Page 115 - Offshore Electrical Engineering Manual

102 CHAPTER 9 Motors

SPEED

The shaft speed of a pump, compressor or fan is critical to its performance.
It is therefore necessary to obtain the required speed or speed range either by
installing a gearbox between the motor and the driven equipment or by selecting a
suitable system frequency and pole configuration for the motor. In the 1930s the US
Navy changed from direct current to alternating current systems and selected 440 V
60 Hz as the operating parameters. Since then the whole of the NATO fleet and the
majority of commercial ship designs have standardised on these parameters. This
has greatly assisted in improving the availability of 60 Hz options on the standard
equipment ranges of most European manufacturers. With small low-voltage motors
the increased efficiency as a result of higher pump speeds at 60 Hz is marginal, but
with motors having ratings of the order of several megawatts the weight and power
savings can be substantial.
With smaller installations, however, operating at 60 Hz may be a disadvantage,
where it is decided to select a reciprocating engine rather than a gas turbine main
generator prime mover. The problem is that the optimum engine speed of around
1500 rpm is better suited to generating at 50 Hz. Reciprocating engines running at
1200 rpm tend to have too low a power-to-weight ratio, and operating at 1800 rpm
leads to short cylinder life or even piston speeds which would be beyond the design
limitations of the engine. The higher synchronous speeds obtainable at 60 Hz also
lead to higher inherent noise levels, although this can be deadened with better mod-
ule sound insulation.
From a machinery standpoint, a major disadvantage in adopting 60 Hz for an offshore
installation is related to testing the equipment before installation on the platform, as full-
load tests cannot be carried out using the British and European national supply networks.
Until recently, tests were mainly carried out at 50 Hz and the results extrapolated to give
projected machine characteristics at the design operating conditions. However, test facili-
ties are now available in the United Kingdom for motors of up to 6 MW at 60 Hz.
For larger machines, where capital investment is high and full-load tests are con-
sidered essential, it is usually possible to arrange full-load tests in conjunction with
testing of the main generators to be installed on the offshore installation. Although
this procedure is usually expensive, the costs should be more than offset by the ben-
efits of adopting the higher frequency.
Once the system frequency has been established, it becomes increasingly expen-
sive to change and hence may no longer be considered a variable after that point
in the system design. It is therefore important to consider the number, rating and
purpose of the larger drives on the installation at an early stage in the power system
design, before the frequency is selected.
However, in many cases the shaft speed available from a motor, even from a
60 Hz two-pole machine (i.e., 3600 rpm), is lower than the required shaft speed, and
it will still be necessary to install a gearbox in the drive string. Once the requirement
for a gearbox has been established, changes in the drive ratio have only minor effects,
and the motor speed may be chosen to give the optimum motor design in terms of
dimensions, weight, reliability, noise emission and so on.