1、Best Practices Entry: Best Practice Info:a71 Committee Approval Date: 2000-03-08a71 Center Point of Contact: KSCa71 Submitted by: Wil HarkinsSubject: Over-Speed Protection System for DC Motor-Driven Cranes Practice: DC drive motor over-speed detection using a voltage sensing relay.Programs that Cert
2、ify Usage: This practice has been used on the KSC Vehicle Assembly Building (VAB) 250-ton and 175-ton Bridge Cranes.Center to Contact for Information: KSCImplementation Method: This Lesson Learned is based on Reliability Practice number PD-ED-1235 from NASA Technical Memorandum 4322A, NASA Reliabili
3、ty Preferred Practices for Design and Test.Benefits:This design employs a simple method of providing protection against the effects of a crane operating at a higher than commanded speed while not introducing unwanted nuisance trips to the crane control system. This improves the reliability of the cr
4、ane control system by preventing the crane from reacting to unwanted commands that are not operator initiated. The improvement allows the crane to be used with a higher degree of confidence that a critical failure will not result in damage to the load suspended from the load hook.Provided by IHSNot
5、for ResaleNo reproduction or networking permitted without license from IHS-,-,-Implementation Method:The design provides protection against damage to a load resulting from a crane drive system experiencing a speed increase caused by an unexpected input created by a failure of the electrical circuitr
6、y used to convey the operator stick input to the drive motors. The voltage sensing relay is set to detect a voltage that is larger than the normal operating voltage of the DC motors in the selected speed (slow, medium, or high). When this voltage is experienced, the relay will shut down the crane op
7、eration and set the brakes. For example, at KSC while performing stacking and mating operations of the Solid Rocket Motors (SRM), External Tank (ET), or Orbiter during Space Shuttle processing, the crane must be operated in the slow speed mode when within close proximity of a structure. The voltage
8、sensing relay is active during this mode to prevent the shuttle components from impacting a surrounding structure as a result of a control circuitry failure. The relay is deactivated while operating in the medium or high speed modes because the critical load is not near an obstruction. This allows t
9、he crane to be operated with a higher degree of confidence and reliability that a critical failure will not result in damage or loss of critical flight hardware during stacking and mating operations.Technical Rationale:With a constant field/varied armature DC motor (more than one motor, the armature
10、s wired in series) the voltage sensing relay coil should be placed in parallel to the motor armature(s) (see Figure 1.). The relay coil should be fed through a bridge rectifier to insure the input to the relay is consistent regardless of the voltage polarity and direction of current flow present in
11、the armatures. The normally open relay contact should be wired in series with the power supplied to the crane and brake controls. When power is applied to the crane the contact should close and enable the drive control circuitry. The contact will remain closed when the voltage in the motor armature
12、is below the predetermined threshold. When the limit is reached, approximately 115% of the full output of the speed range, the contact will open disabling the crane drive and setting the brakes to stop the crane. To avoid the crane shutting down when in the higher speed range, a time delay relay wil
13、l be energized by the speed selector switch and by pass the voltage sensing relay. To prevent nuisance trips when the speed selector switch is repositioned from the higher to the lower speed range while the crane is moving, the time delay relay will disable the voltage sensing relay for a short pred
14、etermined time period when it is de-energized.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-refer to D descriptionD References:1. 69-K-L-11388: “Vehicle Assembly Building 250-Ton Bridge Crane #1 & #2 Electrical“2. SAA09FY12-005: “System Assurance A
15、nalysis of the 250-Ton Bridge Cranes at the Vehicle Assembly Building, High Bays 1, 2, 3 & 4“3. 67-K-L-11348: “Vehicle Assembly Building 175-Ton Bridge“4. Crane Electrical. SAA09FY12-006: “System Assurance Analysis of the 175-Ton Bridge Cranes at the Vehicle Assembly Building“Provided by IHSNot for
16、ResaleNo reproduction or networking permitted without license from IHS-,-,-Impact of Non-Practice: An increased potential would exist for a critical crane failure which could result in damage to flight hardware or serious injury/death to personnel.Related Practices: N/AAdditional Info: Approval Info: a71 Approval Date: 2000-03-08a71 Approval Name: Eric Raynora71 Approval Organization: QSa71 Approval Phone Number: 202-358-4738Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
