This invention relates to a process for production of middle distillates having biodegradability properties. In a first state period during which the motor substantially stops its rotation, a resistance calculation unit calculates a motor resistance value on the basis of a detected value of motor voltage and a detected value of motor current. The controller includes a current limit comparator coupled to terminate the conduction of the power switch during an enabled switching cycle in response to a current through the power switch exceeding a variable current limit. The second terminal may be located within the confines of the annular resistor.