupec]

 

Do you mean these?

 

No, what I meant is: A typical rotor of an induction motor has the highly conductive cage (that is, an array of closed loops). Strong currents are induced in the cage. Why then is so important to prevent eddy currents in the rotor - those eddy currents would simply add to the cage current.

 

One might think that eddy currents are bad because they are flowing through a material (iron) that has higher resistance than the cage (copper/aluminum)... But it is not that obvious (at least not to me) - I know that, in first approximation, a higher resistance of the cage does not reduce the motor efficiency (it only makes it 'softer'). Therefore it is not obvious why would eddy currents in rotor decrease motor efficiency.

 

It might be that, without a laminated rotor, eddy currents would not only be limited to travel in the same 'plane' as cage currents, but may also travel perpendicular to it (say, into the depth of the rotor) and that this is the part of eddy currents that needs to be eliminated.

Edited December 17, 2014 by Danijel Gorupec

[Enthalpy]

An induction motor needs resistance a the rotor to work. Those with contact rings had an external resistor to help starting, squirrel-cages rely on the conductors only.

 

As already noted, the stator's field would induce currents in the rotor that oppose the stator's field, that is, the toro's field would be parallel (and opposed) to the stator's one, which creates zero torque. The rotor has also a leakage inductance that makes the rotor's reaction slightly smaller than the cause, but being inductive also, this leakage brings no help to the phase and angle of the rotor.

 

The rotor's resistance introduces a phase dfference between the stator's field and the rotor's reaction. In the rotating fields, this phase means and angle between both fields which results in a torque. Absolutely necessary.

 

At high slip rate, say when starting, the rotor's reactance augments because its self-inductance sees a higher frequency (the supply frequency minus the rotation frequency, smartly adjusted by the number of poles). Then the loss resistance can't achieve a good angle between the fields and the torque diminishes. So unless the motor rotates a fan, it needs some trick to increase the resistance when starting.

- Some squirrel cage motors have several cages; where the shallow one has a higher resistance to bring torque at start, and the deeper one works only at smaller slip frequency to reduce the resistance and improve the efficiency.

- Ring motors have the external starting resistor, which can be regenerative electronics to save energy.

- Presently, all motors above few kW have drive electronics to produce the optimum three-phase current. Squirrel cage motors are driven with the optimum frequency and voltage that guarantee a big angle between both fields, to obtain the desired torque with minimum currents and losses, even during acceleration and braking - called vector drive. So well that ring motors have disappeared, synchronous motors nearly so.

[CasualKilla]

An induction motor needs resistance a the rotor to work. Those with contact rings had an external resistor to help starting, squirrel-cages rely on the conductors only.

 

As already noted, the stator's field would induce currents in the rotor that oppose the stator's field, that is, the toro's field would be parallel (and opposed) to the stator's one, which creates zero torque. The rotor has also a leakage inductance that makes the rotor's reaction slightly smaller than the cause, but being inductive also, this leakage brings no help to the phase and angle of the rotor.

 

The rotor's resistance introduces a phase dfference between the stator's field and the rotor's reaction. In the rotating fields, this phase means and angle between both fields which results in a torque. Absolutely necessary.

 

At high slip rate, say when starting, the rotor's reactance augments because its self-inductance sees a higher frequency (the supply frequency minus the rotation frequency, smartly adjusted by the number of poles). Then the loss resistance can't achieve a good angle between the fields and the torque diminishes. So unless the motor rotates a fan, it needs some trick to increase the resistance when starting.

- Some squirrel cage motors have several cages; where the shallow one has a higher resistance to bring torque at start, and the deeper one works only at smaller slip frequency to reduce the resistance and improve the efficiency.

- Ring motors have the external starting resistor, which can be regenerative electronics to save energy.

- Presently, all motors above few kW have drive electronics to produce the optimum three-phase current. Squirrel cage motors are driven with the optimum frequency and voltage that guarantee a big angle between both fields, to obtain the desired torque with minimum currents and losses, even during acceleration and braking - called vector drive. So well that ring motors have disappeared, synchronous motors nearly so.

 

"An induction motor needs resistance a the rotor to work."

 

Don't you mean they need resistance to line-start on a constant voltage and frequency line? Why are copper rotor bar more efficient than than aluminium bars, copper has lower resistance for same cross-sectional area.

 

So my questions is, what performance would a motor with super-conducting bars be like near synchronous speed and why?

Edited February 23, 2015 by CasualKilla

[Enthalpy]

As the cage resistance drops, the best efficiency improves, but it happens at a smaller slip frequency (difference to the synchronous frequency). At too big slip frequency, the available torque decreases when the cage resistance decreases.

 

At the extreme case, a perfectly conductive cage (remember there are type I and type II superconductors, type I aren't useable for most applications, type II have a residual resistance) is perfectly unuseable, because it gives zero mean torque at any nonzero slip, so the squirrel cage motor doesn't start, it cogs only.

 

You would have to freeze a non-zero flux in the rotor, which is possible with geometrically separated conductors like a cage, and also with a continuous type II material, but then this is a synchronous machine. Also, permanent magnets do it as well - the benefit of a synchronous superconductor rotor being the higher inductance and coercitive field. Then, at a synchronous machine, you can have a superconductive stator, which lets the coolant flow more easily than the rotor, and may combine with permanent magnets at the rotor. This combination is meaningful for slowly rotating motors like boat propeller pods. Though, my electrostatic motor-alternator will compete with them once someone develops it, search for:

"electrostatic alternator" "Marc Schaefer"

 

Maybe superconducting motors will emerge some day, maybe not. They must replace already good solutions, a hard job for any innovation. My feeling is that people shouldn't try to adapt to superconductors designs that have been optimized for copper, but instead search completely new designs.

[CasualKilla]

I have been looking at some basic induction motor mathematical theory and I think I now understand why the rotor resistance affects the torque (mainly due to the rotor inductance/power factor).

 

At the moment I am wondering if the number of stator teeth must always be equal to simply: "teeth" = "#phases" x 2 x "#pole_pairs_per_phase"

Edited March 5, 2015 by CasualKilla

[Enthalpy]

Electric motors have varied designs, and books use to represent a very simplified vue of them... Because a real stator is a serious mess. Nice intention to help learning, but in my opinion, books should not occult the real world neither - introduced in a second step.

 

Stator windings use to be split. This is important to make the motor run smoothly, and also to reduce the losses, especially at the rotor. Books tell "the sum of three phases make a rotating field" but obviously, the induction varies brutally from the right to the left sides of a slit. Splitting the winding over several slits makes the transitions smoother, so the rotor sees a smaller pulsating field as it crosses the slits. The slits (or the squirrel cage) are also skewed so the transition seen by the rotor spreads over some time.

 

On the other hand, one slit uses to hoist winding paths lines of two phases, which doesn't help counting the slits... The number of conductors at the rotor is also chosen with a complicated ratio with the number of stator slits to further improve the smooth rotation.

 

Every three-phase motor I've seen had split windings and skewed slits. It's conceptually more complicated, but efficiency and smooth run are so important that every designer uses them. In fact, split windings achieve so much with so little complication that it's admirable.

 

Several schemes exist for split windings, and the number of slits changes accordingly, but expect many more slits than one per pole. A realistic picture for just three phases and possibly three pole pairs is there:

http://en.wikipedia.org/wiki/Stator

don't be afraid... If considering it with calm, good drawings and some sheets of clean paper, it's understandable.

 

Then, you have single-phased induction motors that create a second phase using a capacitor, or using a closed conductor around one path of the inductor... So one should distinguish the supplied phase from the added one in that case. Others constructions exist. The induction motor zoo is quite varied, providing exceptions to every simple rule.

 

A good book about motors and generators:

Electric Machinery Fundamentals

Stephen Chapman

it does describe some real things beyond rotating vectors and has nice pictures.

[CasualKilla]

Electric motors have varied designs, and books use to represent a very simplified vue of them... Because a real stator is a serious mess. Nice intention to help learning, but in my opinion, books should not occult the real world neither - introduced in a second step.

 

Stator windings use to be split. This is important to make the motor run smoothly, and also to reduce the losses, especially at the rotor. Books tell "the sum of three phases make a rotating field" but obviously, the induction varies brutally from the right to the left sides of a slit. Splitting the winding over several slits makes the transitions smoother, so the rotor sees a smaller pulsating field as it crosses the slits. The slits (or the squirrel cage) are also skewed so the transition seen by the rotor spreads over some time.

 

On the other hand, one slit uses to hoist winding paths lines of two phases, which doesn't help counting the slits... The number of conductors at the rotor is also chosen with a complicated ratio with the number of stator slits to further improve the smooth rotation.

 

Every three-phase motor I've seen had split windings and skewed slits. It's conceptually more complicated, but efficiency and smooth run are so important that every designer uses them. In fact, split windings achieve so much with so little complication that it's admirable.

 

Several schemes exist for split windings, and the number of slits changes accordingly, but expect many more slits than one per pole. A realistic picture for just three phases and possibly three pole pairs is there:

http://en.wikipedia.org/wiki/Stator

don't be afraid... If considering it with calm, good drawings and some sheets of clean paper, it's understandable.

 

Then, you have single-phased induction motors that create a second phase using a capacitor, or using a closed conductor around one path of the inductor... So one should distinguish the supplied phase from the added one in that case. Others constructions exist. The induction motor zoo is quite varied, providing exceptions to every simple rule.

 

A good book about motors and generators:

Electric Machinery Fundamentals

Stephen Chapman

it does describe some real things beyond rotating vectors and has nice pictures.

Great, that was my course book for electrical drive systems last year, guess time to pull it out of storage and give it a solid read. . I am doing a reluctance synchronous machine design (with squirrel cage for line start) for my thesis, it is one thing to simply use design guidelines, but I don't feel comfortable using a design feature if I don't even understand the theory behind it. I appreciate you sharing your understanding with me, I find it massively valuable.

 

What do you mean by "induction varies brutally from the right to the left sides of a slit", I feel this is important to understand.

[Enthalpy]

The stator current flowing in the slit creates an induction across the air gap (and serves other purposes too). This induction varies quickly, just from one side of the slit to the other, and stays essentially constant between two slits (from the stator or the rotor), since induction has a circulation only around a current..

 

This differs strongly from the model of a rotating induction created by the stator: the induction should vary over time and position as a sine, very smothly. The brutal variation creates cogging at the motor, steps at the electric voltage hence current, and losses since quick induction variations result in stronger eddy currents.

 

Tilting the slits, or the rotor's bars in a squirrel cage, spreads over time the transition of a rotor pole versus a stator pole. The other precaution is to split the coils over several slits, so the induction step spreads over several slits as well. The smart placing of the phases' coils contribute a lot as well.

 

The result is great. A three-phase machine runs very smoothly despite slits would have let it cog brutally like a small DC motor does. Close to a 1400MW generator (=20,000 engines of 95hp) you can talk almost normally. All this is achieved with coils and slits all identical, just by placing them properly. An engineering achievement.