Asynchronous submersible electric motors are currently used most commonly to drive electrical submersible pumps. At the same time, the possibility of improving their performance has been practically exhausted, and the need to reduce power consumption presents specialists with the task of increasing the efficiency of energyintensive technological processes including ESP oil production. Permanent magnet motors, which already occupy a leading position in a number of fields, have better performance characteristics than asynchronous electric motors.
ESP system drives based on permanent magnet motors were not used in the oil industry until recently, but they are currently achieving wider application at the fields of various oil-producing companies.
Many oil industry equipment producers have developments in this field, e.g. RITEK, KB Neftemash, and others. This also includes LLC Borets, which supplies VEDBT 117 mm submersible permanent magnet motors for TNK-Nizhnevartovsk and SNGDU-2.
Permanent Magnet Motor Performance Characteristics Compared to traditional asynchronous electric motors, permanent magnet motors have a number of characteristics that make their application economically attractive. Furthermore, introduction of permanent magnet motors will raise ESP run life and improve health and safety indicators.
It is necessary to regulate and adjust pump-operation parameters during well startup, rate stabilization, and operation. Using a permanent magnet motor as a drive for an ESP system makes it possible to change the pump speed in an expanded range, and thereby react to changes in well flow rate and dynamic level as promptly as possible, without stopping production and performing tripping operations. In such situations, wells equipped with ESPs with conventional AC motors require choking or change of pump running depth, which leads to equipment wear, excessive power consumption, and increased accident probability. The fundamental difference between permanent magnet motors and asynchronous electric motors is that the former are capable of regulating the revolution speed by varying the current intensity, while the revolution speed of an asynchronous motor can be regulated only by changing the current frequency, using frequency converters. Thus, the simpler construction of a permanent magnetic motor makes it possible to regulate the speed of rotation without implementing additional, expensive equipment – variable speed drives.
One of the substantial weaknesses of conventional asynchronous motors is their low efficiency: no more than 85 percent. At the same time, efficiency of permanent magnet motors is much higher – over 90 percent – and it barely changes from fluctuations in voltage from the power source and changes in load on the drive. The minimum idle and operating current values make it possible to measure load on the drive more accurately, optimize ESP operation, and build minimal current protection better.
An important issue in well stabilization is the problem of submersible electric motors overheating. Therefore, it is necessary to periodically stop the unit for cooling when using AC motors. Using a permanent magnet motor as the drive makes it possible to stabilize a well without stopping for cooling – because of their higher efficiency, permanent magnet motors overheat less often, which increases the service life of insulation materials, and makes it possible to reduce the number of failures caused by cables melting at cable input.
Another difference between permanent magnet motors and conventional AC motors is that, with identical power output, they weigh less and have smaller dimensions. This is especially important when operating in wells with complicated wellbore geometry.