Page 114 - Offshore Electrical Engineering Manual

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CHAPTER
9
Motors

INTRODUCTION
Machinery drives for offshore installations range in size from fractional horsepower cabin
ventilation fans to the 15 MW or more machines required for gas injection compressors.
Some concern has been expressed over the practicality of installing very large electric
motors offshore owing to the stresses imposed on the structure, especially during start-
ing. A figure of around 25 MW depending on the application (e.g., less for reciprocating
pump drivers) should be considered as a ceiling figure for offshore electric motor ratings.

VOLTAGE LEVELS

As with onshore industrial plant, motor operating voltage levels correspond to ranges
of motor power ratings in order to keep current and voltage drop magnitudes within
practical limits. For weight saving purposes, the step points at which higher operat-
ing voltages are selected tend to be lower offshore as in the following table.

Voltage Level Rating
415–460 V Up to 150 kW
3300–6600 V Up to 1000 kW
11–13.8 kV Over 1000 kW

Higher voltages such as 22 kV could be utilised provided the economics are
favourable and proven ranges of machine at these higher voltages are available.

STARTING

Where an individual motor load represents a substantial part of the power system
capacity, it is desirable to obtain designs of squirrel-cage motors with the lowest start-
ing current characteristics compatible with the driven equipment. Both reduced voltage
starting of cage motors and the use of slip ring motors will add considerable weight to
the drive package, and therefore direct-on-line low-starting-current machines are pre-
ferred unless technical considerations for selection are overridden by those of cost and
delivery. Motors with starting currents ranging from 3.5 to 4.5, instead of the normal
6–10 times full load, are usually available from the larger manufacturers.

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