1、Best Practices Entry: Best Practice Info:a71 Committee Approval Date: 2000-04-0a71 Submitted by: Wilson HarkinsSubject: DC Drive - Solid State Control Practice: During the design of new (or upgrades to) motor generator set type DC drives, consider the use of solid state assemblies for control functi
2、ons.Programs that Certify Usage: This practice has been used on the National Space Transportation System Shuttle KSC Ground Support Systems.Implementation Method: This Lesson Learned is based on Maintainability Technique number OPS-06 from NASA Technical Memorandum 4628, Recommended Techniques for E
3、ffective Maintainability.Use of solid state controls instead of magnetic amplifiers can improve system restoration time in the event of a failure. Features such as fault detection, modular construction, and packaging can be easily employed. Diagnostics for system health status and problem resolution
4、 can also be readily provided. Incorporation of these features can result in improved system performance and availability.The use of solid state assemblies for control functions represents a great improvement over previous control methods. Historically, the first method of obtaining adjustable speed
5、 using DC motors was the constant potential DC supply using field adjustment. This provided a small range of adjustment. This method was followed by the rotating M-G system of Ward Leonard patented in the 1890s. This drive used an AC motor driving a DC generator to convert AC to DC power. The motor
6、and generator may be combined in a single frame and use a common shaft, or separate coupled units (See Figure 1). The output DC voltage is controlled by adjusting the field excitation of the DC generator. Depending on the accuracy required, armature voltage or a tachometer may be used as a feedback
7、signal in a closed loop system. An important aspect of this drive is that power flow is reversible. The Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-motor acts as a generator, driving the generator as a motor, which drives the AC motor which then
8、pumps power back into the AC lines. This ability, called regeneration, is a useful feature in decelerating large inertias or holding back overhauling loads. This is a very important consideration when replacing the M-G with a conventional packaged silicon-controlled rectifier (SCR) drive.refer to D
9、descriptionD In the late 1940s, electronic tube drives began to replace M-G drives. These used vacuum, thyratron, excitron, or ignitron tubes for armature circuit control. They had limited acceptance because of tube life limits and water cooling requirements on larger ratings. By the early 1960s the
10、 tubes were replaced with the solid state thyristor drives. Magnetic amplifier drives were developed in the mid-1950s when silicon diodes became popular. They were never as widely used because of difficulties of reactor design and acceptable response rate. However, they were rugged and highly reliab
11、le once in satisfactory operation.During the early 1960s the thyristor or SCR became readily available. This device is similar in operation to a thyratron tube. Today it dominates the direct current drive field. Special circuits enable the SCR to regenerate and reverse readily. Larger and less expen
12、sive SCRs have extended the range to well over 1000 HP. Figure 2 illustrates a controlled rectifier drive. Note that the gateing control and SCR bridge have replaced the M-G set of Figure 1, resulting in reduced rotating machinery.Provided by IHSNot for ResaleNo reproduction or networking permitted
13、without license from IHS-,-,-refer to D descriptionD Solid State OperationFigure 3 shows the assemblies comprising a solid state control system for DC drives. A single phase thyristor power converter supplies up to 200 volts positive or negative at 20 amperes to the generator field. A closed-loop co
14、ntroller (speed regulator) provides for armature voltage with IR drop compensation or AC/DC tachometer feedback speed control and linear acceleration and deceleration. A firing circuit provides an isolated gate drive to the power converter. A bi-directional adapter used in conjunction with the firin
15、g circuit assembly provides bi-directional current to the field of a DC generator for contactorless reversing or to regulate to zero output voltage in the presence of residual magnetism of the DC generator. Protective circuitry includes a voltage sensing relay for safety interlocking and an isolator
16、 for isolated armature current feedback.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-refer to D descriptionD Reference:1. KSC Electrical Drawing for VAB 250 Ton Cranes, 250-69-K-L-11388.2. KSC Electrical Drawing for VAB 175 Ton Crane, 175-67-K-L-1
17、1348.Impact of Non-Practice: At KSC the 175- and 250-Ton Bridge Cranes in the Vehicle Assembly Building (VAB) were using metadynes (electromechanical rotating amplifiers) for control function. The metadyne had a long Provided by IHSNot for ResaleNo reproduction or networking permitted without licens
18、e from IHS-,-,-history of maintenance problems because of brush wear, contamination and corrosion. It required extensive pre-operation maintenance attention to support Shuttle processing. In addition, the metadyne units often required maintenance during processing operations impacting processing sch
19、edules. KSC replaced the metadynes with solid state controller units resulting in decreased maintenance actions including pre-operation maintenance and improved system performance and availability. Fault isolation and removal and replacement of failed components is easier and less time consuming. Si
20、nce failures occur at a less frequent rate, the need for numerous operating spares is reduced. Furthermore, the “off equipment“ in-shop maintenance of failed units requires much less time and money to effect a repair. Reduced maint enance and downtime allow for the crane to be ready and operating to
21、 support Shuttle processing in a more timely manner.Related Practices: N/AAdditional Info: Approval Info: a71 Approval Date: 2000-04-0a71 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-,-,-
