Large Motor Protection – Understanding The Most Vulnerable Components for Correct Discrimination.

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Ron Scollay
Principal Engineering Consultant

Motors are robust components, the large power ratings however, manifest as significant energy input during fault conditions and starting. Motors are designed to start direct on line and the winding components, rotor and stator absorb the energy associated with the initial step voltage application. The initial starting current sees the short circuit impedance of the windings and is 5-8 times the full load current rating of the motor with associated I2R heat input. Volt drop at start, acceleration time, the deep bar effect of the rotor cage are aspects that affect how much starting and locked rotor energy is absorbed by the windings, these aspects need to be considered in the design to determine the temperature rise of the motor with respect to protection and operating capability.

This presentation discusses how these design aspects are considered and applied to define operating stipulations, number of consecutive starts and the protection requirements for stall and overload conditions. A case study will demonstrate these parameters, considering a 1MW 11kV double cage machine where the bar rotor, outer cage, rather than the stator is the critical component to protect.

Ron Scollay is a Specialist Electrical Engineer with over 30 years’ experience in the field of machines and auxiliary equipment. This experience covers general design, re-design, construction, testing and application problems of motors, generators and auxiliary equipment. Also technical audits and specialised tests and measurements to qualify condition and cause of failure with recommendations of remedial methods using contemporary design techniques and practices. On behalf of Eskom and Von Roll Isola, Ron conducted research on ‘Life Management of Motors’ and ‘Medium Voltage Insulation for Motors and Generators’ and as a result is well published.