Linear iron-core permanent magnet motor with high force and low acoustic noise

Abstract

Acoustic noise and associated vibration are potentially troubling characteristics of electric machines including variable-reluctance motors, induction machines, and permanent magnet synchronous motors. The severity of this problem depends on the types of motors and their applications. One exemplary case where the vibro-acoustic noise becomes especially detrimental is iron-core linear motors operating at high acceleration and targeted for high accuracy applications. In this doctoral research, we identify root causes of the vibro-acoustic noise of iron-core linear motors, and create magnetic designs paired with control algorithms to achieve high-acceleration with low noise and vibration. Vibro-acoustic noise issues of rotary machines have been researched over the years, but not as much work has been done for linear machines whose major causes of noise generation might be different from rotary motors due to the structural differences. This thesis focuses on the following aspects: 1) Investigate the causes and develop and verify theory for acoustic noise emitted from linear iron-core machines. 2) Design, construct, and test a new linear iron-core motor that can simultaneously provide high force and low noise. 3) Design and construct an experimental linear motor testbed to investigate the noise issue of conventional iron-core motors, and to demonstrate the new motor's desired performance of high force and low noise. We hypothesize and experimentally validate that the acoustic noise and associated vibration of linear iron-core permanent magnet motors are caused by high frequency force harmonics vibrating the moving stage. Such stage vibration can be transmitted through the system structure and can also radiate as acoustic noise, thereby disturbing precision machines. In order to reduce high frequency force harmonic content, our new motor has fine teeth, narrow slots with high slot aspect ratio, five phases, and a moving Halbach magnet array. With our new fine-tooth motor, we significantly reduce the vibro-acoustic noise of linear iron-core motors while substantially enhancing the shear stress density, compared to conventional 3-4 combination iron-core motors. The overall acoustic noise level in Pascals is reduced by 93 % during the acceleration period with non-skewed magnets. In terms of the sound pressure level (SPL), this is a significant noise reduction from 83 dB to 60 dB. The cogging-driven and velocity dependent noise, which is dominant in constant velocity regions, is also significantly reduced in our new motor design. Our fine-tooth motor design reduces the cogging force by a factor of 10-to-1 when using skewed magnets, thereby reducing cogging driven acoustic noise by 90 % in Pascals. We also present in this thesis the force performance of our new fine-tooth motor both in simulations and experiments. Our new motor shows predicted shear stress improvements of 28 % (from 0.090 N/mm2 to 0.115 N/mm 2) at the prototype practical power level of 10 W/mm and 84% (from 0.167 N/mm 2 to 0.308 N/mm2) at an anticipated ultimate RMS (root mean square) current density limit in the coil wires of 50 A/mm2 , relative to a conventional motor. Understanding causes of the vibro-acoustic noise and how to mitigate it in both the design and post-design phases provides useful tools to achieve high-performance and quiet linear motion devices. This research will benefit many industrial applications which require both high throughput and high accuracy.