Can two coaxial motors null each other's inertia?

[activesprockets]

Hi all! I'm new here and hoping someone who knows something about ME or Physics can help me out ... and simple terms would be nice.

I am working with two servo motors and would like to minimize the "kick" they make when starting or stopping since they do it quite frequently. Please excuse my use of the word "kick" ... probably should use something like inertia, momentum, torque, ... if someone could set me straight here, I'd appreciate it.

First question:
If the two motors are identical, the load (rotating mass) attached to their axles are each identical, and the motor/loads are connected back to back so that the axles are coaxial, can I theoretically start and stop the two motors with exactly the same electrical drive but in opposite directions and cancel the "kick" they would have produced individually?

Second question:
If instead of being coaxial (mounted back to back) there was a plate inserted between them and one motor was moved, say, a foot away from the other so their axles remain parallel and in the same plane, what would happen if you energized them exactly as above?

Thanks in advance for helping me reduce the "kicks".

 

[jrmichler]

When started, the motor provides torque to the motor rotor and everything connected to the motor shaft. There is an equal and opposite torque to the motor frame. What you see as a "kick" is the reaction torque when the motor(s) accelerate. When the motor is commanded to go to full speed instantly, it accelerates at the maximum torque that the motor can deliver. When the reaction torque causes the motor to jerk around, you can do some or all of the following:

1) A stiffer motor mounting that does not move.
2) Command the motor to accelerate at a controlled rate such that it does not move.
3) In some cases, the load has a torsional resonance that makes the motor move. Such a system needs:
3a) Stiffer system with higher torsional natural frequency.
or
3b) A programmed rate of torque increase (finite jerk) motion profile.

Adding a second motor will not eliminate motor movement caused by reaction torque, regardless of where you put the second motor because torque is torque, and reaction torque is reaction torque. The amount of reaction torque is determined by the load inertia and how fast the load is accelerated.

All of this assumes that you have servo motors, not induction motors. Servo motors are commanded to move exactly how far and how fast you want, while induction motors are strictly on/off (unless driven by a VFD).

 

[Merlin3189]

Don't quite agree with jrmichler. If the coaxial motors are as specified by OP and driven simultaneously, both will have equal reaction torque at every instant, which must cancel out.

I think that will apply even if they're parallel, but I'm not quite so sure, as I've never thought about that before. Sounds like a twin engine aircraft with contrarotating engines.

Edit: Though a quick check with a more aeronautical friend tells me that contrarotating twins are not so common and there are reasons other than torque reaction for their use.

 

[activesprockets]

Thanks both of you for the replies ... I think I'm getting closer to what I hope to be the answer.

I realize no two motors can be exactly equal nor the loads they rotate. I'll try to give a better example and continue to assume everything is equal, including the opposite electrical excitation.

Lets assume I have a bench grinder mounted on springs with grinding wheels of equal mass on each side. When I spin it up, the motor "kicks" back, opposite to the direction of the grinding wheels.

Now replace the grinder motor with two other "shaft one side" motors. Their type is not important, only that they are coaxial and accelerate exactly the same (theoretically). Mount them so that they are a mirror image of each other, a shaft on the left with it's grinding wheel and the same on the right.

If you spin the motors up exactly the same and same direction, the grinder would function and "kick' back as with the original motor. So keeping in mind the two motors are mounted on springs, I'll repeat the original first question again:

If the two motors are identical, the load (rotating mass) attached to their axles are each identical, and the motor/loads are connected back to back so that the axles are coaxial, can I theoretically start and stop the two motors with exactly the same electrical drive but in opposite directions and cancel the "kick" they would have produced individually?

As for the second question, instead of the ends of the motors opposite their shafts being connected together, a flat bar is inserted between them and the motor on the right is slid back, say, 1 foot. Now bolt both motors to the flat bar while keeping their shafts parallel and in the same plane. If the coaxial arrangement canceled the "kick", will this offset arrangement also?

I'd like to thank you if you've stayed with me this far because my last question is closest to my actual situation. If both motors are mounted on the SAME side of the flat bar, one rotated clockwise and the other equally and simultaneously counterclockwise, would this offset but same side mounting theoretically cause the "kicks" from the two motors to cancel each other?

 

[hutchphd]

Surprisingly I do not agree with @jrmichler either.
case 1) The motors will supply equal and opposite torques exactly cancelling. This of course assumes everything identical and coaxial.
case 2) If (and only if) the plate is supported exactly halfway between the two motors there will be no net torque on the mount. This again assumes perfect alignment and rigidity.

 

[hutchphd]

It is easier to think about these arrangements in zero g. I am going to revise my answer above and remove the stipulation in 2 above so

case 1) The motors will supply equal and opposite torques exactly cancelling. This of course assumes everything identical and coaxial.
case 2) There will be no net torque on the object. This again assumes perfect alignment and rigidity. If it flexes all kinds of things happen
case 3) Again so long as the axles are parallel there should be no kick.

In the last two configurations The angular impulses will create bending stresses and
attempts to rotate the assembly may cause large twisting stresses in the bar from angular momentum direction changes.

Of course I could be totally wrong.

 

[Baluncore]

Counter-rotating coaxial motors would cancel inertia reaction and twist the common platform.
Counter-rotating parallel motors would cancel inertia reaction and fold the common platform.

If the motors were identical simple DC motors where torque is proportional to current, then running the two motors in series, but counter-rotating, would cancel the inertial motor torque during acceleration.

Then comes the problem of transferring motor torque from the motor platform to the load.

Now, how will the energy be shared if one motor has a flywheel, and so turns slower, would the other motor transfer more or less energy?