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ELECTRIC MOTORS 215
Centrifuges may draw up to seven times the full machines commonly generate 2000 to 3000 G's.
load current for approximately 15 seconds at startup. The advent of higher G centrifuges is attributed to
NEMA Design B motors are all rated for 7.5 times improvements in bearing design and manufactur-
full load current for 30 seconds at startup. It is, ing procedures including hard surfacing with tung-
however, considered good practice to limit centri- sten carbide and precision robot welding.
fuge startups to two starts per hour to protect the
motor because the current draw closely approaches
the range limit. Electric Motors for Centrifugal Pumps
Oil field centrifuges do not always use a direct
drive between the motor and centrifuge. Direct The fluid volume necessary to be moved by
drive requires expensive, variable-speed motors many centrifugal pumps is related to the rig cir-
and have restricted availability of replacement culation rate and the specific rig plumbing. Cen-
parts and repair facilities. Most oil field centrifuges trifugal pumps should be sized by the particular
are connected to the motor by a fluid clutch or a application and the maximum anticipated flow
hydraulic drive, which uses a system of adding or rate. Piping friction losses—if lines are reasonably
subtracting motor or hydraulic oil to increase/ short, with few turns or restrictions, and flow
decrease slippage between the driver and driven velocities between 5 and 10 feet per second—are
coupling halves. Some centrifuges use a variable- readily estimated. The pressure or head, which
speed electric motor startup system that brings should be delivered to each piece of equipment
the centrifuge slowly up to operational rotating (for hydrocyclones, typically 75 feet of head), is
speed. It is important that all personnel under- specified by the manufacturer.
stand the manufacturer's recommended startup Horsepower requirements for centrifugal pumps ;
and shutdown procedures. when pumping water or fluids of water-like viscos-
Early centrifuges were capable of generating ity, are well established and published with the per-
500 to perhaps 1500 G's of acceleration. Today, formance curve for each design (see Chapter 11).
