Motor design considerations for medium voltage adjustable speed drive systems in hazardous areas

Designing medium voltage adjustable speed drive systems for safe operation in hazardous areas requires expertise in a number of areas relating to frequency converters, motors, and the way they interact. As the use of different voltage-source inverter (VSI) topologies is increasing in the medium-voltage (MV) range, special care has to be taken to limit temperature rise, sparking, and bearing currents. Temperature rise results from harmonic losses in the motor which are caused by the non-sinusoidal supply from the converter. Several loss mechanisms operate in converter-fed motors, and various steps can be taken in the selection of the converter and design of the motor to mitigate the losses. Temperature rise should be assessed during the factory acceptance tests to ensure that the motor will operate within the permitted temperature range. This is done either by calculation or testing. If not properly mitigated, sparking can occur in both the stators and rotors of converter-fed motors as a result of harmonics and voltage stresses. Prevention of sparking requires special care in the selection and design of the frequency converter and motor. Bearing currents and shaft voltages are caused by the common mode voltage that arises between parasitic capacitances in the motor, an effect typical of converter-fed motors. A number of methods are available to mitigate this phenomenon.