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In variable-speed electrical drives, data transmission between an electric motor and an inverter is required because of sensors installed at the motor. Sensor information is used both in the motor control and in the diagnostics. For example, a data transmission medium can be implemented either by additional cabling or through the power cable. However, additional cabling is neither a cost-effective nor even a reliable solution. Previously, only a few application-specific solutions for data transmission via power cables have been presented. A general method to implement a broadband and Ethernet-based communication medium between a motor and an inverter is proposed here. The method forms an Ethernet-supported packet-based communication medium over the motor power cable. Ethernet supports all kind of protocols implemented above the link layer. The channel and noise characteristics are described. According to these, a coupling interface is developed that allows installing the communication device safely to a three-phase inverter-fed motor power cable. A channel capacity analysis is also performed. Laboratory measurements are carried out and analysed with the proposed method, and two possible applications are discussed.