Page 136 - Offshore Electrical Engineering Manual

Pole Configuration 123

POLE CONFIGURATION

The fastest possible speed of both induction and synchronous motors is with a
two-pole configuration which gives synchronous speeds of 3000 rpm at 50 Hz and
3600 rpm at 60 Hz, respectively. As size and weight penalties are usually incurred
by increasing the number of poles, only two- and four-pole machines are normally
used offshore. The only occasional exception being large reciprocating compres-
sor drivers, where eight-pole (or more) synchronous machines may be used. These
machines, apart from giving the required lower speed, also reduce the current fluc-
tuations caused by the cyclic torque variations associated with reciprocating machin-
ery. The advantages and disadvantages of four-pole against two-pole machines are
listed in the following.
Advantages

a. Energy conversion within the higher speed, two-pole machine is usually
more efficient, giving some reduction in size and weight for a given output.
However, because of the higher rotor speeds, especially at 60 Hz, mechani-
cal forces on the rotor cage, known as hoop stresses, become significant and
limit the maximum dimensions of rotor that may be manufactured using
conventional materials. Metallurgically more exotic materials may be used
to extend this limit with, of course, the accompanying large increases in the
cost of the machine.
b. Manufacturers differ in the application of a practical maximum rating limit

for a two-pole motor, but as a general rule, this is between 3 and 5 MW.
Therefore motors above 5 MW should not be considered as a feasible alter-
native unless the cost of using exotic metals in the rotor is outweighed by the
savings accrued by, for example, the elimination of a gearbox.

c. Below this limit, and within the manufacturer’s normal product ranges, the
use of two-pole machines should, in comparison with an equivalent four-
pole machine, provide dimension and weight savings roughly proportional to
power rating; however, for small machines of only a few kilowatts, there is
little benefit.
Disadvantages

a. Less starting torque is available from two-pole machines, requiring the
driven equipment to have a lower moment of inertia. It may also prove
more difficult to accelerate the machine up to operating speed, where driven
machinery cannot be run up to speed unloaded. The speed/torque character-
istics of pumps in particular should be carefully studied to avoid any prob-
lems. Starting currents are also likely to be higher, and with large machines,
this may lead to unacceptable voltage dips.
b. Irregularities in the core stampings, which are inevitable unless very high

levels of quality control are applied, generate more magnetic noise in two-
pole machines. A characteristic low-frequency (twice slip frequency) growl
can be heard from these motors.