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Information Notice No. 93-74: High Temperatures Reduce Limitorque AC Motor Operator Torque
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 September 16, 1993 NRC INFORMATION NOTICE 93-74: HIGH TEMPERATURES REDUCE LIMITORQUE AC MOTOR OPERATOR TORQUE Addressees All holders of operating licenses or construction permits for nuclear power reactors. Purpose The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to alert addressees that high ambient temperatures can reduce ac motor torque capability in Limitorque motor operators. It is expected that recipients will review the information for applicability to their facilities and consider actions, as appropriate, to avoid similar problems. However, suggestions contained in this information notice are not NRC requirements; therefore, no specific action or written response is required. Background In order to size a motor operator to a given valve, the expected operator torque capable of being developed under design-basis conditions is calculated. The operator torque output is directly related to the torque output of the operator motor. For dc motor operators, Limitorque has published derating factors that are applied to high ambient temperature applications. However, until recently, no derating information was available for Limitorque motor operators equipped with ac motors. Description of Circumstances In response to NRC questions, Limitorque Corporation established a test program to evaluate whether elevated ambient temperatures could significantly affect ac motor output torque. In October of 1992, Limitorque completed testing of five ac motors. This testing indicated that ac motor stall torque values decreased significantly with increasing ambient temperatures. The measured torque reductions ranged from 14 to 25 percent for ambient temperature increases from 24�C [75�F] to 169�C [336�F]. Limitorque contracted with its main motor supplier, Reliance Motor Co., to calculate the expected stall torque reduction for all Limitorque/Reliance ac motor designs. On May 13, 1993, Limitorque issued a report about a potential 10 CFR Part 21 condition. The report contained a tabulation of motor types and expected torque reductions for ambient temperature increases from 25�C [77�F] to 180�C 9309100374. IN 93-74 September 16, 1993 Page 2 of 2 [356�F]. This data is given in Attachment 1. Limitorque has since indicated that the torque reductions would be applicable only for motors in ambient conditions above 40�C [104�F] as the motors are designed to deliver full rated torque up to 40�C [104�F]. Limitorque has stated that the torque reduction is linear with respect to temperature. The Limitorque table also contains information on the expected reduction in locked rotor current at elevated temperatures. The reduced locked rotor current would provide a slightly higher motor terminal voltage. Although this would improve the calculated reduced voltage motor performance, it would not compensate for the torque reduction resulting from an elevated temperature. Limitorque further stated that motor temperature rise as a result of prior motor energization and motor run time (valve stroke time) must be accounted for. Additional information concerning motor operator performance can be found in NRC Inspection Report 99900100/93-01 issued June 28, 1993. This information notice requires no specific action or written response. If you have any questions about the information in this notice, please contact one of the technical contacts listed below or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager. /s/'d by AEChaffee/for Brian K. Grimes, Director Division of Operating Reactor Support Office of Nuclear Reactor Regulation Technical contacts: Jeffrey B. Jacobson, NRR (301) 504-2977 Thomas G. Scarbrough, NRR (301) 504-2794 Attachments: 1. Current/Torque Changes from 25 to 180 Centigrade 2. List of Recently Issued NRC Information Notices .
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