When is a Motor, More Then a Motor?


When is a Motor, More Then a Motor?

For those of us in the noise control field, that’s an easy one!  The answer is, “When it’s also a noise source!”  The smallest motor I ever treated was a 25-HP on a pump, in a laboratory (which was pretty quiet.)  More commonly, we’re usually called for problems with 100-HP and up to thousands of horsepower.  As a Mechanical Engineer, I know a motor has: brushes, rotors, stators, a shaft and …….blah, blah, blah!  What’s more interesting and what we deal with is the noise caused by the cooling fan on that shaft!  These are usually a TEFC (Totally Enclosed, Fan-Cooled) motor.

The fans draw ambient air through an inlet grille in the end-bell and blow it out the back of the bell, along the cooling fins so the motor doesn’t overheat——-and stop!  For larger motors, the air may be blown through the windings inside the motor and then exits out the shaft end.

In either case, the fan, although providing a very important function/benefit, is also the primary source of noise.  For some well intentioned “noise stoppers”, putting an enclosure over the motor may appear to be a logical means to resolve the problem.  It is, however, fraught with several potential long-term side-effects that could prove disastrous.  1) It may limit or discourage maintenance; 2) Because any enclosure, unless properly ventilated, will most likely prevent proper cooling (which is why the fan was added in the first place) or 3) Be such a hassle to remove and re-install, that it often mysteriously disappears after the first or second time the motor requires maintenance.

Some forward thinkers, have utilized a baffle-type silencer, either prior to or built into a plenum that is attached to the inlet grille in the end-bell.  This can work acoustically, without a powered ventilation fan (such as might be needed for an enclosure.)  The problem with this approach is that the airflow area of the silencing elements has to be fairly large to keep the pressure drop across the “silencer”, very low.  The fans on the motors were designed to overcome the pressure needed to capture the ambient still air and move it via a torturous path and along or through the motor.  It doesn’t take too much added, eternal pressure drop to reduce the air flow, which is undesirable.  This can lead to overheating which then requires more frequent maintenance or shorter life-span of the motor……..if not stopping it.

Enter the spiral motor silencer!  This unique silencer improves on the previously described solution by notably reducing the physical size and hence requires far less space, wherever the motor is used.  Models are available for motors mounted in the vertical as well as the horizontal orientation.  The acoustical spirals that replace the conventional baffles, provide a better degree of noise reduction in the mid-to-low frequencies compared to the baffle-type and yet at a lower pressure drop.

The primary disturbing tone is usually related to the “blade-pass frequency”.  That is based on the number of blades on the fan, multiplied by the RPM (revolutions per minute) of the motor and that amount is divided by 60 (sec/min.) to yield the cycles/sec. now referred to as “Hertz.”  As the size of motors increases, the speed or RPM often goes down.  A small fractional horsepower motor may run at 3600 RPM; somewhat larger motors up to perhaps several hundred horsepower, more commonly turn at 1800 RPM.  Much larger motors up to thousands of horsepower may operate at 1200 or 900 RPM.

After running temperature-rise tests on various sized motors with and without the spiral intake silencers, it was found that the differential increase was only 1 or 2-degrees F.  It was also learned that about 60-70% of the fan noise normally comes out through the grille.  Hence, the spiral silencer on the intake often provides enough noise reduction to resolve the problem without having to treat the air exhaust portion.

For any further questions or discussions on a specific application, please contact our office.  www.stanfordassociates.net.