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Dealing with noisy motor: Traditional compensation

Posted: 11 Feb 2016 ?? ?Print Version ?Bookmark and Share

Keywords:vibration? noise? tuning?

If we then add the torque vs. angle perturbation, we still are unable to get rid of the speed variation as shown in figure 5.

Making a feedback-based, only-speed controller more aggressive demonstrates that a high-gain speed controller is not the solution to the vibration and noise problem.

Applications affected by vibration and noise
Any application that uses piston-based compressions or rotating loads most likely will present a degree of vibration or noise. In this article, we are focusing our attention to those appliances that are located inside homes and those applications with a mechanical system that can be damaged by the vibration of the motor. Examples include single-unit air conditioners, split air conditioners, refrigerator compressors, paint sprayers and air compressors, to name a few.

Figure 5: Tuning of the speed controller without vibration compensation (Source: Texas Instruments)

Figure 6: Common applications located inside homes (Source: Texas Instruments)

Traditional compensation: Getting to know your mechanical load
One method of compensation is the traditional method, which compensates a pulsating load based on previous knowledge of the mechanical load attached to the motor shaft. In this traditional compensation method, knowing the mechanical load is critical. This is because any mismatch between the load and what the algorithm thinks the load is could potentially make vibration and noise even worse.

To show how this works, let's reuse the torque vs. angle curve shown in figure 5. The traditional method maps the load profile exactly and uses the information to help the speed controller. The traditional method will work only if the load does not change at all, or if there are very specific speed operating points, each point with a known load profile. If the load does not change, the speed controller can use look-up tables preloaded in the memory of the microcontroller. If the load is known in advance, the load can be fully compensated.

In figure 7, we see two curves: One is without compensation and one with compensation of a known load. Notice how the speed controller combined with the load lookup table is able to compensate the change almost completely. In figure 7, the red line shows the speed without compensation, and the green one with compensation enabled.

Figure 7: Curves with and without compensation of a known load (Source: Texas Instruments)

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