Page 137 - Offshore Electrical Engineering Manual

124 CHAPTER 12 Process Drives and Starting Requirements

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.

COOLING AND INGRESS PROTECTION
As discussed in earlier chapters, only fully enclosed types of motor are normally
considered suitable for offshore installations.
To summarise from PART 2 Chapter 9, the following is recommended:

Machine Design

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

CACA, closed air circuit, air cooled; CACW, closed air circuit, water cooled; TEFC, totally
enclosed fan cooled.

SPECIAL APPLICATIONS
The following section provides some advice on the selection of motors for particular
applications.
Hazardous area topics are discussed in PART 5 Chapter 4 and are not discussed
in detail here. However, readers who are likely to be specifying motors for hazardous
areas are advised to read PART 5 Chapter 4 (see also Fig. 2.12.1).

RECIPROCATING PUMPS AND COMPRESSORS

If an induction motor is used to drive a large reciprocating pump or compressor,
the heavy cyclic torque fluctuations demanded from the motor will in turn demand
heavy current fluctuations from the supply. When the motor load is a significant
part of the installation generating capacity, instability of voltage and power may
result.
An alternative is to use a synchronous motor with a squirrel-cage damping wind-
ing imbedded in the rotor. If a steady torque is being developed by the machine, the