The Rockwell Motors and Drives Laboratory at Clemson University National Science Foundation Workshop on Teaching of Power Electronics And Electric Drives and Their Applications in Power Systems Las Vegas, NV February 20 – 21, 2004 E. R. (Randy) Collins Jr., PhD, PE Associate Professor Department of Electrical and Computer Engineering Clemson University Clemson, SC 29634-0915 firstname.lastname@example.org I. Introduction A generous equipment donation from Rockwell Automation Power Systems has enabled the Department of Electrical and Computer Engineering at Clemson University to develop a new motors and drives laboratory using state-of-the-art industrial motor controls. As a result, a laboratory has been renovated and equipped with three-phase power to house the workstations. The hardware for each of these workstations is mostly complete, but refinements are continuing on the human-machine interface, controls, and data acquisition system. Additionally, laboratory experiments are being developed for the workstations. These experiments will supplement junior- and senior-level undergraduate lecture classes. II. Overview of the Curriculum The Electrical Engineering undergraduate degree program offers three courses in the power systems area and one optional laboratory. During the junior year, EE students are required to take an introductory course on power systems and electric machines. An optional laboratory course can be taken concurrently with the class. At the senior year, students are required to take two technical electives. Two power engineering courses are among these elective choices. One of these is a traditional power system analysis course and the other is a power electronics and drives course. These courses are described in additional detail below. ECE 360 (Energy Conversion) This class is a three-hour required course for juniors majoring in electrical engineering. The topics include a review of three-phase circuit analysis and power computations, magnetic circuits, transformers, dc motors, ac induction motors, ac synchronous motors and generators, and transmission lines. ECE 412 (Energy Conversion Laboratory) This laboratory is a one-credit hour elective lab which is to be taken concurrently with or after ECE 360. This laboratory focuses on experiments with motors and generators, using Lab-Volt brand equipment. The experiments are typical introductory machines experiments: torque-speed characteristics, equivalent circuit parameter determination and verification, synchronizing generators to the line, etc. The motors are all run “across the line” and drives are not discussed. ECE 418 (Power System Analysis) This course is a three-credit hour elective class that is taken by approximately 20% of the EE seniors. Topics covered include power system modeling and the per-unit system, symmetrical components, load-flow, fault analysis and some basics of economic dispatch. It is used as an introduction to the many facets of power engineering and as a foundation for graduate study. ECE 419 (Power Electronics and Drives) This course is a three-credit hour elective class that is taken by about 25% of EE seniors. Topics include power electronic devices (diodes, thyristors, power transistors, etc.), power electronic circuits and converters (rectifiers, inverters, dc/dc, etc.), and the use of the power electronics as drives for motors. Aspects of motor control are discussed, focusing primarily on the basics of ac induction motor control using V/Hz schemes and dc motor control using thyristor-based rectifiers. III. The New Laboratory Rockwell Automation Power Systems, headquartered in Greenville SC, has familiar product lines such as Reliance Electric and Dodge. A few years ago, Rockwell donated funding for a large scholarship endowment and an in-kind equipment gift to Clemson. This gift was targeted specifically for Mechanical Engineering and Electrical and Computer Engineering, with the ultimate goal of equipping a multi-disciplinary senior capstone design project laboratory in mechatronics. As an intermediate step toward that final goal, each department was able to enhance their own laboratories using the equipment gift. The ECE department chose to use the donation to develop a modern motors and drives laboratory to supplement the existing motors lab. The Rockwell equipment was well suited for such a deployment. Since most laboratories have approximately 10 to 15 students per section, we decided to create 5 workstations, with a sixth station as a development prototype and a spare. The original machines lab at Clemson was housed in the basement of our 1927-vintage Riggs Hall. At that time, the basement was the logical place to locate the large motors due to proximity to three-phase power, noise and vibration, and access to the loading dock. In the ensuing years, the large motors were ultimately replaced by the small Lab- Volt brand workstations, but the location of the lab remained the same. Recently, we had an opportunity to move the laboratory to a location on the main teaching floor of Riggs Hall. During the past year, the lab space was renovated to remove an existing senior project lab and house the new power lab. Three-phase 208V power was installed, with twist-lock outlets located around the periphery of the room and at a central location. The facility is highly visible to ECE students and now houses both the older Lab-Volt workstations and the new Rockwell lab stations. It is perhaps the finest teaching lab in the department. IV. The Workstation Details The purpose of the workstations is to enable students to explore the operation of ac and dc motors when fed from adjustable speed drives. The workstations contain ac and dc motors, ac and dc drives, measuring instruments, data acquisition and control, a computer and laser printer, and a digital oscilloscope. Because of the equipment grant, the workstations utilize Reliance Electric and Dodge components almost exclusively. Third party vendors, such as National Instruments, Tektronics, and Eaton/Lebow, were used where comparable components were not available from Rockwell. If the workstations were to be duplicated, equipment from a variety of vendors could be used with similar functionality. Clemson University faculty, undergraduates, and graduate students have designed and constructed these workstations, developed a solid understanding of how they operate and what can be done with them, and are presently debugging the systems and creating laboratory experiments. An electrical engineering Master’s degree student has been instrumental in bringing this project together and wrote an MS thesis on the subject. Another MS student is now working on the completion of the project and the development of the experiments. An overview of the basic functionality of the workstation is as follows. A block diagram of the system is shown in Figure 1. Two small (1 hp) electric motors, one three-phase ac induction motor and one dc motor, are coupled together via a clutch and a rotary torque sensor. The torque sensor also contains a pulse encoder. Mechanical torque and speed are measured using the torque sensor. A dc tach generator is also connected to the system to give an analog speed signal. The induction motor can be line-connected or driven from a variable frequency drive. The dc motor is driven from an adjustable speed dc drive. The dc drive is a thyristor-based, four-quadrant device and is line-regenerative. The ac drive is a 2-quadrant drive and is not capable of regeneration. The drive can be operated in a volts/hertz or vector control mode. Dynamic braking resistors have been added to the ac drive to provide 4-quadrant functionality. Therefore, either machine can be operated as a controllable load (generator) while the opposite machine is used as a motor. The entire workstation is interfaced to a computer using a data acquisition card. The data acquisition and control is performed using National Instruments LabView. The drives’ analog and digital I/O ports are connected to the PC, which acts as a master controller (much like a PLC would operate an industrial system). Therefore, the speed and torque of the motor and the load can be controlled via the LabView Human-Machine interface (HMI) that we designed specifically for use with the workstation. Additionally, electrical parameters (voltages and currents) at various locations on the workstation, as well as torque and speed, can be acquired by LabView. LabView displays these measurements on the computer screen (similar to an oscilloscope) as well as stores the sampled data. Since the computers are on the university’s network, this data can be uploaded to a website or emailed for use by the students outside of the laboratory. 208V Three-Phase Supply Resistive Braking Module Reliance Computer Reliance FlexPak 3000 Control (PLC), GV3000 AC Regenerative HMI, and Data Adjustable DC Adjustable Aquisition Speed Drive Speed Drive using LabView (non-regen) Eaton/Lebow Speed Tacho- Clutch Torque Encoder meter 1.0 hp 1.0 hp Sensor (integrated into Reliance DC torque sensor) Reliance AC Motor with 3 φ Induction Shunt Field Motor Figure 1. Block Diagram of the Workstation. V. Features of the Workstations In this section, some photographs and descriptions of some of the salient features of the workstations are provided. Figure 2 shows an overall view of the workstation, with the drives mounted on the backplane, the motors, clutch, torque sensor and tachometers mounted at the bottom. The computer and printer are located just beneath the countertop, and a LCD monitor on a pivoting arm is mounted to the backplane. Much of the wiring has been placed behind lexan covers so that students can see how the components are interconnected. Safety sockets have been used so that students can connect to these wires with modern “banana” plugs to measure voltages without the risk of contacting exposed conductors. Voltage isolators are used to connect the voltage probes to the data acquisition system. This eliminates the chance of ground loops and other short circuits as well as overvoltage to the input ports. Loops have been brought out at various locations so that students can clamp current transformers to measure currents. DC Drive AC Drive LabView Display Braking Modules Probe Interface Laser Printer Computer DC Motor Torque Clutch AC Motor Transducer Figure 2. A Rockwell Automation Motor Drive Workstation. This workstation consists of a Reliance Flexpak 3000 four-quadrant dc drive, a Reliance GV 3000 ac drive with dynamic braking, a Dell computer using National Instruments data acquisition running LabView, and a Reliance three-phase AC induction motor coupled to a Reliance DC Motor via a Dodge Clutch and Eaton/Lebow torque sensor. A pulse speed encoder and dc tach generator provide speed information. Tektronix voltage isolators and Extech hall effect current probes provide electrical signals to the data acquisition (DAQ) card. A custom-designed input/output interface between the instrumentation and the DAQ card provides overvoltage protection. A Tektronix digital oscilloscope and Fluke multimeter are available for conventional measurements. A laser printer is connected to the computer and oscilloscope for local hardcopies. The computer is connected to the campus network. Lexan Safety Shield Regenerative DC Drive Loops for Current Probe Connections Moveable Current Probe Sockets for Voltage Probe Connections Digital Oscilloscope Figure 3. Details of the DC Drive portion. The left side of the workstation contains the Flexpak 3000 four-quadrant dc drive and access to the input and output electrical wiring. The wiring is behind a lexan cover so that students can see the actual wiring but can only access the live parts via safety plugs mounted on the lexan. Optically isolated voltage probes and current transformers enable students to safely measure voltages and currents at a variety of locations on the digital oscilloscope or on the computer. The dc drive’s input voltage and current on all three phases, the armature voltage and current, and the field voltage and current are accessible for measurement. The speed and torque of the dc motor are also available via transducers. Braking Resistors AC Drive Current Probe Power and Control Connections and Relays Isolated Voltage Probe Figure 4. Details of the AC Drive portion. The right side of the workstation contains the GV 3000 ac drive and access to the input and output electrical wiring. As shown previously, the wiring is behind a lexan cover so that students can see the actual wiring but can only access the live parts via safety plugs mounted on the lexan. The ac drive’s input and output voltage and currents, the dc link voltage, and dynamic braking resistor voltage and current are available for measurement, in addition to the motor’s mechanical parameters. Torque DC Tach Clutch Sensor & Encoder Figure 5. Details of the Motors. The motors are mounted on the bottom of the workstation behind Lexan safety shields to prevent inadvertent contact with rotating parts. The Reliance DC motor is on the left and the Reliance AC induction motor is on the right, with a Dodge Clutch connected between the DC motor and the Eaton/Lebow torque sensor. A Reliance dc tach is mounted to the dc motor and a pulse encoder is contained in the torque sensor. The clutch enables easy no-load testing of the motors and is controlled from the HMI on the computer. Figure 6. HMI Display, showing part of the LabView interface. This screen shows the start, stop, jog, and speed control for the two drives and the oscilloscope shot of the GV 3000 ac drive’s output voltage and current. The data acquisition card can accept 4 channels of analog input and students can choose to connect the voltage and/or current probes to any location on the system for display or data capture. The digital data shown on the graphs can be exported to files for students to use away from the lab. VI. Sample Experiments We are presently developing experiments for students to use with the workstations. During the Spring of 2004, the workstations will be integrated into the ECE 419 Power Electronics and Drives class. The plan is to use the workstations for 4 exercises (outside of normal class-time). These exercises will include: dc drive operation and control, ac drive operation and control, regenerative and dynamic braking, and drive control systems. Initially, we envision the labs being used to primarily demonstrate concepts that have been taught in class and to enable students to explore the drives’ operation. The figures that follow demonstrate some of the items that could be explored. Legend: • Red: AC Input Line Voltage • Green: AC Input Line Current • Blue: PWM Output Voltage of AC Drive • Magenta: AC Motor Current Figure 7. Normal ac drive input and output waveforms. This image was captured from the workstation and shows the input voltage and current waveforms and the output voltage and current waveforms. Concepts such as harmonics could be explored from the data. Legend: • Red: AC Input Line Voltage • Green: AC Input Line Current • Blue: Output Armature Voltage of DC Drive • Magenta: DC Motor Armature Current Figure 8. Normal dc drive input and output waveforms. This image was captured from the workstation and shows the input voltage and current waveforms and the output voltage and current waveforms from the drive. While cluttered, it demonstrates the 4- channel capability of the data acquisition system. Students can choose these or many other parameters, such as field voltage, torque, speed, etc. Switching frequency 8KHz Switching frequency 2KHz 0.6 0.8 AB line voltage (1V/500V) 0.6 AN line current (.1V/A) 0.4 0.4 0.2 0.2 Volts Volts 0.0 0.0 0.015 0.02 0.010 0.025 -0.2 -0.2 -0.4 -0.4 -0.6 -0.6 -0.8 Time (seconds) Time (seconds) Figure 9. Switching frequency effects. These graphs show the ac drive’s output voltage when the switching frequency has been changed from 8 kHz to 2 kHz. The students can see the effect of this change on the voltage PWM waveform and the increase of ripple in the motor’s current waveform, and they can audibly hear the change from the motor and drive. DC Link Voltage 0.9 0.8 0.7 DC Link Volts (pu) 0.6 0.5 0.4 0.3 Resistor Voltage 0.2 0.1 0 -0.1 0 0.01 0.02 0.03 0.04 0.05 0.06 Time (sec) Figure 10. Dynamic Braking Resistor Operation. This figure shows the voltage and current waveforms captured from the dynamic braking resistor when the ac drive is operated in a regenerative mode (being driven by the dc motor). Students can observe changes in this waveform as the loading is increased, and compute the amount of regenerated power, among other things. VII. Conclusion The Rockwell Automation laboratory will provide an excellent opportunity for our students to learn about modern motor control and power electronic applications in a safe and flexible environment. The workstations offer many possibilities for experimentation on motors and drives similar to those that students will encounter in “the real world.” We are just beginning to incorporate these workstations into our classes. We do not anticipate that these workstations will replace the older motor and generator workstations, but they will enable students to learn about the next step in the evolution of modern rotating machinery. Already, we have seen a change in enrollment in our senior-level power electronics and drives class due to these workstations. The mere visibility of these modern platforms, and the digital oscilloscopes, printers, etc., have made students excited about using them. It is our hope that this excitement will go beyond the initial “look through the window” excitement. Indeed, we hope that students will become more interested in modern power engineering, power electronics and drives, and find that power engineering is much more than ugly wires hanging from the poles. VIII. Acknowledgements Special thanks to Mr. Joseph Swann, Joe Razum and others at Rockwell Automation for their generous gift. Thanks to Dr. John Gowdy for providing departmental support for the laboratory space and expenses involved in peripheral equipment, construction of the workstations, and development activities. And, without the assistance of two fine graduate students, Adam Baier and Ryan Yocco, this project would not have been possible. My sincerest thanks to both of you for dedication “above and beyond” the call of duty.
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