Good Vibrations

April 17, 2003
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There are few more frustrating issues to deal with than the problem of noise in electrical and mechanical equipment. There are really two issues involved: the subjective issue of noise perception and the physics of noise itself.

HVACR technicians who learn how to use both subjective and physical information when troubleshooting a “noisy motor” will be able to get to the heart of the problem more quickly.

On the subjective side, the challenge for anyone dealing with the problem of noise is that tolerance of noise varies (often substantially) from person to person. This explains (at least in part) how teenagers can tolerate music at ear-splitting decibel ranges while their parents cannot.

Noise physics includes how to define, measure, and control noise. This can be a much more complex issue than you might think, with variables such as frequency and intensity of harmonics. Complicating matters further is the challenge of dealing with the interaction of the noise-producing device with the physical environment.

All noise is mechanical in origin. For noise to occur, some source must create waves of pressure that are transmitted through either air, liquids, or solid materials and have components within the frequency range discernible to the human ear (generally between 30 cycles and 20,000 cycles for a young person). For the purposes of this article, I’ll concentrate on motor noise (after all, I am the Motor Doctor).

Motor Operation And Noise

Squirrel cage AC induction motors are magnetized by an alternating current. (They were named nearly 100 years ago, from the shape of a rotating part, when people kept squirrels, not hamsters.)

The resulting alternating magnetic field can cause parts to vibrate or hum. Most all HVAC motors, both single and three phase, are squirrel cage induction motors.

Single- and three-phase squirrel cage AC motors have common components:

  • Stator — a doughnut-shaped magnet with wire wound on steel laminations;

  • Rotor — the spinning part, also made with steel laminations. The rotor is not directly connected to the power source. Current flow is induced in its cast aluminum conductors by the magnetic field of the stator. There is a very small gap of air between the rotor and stator. The motor spins due to the magnetic interaction between rotor and stator produced by the shrinking and expanding of the magnetic field at 60 cycles per second.

    There are numerous potential sources of noise in an electric motor. A motor produces so-called “electrical noise,” which is generally at line frequency or a multiple (harmonic) of line frequency, when the magnetized parts of the motor have room to physically move. This movement occurs as these parts are alternately attracted and repelled from one another and is often referred to as “60-cycle hum.” This may be a manufacturing problem, or the result of physical damage, and there is little you can do to alleviate this problem on the jobsite.

    Other noise sources in the motor may result from air disturbances caused by moving parts. This may be a manufacturing issue and, as an installer, you don’t have much control. Vibrations derived from imbalances in spinning parts may be a manufacturing issue, or may result from an imbalanced load, which can be investigated.

    You can have an impact on the third area of noise — the interaction of the motor with the equipment in which it is mounted. Not only is the system a potential source of noise generation, but also of noise amplification. Typically, when you are called to the jobsite, the customer will simply complain of a noisy motor. He or she won’t necessarily describe the 60-cycle hum or excess vibration.

    Any Guitarists?

    The pulsing magnetic field can affect parts within and outside the motor. If the rotor is off center within the stator due to frame damage, the air gap is not uniform around the rotor. This can cause the motor to become noisy. Interaction with the moving system can make the noise objectionable.

    The concept is similar to the behavior of a guitar or harp string: A plucked string vibrates at a certain frequency depending on its length, width, and tension. If something else touches the string while it is vibrating, this can result in an unpleasant string buzz. If the string is depressed in a certain manner while it is plucked, the frequency at which it vibrates changes; it therefore plays at a different pitch.

    Therefore, motor noise that carries audible harmonics could indicate interaction with another part of the system.

    In situations like these, the first thing you’ll need to do is determine if the motor is performing sonically to its designed specifications. Most of the time, the best instrument to test for this doesn’t come in your toolbox. It’s your well-trained ear. The more time you spend in the field, the better you will become at determining if the problem lies with the motor or the application.

    Here are some creative remedies to the problem of noise on the job.

    Probably the first thing to measure is whether the motor has too much voltage on it, causing a condition called magnetic saturation.

    Mechanical isolation is usually the most straightforward action you can take. This is because any noise inherent in the motor transfers very efficiently through metal mounting assemblies.

    Once you’ve established that the motor is the noise source, you can break this sound path in one of three ways:

    1. Separate the motor base from the surface on which it is mounted with a resilient pad.

    2. Couple the motor output shaft to the driven equipment with a “soft coupling” (one that incorporates a rubber bushing).

    3. Make a substitution from a rigid-based fractional horsepower motor to an equivalently rated one with a resilient base.

    But what if these three measures fail to produce the desired outcome? Don’t despair; resolution is still possible, but it will be somewhat more complicated. Your corrective action is based on the concept of harmonics. First a quick definition of the concept. All mechanical objects tend to vibrate at a number of specific frequencies. (For example, when you rub a wet finger over the rim of a glass, you tend to get a specific sound each time from that glass.) These favored frequencies are called harmonic frequencies.

    Now, if by circumstance, design, location, weight, mounting configuration, or any of an endless list of mechanical parameters, a piece of equipment (door operator, pump, fan) has a harmonic frequency that matches the output frequency of a motor, the result will be excessive noise. The good news is that you can attack this problem — and make a dramatic improvement in noise levels — by changing any one or several of that long list of parameters mentioned above.

    As a last resort, you may consider changing out the motor with a new one to see if this resolves the problem. This is anathema for an industry that loathes the term “parts changer,” but on some occasions, it may be warranted for diagnostic purposes.

    While it may be difficult to calculate the exact effect, virtually any change to mass, speed, or separation distance will have some impact on harmonic noise. The trick is to find the most effective course of action. Here are some suggestions:

  • For a belt-driven device, first use your ears; there are diagnostic methods an experienced technician may use. If warranted, try making a slight change in drive speed by varying the pulley sizes.

  • If the application uses rubber isolation (like a soft coupling) but appears ineffective, try changing the density (hardness) of the isolation devices. This may be enough to move the assembly off harmonic frequency.

  • Where space and the application permit, you can try changing the length and configuration of the drive train by adding or subtracting a belt length or chain link.

  • Try to isolate the noise with an audio device, such as an industrial or even medical stethoscope.

    Once you understand the role of harmonics in creating noise, you can direct your efforts in the field to solving the problem by tackling those harmonics. I just wish it were that easy to deal with teenagers and their loud music.

    Neil Simon, a.k.a. the Motor Doctor, is the regional sales manager for A. O. Smith Electrical Products Company, Milwaukee.

    Publication date: 04/21/2003

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