Page 116 - Offshore Electrical Engineering Manual

Pole Configuration 103

POLE CONFIGURATION

At a given frequency, motor synchronous speed is determined by the number of poles
incorporated in the motor stator, and this governs the maximum operating speed of
the machine. 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. An occasional
exception is large reciprocating compressor drivers, where eight-pole (or more) syn-
chronous machines may be used. These machines, apart from giving the required
lower speed, also reduce the current fluctuations caused by the cyclic torque varia-
tions associated with reciprocating machinery.
The advantages and disadvantages of two-pole against four-pole machines are
listed in the following.

1. Advantages of two-pole machines

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, mechanical 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 manufacturers’ normal product ranges, the use of a
two-pole machine 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.
2. Disadvantages of two-pole machines

a. The starting torque of two-pole machines is less, requiring the driven equip-
ment to have a lower moment of inertia. It may also prove more difficult to
accelerate the machine up to operating speed, where driven machinery can-
not be run up to speed unloaded. The speed/torque characteristics of pumps
in particular should be carefully studied to avoid any problems. 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.