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The use of matrix converter technologies for the control of actuators and other auxiliaries onboard work-class, deep-sea, remotely operated vehicles (ROVs) is reported. Key requirements for such systems are the ability to sustain operation at high ambient pressures, up to 300 bar, commensurate with operation of ROVs at depths of 3000 m, and to minimise the number of external connections and cabling mass to improve reliability and reduce drag. Emphasis is given to 3×2 matrix converters for 3φ-1φ AC voltage/frequency for conversion control of system auxiliaries, with experimental results showing circuit functionality during pressure cycling consistent with typical operational duties, and the use of 3×3 matrix converters for control of actuators driven by brushless permanent magnet synchronous machines (PMSMs). A principal feature of the paper is the development of model-based sensorless control methodologies for driving PMSMs using matrix converters. In particular, it is shown that observer-based state-estimation techniques normally employed for sensorless control of PMSMs using voltage source inverters, can be readily exported to matrix converter counterparts with minimal additional computational overhead. Experimental results from a 0.7 kW PMSM driven by a matrix converter, suitable for a subsea actuator pump, are included to demonstrate the ability of the sensorless techniques to provide reliable estimates of machine rotor position under transient load conditions, and the subsequent exploitation for matrix converter/motor combinations is discussed.