Subsea power supply assembly (10, 20) supplying electric power to a motor (11) at a second location (90) from a first location (80), comprising a variable speed drive (13) (VSD) and a step-up transformer (15) connected to it. At a subsea location the assembly comprises a first step-down transformer (17) with input (17a) and output (17b) and an uninterruptable power supply (25) having an input (25a). A step-out cable (19) supplies power from the step-up transformer (15) to the motor (11). The cable (19) connects to the first step-down transformer (17). The speed of the electric motor (11) is proportional to the output frequency of the VSD (13). The power receiving input (25a) of the uninterruptable power supply (25) connects to the output (17b) of the first step-down transformer (17), thereby receiving electrical power with frequency equal to the output frequency of the VSD (13).