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Dealing with noisy motor: Traditional compensation

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

Keywords:vibration? noise? tuning?

Many applications are troubled by vibration and noise since they cause an undesirable end-user experience, as well as mechanical failures due to stress. Many applications contain pulsating loads, which can cause motor vibration and audible noise. In this article, a typical method for compensating the vibration and audible noise will be presented, including the tuning required by this method. In part 2 of this two-part series, a new compensation method will also be covered, showing the details on how it operates and the minimal tuning required.

Causes of vibration and noise
There are several causes of vibration and noise. The main cause that we will focus on is the vibration produced by the load characteristic. We will also focus on the noise that is produced as a result of the mechanical vibration of the system. Two examples of vibration are discussed below: One related to the pulsating load of the motor and the other related to the oscillations of the control loop, even though the load is not pulsating.

Pulsating loads
Mechanical loads present in rotary compressors or the ones used in piston-based compressions are of particular interest in this article. The torque requirement of these compressors varies from no load to full load in one mechanical cycle as the compressor pressurizes and discharges the gas. Figure 1 shows a typical load profile of a reciprocating compressor.

Figure 1: Typical load profile of a reciprocating compressor (Source: Texas Instruments).

With the above torque requirement, it becomes very challenging to maintain a constant speed on a control loop. As a result of a varying speed, mechanical vibration is introduced to the system, causing audible noise. To show how a load like this affects the speed of a motor in a closed-loop system, consider figure 2, showing a PMSM motor in a field-oriented control (FOC) loop:

Figure 2: An FOC loop in a PMSM motor (Source: Texas Instruments)

Figure 3 describes a speed reference of 100 rad/s that switches to a new speed reference of 200 rad/s, once a speed reference step is introduced at 0.5 seconds:

Figure 3: A speed reference step is introduced (Source: Texas Instruments)

Feedback-based controllers
The second source of vibration is due to the oscillation of a speed controller. This oscillation is caused by the feedback of a speed correction when a pulsating load is present. The speed controller will control the speed based on feedback, when in fact the load has already changed as the motor keeps turning. As this occurs every mechanical cycle, the speed controller based solely on a feedback signal creates a vibration that is worse than not having a speed controller at all. For example, if we remove the torque vs. load perturbation and tune the speed controller, the result will be a very sharp response as indicated in figure 4.

Figure 4: Tuning of the speed controller without load (Source: Texas Instruments)

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