ELECTRIC MOTORS FOR SHALE SHAKERS

Shale shaker motors are generally three-phase induction motors that are explosion proof, having NEMA design B or similar characteristics (Table 1). 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.

	U.S. Designation	IEC Designation
Terminology	Explosion – proof	Flame – proof
Hazardous location rating	Class I, Division 1 group D	Eexd Gas Group IIA
Hazardous location rating if hydrogen sulfide is encountered	Class I, Division 1 group C and D	Eexd Gas Group IIB

Table 1. Electric Motor Specifications for Shale Shakers

With the exception of specialized motors for centrifuge feed, practically all AC electric shaker 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.

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 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, lowerspeed 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 1.). 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 2). 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 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.