Understanding Rotor MMF in Induction Generators: Magnetizing or Demagnetizing?

[cnh1995]

In a 3-phase induction motor, rotor current demagnetizes the air gap and hence, stator draws more current (to keep the flux constant), which is the reflection of rotor current. This is like a rotating transformer which consumes active power. An induction generator on the other hand, supplies active power to the grid. So, the stator current will be composed of magnetizing current (same as in motor operation) and an active current. My question is, what will be the nature of rotor mmf? Magnetizing or demagnetizing? Since the generator is supplying active power, will the rotor mmf be magnetizing? Is it consistent with Lenz's law?

 

[Baluncore]

The performance of an induction motor and an induction generator are symmetrical about synchronous speed.

Just what you meant by “magnetizing or demagnetizing” is open to interpretation. You will need to examine a number of different situations and extrapolate your terminology from the motor with slip through to a generator with slip.

If you have a model of a synchronous motor delivering power to the shaft it will have a slip. Reducing the load will reduce the slip. Now drive the motor at synchronous speed with zero slip, the windage losses will need to be provided through the shaft. Now drive it as a generator with say 5% slip.

How do your interpretation of fluxes extrapolate from motor, through the zero slip region, into the generator mode.

 

[cnh1995]

The performance of an induction motor and an induction generator are symmetrical about synchronous speed.

Just what you meant by “magnetizing or demagnetizing” is open to interpretation. You will need to examine a number of different situations and extrapolate your terminology from the motor with slip through to a generator with slip.

If you have a model of a synchronous motor delivering power to the shaft it will have a slip. Reducing the load will reduce the slip. Now drive the motor at synchronous speed with zero slip, the windage losses will need to be provided through the shaft. Now drive it as a generator with say 5% slip.

How do your interpretation of fluxes extrapolate from motor, through the zero slip region, into the generator mode.

I'm not sure if my logic is correct, but here's what I think:
In a transformer, primary supplies power to secondary, so it is responsible for magnetizing current. In induction generator, rotor supplies power to the stator, so it magnetizes the stator. Stator conductors act as secondary and reflected stator current opposes the magnetizing rotor mmf(just like transformer secondary).
In case of induction motor, stator supplies power to rotor, hence, stator acts as primary and rotor acts as secondary. So, rotor mmf demagnetizes the stator. But in case of induction generator, rotor supplies power to the stator, hence rotor mmf magnetizes the stator i.e.it aids the magnetic flux of the stator and reflected stator current(active current) demagnetizes the magnetizing rotor mmf and keeps the main flux(due to stator magnetizing current) constant.
Does this sound correct?

 

[jim hardy]

Here's a conceptual word picture for you

Break your stator current into two components
real, which does the work
and imaginary, which does the magnetizing

imaginary component does not come from the rotor,
it must come from whatever is connected to the stator terminals.
If the stator is connected to a sufficiently large capacitor
enough imaginary current can flow to provide the necessary magnetizing current

that's why you don't compensate an induction motor all the way to unity pf, you don't want it resonant
and you don't want it to become a self excited induction generator should it get somehow motorized(ie spun by its load) with its breaker open.

Any help?

 

[cnh1995]

Here's a conceptual word picture for you

Break your stator current into two components
real, which does the work
and imaginary, which does the magnetizing

imaginary component does not come from the rotor,
it must come from whatever is connected to the stator terminals.
If the stator is connected to a sufficiently large capacitor
enough imaginary current can flow to provide the necessary magnetizing current

that's why you don't compensate an induction motor all the way to unity pf, you don't want it resonant
and you don't want it to become a self excited induction generator should it get somehow motorized(ie spun by its load) with its breaker open.

Any help?

Thanks for the reply! But in induction generator, rotor is given mechanical power. So can I treat rotor as primary and stator as secondary? What is the nature of rotor mmf? Will it assist the stator flux(produced by magnetizing current in the stator) or oppose it(like in case of induction motor)?

 

[jim hardy]

So can I treat rotor as primary and stator as secondary?

I never thought about that. I think it'd be better to stay with stator as primary because the rotor cannot supply the magnetizing current.What is the nature of rotor mmf? Will it assist the stator flux(produced by magnetizing current in the stator) or oppose it(like in case of induction motor)?

How about we apply some real overly-simple thinking ?

Well, at synchronous speed there's no rotor current hence no rotor flux
so let's simplify the machine down to a lossless one , no friction or windage and lossless iron that's infinitely permeable .

Stator supplies magnetizing current required to push flux across the air gap
and that magnetizing current is pure reactive (no watts) meaning it's 90 degrees out of phase with line voltage.

just below synchronous, rotor current flows , causing stator current to acquire a real component that delivers the watts mechanically removed by the shaft
just above synchronous, rotor current flows . causing stator current to acquire a real component that removes the watts mechanically delivered by the shaft
and those real components of stator current being small and perpendicular to the large imaginary component
do not affect the hypotenuse very much at all.
Meaning stator current (hence stator MMF and flux) does not change very much at all.
It just swings slightly away from 90 degrees behind applied voltage.

Now
since it's a vector addition at ~90 degrees
is perhaps a mistake to say rotor flux "adds" or "subtracts" to stator flux? Because that infers direct not quadrature addition?

That's more of a question than an answer... because I'm not sure whether this line of thinking really leads to better understanding.

Seems to me since it's symmetric about zero torque, and stator current increases for either motor or generator operation,
if you say that one case cancels then so must the other.

I wish i'd taught a motors course somewhere along the way so i'd have a better answer for you.

What's your thought ? Is my logic true ? Corrections are welcome.

Can you use this as a launch point for better thinking ? When we can draw a simple phasor diagram we are getting there.
It'll take me quite a while to absorb this guy's attempt
http://yourelectrichome.blogspot.com/2011/08/induction-generator.html

 

[cnh1995]

Seems to me since it's symmetric about zero torque, and stator current increases for either motor or generator operation,

In motor operation, as the load on the shaft increases, stator current increases. This increase in stator current is because of the demagnetizing action of rotor mmf. So, in motor operation, stator current increases to overcome the demagnetizing rotor mmf.
In induction generator, mechanical power is given to the rotor. How does this power reach the stator? The only way is through the air gap, by means of rotor magnetic field. So, I believe the rotor mmf aids the main magnetic flux, which results in transfer of the from rotor the stator. How else would the power be transferred from rotor to stator?

 

[cnh1995]

Stator supplies magnetizing current required to push flux across the air gap
and that magnetizing current is pure reactive (no watts) meaning it's 90 degrees out of phase with line voltage.

just below synchronous, rotor current flows , causing stator current to acquire a real component that delivers the watts mechanically removed by the shaft

Isn't the real component of stator current because of the demagnetizing action of rotor mmf, just like it happens in a transformer?

 

[Baluncore]

Referencing phase to the supply voltage, when you consider crossing the zero slip speed you see that the current in the stator, falls to zero and then changes phase by 180°, so the energy flow direction has changed.
The reactive magnetising current is still supplied to the stator from the supply.
Energy crosses the stator–rotor gap once, but the direction of energy flow is different for motor or generator.

 

[cnh1995]

I believe there are flaws in my understanding of working of an induction motor.

First, magnetizing current in the stator produces its own rotating magnetic field. This field will induce emf in the rotor. Rotor will start moving and attain a steady speed depending on the load. There will be a real component of current in the stator which will be responsible for the active power consumed by the motor. Now, at this steady speed, rotor will have its own current which will have two components: real and imaginary, due to rotor resistance and rotor inductance respectively. Out of these two, which component is responsible for active current in the stator? Real part of rotor current or imaginary part of rotor current?

 

[jim hardy]

Now, at this steady speed, rotor will have its own current which will have two components: real and imaginary, due to rotor resistance and rotor inductance respectively. Out of these two, which component is responsible for active current in the stator? Real part of rotor current or imaginary part of rotor current?

Good question. I lay awake last night trying to figure out rotor current.
Rotor current is at slip frequency not line frequency and I've not yet got a mental picture worthy of mention
what would be your phase reference for rotor current ?

I can't answer the question. Still trying to think it out.

 

[Baluncore]

Now, at this steady speed, rotor will have its own current which will have two components: real and imaginary, due to rotor resistance and rotor inductance respectively. Out of these two, which component is responsible for active current in the stator? Real part of rotor current or imaginary part of rotor current?

Real or imaginary relative to what? Rather than orthogonal components, it is probably better to consider phasors for the traveling waves. Maybe the following will help.

The rotor resistance and reactance are both very low so the resistive current is dominant. The currents induced in the rotor have a low frequency in the rotor material. That is because it is close to synchronous speed. For a 60Hz supply, with 5% slip, it will be about 3Hz. That low frequency further reduces rotor reactance.

The field windings generate a magnetic traveling wave, that induces a current wave in the rotor.
At zero slip, synchronous speed, we define the induced rotor current wave as being at zero phase with the field wave.
When operated as a motor with slip there will be a lag in the rotor wave relative to the field wave.
When operated as a generator with slip there will be a lead in the rotor wave relative to the field wave.

It is the phasor position of the traveling current wave in the rotor, relative to the traveling field wave, that determines the direction of energy flow across the gap between field and rotor.

 

[cnh1995]

When operated as a motor with slip there will be a lag in the rotor wave relative to the field wave.
When operated as a generator with slip there will be a lead in the rotor wave relative to the field wave.

Do you mean rotor mmf wave? In motor operation, rotor mmf wave lags behind stator mmf wave and in generator operation, rotor mmf wave leads stator mmf wave?
Consider a motor with blocked rotor. Since slip is 1, rotor inductance is high. Hence, rotor current will lag the induced rotor voltage by a significant angle, and so will the rotor mmf. So a major component of rotor current will be reactive, hence, it will demagnetize the stator. To maintain the flux constant, magnetizing current of the stator increases and to meet the core losses, stator acquires a real component, which is small compared to the magnetizing component and hence, when the rotor is blocked, the stator current lags behind stator voltage by almost 90°. Is this correct? If yes, then how does resistive component of rotor current cause an active stator current?

 

[jim hardy]

I think to make it intuitive we will have to draw a simplified rotor with only one turn.
It takes me a long time to do those sketches with Paint
but it's how i fight insomnia.

Observe that high rotor resistance shifthe peak torque in direction of more slip
and the answer might come to us from looking at the wound rotor induction motor
which is just an induction motor with adjustable rotor resistance.

I remember running speed-torque tests in college labca 1966 on a wound rotor machine with a resistor bank in the rotor circuit
but I'm afraid i just accepted the formulas instead of really understanding the magnetics. Had i really understood i would still remember.

Judging by my dreams last night i'd say my subconscious is chewing on it.

 

[cnh1995]

I think to make it intuitive we will have to draw a simplified rotor with only one turn.
It takes me a long time to do those sketches with Paint
but it's how i fight insomnia.

Observe that high rotor resistance shifthe peak torque in direction of more slip
and the answer might come to us from looking at the wound rotor induction motor
which is just an induction motor with adjustable rotor resistance.

I remember running speed-torque tests in college labca 1966 on a wound rotor machine with a resistor bank in the rotor circuit
but I'm afraid i just accepted the formulas instead of really understanding the magnetics. Had i really understood i would still remember.

Would it be like this? Resistive rotor current will have its mmf perpendicular to the stator mmf and reactive rotor mmf will be demagnetizing i.e.in direct opposition with the stator mmf. Now, this resistive rotor flux(mmf) links with the stator conductors and to counter that mmf, stator acquires an active current. It has nothing to do with the demagnetizing rotor mmf. Demagnetizing rotor mmf will only increase the magnetizing stator current and not affect the real component of the stator current. This explains the generator action. Only resistive rotor current changes its phase by 180° and hence, active current in the stator changes its phase by 180°. Reactive rotor current is still demagnetizing (and not magnetizing as I was saying in my previous posts). Hence, the total flux (produced by magnetizing current of the stator) is unchanged at any loading condition. Also, this is consistent with the Lenz's law and transformer action.
Does this sound correct?

 

[jim hardy]

Does this sound correct?

We are spiraling in on a simple and concise word picture.

Your description certainly seems plausible

what i fear is this
Lots of people think in pictures (as Einstein said he did) , it's the lucky few who can think in equations.
you have an image in your mind that you describe with words
i read those words and use them to paint an image in my mind
are the two images congruent ?

I'll have to withhold opinion until I've drawn an image .
Do you remember conversations (a few years ago) about synchronous machines with one Bassalisk ? We worked out annotated images that were unambiguous.

Im not ignoring you. Just trying to imagine myself small enough to sit in the airgap holding a flux detector, riding around with the rotor. That way i could see Baluncore's 'travelling waves' I think he probably truly understands what I'm struggling to grasp.
I must've been able to write the formulas sometime in the past for i got "B" in AC machinery course and "A" in the lab.

I'll be along at my plodder's rate. If there's a sprinter reading this , please lead on !old jim

 

[cnh1995]

Would it be like this? Resistive rotor current will have its mmf perpendicular to the stator mmf and reactive rotor mmf will be demagnetizing i.e.in direct opposition with the stator mmf. Now, this resistive rotor flux(mmf) links with the stator conductors and to counter that mmf, stator acquires an active current. It has nothing to do with the demagnetizing rotor mmf. Demagnetizing rotor mmf will only increase the magnetizing stator current and not affect the real component of the stator current. This explains the generator action. Only resistive rotor current changes its phase by 180° and hence, active current in the stator changes its phase by 180°. Reactive rotor current is still demagnetizing (and not magnetizing as I was saying in my previous posts). Hence, the total flux (produced by magnetizing current of the stator) is unchanged at any loading condition. Also, this is consistent with the Lenz's law and transformer action.
Does this sound correct?

If I'm not wrong, then this explains why high rotor resistance shifts maximum torque towards smaller slip. If the rotor were purely inductive (zero resistance), then there would be no starting torque at all, since real component of rotor current would be zero. So, it's the real component of the rotor current that causes the real component of stator current. Reactive component of the rotor current increases the magnetizing current as reactive rotor mmf is in direct opposition with the stator mmf.

 

[Baluncore]

I'm no expert in the detail. If you dig deep enough you will need to do it with mathematics and probably phasors.
The term “demagnetising” relative to “magnetising” is undefined and confusing. It might be better to describe the concept by vector addition of phasors. I think you need to read parts of chapters 7 and 8 of “Alternating Current Machines” by M.G. Say.
Rather than attach selected extracts for your study from my copy of the book, it is probably better for you to get access to a pdf. Last time I looked there was a good open access 11.4 Mbyte pdf with searchable text here. https://archive.org/details/AlternatingCurrentMachines
Start with figure 8.1, 8.4 and 8.8 for rotor currents.

 

[jim hardy]

With any luck i'll find some words and formulas to go with this sketch
i think we all agree 3Φ flux can be represented by this blue phasor rotating CCW at synchronous speed.
So we don't have to draw the stator.
If i imagine myself on the rotor wth a magnetometer i'll see stator flux sweep past slowly.
Since Paint won't let me rotate the squirrel cage
i'll just redraw the phasor in another sketch, pulled slightly ahead,
representing where things will be one mechanical rotation later at low slip,
and try to figure out what currents are induced in the rotor bars (which i color coded) at low slip

Back later

old jim

 

[Tom.G]

Observe that high rotor resistance shifthe peak torque in direction of more slip

If I'm not wrong, then this explains why high rotor resistance shifts maximum torque towards smaller slip.

Please guys, make up my mind, will ya?

 

[cnh1995]

Please guys, make up my mind, will ya?

It should be more slip, not smaller. Slip for maximum torque is equal to r₂/x₂. So for higher r, the slip should be more for maximum torque. What I meant to write was smaller speed.

 

[jim hardy]

Have come across two explanations with a lot of formulas

both make me sigh, must be a better way...

http://www.ewh.ieee.org/soc/es/Nov1998/08/INDMOTOR.HTM
http://electricalstudy.net/lesson/poly-phase-induction-generator-or-asynchronous-generator-2/

i'm trying to come up with a simple step by step pictorial explanation, as much as anything for my own curiosity.

encouragingly it's telling me that when rotor resistance is zero, the machine can operate only at zero slip because X/R is infinite meaning R/X = 0 , so max torque is at zero slip (agrees with your post just above). At any other slip torque is zero.

That had me stymied for two hours but it makes sense .

but I'm worried i'll just wind up proving 0/0= indeterminate. Oh well won't be the first time.

 

[cnh1995]

Have come across two explanations with a lot of formulas

both make me sigh, must be a better way...

http://www.ewh.ieee.org/soc/es/Nov1998/08/INDMOTOR.HTM
http://electricalstudy.net/lesson/poly-phase-induction-generator-or-asynchronous-generator-2/

i'm trying to come up with a simple step by step pictorial explanation, as much as anything for my own curiosity.

encouragingly it's telling me that when rotor resistance is zero, the machine can operate only at zero slip because X/R is infinite meaning R/X = 0 , so max torque is at zero slip (agrees with your post just above). At any other slip torque is zero.

That had me stymied for two hours but it makes sense .

but I'm worried i'll just wind up proving 0/0= indeterminate. Oh well won't be the first time.

Well, I'm really grateful to you Jim, for the time you are spending on this thread! It's so great to see a man with such a curiosity at this age! Take a bow!

 

[anorlunda]

Doesn't this animation illustrate what members have been trying to say?


Then add

If an induced current flows, its direction is always such that it will oppose the change which produced it.

As @jim hardy suggested, start with zero slip, zero torque, zero rotor current. Then let the rotor slow down a bit. Lenz's Law says that the induced currents will try to make it speed up again (i.e. a motor). Starting again from zero slip, let the rotor speed up a bit. Lenz's Law says that the induced currents will try to make it slow down again (i.e. a generator).

It is certainly true that we have a communications gap between those who think in equations as opposed to words. To even understand what the question was in the OP, I had to try to decompose it into equations.

To me, a fatal flaw in verbal explanations is the ambiguity in natural language. Take a simple phrase, 'current increases in both directions" Yes, current magnitude increases, but the direction changes. Does the person saying "increase" mean magnitude only or magnitude plus sign. In a thread with many participants, some people might be using increase with one definition, while others are using a second definition of increase in the same conversation. Everyone wastes their time on miscommunication. That is one of the reasons I prefer equations.

 

[jim hardy]

To me, a fatal flaw in verbal explanations is the ambiguity in natural language.

yes, I'm having that trouble. Math is unambiguous . But i need a mechanical picture to go with the math so i can figure out the formula..

That's a really nice animation.

AT 2:10 he says "Lorentz" which is the key, QV cross B right inside the conductor.
His image at 2:30 is a great thought tool. You don't even have to draw the stator, just make a rotating DC field for stator .
Then let rotor loop rotate sightly slower and figure rotor current from dΦ/dt divided by Zloop
that makes a dipole which (if i understand correctly) is the torque on that loop in your known stator field
I'm trying to figure the forces on my ten bar rotor bars in that sketch i posted. Got the bar currents .
Once it works for motor it's a snap to overspeed and solve for generator.
As you say just direction of current in rotor bars will reverse because relative velocity V in QV cross B has reversed.

my drafting is so awkward (my algebra too) I'm having a time with it.
It has helped to pick my dimensions with pi in them , like rotor with area 1/2π m² and air gap of π millimeters.
I have to work that way - get a model that gives credible numbers then turn that cipher into equations.

Thanks anorlunda that's the best animation I've seen.

If my method works out for plausible rotor L and R values i'll post it. Showing my clumsy algebra.

Lavoisier: ""We think only through the medium of words. --Languages are true analytical methods. --Algebra, which is adapted to its purpose in every species of expression, in the most simple, most exact, and best manner possible, is at the same time a language and an analytical method. --The art of reasoning is nothing more than a language well arranged.""
Not bad for 1789, eh ? A great read still. http://web.lemoyne.edu/giunta/EA/LAVPREFann.HTML

old jim