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Study Unit Controlling Industrial Motors Study Unit Controlling Industrial Motors By Robert L. Cecci Technical Writer There are many different types of electric motors used in today’s industrial plants. A machine that’s used to mill slots in a metal part will use stepper, DC servo, or brushless motors to drive the cutting tool. A conveyor system in the same plant P r e v i e w may control the speed of that conveyor with an AC motor that’s P r e v i e w controlled by a frequency inverter. With the wide range of controller manufacturers and systems used in industry, it would be impossible to cover every type of controller in this text. However, this text will study the operation, electrical connections, and troubleshooting of generic controllers in detail, allowing you to easily translate your knowledge to most motor controllers used today. When you complete this study unit, you’ll be able to • Explain how stepper motors operate and how they’re electronically controlled • List the steps used to troubleshoot stepper motors and controllers • Define how an AC motor rotates in synchronous speed to the AC line frequency • Explain how a frequency inverter can alter the three- phase output frequency and thereby control motor speed • Identify proper troubleshooting procedures to use when working on AC inverter systems • Describe how pulse width modulation is used to control a servo motor and how to find the causes of servo sys- tem problems such as inaccuracy and oscillation • Explain how a brushless motor operates and how the controller commutates the motor to provide a precise positioning of the motor’s shaft • List the steps to use when troubleshooting brushless motor and controller systems Remember to regularly check your student portal. Your instructor may post additional resources that you can access to enhance your learn- ing experience. iii INTRODUCTION TO CONTROL SYSTEM BASICS 1 The Mechanical Movement Components The Controller and Motor Drivers C o n t e n t s C o n t e n t s Control Loops Controller System Packaging Other Functions of Controllers STEPPER MOTOR CONTROLLERS 13 Stepper Motors Unipolar Stepper Motors Bipolar Stepper Motors Unipolar and Bipolar Stepper Motor Control Systems CNC Controllers Troubleshooting Stepper Motor Controllers DC SERVO MOTOR CONTROLLERS 28 Motor and System Basics Control System Basics DC Servo Driver Adjustments The H-Bridge Driver An Example Servo System Troubleshooting DC Servo Motor Systems BRUSHLESS DC MOTOR CONTROLLERS 43 Controller Basics Motor Commutation The Brushless Servo Motor Brushless Motor Commutation The Brushless Motor Controller Brushless Driver Adjustments Troubleshooting Brushless Motor Driver Systems AC FREQUENCY INVERTERS 60 Frequency Inverter Basics Principles of Operation Frequency Inverter Parameters Frequency Inverter Circuits Troubleshooting Frequency Inverters POWER CHECK ANSWERS 83 v Controlling Industrial Motors INTRODUCTION TO CONTROL SYSTEM BASICS As long as there’s been a need to move an object for a manu- facturing process, there’s been a need to control that motion. Before electrical and electronic controls became popular, the machine’s operator would manually move the handles of the machine to provide the necessary motion. The dials near the handles of the machine identified the distances that were moved. For more accuracy, dial indicators were mounted on stops and the operator turned the machine’s handles until the proper numbers appeared on the indicators, displaying that the machine moved the proper amount. In either case, the accuracy of the movement of the machine was entirely dependent upon the skill of the operator, and a skilled operator could produce parts with a high degree of accuracy. However this process is very time consuming and the repeata- bility of each dimension from part to part is always in question when the part is made manually. The first generation control systems were called indexers. A typical indexer is shown in Figure 1. The indexer has a front panel with five or six dials that are turned to define how far the machine is to move. For exam- ple, if you wanted a 2.035 inch move from right to left, you would turn the dials to 0, 2, 0, 3, and 5 and set the direction to positive and press the X axis button. The stepper motor on the X axis would turn enough times to move the machine 2.035 inches. Each movement of the machine had to be dialed in by hand, allowing for dial placement errors and 1 FIGURE 1—Early industrial motor controllers were called indexers. Imagine having to dial in each move of a complex program for a complicated part. therefore positioning errors on the manufactured part. This was a time-consuming process that did, however, produce very repeatable parts. The next improvement in control systems used an indexer- style controller. But instead of turning the dials on the front of the controller, the “program” was fed to the indexer by means of punched paper tape. A small section of punched paper tape is shown in Figure 2. UP TO SPROCKET 8 HOLES WIDE HOLES (8 BITS) FIGURE 2—Paper tape was punched with holes to code a motion-control program. This method is very similar to the punched paper card systems that were used by early computers. The equally-spaced holes near the center of the tape were for the drive sprocket of the paper tape reader that was driven by a small stepper motor. The other holes in the tape were 2 Controlling Industrial Motors used for the information such as “X ϩ 2.035” or “Z Ϫ 4.028.” This system worked well for its time. However, any program changes required that a new tape be punched and tested in the machine. The tapes also sometimes ripped or were caught in the reader or the reader itself would fail. Some readers used fine spring steel wires that made contact to a bottom- grounded plate to read each hole in the tape. Paper fibers would often keep the fingers from properly reading a hole, even when one was there. Some readers used optical light sources and phototransistors or solar cells. Paper fibers often blocked these too. By the time technology had surpassed the usefulness of the common indexer, paper tape had evolved to a polyester tape, a tape without the iron oxide coating similar to the base material used in today’s VCR or cassette tapes. The introduction, development, and widespread use of the microprocessor was the next main improvement in industrial control systems. A microprocessor offered many advantages to the indexer-type controller. First, the manufacturing pro- gram could be stored in the computer’s memory instead of on a paper tape. Early systems used audio cassette tapes for program storage. Later, the programs could be stored on floppy disks at the controller or at an off-line programmer. Second, on-line changes could be made in the computer pro- gram without the need to load a complete new program on tape or disk. Third, the computer could be preprogrammed with special “canned cycles,” such as a peck-drilling cycle or a bolt hole pattern cycle that made system programming very simple. The first microprocessors used were the 6502 and the Z80 series. Today’s industrial control systems follow current trends of home personal computers and use the latest microprocessor chips. These powerful microprocessors offer such great speed and processing power that the control systems include fea- tures that were once only dreamed of by system developers. Motor and motor driver technology has also improved right along with the controller technology. At first, stepper motors were used for precise positioning of systems. These motors soon gave way to DC servo motors that were controlled by an analog signal from the computer controller. Next the use of digital drive control came about due to the precise “tuning of Controlling Industrial Motors 3 the drives” required for analog control and due to the thermal drift of the drives and the controller. Now the use of brush- less motors and digital drive and controller packages is standard in all new industrial motor control systems. Let’s begin our study of industrial motion control systems by looking at the components of these systems. The Mechanical Movement Components To precisely move an industrial machine, the drive motor of the control system must be coupled to a mechanical system on the machine. Figure 3 displays a typical mechanical sys- tem that’s often used to provide linear or straight-line motion. The table, in most cases, is the work surface of the industrial machine. In a boring, drilling, or milling machine, this is the table on which a vise or fixture is mounted. The table is free to move back and forth on precision ground rods or specially TABLE ROD NUT LEAD BALL BUSHING SCREW SHORT SIDE VIEW TABLE MOTOR ROD BASE NUT BALL LEAD SCREW BUSHING LONG SIDE VIEW FIGURE 3—A mechanical movement can be precisely positioned by using an assembly such as the one shown here. 4 Controlling Industrial Motors shaped and ground ways. Rods will support the table on special linear bearings called ball bushings. Ground ways allow for metal-to-metal surface contact with an adjustable device known as a gib between the way surfaces of the table and the base. This gib can be adjusted to take up for way surface wear. A lubricant known as way lube is usually pumped under low pressure to these surfaces to prevent excess friction and way surface wear. Beneath the table or movement is a set of devices known as a lead screw and nut. There are two types of lead screws and nuts.
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