Information Notice No. 93-74: High Temperatures Reduce Limitorque AC Motor Operator Torque
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
All holders of operating licenses or construction permits for nuclear power
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.
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
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
Thomas G. Scarbrough, NRR
1. Current/Torque Changes from 25 to 180 Centigrade
2. List of Recently Issued NRC Information Notices
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