Page 118 - Offshore Electrical Engineering Manual

Particular Applications 105

through the inside of the motor to remove surplus heat. With the majority of
CACA types found offshore, air is usually forced through the windings by a
rotor-mounted fan within the motor casing. As the machine is still required
to be enclosed, this cooling air is recirculated in a closed circuit through
the machine, and heat is extracted by an air-to-air heat exchanger mounted
on top of the motor. A second rotor-driven fan is often required to force the
external air over the heat exchanger.
b. The CACA design suffers from the same noise and weight problems as the

totally enclosed fan cooled (TEFC) type, and the air-to-air heat exchanger
only adds to weight and bulk. Nevertheless, it does provide an adequate
and simple method of cooling the larger motors and, important from the
reliability point of view, requires no external services in order to continue
operating.
3. Closed air circuit, water cooled (CACW)
a. If the air-to-air heat exchanger on the CACA machine is replaced by an

air-to-water unit, we then have a CACW machine. The machine is depen-
dent on an adequate supply of cooling water for continued operation. The
benefits of this arrangement are that the bulk of the heat exchanger is very
much reduced, and there is no requirement for the secondary cooling fan and
consequently there is a substantial reduction in noise.
b. The disadvantages are the machine depends on the cooling water supply and

hence there is a reduction in reliability and the presence of water under pres-
sure around the machine is a hazard.

To summarise, the following is recommended for enclosure selection:

Cooling Type Application
TEFC Smaller low-voltage machines
CACW Where the TEFC design is not practicable, i.e., with larger
3.3/4.16 kV and all higher-voltage machines
CACA Larger machines where the cooling water supply is uneconomic or
the machine must operate during a cooling system outage

PARTICULAR APPLICATIONS
These are covered in PART 2 Chapter 12.
Hazardous area topics are discussed in PART 5 Chapter 4 and are not presented
here. Readers who are likely to be specifying motors for hazardous areas would be
advised to read PART 5 Chapter 4.