Home > NRC Library > Document Collections > Generic Communications > Information Notices > 1993 > IN 93­

74

Information Notice No. 93­74: 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

.

Page Last Reviewed/Updated Tuesday, November 12, 2013