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 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:

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