United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 89-11: Failure of DC Motor-Operated Valves to Develop Rated Torque Because of Improper Cable Sizing

                                  UNITED STATES
                          NUCLEAR REGULATORY COMMISSION
                             WASHINGTON, D.C.  20555

                                February 2, 1989

Information Notice No. 89-11:  FAILURE OF DC MOTOR-OPERATED VALVES TO 
                                   DEVELOP RATED TORQUE BECAUSE OF IMPROPER
                                   CABLE SIZING


All holders of operating licenses or construction permits for nuclear power 


This information notice is being provided to alert addressees to potential 
problems resulting from motor-operated valves (MOVs) not developing rated 
torque because of the failure of the original cable sizing calculations to 
include the proper current values and cable resistances.  It is expected 
that recipients will review the information for applicability to their faci-
lities and consider actions, as appropriate, to avoid similar problems.  How-
ever, suggestions contained in this information notice do not constitute NRC 
requirements; therefore, no specific action or written response is required.

Description of Circumstances: 

As a part of the Equipment Qualification and Nuclear Plant Aging Studies, the 
NRC obtained a motor-operated wedge-gate valve from the Shippingport Atomic 
Power Station following its decommissioning.  The valve had been installed 
in a secondary system for the 25-year life of the plant.  A review of the 
maintenance records for the valve and the operator indicated that there were 
no reportable incidents associated with either the valve or the operator.  
Following removal, the valve and the operator were refurbished by an NRC 
contractor and tested to demonstrate the valve's ability to operate at the 
conditions expected during the forthcoming testing.  The valve, the operator, 
and the controller were then transported to Germany where they were installed 
in the Heissdampfreactor (HDR) decommissioned reactor test facility as part of 
a jointly sponsored seismic test research program.  With the exception of the 
use of equivalent metric-sized cable, the electrical installation of the 
operator was identical to that at Shippingport.

Each test of the valve consisted of a closure cycle, an opening cycle, and 
a final closure cycle.  When the data from these tests was analyzed, it was 
determined that the valve had routinely failed to fully seat during the second 
closing cycle of each test which had subjected the valve to system pressure 
and flow and ambient temperature conditions.  In addition, the valve had 

..                                                            IN 89-11
                                                            February 2, 1989
                                                            Page 2 of 3

failed to fully seat during both closing cycles of each test which had sub-
jected the valve to system pressure, flow, and elevated temperature 
conditions.  Further evaluation of the data indicated that (1) although the 
motor had stalled, the output torque had not been sufficient to open the 
torque switch; (2) the maximum recorded current appeared to be significantly 
less than that expected for a stalled motor; and (3) the motor voltage at 
stall was only slightly less than the running voltage.  In response to a 
request from NRC, the contractor conducted a second series of tests at HDR.  
Because of the concern that degraded voltage might have been a contributor to 
the failures, the electrical instrumentation inputs were moved from the motor 
control center to the junction strip in the switch compartment of the motor 

The second test series produced the same valve failure pattern that had been 
obtained in the initial test series.  As the second test series was thought 
to have eliminated degraded voltage as a cause, it appeared that the problem 
might be improper motor operator sizing.  The NRC requested the contractor to 
conduct additional testing to determine whether the motor and the operator had 
been properly sized initially or whether they had degraded during their 
service at Shippingport.  To accomplish this testing, the motor operator and 
the control-ler were removed from the HDR decommissioned reactor test facility 
and shipped to the Limitorque manufacturing facilities for dynamometer 

The dynamometer testing of the motor operator by Limitorque demonstrated that 
the performance of the motor operator was better than that at HDR, but that 
the motor performance was less than that originally specified.  It was 
therefore decided to perform motor dynamometer testing at the motor 
manufacturer's facilities.  To accomplish this testing, the motor was removed 
from the motor operator and shipped to the motor manufacturer.  The motor 
dynamometer testing by the motor manufacturer demonstrated that the motor's 
performance did not differ appreciably from that of a new motor.

However, before any additional evaluations could be performed, the NRC contrac-
tor became aware of the recent MOV failures at Brunswick Steam Electric Plant, 
Unit 1, which had been described in NRC Information Notice 88-72, "Inadequacies 
in the Design of DC Motor-Operated Valves."  Because of the similarity of 
conditions noted in the HDR and Brunswick failures, the NRC contractor, 
working with Limitorque, focused on the analysis of the cable sizing for the 
MOV.  This analysis pointed out the following two factors:

(1)  The first factor concerned the current values that should be used to 
     establish the cable size.  As a motor operator is not a continuous-duty 
     device, the normal operating voltage and current data contained on the 
     nameplate are of only limited applicability to cable sizing.  Rather, it 
     is the voltage and current required to develop the rated output torque of 
     the motor operator that is important.  As the current required for this 
     rated torque must be developed during locked rotor conditions, the cable 
     should be sized on the locked rotor current instead of on the full load 
     current.  The ratio of these currents is quite large as the locked rotor 
     current is typically in excess of five times the full load current stated 
     on the nameplate.

..                                                            IN 89-11
                                                            February 2, 1989
                                                            Page 3 of 3

(2)  The second factor concerned the cable resistance that should be 
     considered in the analysis.  The wiring diagram for a typical motor 
     operator is shown in Attachment 1.  Significant cable resistances and the 
     location of the voltage measurement during the second HDR tests are also 
     shown.  Attach-ment 2 presents a simplified schematic of the same circuit.  
     As can be seen, any attempt to measure the voltage drop at the power 
     supply bus (i.e., the sum of the voltage drops across the armature and 
     the series field) includes the resistance contribution from four cables.  
     Even taking the voltage measurement at the motor (as was done during the 
     second HDR tests) will result in including the voltage drop across three 

From the above information, it can be seen that the required current for the 
MOV to develop the design torque should be based on the motor resistance and 
the cable resistance (i.e., four times that of a single cable) under a minimum 
postulated bus voltage.

As indicated in their August 17, 1988, maintenance update, Limitorque recom-
mends that the cable be sized to allow for five times full load current flow 
at the minimum voltage condition.  This maintenance update also contained a 
method for calculating the maximum current draw at locked rotor conditions.

No specific action or written response is required by this information notice.  
If you have any questions about this matter, please contact one of the techni-
cal contacts listed below or the Regional Administrator of the appropriate 
regional office.

                                   Charles E. Rossi, Director
                                   Division of Operational Events Assessment
                                   Office of Nuclear Reactor Regulation

Technical Contacts:  Yun-Seng Huang, NRR
                     (301) 492-0921

                     Peter J. Kang, NRR
                     (301) 492-0812

                     Richard J. Kiessel, NRR
                     (301) 492-1154

1.  Cable Resistances and Voltage Measurements Identified
2.  Motor Functional Schematic
3.  List of Recently Issued NRC Information Notices
..                                                            Attachment 3 
                                                            IN 89-11
                                                            February 2, 1989
                                                            Page 1 of 1

                             LIST OF RECENTLY ISSUED
                             NRC INFORMATION NOTICES
Information                                  Date of 
Notice No._____Subject_______________________Issuance_______Issued to________

89-10          Undetected Installation       1/27/89        All holders of OLs
               Errors In Main Steam Line                    or CPs for BWRs.
               Pipe Tunnel Differential
               Temperature-Sensing Elements
               at Boiling Water Reactors.

89-09          Credit for Control Rods       1/26/89        All holders of OLs
               Without Scram Capability                     or CPs for test and
               in the Calculation of the                    research reactors.
               Shutdown Margin

89-08          Pump Damage Caused by         1/26/89        All holders of OLs
               Low-Flow Operation                           or CPs for nuclear
                                                            power reactors.

89-07          Failures of Small-Diameter    1/25/89        All holders of OLs
               Tubing in Control Air, Fuel                  or CPs for nuclear
               Oil, and Lube Oil Systems                    power reactors.
               Which Render Emergency Diesel
               Generators Inoperable

89-06          Bent Anchor Bolts in          1/24/89        All holders of OLs
               Boiling Water Reactor                        or CPs for BWRs 
               Torus Supports                               with Mark I steel 
                                                            torus shells. 

89-05          Use of Deadly Force by        1/19/89        All holders of OLs
               Guards Protecting Nuclear                    for nuclear power
               Power Reactors Against                       reactors.
               Radiological Sabotage

89-04          Potential Problems from       1/17/89        All holders of OLs
               the Use of Space Heaters                     or CPs for nuclear
                                                            power reactors and
                                                            test and research

89-03          Potential Electrical          1/11/89        All fuel cycle and
               Equipment Problems                           major nuclear 
OL = Operating License
CP = Construction Permit 
Page Last Reviewed/Updated Tuesday, November 12, 2013