ESP motors

The ESP motor is a two-pole, three-phase, squirrel cage, induction design. A two-pole design means that it runs at 3,600-rpm synchronous speed at 60-Hz power or roughly 3,500-rpm actual operating speed. It operates on three-phase power at voltages as low as 230 and as high as 5,000, with amperages between 12 and 200. Generally, the length and diameter determines the motor's horsepower (HP) rating. Because the motor does not have the power cable running along its length, it can be manufactured in diameters slightly larger than the pumps and seal-chamber sections and still fit in the same casing bores. Typical diameters and rated HP ranges are shown in Table 1. A cross section of a motor is shown in Fig. 1.

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  Table 1

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  Fig. 1-ESP motor (after RP11S1).

Functional features

Wound stator

A wound stator comprises an unwound stator, electrical windings, and insulation and encapsulation systems. The unwound stator has thousands of electrical-grade steel laminations stacked in the housing and is compressed to hold them aligned and stationary. The laminations are die-punched with a center bore for the rotating components to fit into and 18 winding slots for the winding wire. Each slot is insulated with a very-high-dielectric-strength polyamide insulation material. This slot insulation provides winding-to-stator (turn-to-ground) electrical protection.

Insulated copper wire called "magnet" wire or "mag" wire is then wound into each slot to form three separate phase coils displaced at 120° intervals. The insulation on the mag wire provides wire-to-wire (turn-to-turn) electrical protection. Also, at the end of the lamination stack, where the coil has to make a 180° winding turn ("end turn"), insulation is placed between the first winding phase and the motor housing and then between each phase. This protects for phase-to-phase faults.

After the mag-wire winding and insulation is complete, the wound stator is then encapsulated with either a solid-fill epoxy or varnish coating. The encapsulation process fills the voids left in the slots and around the end-turn coils. This provides several important functions. First, it mechanically holds the windings to resist movement that causes wire-to-wire rubbing and possible damage to the wire’s insulation. Second, it adds dielectric strength to the slot winding and end turns. Third, it significantly improves the overall thermal conductivity for better heat dissipation from the motor core through the slots to the motor housing skin. And last, it protects the winding from an attack by contaminates such as wellbore fluid. The last two are less significant for the varnish coating method. As its name implies, it is just a thin coating, mainly on the surfaces of the lamination slots and the mag wire, and has voids where motor lubricating oil accumulates, reducing both the thermal conductivity and the dielectric strength.

The length of the wound stator determines the number of rotors, which also determines the nameplate HP for a given-diameter motor. Within each given length or HP, there are numerous voltage/amperage combinations. Typically, there are various selections running from low voltage/high amperage to high voltage/low amperage. Voltages range from 440 to 4,000+, and amperages typically range from 15 to 150+ amps. The relationship of the HP, voltage, and amperage is

....................(1)

Shaft

The shaft transmits the torque produced by the rotors, keeps all the rotating components aligned axially, and provides a path for the cooling and lubricating oil to the radial bearings and rotors. The shaft is generally tubular material, and the hollow core allows for the motor oil to communicate from the motor head and base areas to the hotter radial bearing and rotor areas. Because the shaft is completely immersed in clean oil, exotic corrosion-resistant materials are not required. Typically, the shaft material is alloyed carbon steel. Its straightness is also critical because of its close rotating clearances and high speed.

Rotor

Ideally, the rotor should be one continuous component that runs the length of the stator lamination bore. This would cause tremendous dynamic-instability problems because of the very large rotor length-to-diameter ratio. Therefore, the rotors are constructed in short segments with radial support bearings placed between them for dynamic stability. Rotors are constructed by stacking hundreds of thin, electrical-grade laminations between two metal end rings. Copper rotor bars are inserted into the lamination slots, the whole stack is compressed, and the rotor bar’s ends are mechanically bonded to the end rings. This results in the "squirrel cage" rotor. The center bore of the rotor has an axial-keyway groove for engaging the axial key stock mounted on the motor shaft. This locks the rotor to the shaft for torque transmission but allows axial movement for thermal growth.

Radial bearings

A sleeve-type-bearing system provides the alignment and radial support for the long shaft and rotor assembly. The sleeve part of the journal is keyed to the shaft and rotates with the shaft. The stationary part of the bearing has a bore in which the sleeve runs. It has an outside diameter (OD) that has a small clearance with the stator-lamination inside diameter. Also, the stator laminations at the bearing locations are made of nonmagnetic material to reduce the rotating magnetic field and the rotational forces tending to rotate the radial bearing. In some designs, an elastomer ring or locking key is located between the bearing OD and the stator inside diameter (ID) to prevent or retard any relative rotation. If rotation does occur, the bearing may start wearing into the stator until contact with the phase mag wires causes an electrical short.

Motor head

The motor head contains the electrical termination for the connection of the three-phase windings to the electrical power cable. This connection is made in an insulated cavity either by a male/female plug-in design or a motor-wire to power-cable-wire splice. Also, a small thrust bearing is located in the head. It is designed to carry the weight of the shaft and rotor stack during startup and maintains the axial position of the rotors and radial bearings relative to the stator.

Performance characteristics

The performance of a submersible motor is usually characterized by the manufacturer’s performance curve. An example is shown in Fig. 2. The curve represents typical motor performance for a given motor diameter, based on the average of several tests. To get the curve data, a motor is loaded across a broad HP load range with a dynamometer. A detailed description of these tests is given in Cahsmore. Data collected include: three-phase voltage, amperage, kilowatts, speed or rpm, motor torque, motor temperature rise, and fluid velocity past the motor. The motor amperage, rpm, efficiency, and temperature rise are especially important for the proper application of any motor. Even though the motor temperature rise is measured during the dynamometer test, it is not generally plotted on the motor characteristic curve. This is because it is a critical parameter in the proper application of the motor, and its value is affected by several application conditions.

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  Fig. 2-ESP-motor performance curve-example [after Centrilift Graphics, Oklahoma (1995)].

Amperage

The motor current is nearly linear with HP loading and is one of the easiest parameters to measure. Because of this, it is the most useful for determining the actual loading of the motor. On the basis of nameplate current rating of the motor and the amperage curve of the motor characteristics, an output HP can be determined. Calculate the percentage of nameplate amps in which the motor runs, and determine the percent of nameplate HP the motor is developing.

Revolutions per minute (RPM)

The rotational speed or RPM of the motor at its application load point is very important in determining the operating point or output of the pump. The pump-performance curve used in determining the head and flow output of the pump for its application is based on a pump-motor speed of 3,500 RPM. If the RPM varies from 3,500, the pump flow will vary with the ratio of the speed, and the flow rate will vary with the ratio of the speed squared. (See Eqs. 2 and 3.) By knowing the percent of nameplate amps, the motor speed can be read from the motor characteristic curve. Even though this RPM change is usually small, it can still impact the final motor and pump operating point for a particular application. When the pump-performance point is modified, because of the motor RPM, the pump head and flow rate change; therefore, the load on the motor is changed. Determining the final pump operating point and motor loading point becomes an iterative process.

....................(2)

....................(3)

Efficiency

Because power costs are a major part of the overall expense of operating an ESP, the efficiency of the motor is an important factor. The efficiency curve for a submersible motor has a fairly flat peak through its normal operating range but starts dropping off significantly at less than 50% loading. Note that this efficiency curve is based on the nameplate voltage being maintained at the motor. If the surface power is not optimized, the voltage delivered to the motor can vary, and the efficiency drops off. Fig. 3 shows the constant motor HP plotted as a function of current and voltage. It indicates that as the motor voltage is increased or decreased away from its nameplate rating, the current increases, resulting in a decrease in efficiency. Therefore, the ESP-motor operating efficiency can be optimized by adjusting the surface voltage and monitoring the motor amperage until the bottom of the current or amps curve is found.

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  Fig. 3-Motor horsepower vs. voltage and current [after Centrilift Graphics, Claremore, Oklahoma (1985)].

Motor temperature rise

The temperature-rise data of the motor, where provided, are an indication of the average winding temperature rise above the ambient motor temperature. At test conditions, with water circulating by the motor at 1 ft/sec, submersible motors typically have rises of 50 to 100°F (10 to 38°C). Under wellbore-application conditions, the temperature rise is affected by various parameters, including: the velocity and thermal-conductivity characteristics of the production fluid flowing past the motor skin, API gravity of the crude, water cut, the percentage of free gas, fluid emulsions, fluid scaling tendencies, voltage imbalance at the motor terminals, and the use of a variable-speed drive. Typically, the industry guideline has been a 1-ft/sec flow by the motor, but there are many applications with velocities below this. The manufacturers have a method for calculating or estimating the impact of these parameters on the heat rise of their motors.

The rating of a motor or its nameplate HP is determined by its designer, on the basis of these same performance-test values. Specifically, the designer is interested in the voltage, amperage, and HP ratings that provide the best motor performance for general operating conditions. Additionally, there are only three absolute limits that also influence the nameplate HP rating. These limits include mechanical, torque, and temperature.

Mechanical limit. The mechanical constraints applied to the motor rating are determined by the maximum torsional-load capability of the design and materials. This limit is based on the mechanical strength properties and the geometry of the shaft.

Torque limit. Here, the designer is looking at the maximum torque of the motor at rated voltage. For a particular motor design, a motor can produce only a given amount of torque for the volume of available active material. The active material is the material that contributes to producing magnetic flux. The maximum amount of torque a motor can produce is called breakdown or pullout torque. The breakdown torque of the motor is usually greater than 2.5 times the existing running torque, which poses no practical limit to the HP rating.

Changing the frequency of the electrical power can also vary the torque or HP rating of the motor. Generally, the motor’s HP rating is based on either 50- or 60-Hz power. A fixed frequency motor has a specified full-load nameplate HP at the specified nameplate voltage, as stated earlier. This same torque can be achieved at other speeds by varying the voltage in proportion to the frequency. This maintains a constant magnetizing current and flux density, which provides a constant available torque. Therefore, the HP output rating of the motor is directly proportional to the frequency or speed (Eq. 4) because power rating is a function of torque (ft-lbf) multiplied by speed (Eq. 5).

....................(4)

and

....................(5)

Temperature limit. For this limit, the designer is interested in the maximum temperature rating of the insulation system and the motor bearing lubrication system. The high-tech insulation used in today’s ESP motors allows an insulation temperature rating in excess of 500°F (260°C). The limiting factor is the motor bearing system. Even though significant advances have been made in bearing design and motor oil formulations, the maximum recommended operating temperature of an ESP motor is around 400°F (205°C). There have been application incursions above this, but they have generally been made with experimental designs or in applications where a reduced ESP run-life has been accepted.

An important application point is that the proper motor oil lubricating viscosity must be maintained at the motor operating temperature. Therefore, the manufacturers provide and specify several grades of dielectric motor oils to cover the range of motor operating temperatures. Each type of oil has a minimum and maximum recommended motor operating temperature.

Nomenclature

C	=	constant = 3,960, where Q is in gal/min, and TDH is in ft [= 6,750, where Q is in m3/D, and TDH is in m]
slope
N	=	rotating speed, rev/min
T	=	torque, ft-lbf
Q	=	flow rate, B/D [m3/d]
ηp	=	pump efficiency

Noteworthy papers in OnePetro

Blanksby, J. M., Hicking, S., & Milne, W. H. (2005, January 1). Deployment of High Horsepower ESPs to Extend Brent Field Life. Society of Petroleum Engineers. doi:10.2118/96797-MS

Waldner, L. B., Wonitoy, K., Klaczek, W., & Noonan, S. G. (2012, January 1). Thermal Performance Testing of a High Temperature ESP Motor for SAGD Applications. Society of Petroleum Engineers. doi:10.2118/160317-MS

External links

Use this section to provide links to relevant material on websites other than PetroWiki and OnePetro

See also

Electrical submersible pumps

ESP centrifugal pump

ESP seal section

ESP power cable

ESP surface motor controllers

ESP optional components

PEH:Electrical_Submersible_Pumps

Page champions

Jose Caridad, BSME & MSc ME