As quietly as possible, please: motors for marine pumps pass acoustic and vibration tests.
A customer that manufactures pumps for naval vessels required electric motors in frame sizes 132 and 160 that would operate as quietly as possible in continuous duty S1. Acoustic category D in accordance with MIL-STD-147-E was required, with a limit of 70 dB in the frequency range from 32 to 8,000 Hz.
To verify compliance with the noise emission requirements, we carried out an acoustic test and vibration measurements at our facility in partnership with an accredited testing laboratory. The vibration measurement to determine the sound pressure level – or, more precisely, the measurement of the vibration acceleration of structure-borne noise – was carried out in the usual way on our test bench: each motor was coupled to the load machine and fitted with vibration sensors at five positions in three directions (vertical, horizontal, and axial). During the measurement, in which the motors ran at their rated speed, the readings were below the predefined target curve by the required 5 dB in all positions, directions, and frequencies. The motors therefore passed the test.
Test setup in the yard at BEN Buchele
An unusual test setup was chosen for the acoustic test: in our yard, the external inspector positioned five microphones around each motor at a distance of one meter. Ambient noise was filtered out so that only the motor noise would be assessed. In total, each motor ran at rated speed for three three-minute intervals, which is sufficient for assessing the noise level in continuous duty S1. Both motors fell below the 70 dB limit across all frequency ranges; the frame size 132 model did so by such a wide margin that it is also suitable for the more stringent acoustic category E.
Transferable test result
Once the acoustic test was passed, we delivered four motors of each frame size. However, it is neither necessary nor cost-effective to subject every single motor to an acoustic test. Since the motors are manufactured to identical specifications, the test result can be applied to these motors as well. Additional motors in both frame sizes have also already been ordered for marine pumps. A new acoustic test is not required for these motors either.