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Bearing friction damping in a stepper motor

Motors and generators with MagNet

This stepper motor consists of a stator with eccentric pole faces and a rotor made of samarium cobalt permanent magnet that is magnetized in a fixed direction. The rotor rotates in steps of 180 degrees. Each step in the rotor is due to a short pulse from a current source.

To obtain a uni-directional motion, the current pulse is alternating. In this example, the response due to one pulse is examined. The damping due to bearing friction is taken into account in the simulation. Due to geometric and electromagnetic symmetry, only half of the device needs to be modeled.

The simulation and analysis of this stepper motor is performed with MagNet.

permanent magnet stepper motor

METHODS and RESULTS

AUTOMATICALLY REMESHING AROUND THE ROTOR as it MOVES

The air region surrounding the rotor is automatically re-meshed as it changes position. In order to reduce the size of the re-meshed region, the user can create an air region that is slightly larger than the rotor. Re-meshing this relatively small and simple region is quick, and with this approach, no additional constraint equations need to be solved, which keeps solution times short and memory requirements low.

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REFINING the MESH

In regions where the direction or magnitude of the field is changing rapidly, high accuracy requires small elements. One method of refining the mesh is increasing the density by setting the maximum element size for a component volume or specific faces of a component.

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ONE CURRENT PULSE

This figure shows one current pulse for which the simulation is performed. The pulse is switched on at 0.2 mSec and switched off at 4.2 mSec and remains off until the next switch time.

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ANGULAR POSITION of the ROTOR

The angular position of the rotor is shown as a function of time. It can be seen from this graph that the rotor rotates by 180 degrees for one current pulse. Each 180-degree switch involves minor oscillations before the rotor becomes stable in the new position.

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MAGNETIC TORQUE in the STEPPER MOTOR

The graph of the magnetic torque vs time shows both the forward and backward torque that the rotor experiences for one pulse before it settles down in the new position. Initially, the magnetic torque is caused when the pulse is switched on. After the pulse is complete, there is a magnetic torque due to the permanent magnetism of the rotor trying to align itself with the stator pole faces. The oscillations seen in the graph are due to the overshoots, but there is a damping in the oscillations due to the bearing friction that is modeled in this simulation.

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VELOCITY and ACCELERATION of the ROTOR

Other quantities of interest to the user are the velocity of the rotor and its acceleration as a function of time which is readily available once the solution is complete.

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MAGNITUDE of the FLUX DENSITY

All field quantities can be animated and displayed in MagNet or exported to other video players. The illustration shown here is created from the magnitude of the flux density in a view that is zoomed into the rotor area.

When playing this animation, one can see the field changing as a function of time for different rotor positions. The notch in the shaft can be used to track how the rotor is changing position.

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