Possible feedback problems of a uni-directional torque

[chazemz]

Hi, I wonder if someone can help with the following problem? We have a sealed box in space and inside the box is an electric motor with the stator attached to the box. The rotor arm is attached to the inner race of a bearing and the outer race of the bearing is also attached to the box.

There is an internal power supply. We know that when the power is applied to the motor, the stator will begin to rotate in the opposite direction to the rotor arm and the outer race will rotate in the opposite direction to the inner race and so will apply a torque to the box in the same direction as the stator.

Will the stator and rotor rotate in balance?

Will the rotor arm rotate faster than the stator so that the outer race will always apply a small torque to the box?

Will the rotor arm slow so that the outer race now starts a reverse effect on the rotor arm?

None of the above.

 

[berkeman]

Is this related to your Reactionless Drive threads here back in 2017?

 

[chazemz]

No

 

[chazemz]

May I add that the stator and outer race will act as described they are well known. What I would like to know is how the rotor and stator interact as the box, stator and outer race all rotate in the opposite direction to the rotor.

 

[Lnewqban]

At least during initial acceleration time, it seems to be a struggle among moments of inertia (of rotor and stator), polar moment of inertia (perpendicular to plane of rotation) and gyroscopic effect.
Any unbalanced mass may cause additional rotational movements in other planes.
If feasible, experimentation seems to be the simplest way to an answer.

 

[berkeman]

May I add that the stator and outer race will act as described they are well known. What I would like to know is how the rotor and stator interact as the box, stator and outer race all rotate in the opposite direction to the rotor.

Can you upload a diagram? That would be a big help.

I'll also move this thread to the ME forum for now...

 

[chazemz]

No problem. I will upload a diagram tomorrow morning.

 

[Dale]

Will the stator and rotor rotate in balance?

Total angular momentum is conserved. So everything together will rotate in balance. Not necessarily the stator and rotor, since there are other parts too.

 

[jrmichler]

That's how they aim orbiting telescopes and other satellites. A reaction wheel turns one way, and the satellite turns the other way. Search terms that lead to good links include satellite angular positioning reaction wheel and satellite angular positioning flywheel. Here's one of a number of good links: https://aerospace.honeywell.com/us/en/about-us/blogs/understanding-reaction-wheels. And a quote from that link: They can be used to control the position and attitude of a satellite without requiring thrusters or any other external applicators of torque. This brings multiple benefits, including a significant reduction in the payload fraction needed for fuel.

 

[berkeman]

And a quote from that link: They can be used to control the position and attitude of a satellite without requiring thrusters or any other external applicators of torque. This brings multiple benefits, including a significant reduction in the payload fraction needed for fuel.

(I fixed that for them)

 

[chazemz]

Diagram attached

 

[Dale]

So the stator, box, and outer race are all rigidly attached to each other, and the rotor and inner race are rigidly attached to each other?

 

[chazemz]

That is correct.

 

[Dale]

Will the stator and rotor rotate in balance?

So the stator + box + outer race will rotate in balance with the rotor + inner race.

 

[chazemz]

The stator of an electric motor is usually attached to an object that does not rotate. In this situation it is easy to say that as long as the rotor spins faster than the object a uni- directional torque will be applied to the object. The physics has no problem with this. By putting the box in space means that the box can counter rotate with regard to the rotor with ease. If the box can counter rotate faster than the rotor, a crossover point should be reached and the bearing will now apply a toque to the rotor not the box. What I am interested in is how this would change the dynamics between the rotor and stator?

 

[Dale]

a uni- directional torque will be applied to the object

Angular momentum is conserved. So there is no uni-directional torque. Any torque is accompanied by an equal and opposite torque. Just like forces with newton’s 3rd law

If the box can counter rotate faster than the rotor, a crossover point should be reached and the bearing will now apply a toque to the rotor not the box.

I don’t think so. I cannot see any physical justification for such a crossover point

 

[chazemz]

The ball bearings between the inner and outer races change the direction of the torque of the rotor. It is impossible for the rotor in the diagram to apply a torque to the box. Even though everything is connected to each other a torque is applied to the box in one direction only. May I also add that the box will spin and therefore interact with its surrounding which will introduce an exterior force. Conservation of angular momentum applies in a closed system.

 

[berkeman]

the box will spin and therefore interact with its surrounding which will introduce an exterior force.

Interact with the vacuum of space?

Is this related to your Reactionless Drive threads here back in 2017?

No

Short leash...

 

[Dale]

We have a sealed box in space

May I also add that the box will spin and therefore interact with its surrounding which will introduce an exterior force. Conservation of angular momentum applies in a closed system.

This is a classic closed system. There is no external torque, no exterior force, and no interaction with the surrounding. All by your own stipulation that this is a sealed box in space.

 

[jrmichler]

The ball bearings between the inner and outer races change the direction of the torque of the rotor.

No. Ball bearings allow inner and outer races to rotate relative to each other with minimal friction between them. That's all that they do.

It is impossible for the rotor in the diagram to apply a torque to the box.

No. A motor inside a box that is applying a torque, will apply a torque to the box. The direction of that torque depends on whether the box is connected to the motor rotor or motor stator. The amount of that torque depends on the rate of acceleration and the relative inertias.

May I also add that the box will spin and therefore interact with its surrounding which will introduce an exterior force.

We have a sealed box in space and inside the box is an electric motor with the stator attached to the box.

A sealed box in space, by definition, has no exterior force. In a physics problem such as this box in space, we assume zero friction, zero magnetic fields, zero atmosphere, zero sunlight pressure, and zero external influences. Any exceptions are clearly and specifically stated.

In this situation it is easy to say that as long as the rotor spins faster than the object a uni- directional torque will be applied to the object.

Pure nonsense. A motor can accelerate in either direction, therefore motor torque is bidirectional. A motor that applied unidirectional torque, by definition, would accelerate to a speed and never slow down.

What I am interested in is how this would change the dynamics between the rotor and stator?

The dynamics are simple. Apply current >> motor accelerates or decelerates depending on how the current is applied. An observer sitting on the stator will see the rotor change velocity, but will not know if the rotor, stator, or both are rotating with respect to the outside world.

Conservation of angular momentum applies in a closed system.

Yes. Unfortunately, the rest of your posts ignore this fact. This raises the question of whether you do not understand basic physics, or are trolling us. Please clarify which is the case. If you do not understand basic physics, say so, and we can help you. Otherwise this thread will be locked.

 

[chazemz]

Now we are getting somewhere.

There is no such thing as a perfect vacuum.
The inner and outer races of a bearing rotate in opposite directions. That is how they work.
It is true that a motor can accelerate in both directions but it can only rotate in one direction at a time.

The basic physics here are that a stator will rotate in the opposite direction to the rotor and the inner race of a bearing will rotate in the opposite direction to the outer race. These are facts.

In your rush to ignore these facts you are missing the point of the title. The feedback of the bearing is very important when the motor is switched off and the system begins to slow. Whether the box comes to rest at the end is unknown due to the exterior force. Can you please stop using the word reactionless, both the stator and the bearing rely on an action/ reaction exchange of angular momentum.

Am I right in thinking that if a higher friction bearing is used the torque reaction of the stator will increase due to the moment of inertia increasing. This will cause the box to spin faster than if using a low friction bearing.

 

[Dale]

Now we are getting somewhere.
There is no such thing as a perfect vacuum.

It doesn’t have to be perfect to produce negligible drag. The phrase “in space” in a classical physics discussion means “I am assuming that all external interactions are negligible”. What you are doing here is called a “bait and switch”.

For all practical purposes, the answer remains as we have described above. Uncertainties in the amount of energy in the battery, efficiency in the motor, and friction in the bearings will be orders of magnitude greater than any external torque due to drag in the interplanetary medium.

The inner and outer races of a bearing rotate in opposite directions. That is how they work.

That is not at all how they work. Bearings work to not transmit frictional torque. They are ideally a connection with 0 frictional torque. That in no way implies that they rotate in opposite directions.

The feedback of the bearing is very important when the motor is switched off and the system begins to slow. Whether the box comes to rest at the end is unknown due to the exterior force.

Since the exterior force is negligible by your original stipulation, all of the replies assumed that. With that assumption it is not unknown. The system does come to rest at the end.

With non negligible drag the system comes to rest with respect to the fluid at the end.

Am I right in thinking that if a higher friction bearing is used the torque reaction of the stator will increase due to the moment of inertia increasing. This will cause the box to spin faster than if using a low friction bearing.

A bad bearing will just mean that the battery wastes some additional energy. The final state will be reached faster, but won’t be different.

 

[jrmichler]

Three different Mentors have explained the physics of angular momentum, and the OP refused to learn. This thread is locked.