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

#### Neumahn

Summary
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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#### Baluncore

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?

#### Neumahn

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.

#### Tom.G

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 similiar 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.

#### Baluncore

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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