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 HP 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, which have been determined using different techniques, are interpreted in the same way, this can result in protection levels, which either limit the availability or reduce the motor lifetime. Calculation methods for protection-relevant data are compared with measurements and discussed. Especially for large HP 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