Lenz's Law for a rotating PM motor rotor spinning in a thin CU tube

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Discussion Overview

The discussion revolves around the implementation of a damping mechanism in a permanent magnet (PM) motor, specifically focusing on the use of a thin-walled copper tube within the stator bore to achieve this. Participants explore the implications of this design choice on motor performance, particularly in relation to resonance frequency and operational characteristics.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Exploratory

Main Points Raised

  • Neumahn inquires about calculating the damping torque as a function of motor speed when installing a copper tube with a specified air-gap.
  • One participant suggests that the existing clearance is necessary for bearing movement and questions whether the copper sleeve would interfere with motor operation, proposing the idea of short-circuiting existing windings for damping.
  • Neumahn clarifies that the electronics for the redundant winding are open, making it impractical to control them, and mentions the challenging option of creating new stators with a damping coil.
  • Another participant discusses the potential of the copper sleeve acting like a shorted-turn secondary and suggests ramping the stator current to achieve a similar damping effect without modifying the motor.
  • There is a proposal to use circuitry to switch a resistor across non-energized windings to function as a damping winding, noted as a simpler option.
  • A later reply emphasizes that a squirrel cage of damping windings should be installed in the PM rotor rather than in the stator poles, indicating a different approach to the damping mechanism.

Areas of Agreement / Disagreement

Participants express differing views on the effectiveness and practicality of using a copper tube for damping, with some proposing alternative methods and configurations. The discussion remains unresolved regarding the optimal approach to achieve the desired damping without significant modifications to the motor.

Contextual Notes

Participants highlight various assumptions about motor operation and the effects of modifications, including potential interference with magnetic characteristics and the implications of using a copper sleeve versus other damping methods.

Neumahn
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TL;DR
I am trying to determine the damping generated by spinning a motor rotor in a thin-walled copper tube. This is a stepper motor with 40 magnets radially charged along the circumference.
The motor is required to operate at its resonance frequency and I am looking to add a thin-walled (0.010") copper tube inside the stator bore to add some damping. The current motor air-gap is 0.015". If I install a copper tube in the stator bore bonded to the stator and leave a 0.005" air-gap between the tube and the rotor how would I calculate the damping torque as a function of motor speed?

Normally we could install a damping winding but these motors are already built and there is no room for a damping coil.

Thank you,

Neumahn
 
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Welcome to PF.

The 15 thou clearance is probably needed for bearing movement and wear. You will probably be left with insufficient clearance, so the motor poles will connect with the copper insert.

With a copper sleeve in the gap the motor will probably not operate like a stepper motor. The copper sleeve would be driven like an induction motor. Could you short circuit the existing windings while they are not being driven to provide the required damping?
 
No, the electronics are open on the redundant winding and we have no control over them. The other option being considered is making new stators and adding a third damping coil. This is a painful option as it requires considerable schedule.
 
You could probably get away with the Copper sleeve but only at slow step rates. The Copper will act like a single shorted-turn secondary of the stator windings. Since a transformer operates only on a varying current, once the current settles the Cu will have no effect on the field windings and will allow the the static magnetic field to reach the rotor.

You could get a similar effect by slowly ramping the stator current. This would not necessarily require a change in the drive electronics, as you could put an Integrator circuit between the windings and the existing electronics. That may or may not be cheaper than modifying the motor.

Alternatively, you could put circuitry between the windings and the existing electronics to switch a resistor across the non-energized windings, using them as a damping winding.

The above has been a stream-of-consciousness. The last option seems the simplest.

Cheers,
Tom

p.s. be aware that dis-assembling a stepper motor often means a significant decrease performance. Something to do with altering the magnetic characteristics.
 
Neumahn said:
If I install a copper tube in the stator bore bonded to the stator
As I understand it, a two or three phase stepper motor is being run as a synchronous motor.

The damping windings are used to start rotation as an induction motor until the PM poles of the rotor synchronously lock to the rotating stator field.

That suggests a squirrel cage of damping windings should be installed in the PM rotor, NOT in the stator poles of the phase windings.
 

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