Page 117 - Offshore Electrical Engineering Manual

104 CHAPTER 9 Motors

c. Rotor imbalance is more likely and can cause more vibration on two-pole

machines.
d. The higher rotor speeds associated with two-pole machines will shorten

bearing life or require a more expensive higher-performance bearing.
e. Poorer heat dissipation within the rotor necessitates increased cooling

airflow rates in two-pole machines. The effect of higher fan speeds and
increased airflow rates is to increase noise emission from the machine.

f. From the above-mentioned points, it can be seen that the two-pole machine
generally scores on weight and dimensions but suffers from the design
limitations associated with higher speeds. Certain applications such as
direct drive ventilation fans and axial compressors can often take consider-
able advantage of the higher-speed machine. Nevertheless, if no significant
benefit is obtained from the higher speed then the four-pole machine should
be used.

COOLING AND INGRESS PROTECTION

As discussed in earlier chapters, only fully enclosed types of motor are normally
considered suitable for offshore installations.
Three typical enclosure types are to be found offshore.

1. Totally enclosed fan ventilated

a. In this motor type, the stator windings are enclosed within a finned motor
casing. Cooling is achieved by the mounting of a fan on the nondrive end
of the rotor shaft, external to the stator end plate, in order to blow air over
the external cooling fins. The fan fits inside a cowling which deflects the
air over the fins. An ingress protection (IP) rating of IP55 is typically
achieved with this design, which provides a high level of physical protec-
tion. The majority of low-voltage machines, especially in hazardous areas,
are of this type.

b. The chief disadvantage with this type of motor is the inherently high
noise level produced by the external fan. Methods are, however, avail-
able to reduce this noise, such as the use of acoustically treated fan cowls
or the use of a machine oversized for the application but fitted with an
undersized fan.

c. This design is unsuitable for the larger medium-voltage machines but can be
used advantageously with the smaller 3.3 or 4.16 kV machines.
2. Closed air circuit, air cooled (CACA)

a. As motor sizes increase, it becomes less and less likely that heat generated
in the rotor and stator windings can be dissipated to the machine casing
at a fast-enough rate to prevent design insulation temperature rises being
exceeded. It then becomes necessary to pass a cooling medium (usually air)