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106 SHALE SHAKERS AND DRILLING FLUID SYSTEMS
The rig power supply should provide constant For example, a shale shaker is designed to op-
voltage and frequency to all electrical components erate at 4 G's and has an angular displacement,
on the rig. Most drilling rigs generate 460 VAC, or stroke, of 0.09 inch. This shaker must vibrate
60 Hz, 3-phase power or 380 VAC, 50 Hz, 3-phase at 1750 rpm to produce 4.1 G's. At 60 Hz, the
power. Other common voltages include 230 VAC, motor turns at 1750 rpm, which yields a "G"-
190 VAC, and 575 VAC. Through transformers and factor of 4.1, just as designed. If the frequency drops
other controls, a single power source can supply to 55 Hz, the motor speed reduces to 1650 rpm,
a variety of electrical power to accommodate the resulting in a "G"-factor of 3.5. Further reduction
requirements of different rig components. of frequency to 50 Hz results in 1500 rpm and a
Shale shakers should be provided with motors "G"-factor of 2.9.
and starters that match the rig generator output. Most rigs provide 460 VAC, 60 Hz power and
Most motors are dual-wound. These may be wired most shale shakers are designed to operate with
to accommodate two voltages and starter configu- this power supply. However, many drilling rigs are
rations. For example, some dual-wind motors op- designed for 380 VAC, 50 Hz electrical systems. To
erate at 230/460 VAC, while others operate at provide proper "G"-factors for 50 Hz operations,
190/380 VAC. Dual-wind motors allow the shaker shale shaker manufacturers rely on two methods:
to operate properly with either power supply af- increasing stroke length or through the use of
ter relatively simple rewiring. Care must be taken, voltage/frequency inverters (transformers).
however, to make certain that the proper voltage A motor designed for 50 Hz applications rotates
is used. Additionally, electrical motors are de- at 1500 rpm. At 0.09 inch stroke a shale shaker
signed to rotate with a specific speed. Typically the will produce 2.9 G's. Increasing the stroke length
rotational speed is 1800 rpm for 60 Hz applications to 0.13 inch provides 4.1 G's, similar to the ori-
and 1500 rpm for 50 Hz applications. ginal 60 Hz design. However, the longer stroke
Shale shakers use a vibrating screen surface to length and slower speed will produce different
conserve the drilling fluid and reject drilled solids. solids separation and conveyance performance. At
The effects of this vibration are described in terms the longer stroke lengths, shakers will probably
of the "G"-factor, or the function of the angular convey more solids and have a higher fluid capac-
displacement of a screen surface and the square ity. If the stroke length is not increased, some
of the rotational speed. Angular displacement is manufacturers use voltage inverters to provide
achieved by rotating an eccentric mass. Most shale 460 VAC, 60 Hz output power from a 380 VAC,
shakers are designed to be operated at a specific, 50 Hz supply.
fixed "G"-factor by matching the stroke to a given Constant electrical power is necessary for proper,
machine rotational speed. It follows that any consistent shale shaker performance. Tables 3-2
deviation in speed will affect the "G"-factor and in- and 3-3 below assist in designing a satisfactory
fluence performance. electrical distribution system. Alternating current
Deviations in speed may be caused by one or (AC) motors are common on most shale shakers.
more factors. Typically, they are caused by fluctua- The motor rating indicates the amount of electri-
tions in voltage or the frequency of the alternat- cal current required to operate the motor. The
ing current. If the voltage drops, the motor can- values in Table 3-2 provide some guidelines for
not produce the rated horsepower and may not be various motors. The manufacturer's recommenda-
able to sustain the velocity needed to keep the tion should always take precedence over the gen-
eccentric mass moving properly. Low voltage also eralized values in these tables. The amount of
f
reduces the life of electrical components. Devia- electric current that a conduc ^r wire) can
tions in frequency result in the motor turning carry increases as the diameter of the wire in-
faster (frequencies higher than normal) or slower creases. Common approximate values for currents
(frequencies lower than normal). This directly in- are presented with the corresponding wire size
fluences rpm and shaker performance. designation in Table 3-3. Conductors, or even rela-
Slower rpm reduces the "G"-factor, causes poor tively large-diameter wire, still have some resis-
separation, and poor conveyance. Faster rpm in- tance to the flow of electric current. This resis-
creases the "G"-factor and, although it may im- tance results in a line voltage drop. When an
prove conveyance and separation, often increases electric motor is located in a remote area relative
screen fatigue failures. In extreme cases, higher to the generator, the line voltage drop may de-
rpm may cause structural damage to the shaker. crease the motor voltage to unacceptably low val-
Thus, it is important to provide proper power to ues. Wire diameter guidelines necessary to maintain
the shale shaker. the voltage drop to 3% are presented in Table 3-4.
