Page 314 - Offshore Electrical Engineering Manual

Motors 301

MOTORS

Unless the predominant noise is from the driven equipment, sound insulation of the
module is not usually provided, as it should be possible for the motor manufacturer to
control noise in the machine concerned. The specifier, however, may assist by avoid-
ing the use of two-pole motors wherever practicable.
The sources of induction motor noise are as follows:

1. low-frequency magnetic noise from stator core laminations (100 Hz),
2. fan and airflow noise (400–2000 Hz),
3. core vibration noise (1000–2000 Hz),
4. slot tooth vibration noise (2500–4000 Hz),
5. bearing noise (6000–8000 Hz).

The magnetic noise can never be totally eliminated, as it is dependent on a large
number of variables, which the machine designer must consider in order to trade
off minimum noise against optimum performance and efficiency. Fan noise can be
reduced by fitting a smaller fan on an oversized motor and also by employing closed
air circuit, air cooled (CACA) cooling with an air-to-air heat exchanger. Silencing
material may be fitted to the cooling air circuit, but this needs to be of a type suitable
for the environment in that it should not absorb dirt, oil and sea spray as well as it
does sound. The ingress of dirt, etc. is much less with CACA machines.
If a cooling water supply is available, then a substantial noise reduction may
be obtained by replacing the air-to-air heat exchanger with an air-to-water type.
However, the availability of the machine will then be dependent on the availability of
the cooling water supply, which may itself be dependent on the satisfactory operation
of a number of seawater lift and circulating pumps.