Control systems and methods for increasing efficiency of a compressor while the compressor capacity exceeds compressor load, by using a mechanical unloader such as a slide valve to reduce the internal volume ratio of the compressor and allow a more efficient speed to be maintained by a variable frequency drive (VFD) while reducing the compressor capacity based on the load. Control systems include the VFD, a controller for the VFD and the mechanical unloader, and temperature sensors. Compressor embodiments further include one or more compressors and mechanical unloaders operated by the control systems.

A method of protecting a hydrocarbon pump (6) from excessive flow rates in a system for pumping a hydrocarbon fluid, which system comprises said pump and an electrical motor (10) for driving the pump. The method comprises the steps of: for each of a plurality of gas volume fraction values of the hydrocarbon fluid, establishing a maximum torque limit for the pump by mapping the maximum allowable torque of the pump as a function of the differential pressure across the pump, thereby creating a plurality of maximum torque curves (4), each representing the maximum torque limit for a unique gas volume fraction value; from the plurality of maximum torque curves (4), establishing a master maximum torque curve (5) which represents the maximum torque limit for all gas volume fraction values; monitoring the torque of the pump and the differential pressure across the pump; based on the monitored differential pressure (DP) and using the master maximum torque curve, establishing a maximum allowable torque (T) for the pump; and taking a predetermined action if the monitored torque exceeds the established maximum allowable torque (T), e.g. raising an alarm and/or shutting down the system.

A method for controlling a permanent magnet (PM) synchronous motor in an ESP application is provided. A load angle of the PM motor is estimated. A voltage adjustment value is determined for the PM motor based at least on the estimated load angle of the PM motor. A voltage to be applied to the PM motor is determined based on the voltage adjustment value.