How Will AC Motor Run Under This Condition?

Interview question, or home work?

Certainly not!

What - neither of those?

correction:

should be "...what happen if you force feed 3 phase 50Hz supply to the rotor at 1/2 line freq OR 1/1 line freq...

..."

1. In the 3-phase induction motor the synchronous speed or the speed of rotation of flux depends directly on the supply frequency. So if you are changing the supply frequency you are changing the speed of rotation of the flux wave.
2. The speed of rotation of the rotor (the shaft) is (1-s) * synchronous speed, where s - is the slip (it should be small in oder to the induction motor operates efficiently).
3. The slip s depends on the amplitude of the rotating flux wave - the higher the flux, the smaller the slip is for a given torque. So you need to have the flux is at its full (rated) level regardless of the speed of rotation of the rotor. And the flux depends directly on the supply voltage and inversely on the supply frequency. If you decreased the supply
4. frequency only at full supply voltage the flux were above its rated value. As result the iron of the magnetic circuit of the motor would be saturated and the motor overheated.
5. So you need to use so called V/f=const control strategy (up to base frequency 50 or 60Hz): voltage varies in direct proportion to the frequency, the flux wave will have a constant amplitude."...

..."As the electromagnetic torque is proportional to the square of the stator voltage any change in voltage will change the torque accordingly. Any drop in voltage will be far worse than an increase in voltage.

In terms of any change in frequency it will simply change the speed of the machine.

Any 3-phase induction motor basically has only two inputs that one may control namely Voltage and Frequency."...

https://www.quora.com/What-is-the-effect-of-variation-of-stator-voltage-and-frequency-of-3-phase-induction-motor

Sorry I realise my question was not clear enough. I meant this, the stator remains supplied with normal line voltage and frequency, the rotor's frequency is changed to 1/2 line freq , the rotor voltage is adjusted to appropriate level to avoid over current (now added. )

..."It depends on the motor itself, and the load. When motoring the rotor frequency is always lower than the stator frequency. The difference between these two frequencies is called the slip. The amount of slip depends on the input power, the applied load, and the characteristics of the specific motor.

The equation is

This video should help give a simple explanation of it."...

https://electronics.stackexchange.com/questions/307063/relationship-of-induction-motor-frequencies

This does not answer my question. Your answer applies to normal induction motor working priciple. My question is : when the rotor is force fed with an AC source with a certain freqnecy, there is no more induction, what will be the rotor rpm then?

You could have explained it better. In your post it sounded like a standard 3-phase induction motor, where you can't apply anything to thr rotor.

I still can't answer your qustion, but it doesn't seem a good idea!

_{In my question, I said "a} 3 phase induction motor with wounded rotor _{...", you can apply something to the rotor.}

Yes, you did, my bad. I noticed when I first read it, then forgot. Noticed it because it should be wound, not wounded. In case you're a non-native English speaker, wounded means injured, harmed or damaged etc.

Yes wound instead of wounded, thx for correcting.

Then it is no longer an induction motor and you're in experimental territory...I imagine it would take some major tweaking just to get it to run....It's not likely that a motor designed to run one way, will run another way...very long anyway...A lot of engineering goes into making a motor work properly, it's not likely you can just roll the dice and have something that works properly....in fact I would say it's impossible...motors are designed from the ground up, everything is for purpose...

How is the forum expected to answer this question when there is no information about the load it is trying to turn (rhetorical question - NNTR)? The outcome depends upon what it is doing. Full motor and load numerical information is needed to produce a concise answer.

An alternative is to pick up the and discuss the problem with the motor manufacturer's Technical Helpline. That usually works, and it is certainly quicker than asking a bunch of strangers in an anonymous internet Engineering forum.

So you want to force feed the rotor a different frequency than the armature?

I don't follow but I'm curious to learn more...

Do tell....

By definition an induction motor couples the stator magnetic field into the rotor core. The induction motors with a slip ring are designed to allow a large amount of slip to be tolerated without overheating by including a series resistance to the rotor windings. So applying a voltage to the rotor is using this motor in an atypical fashion. (IMHO this is a very good interview question.)

I expect driving the rotor field with half frequency external power would make the effective synchronous rotation rate to be half of whatever it previously was. Possibly this will cause overheating for a variety of unknown reasons. This motor was designed to cope with a large slip condition by putting heat into an external resistance. As presented we know nothing of the wire resistances in the stator or rotor to estimate how much heat each can be producing but if stator winding heat is cooled now by a fan operating at half speed then this alone might cause an overheating condition.

Can this approach allow for effective speed control without overheating? I say that it can but more needs to be known to draw a conclusion.

What you are describing is known as a Doubly Fed Induction Motor/Machine (DFIM). There's plenty of info on the web, just use your favorite search engine on that phrase. You can start here.

That has value as a form of induction GENERATOR (DFIG), but I don’t know of anyone that has used it as a MOTOR and I can’t see why you would.

PS: ok, I scanned through the abstracts of a couple of those papers in the link, I guess it has some value, but likely only to someone who ALREADY has a WRIM and wants more precise speed control of it by using a smaller VFD based system for only the rotor. I can’t see going to the expense of having a new WRIM made and getting a highly specialized rotor power control system just to avoid buying a standard VFD and a standard motor.

Yes, thanks for your understanding. I was asking a theoretical question (obvious isn't it?). Conventional way of VFD has to take the full load current. I was thinking, if it is possible to control motor speed by just controlling the rotor current. It seems very possible on the surface. THe rotor power is just a small fraction of total input power in the stator, because stator power ( or air gap power) is split into mechanical output power and the rotor power.

There is very little info DFIM (motoring), most literature are for DFIG.

Someone must have tried it before trying to control IM speed via rotor injection. I am hoping that someone can point out if there is any theoretical reason why this cannot be done .

It was obvious to someone who has researched, taught about, and worked on such machines. If you think your idea is unique, then I'm afraid that you're late to the party, there are quite a few implementations of speed control via current injection.

You can see some of them by using your favorite search engine and your own phrase, "control IM speed via rotor injection". Typically they are named after the person who originally devised/patented the idea. Some examples can be found here.

For example, "...Injection of EMF in the rotor circuit: Consider a three-phase induction motor in which the power is supplied through both stator and rotor. It is assumed that the machine is running at constant torque load. The speed control of three-phase slip-ring induction motor can be done using injected EMF in the rotor circuit. In the Schrage motor slip frequency EMF is produced and injected into secondary winding on the stator by means of brushes..." The author of this paper goes on to describe two other schemes, and includes plenty of math which can be used to prove Rixter's assertion that the machine can be operated below/above/at synchronous speed (physical constraints allowing).

I'm thinking that if you energize the rotor with AC, you will be turning it into a synchronous motor. The rotor will have a rotating magnetic field, just as the stator does. The two rotating fields will want to be locked together.

1. If the rotor is fed with 1/2 frequency, same rotation direction, then it will have to turn at half synchronous speed so that its field will be rotating at full synchronous speed, the same as the stator.
2. If the rotor is fed with 1/1 frequency, same rotation, both fields will be rotating in the same direction at the same speed. The rotor will not rotate at all, just remain locked stationary.
3. If the rotor is fed with 1/2 frequency, opposite rotation, then the rotor will turn at 3/2 synchronous speed so that the rotor's field is turning at synchronous speed of the stator field.
4. Finally, if the rotor is fed at 1/1 frequency, opposite rotation, then it will turn at twice the synchronous speed so that its field will be rotating at synchronous speed, the same as the stator field.

In all cases except #2 where it is stopped, the rotor will rotate in the direction of the stator field.

This is all assuming that the rotor can be pulled into synchronism, which depends on the load, moment of inertia of the rotor, and the field strength.

You want details? Shame on you.

Your answers is based on this assumption : "The two rotating fields will want to be locked together." This assumption is incorrect due to the reason below:

From Induction motor working principle, we know IM will rotate with a wide range of slip value (therefore different motor rpm). We also know the rotor current frequency is given by s*f_stator . Therefore, from this, we know the rotor field need not be locked with the stator field.

Wanting and having are two different things....You can't determine the %slip without all the details...

The rotor current frequency and rotor field frequency are not the same. You have to remember that the rotor is rotating. So the rotor field is rotating with respect to the rotor at the rotor current frequency but with respect to the stator at the rotor current frequency plus the rotation rate of the rotor.

The motor torque comes from the interaction of the stator field and rotor field. They both have to rotate at the same rate with the stator field slightly leading the rotor field to produce this torque.

When the stator field leads the rotor field the torque will be in the same angular direction as the rotor motion.(Accelerating) When the stator field lags the torque will be opposite of the rotor motion. (Braking or generating)

Just to be a meticulous PIA.

Yup, I was assuming some friction, windage, any kind of mechanical load.

You're forgetting something. There are TWO fields/currents in that one rotor. One is produced by the applied voltage to the windings (constant). The other's produced by the induction from the stator (dependent on slip). These two fields/currents will not be in sync with each other, and are out of phase. That conflict will prevent the motor from acting as either a synchronous motor or an induction motor. It won't be able to produce any usable torque.

There are TWO fields/currents in that one rotor. One is produced by the applied voltage to the windings (constant). The other's produced by the induction from the stator (dependent on slip).

You are probably right on this one, and you can't have any usable torque under this condition.

To have usable torque, you have to control both stator and rotor.

No, there is only one magnetic field produced in any motor. Most of that field lies in the gap between stator and rotor cores. That field is mathematically defined by the sum of the of the currents running in the stator, rotor, the magnetic permeability of the material (cores) in proximity to these currents and their spatial geometry. By definition the stator is presumed stationary but by definition the rotor is presumed movable. One must consider the kinematics of the rotor.

Don't forget that the currents running through all of these conductors are determined not just by the external voltage source applied but by the back EMF, too. That back EMF is proportional to the rotor angular velocity, too.

Each field is comprised of its own vector components. When combined, they produce a single result. But don't confuse that single result with its component parts.

"No, there is only one magnetic field produced in any motor." That one combined field is composed of two fields to make one.

You are missing a critical point here by insisting to analyze the field vectors only independently. Only the stator field is spatially fixed but it will still change in time. The rotor field must move in space and time and that movement can change the magnitude of its field strength. Nonetheless the magnetic field is one single time varying field predominantly residing in the gaps between these cores. I don't dispute that it is easier to derive each field independently but one should find that each dimensional field strength equation will ultimately include the induced voltage from the "other" field. There is only one field.

Depends on how you look at it. That "one field" could even be broken down into each pole, of each phase, of both stator and rotor. There must be at least be the separation between the stator and rotor field to produce a torque-producing reaction/transfer-of-power from stator to rotor to mechanical load. If you break it down farther (to even each turn of each winding of each pole of each phase, there could be a countless number of fields moving around in countless ways. But the whole point was to say/show that the field developed by the applied voltage to the rotor is not the only field produced by the rotor, and the field induced into the rotor from the stator is not the only field produced in the rotor. By themselves, one would make it a synchronous motor, and the other would make it an induction motor. But, combined, you'd just get a humming motor in need of defibrillation.

Sorry, if there's a conductor carrying current, then there is a field around it. If there's a conductor moving in a field, then there is a voltage induced in it. It is the combination of these multiple MMFs and EMFs that make the rotor move relative to the stator.

If you really want to understand the nature of these time-varying, non-linear, dynamic-positioning MMFs, EMFs, forces, and their interactions, that occur in AC electrical machines, then search on the Clarke and Park Transformations as detailed in the Unified Electrical Machine Theory and Generalized Machine Theory. Here's a place to start that isn't too math-heavy.

I think you might be replying to the wrong post. I fail to see what error this answer addresses. I certainly don't disagree with anything you said here. What am I supposed with this?

To be realistic, there are 2 e.m.f.s in your rotor, but just one current in each rotor winding phase. This assumes that the currents induced in the rotor metal by the stator field can be ignored as causing negligible loss, suggesting a laminated rotor.

As has been written, extensive theory is available for induction generators with wound rotors connected to allow continuous non-synchronous rotation & changing the sign of the slip etc gives the motor behaviour.

Without parameter & wave form/phase information, that theory cannot tell much, but one can suppose some basic behaviour from your proposal to apply 3 ph 50 Hz to rotor.

At half speed, the rotor voltage induced from stator will be 25 Hz. If there is no mechanical load the rotor will accelerate. If your 50Hz is a low impedance source, there is minimal rotor resistance, lots of current & about twice rated torque causing acceleration - it is a wound rotor machine with zero external resistance.

Putting in the magnitude of your 50Hz rotor voltage, supposing it is phase locked with stator, it is going to aid or oppose the natural 25 Hz.

The glory of Mr Tesla's symmetrical 3 phase motor/generator is that the rotor torque is theoretically constant with a sinusoidal flux.

I feel, however, that a 50Hz fluctuation in torque will result, dependent on injected rotor voltage, which does not make a good motor. I suppose it could behave like an induction motor with high supply voltage harmonics or flux harmonics which is all trouble, with - for example - lock-in at sub-synchronous speeds.

Is this really a motor or is it an experiment to turn it into a convector heater?

Wound-rotor AC motors are synchronous motors. When a synchronous motor is over-loaded, it slips a pole, and loses torque. When the rotor and stator are at different frequencies, it's never in sync, and is continuously slipping poles. It will never develop any usable torque and draw locked-rotor starting amps.

Really? Where does the excitation power come from? Better not tell that to anybody who has experience with variable resistance speed control of three phase slip ring induction motors.

From the same Wikipedia article

I assumed excitation power was required for this specific motor. Why else would he ask about applying 50hz to it? Maybe there are non-sychronous wound-rotor motors. But I didn't think it applied here. Sorry.

Sir,

The fairly large synchronous motors I worked with had two slip rings on the rotor. As the motor started, the controller was treating this motor as a standard induction motor and monitoring the difference in frequency between the lines and the rotor. When this frequency difference was small enough the controller then applied a DC field to the rotor via the slip rings to pull the motor into synchronization with the line frequency.

The various wound rotor motors I worked with had three slip rings on the rotor with external variable resistance between them. With no external resistance they ran very close to line frequency, but as the external resistance was added they would run slower, even as low as 1/4 line frequency (or lower), but with a considerable amount of waste heat in the external resistance.

The amazing Schrag motors I worked with were "inside out" with the main winding being on the rotor instead of the stator, using three slip rings to bring the power to the rotor. At the other end of the rotor there was a very wide commutator with two sets of three brushes and a method to counter-rotate them. These motors had the ability to produce full power at any speed from 10% of line frequency up beyond synchronous speed to over 150% of line frequency, all at very high power factor. For information on them, Dreisilker in the upper midwest USA can send you a couple technical papers on theory, adjustment, etc.

The original poster's question does not sound like any of the above. I do not know what would happen. I suggest that he fully instrument a motor (voltage and current on all incoming phases as well as on the rotor's slip rings), along with embedded thermal detectors (or a continuously viewable IR camera). Then apply power at frequencies he has mentioned to each set of leads. Since the rotor is generating voltage on its set of leads, they must be connected to some sort of a circuit that would appear to be of less than infinite resistance, or the motor will not turn. Imposing voltage on them at a fixed 50% frequency will be "interesting".

--JMM

Ok. I have no problem with paragraph 1. A controlled transition from induction to synchronous presents no problem.

Paragraph 2 indicates an induction motor.

Paragraph 3 has too little info to determine what kind of motor it is. It seems like it could be a hybrid "universal" motor designed for AC . But, the variable speed capability indicates a non-synchronous motor (to me).

As far as monitoring the voltage/current, an oscilloscope would be required to get a visual wave form. I don't really know about the rest of the paragraph; perhaps it would be an interesting experiment. It would better on a small motor tho, just to avoid catastrophes.

Dennis,

A synchronous motor is NOT an induction motor. The current in the rotor, when it is synchronized, is externally applied, instead of induced by the stator.

The Schrage motor is not a universal motor. For any setting of the adjustable commutator poles, the speed is constant regardless of load. It is not capable of "running away" at any combination of load or speed. I made one mistake in describing it--the motor produces full torque (not full power) at all speeds.

--JMM

"A synchronous motor is NOT an induction motor. The current in the rotor, when it is synchronized, is externally applied, instead of induced by the stator."

That's exactly what I was trying to say. See post #33.

It just seems somewhere somebody went off on a tangent. Thanks for for bringing it back to my point.

The "controlled transition" that you note has little to do with the controller, the reason that the motor starts and runs up to near synchronous frequency, is a result of its construction. It has an amortisseur winding (aka squirrel cage winding); i.e., it IS an induction motor during the acceleration phase. The controller monitors the slip frequency (which is easily accessible from the field winding via the slip rings), and as it approaches zero, applies the DC excitation power. Since the rotor is nearly at synchronous speed (and usually unloaded), it is pulled into synchronism with minimal disturbance to its supply.

Once synchronized, the amortisseur windings have no function (the slip frequency is 0) except when there is a disturbance that causes dynamic oscillations in the rotor. If that occurs, the armortisseur windings take on the function of their more common name, damper bars, which help to minimize the swings in the rotor torque/power angle.

The key thing that distinguishes a wound rotor induction machine from a wound rotor synchronous machine, is the number of slip rings.

The OP's question is clearly about a WRIM because he postulates injecting a "... 3 phase 50Hz supply to the rotor...", something that is pretty difficult to accomplish with only 2 slip rings.

When I said injecting “3 phase 50Hz supply to the rotor..." , of course it is implied that the windings is a 3 phase winding with 3 slip rings.

I totally agree with everything you said. But, the last two paragraphs is what I missed in the first place. I should have recognized that the whole synchronous motor thing did not apply. Sorry for the mistake. I stand corrected.

On second thought, I think a reconsideration of post #17 would be good. There might be some validity in that.

See:

https://en.wikipedia.org/wiki/Doubly-fed_electric_machine

For answers to OP's questions, look here.