System and method for controlling muliphase electric motors

#This disclosure relates to electrical motors and in particular for electrical motors used in electrical pumps for high power offshore submersible pump applications.

#Electrical motors for use in high power offshore submersible pump applications, such as oil wells, typically use three-phase asynchronous induction motors with two poles and a distributed winding. The low pole number enables high speed operation at around 3500 rpm when using 60 Hz supply. The induction motors are also typically driven by a variable frequency drive that enables operations at varying speeds depending on the required flow rates through a submersed pipe.

#Due to the high temperature in the well improper cooling and overloading can result in breakdown of ground wall insulation leading to stator inter-turn and phase-to-ground faults. Additionally, due to the long length of the electric cable providing power to the electric motor, there is an increased risk of a reflected voltage wave increasing the instantaneous voltage at the motor terminals above their rated value, especially during start-up of the motor. This phenomenon can also lead to insulation breakdown and cause phase-to-phase or phase-to-ground faults. Such faults lead to the loss of one or more phases, rendering the motor inoperable and in need of replacement. Typical motors, have overlapping end windings allowing faults to migrate to healthy phases leading to phase-to-phase faults.

#FIG. 1 illustrates a system for controlling a multiphase electric motor, in accordance with one or more implementations;

#FIG. 2 illustrates a circuit diagram for a multiphase current regulated PWM motor drive, in accordance with one or more implementations;

#FIGS. 3A-3C illustrate phase shifts for a multiphase motor under different operating configurations, in accordance with one or more implementations; and,

#Increasing the number of phases for the electric motor in an electrical pump allows the electrical pump to perform after one or more phases has failed. Additionally, avoiding overlap of the phases, such as with a fractional slot per pole per phase (SPP) ratio, which produces windings with minimal to no overlap between the phases, may reduce phase faults from propagating to the other phases. Such windings may be used with induction, permanent magnet synchronous motors (PMSM), synchronous reluctance motors (SynRM) and permanent magnet assisted synchronous (PMa-SynRM) reluctance motors.

#When one or two phases of a high number phase motor are in fault, it is possible to continue operation of the motor using the remaining phases. For example, if the electric motor is a five-phase electric motor, the motor may still operate when one phase, or even two phases, are in fault. Such motors may have a minimum number of phases not in fault in order to remain operational.

#When one or more phases are in fault, the phase shift between the remaining healthy phases is determined to satisfy certain constraints. Such constraints may include: configuring the system so that the total instantaneous current is approximately zero, ensuring that there is no zero sequence current present which can lead to bearing failure losses; and/or, configuring the second harmonic torque ripple to be approximately zero. For permanent magnet type motors an analytical field solution of the air gap flux density due to the magnets may be used to calculate the required phase shifts between the currents in the healthy phases to reduce the second harmonic torque. A more accurate compensation may be achieved by injecting third harmonic currents to additionally reduce the fourth harmonic torque ripple that is created during phase faults. For induction and SynRM motors a more accurate compensation may be achieved by including the effect of third harmonic magneto-motive force (MMF) produced by the stator winding. By injecting third harmonic currents it may be possible to reduce the second harmonic torque when operating under fault and achieve higher average torques.