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Quiet is key for air filtration

Quiet is key for air filtration

Torrington, CT-Airflow design engineers at Torrington Research Company had to develop an air filtration system to clean a large volume of air, but still be quiet enough for use in a bedroom. The product would be geared for the Asian market, particularly Japan, where homes are inherently smaller and more confined thus maintaining air quality is important for many people. So much so they would pay more for better systems.

Kollmorgen's filtering of drive motor current smoothes motor torque transitions, thus
reducing physical vibrations that generate acoustic noise.

One key problem arose from the plastic case housing the system. The plastic acted as a sounding board for any vibration--increasing noise heard by the customer. Thus vibrations originating in the motor/drive system had to be controlled while still delivering up to more than 225 ft3/min of clean air. "We developed a double-centrifugal design that delivered required performance, but at substantially reduced noise levels," says Torrington President Roger Dickinson. "The mechanical package is completely open, but the motor had to meet stringent noise requirements at four key speed/ torque points".

The company's designers turned to Kollmorgen (Radford, VA) to supply the needed motor--not just on the basis of their low-noise, brushless dc motor engineering expertise but also their ability to source internationally (Asia was the key here) and history of design-responsibility partnering on projects. Torrington wanted a single vendor to be accountable for the motor and drive--a dual-shaft configuration with the electronics mounted on the motor body to conserve space. Lower-cost ac induction or dc brush motors would have higher torque ripple, generating vibration. But engineers can now easily integrate brushless motors with readily available, low-cost driver chips, closing the cost gap.

Kollmorgen designers chose Motorola's 33033P chip for their motor-drive system to govern the four airflow levels (and corresponding motor torque points) the filtration system would provide. Hall effect sensors would generate signals for the six-step commutation scheme. And they laid out the drive PCB for minimal space and component count.

Without filtering and waveform shaping, even a standard brushless drive on the air filtration system at close to its highest speed (2,230 rpm) shows sound levels (at 1.6, 2.0, and 20k) more than 10 dB above neighboring frequencies, a source of annoying sound. A log scale, a 5 dB difference doubles the sound intensity.

In addition to the basic brushless design for the motor, Rob St. Germain, Kollmorgen national accounts executive, emphasizes other key technologies such as efficient magnet materials and bearings. He notes that while several manufacturers could supply the specified ABEC 3 ball bearings, the designers selected ones from NMB Technologies (Chatsworth, CA) as most consistently having the lowest noise (see Design News 6/21/99, p. 97).

Since switching of a commutating motor is a source of vibrations, those from the brushless motor were further cut by skewing the motor magnets to share and ramp up magnetic flux more smoothly in rotating between poles. Inductance filters in the circuits smooth electric current and thus the abrupt drive torque transitions which cause vibration. And rubber motor mounts also serve as the last line of vibration reduction.

Finally, as part of maintaining overall quality, St. Germain says every motor is tested for mechanical vibrations in axial and radial planes--which is quite a task given a production volume of more than 150,000 motors annually. Kollmorgen uses a Bruel & Kjaer (Decatur, GA) Model 2144 dual-channel, real-time frequency analyzer for its multifrequency capabilities to test each motor in less than 30 seconds. A probe placed on the motor measures 28 frequencies in both the radial and axial planes. In addition to having to hit an acceptable "window," the data is logged for trends, and Torrington again tests the motors prior to installation. Excessive amplitude at given frequencies can indicate specific problems. For example, 2,500 Hz is indicative of a bearing problem. The offending component may be removed, repaired, and reused.

How successfully has the filtration system been received? In two-plus years, over 300,000 have been sold, with less than 0.5% returned.

Other applications for brushless dc motors

  • Computer peripherals: cooling computers and electronics

  • Medical equipment: heart pump motors

  • Motion control: silicon IC fabrication--slice, dice, and polish

Brushless basics

  1. A brushless dc motor consists of a permanent-magnet motor, a wound stator, and a rotor-position sensing scheme.

  2. Information from the rotor-position feedback is applied relative to the back EMF waveform to provide proper commutation.

  3. The amount of torque available from a motor during the commutation interval is a function of the back EMF waveshape and the applied current waveform.

  4. Elimination of brushes provides these advantages:

  • Increased speed because no brush "flashover"

  • High torque-to-inertia ratio (improved efficiency)

  • Reduced weight and size

  • Smaller size due to more efficient heat dissipation with the outer wound stator

  • Reduced acoustic and electrical noise

  • Less debris generation

  • Improved MTBF

  • Less wear for reduced maintenance

5. With the cost of brushless dc motors dropping over the past few years (because of cheaper electronic control chips and more efficient neodymium magnet materials), they are seeing increased use in high-volume, cost-sensitive applications.

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