Starting of large medium voltage motors: design, protection, and safety aspects

Large induction and synchronous electric motors play an essential role in petrochemical installations. The availability and operating safety of large horsepower motor drives are influenced by the frequently encountered difficult starting conditions and the correct selection and setting of protective devices. Motor data, required for monitoring functions, such as locked-rotor time, starting time, or starting cycle, are determined using different calculation methods and criteria, as motor manufacturers apply various techniques and limit values and no uniform standards exist. If these data have been determined by using different techniques, and are interpreted in the same way, this can result in protection levels, which either limit the availability or reduce the motor life. Calculation methods for protection-relevant data are compared with measurements and discussed. Especially for large horsepower motors, only state-of-the-art computer based calculations are able to provide results, realistically defining the actual motor stress levels. Drives in classified locations require special attention when selecting protection devices for starting and operation