Behaviour of rotating Electric Machines in Free Space

[b.shahvir]

Hi Guys,

This somewhat abstract concept just happened to pop in my mind.

CASE 1 :-
Assume a 3 phase Induction motor to be placed in free space such that both the stator and rotor are free to move relative to each other i.e. neither the stator (nor the rotor for that matter) is bolted or fixed to any hard supporting platform or ground surface. Now, 3 phase supply is given to the stator coils. Assuming rotation of rotor to be in clockwise (CW) direction, the stator will move in counter-clockwise (CCW) direction (since both stator and rotor are assumed to be able to to rotate freely in either directions as mentioned earlier.

So, will both the stator and rotor continue to rotate in oppisite directions relative to each other, or will they achieve electromagnetic equilibrium and stall? In my opinion, if both do continue to rotate in opposite directions, they would rotate at half the synchronous speed of the machine.

CASE 2:-This is same as above but i consider a separately Excited DC motor (stator excitation is thru DC coils). Since both stator and armature are free to rotate in either directions, they will rotate in opposite directions relative to each other but stall when both will reach the Magnetic Neutral Axis (MNA) due to zero electromagnetic torque at MNA... or at a position wherein the distorted magnetic lines of force linking the rotor/armature conductors would straighten out.

Also, in both the above cases, the back EMFs should develop in stator as well as rotor/armature conductors, since both are moving in relatively different directions in each other's magnetic fields. The explanation to the above cases would also be valid for DC Generators/Alternators as electric machine working principles are interchangeable!

I feel torque equilibrium might not be achieved in CASE 1 due to absence of MNA as Induction motor has a synchronously revolving magnetic field,and torque depends on the existence of relative motion between stator and rotor.

PS:- The magnetic field densities due to both the stator and rotor/armature coils are assumed to be same, hence theoretically, both can be considered to be interchangeable.

Kindly guide me as regards the behaviour of rotating electric machines in above 2 cases. Pls. help!

Kind Regards,
Shahvir

 

[sophiecentaur]

Angular momentum must be conserved so I should expect them to rotate in opposite directions and the light rotor to be rotating faster than the massive stator and frame. (Speeds inversely proportional to moment of inertia) The terminal relative angular speed would be the same as if the frame were bolted down to the Earth.
This should apply irrespective of the type of motor involved.

 

[b.shahvir]

Angular momentum must be conserved so I should expect them to rotate in opposite directions and the light rotor to be rotating faster than the massive stator and frame. .

But what if the mass of stator and rotor/armature is assumed to be same?

 

[sophiecentaur]

Half speed in either direction means a relative speed of one, surely.

 

[b.shahvir]

Half speed in either direction means a relative speed of one, surely.

How would this concept relate to machines with rotating magnetic fields such as the Induction motor. Since magnetic field is also rotating alongwith the spinning stator and rotor. What about DC machines?..will both AC and DC machines behave in same manner?

 

[sophiecentaur]

The magnetic field of one part is rotating relative to the other part, surely?
You could possibly suggest that the effect of rotation could affect the torque but I can't see that you could violate conservation of angular momentum. But why should it affect torque?

 

[b.shahvir]

My main focus is on magnetic lines of force emanating from the stator linking the rotor or armature conductiors. Since torque depends on the amount of distortion of the elastic magnetic lines of force which would eventually straighten out if both stator and rotor move in opposite directions relative to each other. If that happens, torque should become zero as there is no distortion of stator magnetic field. This logic somehow eludes me.

 

[sophiecentaur]

My main focus is on magnetic lines of force emanating from the stator linking the rotor or armature conductiors. Since torque depends on the amount of distortion of the elastic magnetic lines of force which would eventually straighten out if both stator and rotor move in opposite directions relative to each other. If that happens, torque should become zero as there is no distortion of stator magnetic field. This logic somehow eludes me.

But is it not true to say that when the motor is at its maximum no-load speed, this 'distortion' is minimal, in any case.

 

[b.shahvir]

But is it not true to say that when the motor is at its maximum no-load speed, this 'distortion' is minimal, in any case.

So, if we do conclude that the stator and rotor/armature were to spin continuously in opposite directions relative to each other, how would it actually behave in practice?...would torque and speed be reduced to one half the value as compared to a normal motor whose stator is fixed on a ground surface and rotor/armature is free to rotate normally?..or will it behave in some weird manner, the physics of which is never witnessed in normal operation.

What about reflected currents and induced back EMFs in stator coils? will the value be different from that of the normal motor... the point being that in the present case, the spinning stator conductors are also linked by the magnetic field of the rotor/armature coils!

Regards,
Shahvir

 

[sophiecentaur]

Why are you looking for something extra on top of the normal operation of the motor? Why should its magnetic circuit behave any different just because it's rotating?

The 'torque and speed' you refer to. What do you mean? Are you 'using' the motor in some way? If you try to drive something from the rotor with the stator mounted in this way, you could find the stator is the only thing rotating (conservation of momentum and all that) and the rotor might not move at all.

 

[b.shahvir]

The ambiguity has risen since I've assumed stator and rotor to be able to freely rotate in opposite directions relative to each other

 

[sophiecentaur]

So you have to resolve it if you want an answer.
With 'nothing to push against' the motor can do no work.

 

[b.shahvir]

So you have to resolve it if you want an answer.
With 'nothing to push against' the motor can do no work.

That's exactly why I'm considering motor on no-load! I'm only interested, currently, in the electromagnetic behaviour of AC/DC rotating machines under the stated conditions...the reflected current or the back EMF magnitudes induced in then stator coils, etc

 

[sophiecentaur]

OK - but, firstly, why should you consider that rotating would make any difference? and,secondly, the relative moments of inertia will affect the relative rotation rates.
If you are really convinced that rotation makes a difference then you would have to specify the actual moments of inertia so that you can tell how the 'relevant' field of which part is rotating.

 

[m.s.j]

Electromagnetic behavior of whole system will be same, because bolting of stator or rotor doesn't appear in machines basic electrometrical relationships except total rotating inertia of rotating mass and negligible rotating frictions and related losses.

--------------------------------
Creative thinking is breezy, Then think about your surrounding things and other thought products. http://electrical-riddles.com

 

[b.shahvir]

OK, this is what I've deduced.Assuming mass and moment of Inertia of stator and rotor to be same and also assuming shaft bearing friction as the only load on the machine, rotation of each in opposite direction will reduce speed of stator and/or rotor to one half the value compared to when one is rotating (rotor) and the other is fixed (stator). The torque developed by each will be one half of the total torque developed by the motor, compared to when stator is fixed and rotor is rotating. Pls. guide whether my deductions are correct under the conditions specified above.

Thanks,
Shahvir

 

[b.shahvir]

Actually I was referring to counter rotating (contra rotating) electric machines.. wherein stator and rotor rotate in opposite directions relative to each other.

 

[sophiecentaur]

I really don't understand where you think you are going with this thread.
If you are planning to measure torque then you could measure it between rotor and stator - with some sort of 'floating' device. That will give exactly the same answer as you would get with the stator held static because the electromagnetic situation is exactly the same, however you choose to rotate the unloaded motor. (You keep ignoring this, apparently)
If you measure the torque, wrt a 'stationary frame', of each by using two (exactly matching) torque meters, then you will get one half for each (in opposite senses) - which will add up to the same as the total torque of stator against rotor.
Is there anything else to say on the matter?

 

[b.shahvir]

OK I understand, so in that case, the speed/torque of the individual stator or rotor would be one half of the total value if either one was fixed, correct? will the same logic apply to a AC/DC generator? will the value of back emf be one half in case of motor ?

 

[sophiecentaur]

The back emf relates to the relative rotation of rotor and stator, doesn't it? So why should the situation be any different for an AC DC motor (you meant??). You still seem to be ignoring the relative thing.

 

[b.shahvir]

Here's an interesting link to thread on contra rotating electric motor incidently on physicsforums itself ;

https://www.physicsforums.com/showthread.php?t=99907

On most of my online searches I've found contra rotating propellers actually add up or double the output thrust or torque and improves efficiency. If I were to consider one propeller on rotor and the other on stator, both spinning in opposite directions, will the speed/torque of each propeller be one half as compared to total speed/torque of, say, a single propeller connected to either stator or rotor with either of them fixed as is normal in case of unidirectional machine?

 

[sophiecentaur]

That's interesting but any possible advantage really relates to the fluid dynamics of the situation rather than to electric motors. We can't go down the road of 'doubling efficiency' can we?

 

[b.shahvir]

If I were to consider one propeller on rotor and the other on stator, both spinning in opposite directions, will the speed/torque of each propeller be one half as compared to total speed/torque of, say, a single propeller connected to either stator or rotor with either of them fixed as is normal in case of unidirectional machine?

So, in your opinion, how would the machine behave in terms of individual stator-rotor speed/torque charac. as mentioned above? will there be any added advantage in terms of improved torque etc, or the net system performance (torque/speed, etc) will remain same as under normal conditions?

 

[m.s.j]

The torque developed by each will be one half of the total torque developed by the motor, compared to when stator is fixed and rotor is rotating.

According to Newton's third law, for every action force there is an equal (in size) and opposite (in direction) reaction force. Forces always come in pairs - known as "action-reaction force pairs."
Indeed there are one developed torque and its reaction, T = K.Ør. Øs.SinΘ


--------------------------------
Creative thinking is breezy, Then think about your surrounding things and other thought products. http://electrical-riddles.com

 

[sophiecentaur]

According to Newton's third law, for every action force there is an equal (in size) and opposite (in direction) reaction force. Forces always come in pairs - known as "action-reaction force pairs."
Indeed there are one developed torque and its reaction, T = K.Ør. Øs.SinΘ--------------------------------
Creative thinking is breezy, Then think about your surrounding things and other thought products. http://electrical-riddles.com

Of course, but it is also quite valid to add forces (and moments) vectorially. The motor works by the mutual effect of stator and rotor but you can disconnect them from the bench and then, still, measure the torque between them by measuring torques relative to some other frame and then adding them up. Those two torques are 'really there' because they will show up on a meter.