Behaviour of rotating Electric Machines in Free Space

b.shahvir

Joined Jan 6, 2009
457
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 Seperately 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

Kermit2

Joined Feb 5, 2010
4,162
The magnetic forces would pull the rotor face ONTO the surface of the stator iron and the rotor would "ROLL" around in contact with the stator iron. Imagine a planetary gear rotating inside of a toothed outer ring.

You HAVE to have the rotor fixed in position relative to the stator in some form or fashion or the magnetic pieces of iron in the two WILL come together with force and stay together until forcefully removed.

b.shahvir

Joined Jan 6, 2009
457
The magnetic forces would pull the rotor face ONTO the surface of the stator iron and the rotor would "ROLL" around in contact with the stator iron. Imagine a planetary gear rotating inside of a toothed outer ring.
The rotor cannot come in contact with the stator since bearings/bushings are in place. Besides, it would have something to do with the distorted magnetic lines of force acieving it's normal undistorted state. Still somehow, the concept seems unclear.

Kermit2

Joined Feb 5, 2010
4,162
Hi Guys,
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...
Sorry if I misinterpreted your vision. I based my reply on the facts you presented in the above sentence. "free to move relative to each other", indicated to me that there was no "fixed" distance that would keep them separate.

b.shahvir

Joined Jan 6, 2009
457
Actually, I was referring to counter rotating (contra rotating) electric machines..wherein stator and rotor spins in opposite directions relative to each other.

GetDeviceInfo

Joined Jun 7, 2009
2,180
In either machine, frictional losses still occur, so in the AC machine you would near synchronous speed, while the DC machine would likely blow up.