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CHAPTE R TWELV E
Electric Motors
This chapter discusses electric motors, electric The motor's internal heat is a function of load
motor standards, motor enclosure and frame desig- conditions, motor design, and ventilation condi-
nations, hazardous location ratings, and specific tions. Heat produced internally by the motor raises
motor applications. operating temperature and adversely affects insu-
Continuous duty electric motors are an integral lation used to isolate electrical conductors from
part of the drilling rigs' solids control and process- each other and from the motor frame. Insulation
ing systems. Centrifugal pumps that feed hydro- materials are rated based on thermal capacity, or
cyclones, circulate mud for mixing, transfer mud the ability to withstand heat effects. High-quality
to and from reserve, and also into the trip tank, insulation systems with high thermal capacity can
are powered by electric motors. Shale shakers, withstand relatively high temperature increases
mud cleaners, and pit agitators are also driven by and deliver a long motor service life at rated per-
electric motors while they rotate centrifuges. formance. Because motors may be operating prop-
Continuous duty electric motors meet well- erly and still be too hot to touch, it is important
defined performance standards. Motors are designed to check the manufacturer's guidelines.
with conductor, frame, and insulating materials to
continuously deliver rated horsepower and not
exceed the insulation's temperature limits. A ser- VOLTAGE
vice factor rating defines the ability of the motor
to continuously withstand prolonged overload con- Motors are rated for operation at specific volt-
ditions while remaining within the temperature ages. Motor performance is affected when the
limitations of the insulating material. supply voltage varies from the motor's rated volt-
The criterion for sizing and selection of any age. Motors generally operate satisfactorily with
motor is its ability to deliver startup power under voltage variations within ±10%. However, equipment
the process load, and to then provide power that connected to the motor may not always function
drives the equipment throughout operation. Ad- properly with such variations.
equate torque must be developed to overcome Surge voltage is any higher-than-normal voltage
inertia during startup. The load must then be ac- that temporarily exists on one or more of the power
celerated to the desired operating speed and full lines of a three-phase motor. A surge causes a large
load power requirements supplied without over- voltage rise during an extremely short period of
heating. These parameters depend on motor de- time. Surges are of concern because the higher volt-
sign and the full load rating (output horsepower). age is impressed on the first few turns of the motor
Electric motor operating efficiency is the ratio windings. The lead insulation may be destroyed and
of output power to input power. The power loss the motor burned out. Frequent voltage surging can
is the difference between the power into the motor result from line switching of large generators.
and the power output of the motor. This power Under-voltage at the motor terminals can result
loss is caused by: when large current demands are placed on the
generator, such as starting the top drive motor.
• Heat from the electrical resistance of motor Operation below 10% of the marked motor volt-
windings and rotor age will generally result in excessive overheating
• Windage losses from cooling fans or rotor fins and torque reduction. Overheating prematurely
• Magnetic and core losses from currents in- deteriorates the insulation system. Torque reduc-
duced in the laminations of frame and stator tion may result in the motor stalling or, in the case
• Friction losses from shaft bearings of shale shakers, may result in poor performance.
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