Continuous-duty electric motors are an integral part of a drillings rig’s solids-control and processing systems. Centrifugal pumps that feed hydrocyclones, circulate mud for mixing, and transfer mud to and from reserve and also into the trip tank are powered by electric motors. Shale shakers, mud cleaners, centrifuges, and pit agitators are also driven by electric motors.
Continuous-duty electric motors meet well-defined performance standards. Motors are designed with conductor, frame, and insulating materials to continuously deliver rated horsepower and not exceed the insulation’s temperature limits. A service factor rating defines the ability of the motor to continuously withstand prolonged overload conditions while remaining within the temperature limitations of the insulating material.
A statement of operating limits of each commercial machine is provided by the manufacturer in the form of data stamped on a nameplate fastened to the machine frame. These data usually include the hp output, speed, voltage, and current assigned to the machine by the manufacturer. These data constitute the rating of the machine, and the various parameters are referred to as the rated voltage, rated current, etc.
The rating of a machine is an arbitrarily specified safe operating limit for the machine, determined in accordance with accepted standards and procedures. The rating specifies the operating limit that the machine cannot ordinarily exceed for a considerable length of time without some damage occurring to it, or at least causing an accelerated rate of wear in one or more of its parts. Unless stated otherwise, generators and motors are rated for continuous service. Their specified loads may be carried for an unlimited period of time. Machines that operate on intermittent, varying, or periodic duty are given a short time rating such as 5, 10, 15, 30, 60, or 120 minutes.
The rated output of a generator is expressed in kilowatts (kW) available at the machine terminals. A generator whose rating is 25 kW at 125 V and 1000 rpm will, when operated at 1000 rpm, supply an output current of (25,000/125)=200 A. The value of the current that appears on the nameplate is 200 amperes.
The rated output of a motor can be given in kilowatts but is more often given in horsepower available at the shaft when the specified voltage is impressed and the motor runs at rated speed. The motor input is greater than the motor output because there are certain friction and other heat losses that must be supplied from the electrical input in addition to the useful work, which the motor does.
Heating is the main factor affecting the ratings of motors and generators. There is no definite load a machine can carry in the sense that a 5-gallon bucket holds a certain maximum amount of liquid and no more. A machine may exceed its rating by 25% or even 50%, but if the excess load is carried for a considerable period of time, the temperature will rise to a value that will result in permanent damage to the insulation.
Voltage regulation, in the case of a generator, and speed regulation, in the case of a motor, also influence rating. The full-load and no-load voltages of a generator are often specified, as are the full-load and no-load speeds of a motor.
I²R losses (both stator and rotor), friction losses, hysteresis, and eddy currents are electrical-energy phenomena converted into heat that must be radiated away by currents of air.
Heat energy can flow only if a difference in temperature of the heated part or surface and the surrounding air exists. There is a definite temperature limit that no winding can exceed without incurring permanent damage to the machine (generator or motor). Heat losses should be kept to a minimum. Good design allows for the transfer of heat away from the motor frame—typically through conduction or convection.
Generators and motors are sometimes enclosed and provided with a forced ventilating system. Many others are open and have free access to the air for cooling. Dirt and dust accumulating in the air ducts may impair ventilation to such a degree that a machine may overheat even though its rated load is not exceeded.
Altitude affects heating. Air is a better cooling medium at sea level than at elevations. Standard ratings apply to altitudes of 3300 feet (1000 m) or less.
Excessive temperatures deteriorate and finally destroy the insulating properties of the materials used to insulate the windings of motors and generators. The highest temperature to which an insulating material may be subjected continuously is the maximum rated temperature for that material. Exceeding the maximum rated temperature of the insulating material will result in premature breakdown of that material.
The criterion for sizing and selection of any motor is its ability to deliver startup power under the process load and to then provide power that drives the equipment throughout operation. Adequate torque must be developed to overcome inertia during startup. The load must then be accelerated to the desired operating speed and full-load power requirements supplied without overheating. These parameters depend on motor design and the full-load rating (output hp).
Electric-motor operating efficiency is the ratio of output power to input power. The power loss is the difference between the power into the motor and the power out of the motor. This power loss is caused by:
- Heat from the electrical resistance of motor windings and rotor.
- Windage losses from cooling fans or rotor fins.
- Magnetic and core losses from currents induced in the laminations of
frame and stator.
- Friction losses from shaft bearings.
The motor’s internal heat is a function of load conditions, motor design, and ventilation conditions. Heat produced internally by the motor raises operating temperature and adversely affects insulation used to isolate electrical conductors from each other and from the motor frame. Insulation materials are rated based on thermal capacity, or the ability to
withstand heat effects. High-quality insulation systems with high thermal
capacity can withstand relatively high temperature increases and deliver a long motor service life at rated performance. Because motors may be operating properly and still be too hot to touch, it is important to check the manufacturer’s guidelines.
Motors are rated for operation at specific voltages. Motor performance is affected when the supply voltage varies from the motor’s rated voltage. Motors generally operate satisfactorily with voltage variations within ±10%. However, equipment connected to the motor may not always function properly with such variations.
Surge voltage is any higher-than-normal voltage that temporarily exists on one or more of the power lines of a three-phase motor. A surge causes a large voltage rise during an extremely short period of time. Surges are of concern because the higher voltage is impressed on the first few turns of the motor windings. The winding wire insulation may be destroyed, causing the motor to fail. Frequent voltage surging can result from line switching of large generators.
Undervoltage at the motor terminals can result when large current demands are placed on the generator, such as starting the top drive motor. Operation below 10% of the marked motor voltage will generally result in excessive overheating and torque reduction. Overheating prematurely deteriorates the insulation system. Torque reduction may result in the motor stalling or, in the case of shale shaker vibrators, in poor performance.
Figure 1. provides general guidelines for the effects on induction motors of voltage variation and the effects of voltage unbalance on motor performance.