Voice Coil actuator or Linear Motor ?
Posted: Wed Nov 22, 2023 4:23 pm
The first motor I designed was a Voice Coil Actuator. This has the same basic components as a loudspeaker. What is great about a voice coil actuator is that within a certain deflection, the force generated by the electromagnet is linear. This linear response is required for loudspeakers, so that it can faithfully reproduce the sound waveform. For my application, I required a clean 20hz movement in order to make sure the fan is kept in the infrasonic region (and thus silent). Any harmonics would be audible.
While designing the VCA, I learned the major drawback of this type of motor. The magnetic flux has to pass through the voice coil (coil of copper wire) . Magnetic flux does not like to travel through air or copper. The result is that it is hard to achieve high magnetic flux density and thus motor efficiency. One way to increase the force of the motor, is to increase the number of copper windings. However, this requires you to increase the air gap, which in turn, reduces the magnetic flux density and thus performance. I had to resort to many neodymium magnetics in order to achieve the performance target.
My second design was a linear motor. The major advantage is that the only air gap that is required for the tollerance of your machining and stiffness of the spider. This dramatically reduced the amount of magnet and copper material required to meet the performance target. A disadvantage of a linear motor is that it has an iron core, and thus cogs. The moving part of the motor is attracted to the stator independently of whether the electromagnet is energized or not. Thus a linear motor does not naturally have the desirable linear response of a VCA.
In order to overcome this problem, I designed the linear motor to have a single pair of poles. The Iron and magents were shaped so that the magnets increased their restoring force linearly with deflection from the middle of the motor. So, the magnets have a restoring force that mimics the spring that the spider normally provides. The resulting design was much more efficient than the VCA.
I am an amateur at motor design. I have no idea if using the cogging to your advantage is a novel idea. I can imagine every possible motor design has now been explored. I would be grateful if the wider community can comment on this. If this is novel, it could be used to increase the efficiency of loudspeakers (if they don’t mind the added restoring force).
One thing I did not do is laminate the core. I did one quick estimate and *thought* that eddy current losses would be insignificant, due to the relatively low 20hz switching frequency, but I could easily have been mistaken.
While designing the VCA, I learned the major drawback of this type of motor. The magnetic flux has to pass through the voice coil (coil of copper wire) . Magnetic flux does not like to travel through air or copper. The result is that it is hard to achieve high magnetic flux density and thus motor efficiency. One way to increase the force of the motor, is to increase the number of copper windings. However, this requires you to increase the air gap, which in turn, reduces the magnetic flux density and thus performance. I had to resort to many neodymium magnetics in order to achieve the performance target.
My second design was a linear motor. The major advantage is that the only air gap that is required for the tollerance of your machining and stiffness of the spider. This dramatically reduced the amount of magnet and copper material required to meet the performance target. A disadvantage of a linear motor is that it has an iron core, and thus cogs. The moving part of the motor is attracted to the stator independently of whether the electromagnet is energized or not. Thus a linear motor does not naturally have the desirable linear response of a VCA.
In order to overcome this problem, I designed the linear motor to have a single pair of poles. The Iron and magents were shaped so that the magnets increased their restoring force linearly with deflection from the middle of the motor. So, the magnets have a restoring force that mimics the spring that the spider normally provides. The resulting design was much more efficient than the VCA.
I am an amateur at motor design. I have no idea if using the cogging to your advantage is a novel idea. I can imagine every possible motor design has now been explored. I would be grateful if the wider community can comment on this. If this is novel, it could be used to increase the efficiency of loudspeakers (if they don’t mind the added restoring force).
One thing I did not do is laminate the core. I did one quick estimate and *thought* that eddy current losses would be insignificant, due to the relatively low 20hz switching frequency, but I could easily have been mistaken.