Electric Motors For Shale Shakers

With the exception of specialized motors for centrifuge feed, practically all AC electric motors encountered in drilling-fluid operations are integral-horsepower, across-the-line start, horizontal squirrel-cage motors. Across-the-line motors are the simplest and lowest cost. The motor is connected directly to the input power through a starter switch. Full current and torque are realized at startup. This is acceptable with solids-control and processing equipment; however, it is suggested that centrifugal pumps be started with the discharge valve partially closed to restrict initial pump output and load demand on the motor.

Shale shaker motors are generally three-phase induction motors that are explosion proof, having NEMA design B or similar characteristics (Table 17.10). The number of magnetic poles in a shale shaker motor can be four (1800 rpm synchronous shaft speed at 60 Hz), six (1200 rpm), or two (3600 rpm). The motor should have independent, third-party markings indicating its suitability in explosive or potentially explosive environments. It is recommended that these motors be suitable for Class I, Division 1, Groups C and D, and Group IIB atmospheres. The motor also should have the proper operating temperature or code designation for the anticipated ambient temperature.

A 50-Hz motor driving a shale shaker vibrator should not be operated at 60 Hz, since the centrifugal force output will increase by 44%. This will likely damage the bearings, the vibrating screens, and the shale shaker. A 60-Hz motor driving a shale shaker vibrator can be operated at 50 Hz with the understanding that the centrifugal force output will decrease

Table 17.9

Classification by Group and Category According to Intended Use (Surface Industry)

Table 17.9

Classification by Group and Category According to Intended Use (Surface Industry)

Category of

Presence or Duration of

Inflammable

Comparison with

Area

Equipment

Explosive Atmosphere

Substances

Level of Protection Faults to

Present Practice

e

Equipment

1

Continuous presence

Gas, vapors,

Allow for very high level of pro

Group II

A O

Group II (surface)

Long Period

mist, dust

tection: 2 types of protection

Zone 0 (gas)

n

Frequent

or 2 independent faults

Zone 20 (dust)

c t

2

Likely to occur

Gas, vapors,

High level of protection: 1 type of

Group II

S*

mist, dust

protection

Zone 1 (gas)

M

Habitual frequent malfunction

Zone 21 (dust)

t o

3

Unlikely to occur

Gas, vapors,

Normal protection: Required level

Group II

Present for a

mist, dust

of protection

Zone 2 (gas)

short period

Zone 22 (dust)

Table 17.10 Electric Motor Specifications for Shale Shakers

U.S. Designation IEC Designation

Terminology Hazardous location rating Hazardous location rating if hydrogen sulfide is encountered

Explosion-proof Class I, Division 1 group D

Class I, Division 1 groups C and D

Flameproof

Eexd Gas Group IIA

Eexd Gas Group IIB

Centrifugal Force Centrifuge
Figure 17.13. Eccentric weight unbalance.

by 31%. If, at 60 Hz, the centrifugal force is 1000 lb, the centrifugal force will only be 690 lb at 50 Hz.

For a given frame size, higher-speed motors will have high hp ratings, low slip, high starting torque, and low bearing life. Conversely, lower-speed motors will have lower hp ratings, high slip, low starting torque, and long bearing life.

Electric industrial vibrators are rated in centrifugal force output, frequency, unbalance (static moment), and hp. Centrifugal force is caused by torque resultant from the offset eccentric weight acting through the moment arm (the distance from the shaft center to the center of gravity of the weight) (see Figure 17.13). This torque is referred to as unbalance or static moment. The unbalance provides the amplitude at which the vibrating screen will move.

Two counterrotating shale shaker motors will produce a linear force that should be located through the center of gravity of the shaker basket (see Chapter 7 on Shale Shakers). The resultant motion is perpendicular to a plane drawn between the rotating shafts directed through the center of gravity of the machine (see Figure 17.14). The shale shaker motor should be selected to meet or exceed the desired stroke of the machine, centrifugal force, and acceleration (g's). Adequate hp is required to perform the work and to ensure synchronization. Synchronization results in opposing forces from two counterrotating vibrators that cancel each other and double directional forces.

Stroke, which is independent of motor speed, is the peak-to-peak displacement imparted to the machine. Dampening may occur in the system affecting the total stroke. Stroke is a function of the unbalance (or static moment) of the motor and of the total weight of the shaker basket, including the weight of the motors and the live load. The stroke

POSITION A

POSITION A

POSITION B

POSITION B

POSITION C

POSITION C

POSITION D

NOTE: FORCE VECTOR SHOWN PARALLEL TO VELOCITY VECTOR Figure 17.14. Movement of two counterrotating shale shaker motors.

POSITION D

NOTE: FORCE VECTOR SHOWN PARALLEL TO VELOCITY VECTOR Figure 17.14. Movement of two counterrotating shale shaker motors.

Shale Shaker Motor
B
Centrifugal Force Engine

equals two times the motor unbalance, multiplied by the number of motors, divided by the total weight. Note that the motor unbalance is a function of the eccentric weight setting. For example, the motor unbalance is 50% of the maximum unbalance if the eccentric weights are set at 50%.

The centrifugal-force output of the vibrating motor (lb) is equal to the shaft speed squared times the unbalance (in.-lb), divided by 35,211. Once again, the vibrating motor's centrifugal-force output is a function of the eccentric weight setting. For example, the centrifugal force is 50% of the maximum centrifugal force if the eccentric weights are set at 50%. Typical acceleration rates for vibrating screens are 4 to 8 g's.

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Responses

  • Jens
    Why do shaker motors synchronize?
    8 years ago
  • sergio
    How should shale shaker motor weights be set?
    6 years ago
  • carita tuikka
    What types of motor are used in shale shaker in rig system?
    6 months ago

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