Expanded Field Data Analysis in Support of a Torque-Based Motor Bus Transfer Criterion

The ANSI/NEMA C50.41 Standard, “Polyphase Induction Motors for Power Generating Stations” states, “Induction motors are inherently capable of developing transient current and torque considerably in excess of rated current and torque when exposed to out-of-phase bus transfer.” Further, “The magnitude of this transient current and torque may range from 2 to 20 times rated.” This suggests a criterion related to a multiple of rated full load torque. Since a motor bus is comprised of an aggregate of different motors and loads, it is unrealistic to create an industry criterion for measuring the severity of the transfer of the aggregate bus based on individual motor rated torques. Data from several live motor bus transfer events at industrial facilities were analyzed with a new transfer metric based on the ratio of the aggregate peak torque at transfer to the aggregate running torque just before transfer. These torque ratio results were scrutinized based on the C50.41 range from 2 to 20 times rated and then compared with the C50.41 per-unit volts/hertz criterion, calculated at transfer, which is presently used to determine an acceptable transfer. The calculation method is presented with the theory and mathematics employed, and is verified with simulation of a representative induction motor. Analyzing field cases, calculating torque ratio (T_PK/T_L), and comparing with the per-unit volts/hertz value, demonstrate the validity of this torque ratio metric for determining an acceptable limit for transfer. The periodical version of this paper contains updated material not found in the original conference record edition.