A system (100; 200; 400) comprising: an above-surface control unit (120; 220; 420) comprising: a variable speed motor control circuit (122; 213; 413) configured to generate at least one switch control signal; and a first communications circuit (114; 224; 424) configured to transmit the at least one switch control signal over a communications medium (130; 230; 430); and a subsea motor drive unit (110; 210; 410) comprising: a driver circuit (112; 214; 414) configured to be coupled to an electric motor and comprising at least one semiconductor switch; and a second communications circuit (124; 212; 412) coupled to the first communications circuit (114; 224; 424) via the communications medium (130; 230; 430) and configured to recover the transmitted at least one switch control signal and to apply the recovered at least one switch control signal to the driver circuit (112; 214; 414); wherein the second communications circuit (124; 212; 412) is configured to transmit at least one sensor signal over the communications medium (130; 230; 430), wherein the first communications circuit is configured to receive the transmitted at least one sensor signal; characterised in that the variable speed motor control circuit (122; 213; 413) is configured to generate the at least one switch control signal responsive to the received at least one sensor signal. an above-surface control unit (120; 220; 420) comprising: a variable speed motor control circuit (122; 213; 413) configured to generate at least one switch control signal; and a first communications circuit (114; 224; 424) configured to transmit the at least one switch control signal over a communications medium (130; 230; 430); and a subsea motor drive unit (110; 210; 410) comprising: a driver circuit (112; 214; 414) configured to be coupled to an electric motor and comprising at least one semiconductor switch; and a second communications circuit (124; 212; 412) coupled to the first communications circuit (114; 224; 424) via the communications medium (130; 230; 430) and configured to recover the transmitted at least one switch control signal and to apply the recovered at least one switch control signal to the driver circuit (112; 214; 414); wherein the second communications circuit (124; 212; 412) is configured to transmit at least one sensor signal over the communications medium (130; 230; 430), wherein the first communications circuit is configured to receive the transmitted at least one sensor signal; a variable speed motor control circuit (122; 213; 413) configured to generate at least one switch control signal; and a first communications circuit (114; 224; 424) configured to transmit the at least one switch control signal over a communications medium (130; 230; 430); and a driver circuit (112; 214; 414) configured to be coupled to an electric motor and comprising at least one semiconductor switch; and a second communications circuit (124; 212; 412) coupled to the first communications circuit (114; 224; 424) via the communications medium (130; 230; 430) and configured to recover the transmitted at least one switch control signal and to apply the recovered at least one switch control signal to the driver circuit (112; 214; 414); wherein the second communications circuit (124; 212; 412) is configured to transmit at least one sensor signal over the communications medium (130; 230; 430), wherein the first communications circuit is configured to receive the transmitted at least one sensor signal; characterised in that the variable speed motor control circuit (122; 213; 413) is configured to generate the at least one switch control signal responsive to the received at least one sensor signal.