DC Motor Drive System Control Methods

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Unformatted text preview: Motors&Drives.book Page 231 Monday, January 10, 2005 1:54 PM Chapter 6: Drive System Control Methods 231 Drive System Control Methods Introduction Up to this point, basic drive theory, component hardware, and interface devices have been discussed. It is now time to put the basics to work to develop a drive system. The following information will help tie the components together into a coordinated control system. All systems configurations would be closed loop because of the precise speed and torque regulation required. In actuality, there would be many more pieces to the system “puzzle” than what is presented here. However, this section is meant to present a general outline of drive systems and how the pieces work together in an automated environment. Coordinated Drive Systems It is helpful to start with what could be considered a “simple” system and move to the more complex. Figure 6-1 indicates one such simple closedloop system. As seen in Figure 6-1, this is a “widget” manufacturing facility. This section is the “finishing” section of the system, with proximity sensors strategically placed along the out-feed conveyor. All of the sensors are connected to an amplifier unit that sends contact signals to the drive. The drive needs to know where the widget is at all stages of the system. Therefore, the job of the proximity sensor is to send a contact closure signal to the drive. This would be considered a digital input (DI). In this case, the drive does not need to know how big, or how long the widget is, just that fact that it has arrived at a particular station on the conveyor. Once the drive has determined the widget has finished all processes, it can then send a relay output signal to the warehouse, alerting that the widget is on its way. Motors&Drives.book Page 232 Monday, January 10, 2005 1:54 PM 232 Motors and Drives Proximity Sensors Input to Control System Amp Proximity Sensor Widget Labeling Sorting To Warehouse Speed Ref Drive M Figure 6-1. Sensor control system In this case, the operator sends a speed reference to the drive. The drive operates the motor at that speed until the widget reaches the first proximity sensor. At that point, the contact closure signal indicates the widget has arrived at the labeling section. The drive takes the contact closure and operates at a preset speed 1 (slower speed), so the label section has time to perform its function. Once the widget exits the label section, another proximity sensor “opens” the preset speed contact, and the conveyor returns to normal speed. The process is repeated when the widget enters the sorting section. At this point, another digital input is closed, which signals the drive to switch to preset speed 2. At the output of the sorting section, another proximity sensor closes. This indicates that the widget is ready for the warehouse, and the drive returns the conveyor to normal speed. This is a somewhat crude system but could be considered a coordinated system. The drive could be either DC or AC, along with the corresponding motor. Similar configurations would be seen on packaging systems, food processing systems, and any application where process speed may differ from part movement speed. Later in this chapter, a view of a more automated system will be presented. Figure 6-2 indicates a simple automatic AC drive pumping system. The heart of this system is proportional integral derivative (PID) control. In Figure 6-2, the high level is set for 10 feet and the low level (danger) is set for 2 feet. This automatic pumping system would be set to have a constant level of 10 feet at all times. If the actual value (level) is less than 10 feet, the drive responds to the corresponding “error” signal and activates the pump at a designated speed. In this case, the level is only at 90%, with a feedback voltage of –9 V, indicating a 1-V error and a corresponding increase in speed. Once the error is at zero (feedback at –10 V), the drive slows to zero speed, meaning no pumping action is required. Motors&Drives.book Page 233 Monday, January 10, 2005 1:54 PM Chapter 6: Drive System Control Methods — Coordinated Drive Systems 233 Drive Feedback information M Input P Surface level measurement Specifications: High: 10 feet (10V) Low: 2 feet (2V) Feedback voltage: -0 to 10VDC Actual values: Percent of total: 90% Feedback Voltage: -9V Output Figure 6-2. Automated pumping application—PID If this were a duplex pumping system, the drive could be programmed to bring online a fixed-speed “lag” pump, if the demand required it. Several of the latest AC drives on the market include the software intelligence to operate this type of system automatically. Relay outputs can be programmed for a low-level pump start and a high-level pump stop. In addition, a sleep function can be set when the drive stops pumping. The sleep function would cause the drive to release the start command but keep the microprocessor alive, waiting for the next pumping cycle. This function saves additional energy, since the IGBTs are off, as well as any other power electronics circuits. In Figure 6-2, when the level reached a “critical” stage (2-foot level), the drive could be programmed to take emergency action, such as sound an alarm. Some type of level sensor would be set at the 2-foot level. It would send a contact closure to a digital input that would be programmed for an emergency function (e.g., sound an alarm, engage full speed pumping, etc.). The sensor could also send a contact closure to a programmable logic controller (PLC), if that’s what was controlling the application. In Chapter 5, information on PID control was presented. Figure 6-3 is a graphic representation of PID system response and how it might be programmed for the drive shown in Figure 6-2. Though indicating meters instead of feet, the graph indicates the response of the drive to a feedback error signal. If the drive was programmed as seen in the graph, it would take about 1 s for the drive/pump to stabilize. A combination of gain, integration time, and derivative rate are needed to effectively tune this pumping system. By looking at the graph, the drive gain may be set too high, causing the pump to overspeed (overshoot) the desired level. An oscillation would occur until the system stabilized in about 1 s. The integration time would also need review, since the drive may Motors&Drives.book Page 234 Monday, January 10, 2005 1:54 PM 234 Motors and Drives Closed - Loop Step: Kp=300 1.4 1.2 Displacement (m) 1 0.8 0.6 0.4 0.2 0 0.5 1 Time (sec) 1.5 2 0 Figure 6-3. PID control—system response be set to achieve the desired level too quickly for the conditions that exist. The derivative function could also be reviewed, since it dictates how the amount of error is to be corrected per unit time (i.e., 10% correction per second). Figure 6-4 indicates a device called a scanner used to check the quality of output from a paper machine. The scanner would check paper-quality items such as thickness, coloration, moisture content, surface smoothness, and fiber content. (Top View) (Front View) Upper Head Scanner Head Paper Web Lower Head Paper Web Feed Direction Figure 6-4. Paper machine scanner system As one unit, the scanner could be considered a sensor that feeds vital information back to the main system control. The scanner would be the input device to the main control, which would signal the drives to change speed- or torque-control output to correct for errors in quality. Motors&Drives.book Page 235 Monday, January 10, 2005 1:54 PM Chapter 6: Drive System Control Methods — Coordinated Drive Systems 235 Included in the scanner assembly would be a small AC drive that controls the scanner head. The scanner head-speed rate (back and forth motion) would be controlled by the small AC drive (as low as 1 HP). The upper and lower head are connected through mechanical linkages that cause both pieces to move together. The transmitting devices would be in the upper head, and the receiving sensors would be in the lower head, for example. Tension Control Before the web of paper arrived at this point, a series of coordinated devices would be in action. Paper, plastic, foil, or any other type of web system would require the use of tension control. Figure 6-5 illustrates a basic tension-control system. Tension Control Load Cell Master Speed Ramped Reference Draw Control (Dr. 1) Tension Control (Dr. 2) Tension Feedback Tension Set Point Figure 6-5. Tension-control characteristics The tension control system shown Figure 6-5 would be part of a webwinding system, just ahead of a winder unit. The web material is fed through the in-feed rollers, under the tension control load cell and out through the out-feed rollers. The purpose of tension regulation is to control the surface web tension as material is wound. The same would be true if the material was being unwound. Tension regulation is achieved by using load cell tension feedback. A load cell is a pressure-sensitive sensor that decreases in resistance as pressure on the cell increases. Differences between the tension set point and the load-cell feedback allow the tension PI regulator (proportional integral) to develop the error correction needed to maintain tension by trimming either speed or torque of the driven section. The tension PI regulator is updated in millisecond timeframes to produce very responsive trim (fine tuning). The proportional gain of the tension regulator can be adapted to accommodate changes in roll diameter. Motors&Drives.book Page 236 Monday, January 10, 2005 1:54 PM 236 Motors and Drives As the diameter of the roll increases, the proportional gain can be set to increase from a minimum gain at core diameter, to a maximum gain at full roll. A winder application is operated in torque control. The torque-control drive will operate at any speed necessary, as long as the torque (tension) of the web is satisfied. When operating in torque control an encoder is typically required. When using this type of control, accurate web material information is required for calculating the WK² torque value of the roll. These calculations will vary depending on the density and tensile strength of materials. If tuned properly, torque control may result in stable steady-state performance. Dancer control is similar to tension control. The dancer unit changes output resistance according to the tension on the web that is wrapped over the dancer roll. The changes in resistance and corresponding voltage change are fed back to the drive so proper torque control adjustments can be made. Figure 6-6 shows a dancer-control scheme. Master Speed Reference Dancer Position Pot. M Ramped Ref. Draw Control (Dr. 1) Dancer Control (Dr. 2) M Figure 6-6. Dancer position (tension) control The purpose of dancer regulation is to control the surface-web tension as material is wound or unwound. This is done by monitoring the position of the dancer feedback device, similar to a variable resistor or pot. The dancer is loaded with web material from the in-feed rolls or from another area of the system. The load on the dancer is monitored with the feedback sent to the dancer control drive. The output of the drive is automatically adjusted to achieve desired web tension at the out-feed rolls of the system. Web tension variations are absorbed by the dancer and cause the position of the dancer to change. The difference in dancer position feedback and the dancer position set point allow the dancer PI regulator (drive 2) to develop the error correction needed to return the dancer to the set point position by trimming the speed of the section. The proportional gain of the dancer regulator can be adapted for roll diameter changes. As the diameter of the roll increases, the proportional gain can be set to increase from a minimum gain at core diameter to a maximum gain at full roll. The set point position for the dancer is defaulted to the center of the Motors&Drives.book Page 237 Monday, January 10, 2005 1:54 PM Chapter 6: Drive System Control Methods — Coordinated Drive Systems 237 total dancer movement. The regulation position of the dancer can be adjusted by the operator by way of a drive parameter. The dancer PI regulator is updated in millisecond timeframes to produce very responsive trim (fine tuning). This allows for very stable dancer position control over the entire speed range. Remote Operator Interface In many cases, the actual drive must be located at a distance from the motor and driven machine. This is because the drive must reside in a somewhat clean atmosphere, free from dust particles and other contaminants that may be present in the factory. Also, in many cases, the operator needs to be close to the application to verify system operation or to make machine adjustments under safe conditions. For these reasons, some type of remote-control capability is more often than not necessary. The simplest remote operator device would be a remote start/stop pushbutton, along with an analog speed pot. A step up from the simplest devices would be an operator console, with pushbuttons for preset speeds, jog, forward/reverse, drive reset, speed increase, and speed decrease. It could be as elaborate as pilot lights to indicate each production stage, status indicators, and a full-color monitor and touch screen to enable the complete system. For purposes of this text, the focus will be on standard hardwired control devices, followed by automated controls with serial and fiber optic communications. Figure 6-7 indicates a remote operator station and an operator console. START FAULT RES SPEED STOP E-STOP Figure 6-7. Remote operator controls Any drive, AC or DC, needs two items to be satisfied—start command and speed reference. These remote devices would be wired into the drive’s digital input and analog input terminals. Additional controls may be needed Motors&Drives.book Page 238 Monday, January 10, 2005 1:54 PM 238 Motors and Drives such as the ones listed above, but basic drive operation would require start and speed command signals. Most drives have a control terminal block where standard I/O is connected. In addition, terminals or a removable connector or terminal block may be available for future connection to serial communications. Figure 68 indicates standard drive I/O connections. RS-485 Serial Comm’s X3 1 5 Analog Inputs (0 - 10VDC) or (4 - 20 ma) Analog Output X1 1 8 9 Digital Inputs 16 Relay Output 1 (SPDT) 17 19 20 22 Relay Output 2 (SPDT) Figure 6-8. Standard drive I/O connections As shown in Figure 6-8, in many cases the control connections are easily identified and are laid out on the control board in a logical manner in a sequential number scheme. The analog input circuit must be matched with the signal type that is connected. In many cases, small terminal jumpers function to match either voltage-reference or current-reference inputs. Some drives include a DIP switch or rocker switch for signal matching. To function properly, this matching jumper or switch must be located in the proper location. If it is not, unstable control could result, or in the worst case, the drive would operate at maximum speed, with no speed control. The user’s manual and the silk screen indication on the board are the likely places to find the correct settings. Motors&Drives.book Page 239 Monday, January 10, 2005 1:54 PM Chapter 6: Drive System Control Methods — Coordinated Drive Systems 239 The analog output is normally a connection for an external analog meter. This meter could be a separately purchased and mounted device or an item included on an operator console. The normal output would be 0–20 mA and scalable to many types of values. Digital inputs receive their name from the fact that the input is either on or off. Some drives operate on 12 VDC logic, some on 24 VDC, and still others use a 120 VAC interface option. Typically all the drive needs to see at a digital input terminal is control voltage, which is accomplished through a contact closure (a manual switch, limit switch, auxiliary contact, etc.). Once the drive sees the control-logic voltage at the terminal, the drive performs the operation connected with that digital input (i.e., start, stop, preset speed, reverse, etc.). When control voltage is removed from that terminal, the function stops. A word about control logic would be appropriate here. Many drives use what is called source control. That is, the terminals on the control board must see a control voltage before a function occurs. In other words, voltage must be sourced to the drive. Some drive manufacturers term this control “PNP” logic, in reference to the transistor regulator types involved with the control logic inside the drive. Figure 6-9 illustrates this type of logic control. In source logic, all of the circuit commons are tied together. The circuit is complete when the control voltage is applied to an appropriate DI. The converse logic control is termed sink control. In this type of control, the logic voltage is actually tied to circuit common. The control logic voltage sinks to circuit common. When a contact closure is made with a DI, a circuit is closed between the terminal and circuit ground. The internal control logic energizes the function. This type of control is required by some PLC controllers, where TTL (Transistor-Transistor Logic) outputs, or external voltage sources, must be applied for control. This is also used where circuit ground is something other than earth ground. Figure 6-10 shows this type of control logic. If there is a possibility for an unsafe condition to exist, it would be in the fact that inadvertent grounding (connecting to common) of an external DI switch or contact would cause a function to occur. If for some reason the start switch was connected to ground during routine maintenance of the drive, the drive would accidentally start. Many codes or industrial control schemes require a positive voltage at the terminal block before any operation occurs. In source logic, if the start switch were connected to ground, the logic voltage would be shorted to ground and the operation would stop. It should be noted that some drive manufacturers include a logic-control power supply on the control board. No external power source is needed, only a contact closure to engage a function. If an external source is required by the drive, care must be taken to ensure that polarity and Motors&Drives.book Page 240 Monday, January 10, 2005 1:54 PM 240 Motors and Drives SOURCE Logic (PNP) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 SCR Al1 AGND +10V A12 AGND AO1 AGND +24 V DCOM1 Dl1 Dl2 Dl3 Dl4 Dl5 DCOM2 RO1C RO1A RO1B RO2C RO2A RO2B J1 Analogue Inputs Al1 Al2 Al1: 0-10V Al2: 0-10V Figure 6-9. Source control logic connections grounding of the external source matches that of the drive. Ground loops and voltage mismatch can cause many aggravating situations, especially at the low voltage or current values being used. Many drives include several relay outputs as external-control contacts. These contacts may be dry (not carrying a voltage) or Form C contacts. They can be programmed for a multitude of operations. For example, the contacts may be programmed to close when a preset speed is selected, or a set speed is reached, or reverse is commanded. In addition, some drives have the capability of monitoring any type of read only information using the relay outputs. Some manufacturers call this the supervision function. Quantities such as current, hertz, output voltage, DC bus voltage, and calculated torque can be internally connected to a relay output. When a programmed value is obtained, or exceeded, the relay would change state or energize. The SPDT (Single Pole, Double Throw) contact can be wired into an indicator light, an alarm circuit, or fed back to a system controller like a PLC. With this type of function, the drive can actually be used as a monitoring tool. It has Motors&Drives.book Page 241 Monday, January 10, 2005 1:54 PM Chapter 6: Drive System Control Methods — Coordinated Drive Systems 241 SINK Logic (NPN) 1 2 3 4 0.20 mA 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 SCR Al1 AGND +10V A12 AGND AO1 AGND +24 V DCOM1 Dl1 Dl2 Dl3 Dl4 Dl5 DCOM2 RO1C RO1A RO1B RO2C RO2A RO2B J1 Analogue Inputs Al1: 0-10V Al1 Al2: 0(4)-20 mA Al2 Figure 6-10. Sink control logic connections the intelligence to compare values and make indications when values are not reached, when they are exceeded, or when they occur. Some manufacturers use digital outputs instead of relay outputs. A digital output would supply a voltage or current value to an external device, when a programmed value is met. Serial and Ethernet Communications During recent years, the push has been to automate many operations to improve product quality or to maximize the efficiency of the system. Communications is a necessary factor in automated systems. Generally speaking, the most common type of drive communication is through a serial link. In this scheme, the data is transmitted in a serial fashion (bits are transferred sequentially, one after the other). When talking about drive communications, three modes currently apply: serial, fiber optic, and Ethernet (intranet) communications. The drive control schemes using each of these modes will be explored, as well as their connections to higher level control systems. Motors&Drives.book Page 242 Monday, January 10, 2005 1:54 PM 242 Motors and Drives Since serial communications involves sequentially transmitted data, it would not be considered high-speed. Typical communication rates include 4800, 9600, and 19,200 baud (bits per second). This transmission speed is acceptable when communicating with an air-handling unit, which moves volumes of air in and out of the building. This speed would not be acceptable in a paper machine, where high-speed data transmission is critical to the success of paper density, coating, and composition. In many cases, this type of communication is ideal for 24-hour monitoring of drive operation. Figure 6-11 indicates this type of monitoring function. Computer Controller RS-232 / 485 Converter Serial Link connecting each drive to Computer Controller Figure 6-11. Serial communications setup In most cases, the drive serial link is an RS-485 configuration. With this type of connection, the total communication network length can be ~4000 feet. To accomplish this network matching, the computer must have an RS-485 output. If it does not, then a converter such as the one listed in Figure 6-11 would be needed. There are many different languages (protocols) available for industrial applications. The HVAC industry also has several protocols available specifically designed for air handling, cooling tower, chiller, and pumping applications. Several companies have been pioneers in the PLC market and have developed their own specific protocol. The drives pictured in Figure 6-11 have the same protocol as that available in the computer. If the protocol matches, then communication is possible after the computer and all drives are programmed to accept serial communications. In many cases, because of transmission speed and protocol limitations, the total number of drives on a serial link may be as high as 32. In practical terms, if communication speed is at issue, the total number of drives may Motors&Drives.book Page 243 Monday, January 10, 2005 1:54 PM Chapter 6: Drive System Control Methods — Coordinated Drive Systems 243 be around half of that. With a 9600 baud rate, the communication speed starts at 100 ms and would drop from that point, with every drive added to the network. Typical programming for the drive would include ID number, baud rate, protocol selection, fault function, and communication time out. The computer would also be programmed in a similar fashion to match the drive communication speed and inputs. Connection to a building’s Ethernet or network system is now becoming more popular. There are definite advantages to this type of communication connection. Information gathered from the drives can be easily downloaded to a mainframe network computer. Trends can be identified and analysis can quickly be done, as opposed to transferring data to an independent system and then transferring to the network. In addition, connecting to a building’s drive system can easily be accomplished though the Internet, which would be available through a modem connection. Several software companies offer software that allows the user to develop customized screens that indicate parts of the process. This would include drive operation, inputs to the system, and outputs from the system. Color graphics are fast becoming the interface media of choice. Figure 6-12 gives an example of this type of network/drive communications. Higher Level Host Server TCP / IP (Internet) Local Building Network Server Figure 6-12. Ethernet/drive communications The Ethernet or network communication speed is higher than that of the serial link. Though the network speed is high (in the low mega-baud range), that doesn’t necessarily indicate a rapid transfer of data from network to drive, or vice versa. Information from the network must be transmitted to the drive, and the drive’s microprocessor must be able to Motors&Drives.book Page 244 Monday, January 10, 2005 1:54 PM 244 Motors and Drives efficiently decode the information. With the drive’s slower serial link rate, only a limited amount of information can be handled per second. The processing speed of many drives would be in the 25- to 50-MHz range, with 2–5 MB of memory available. By today’s computer standards, this rate would be extremely slow. However, by today’s drive microprocessor standards, this speed is quite adequate for processing all internal drive information. As drive microprocessor technology improves, internal processing speed will also improve. When connecting the serial link to the drive, it is done in a “daisy chain” fashion. Figure 6-13 indicates this procedure. From Previous Drive Shield A B Common To Next Drive Shield Figure 6-13. Serial link connections When wiring the drives, care should be taken not to “daisy chain” the shields. Many drive manufacturers include termination resistors on the control board. These resistors reduce electrical noise on the communication network. Termination resistors are to be included on the first and last drive in the network. The inner drives are not to be terminated. (Too many resistors in-series with the communications devices may cause the network to malfunction.) Termination resistors are typically placed “in circuit” by moving a pair of jumpers into position across several contact points. Fiber-Optic Communications The use of fiber-optic communications has steadily increased over the last few years. Optical fibers or “light pipes” are highly immune to electrical Motors&Drives.book Page 245 Monday, January 10, 2005 1:54 PM Chapter 6: Drive System Control Methods — Coordinated Drive Systems 245 noise (EMI and RFI) and have the capability of transmitting data over long distances. Glass-constructed optical fibers, along with periodically placed amplifiers, would allow transmission over thousands of feet. The glasstype of optic fiber is quite costly compared with plastic. When comparing optical fiber with hard-wired control, the user must decide between high noise immunity and higher installation costs and lower immunity and lower installation costs. In addition to drive-to-control fiber-optic communication, an increased use of optical fibers is seen in modern drive circuitry. The advantages of fiber optics for building communications certainly holds true for internal drive control and communications. Figure 6-14 illustrates this type of internal communications. Main Control Board Ch Ch Ch Ch INT Input Control Board Option Module Option Module Interface Board Gate Driver I/O Control Board Serial Comm M Figure 6-14. Internal fiber-optic communications In Figure 6-14, the fiber-optic cables interconnect between the main control board and the other definite purpose boards. In addition to the internal communications, fiber optics can also be used to interface with monitor and programming software, as well as optional PLC modules. In a higher-level system, fiber optics is the normal. Its high speed (~4+ mega baud) communication makes this format ideally suited for industrial applications. Figure 6-15 shows an external drive interface and a PLC. As seen in Figure 6-15, the drives are connected by fiber optics in a “ring” structure. The fieldbus option module is purchased from the drive vendor. It changes the protocol of the PLC into a language that the drive can understand. Motors&Drives.book Page 246 Monday, January 10, 2005 1:54 PM 246 Motors and Drives PLC Protocol Fieldbus Option Module (purchased from Drive Vendor) (Fiber Optic Link) Drive Protocol Figure 6-15. Drive and PLC interface The only possible disadvantage to this type of communications is the fact that if one drive goes down on a fault, the entire communication network goes down with it. Programming is similar to that of serial communications. Each drive on the network needs a unique ID, as well as baud rate, communication fault function, and communication time out. When installing a fieldbus module to a network, care must be taken in connecting the transmit and receive fibers in the correct positions. Figure 616 illustrates this procedure. In Figure 6-16, output from the drive (TX) must be the input to the Fieldbus module (RX). The same holds true for the receiving optic cable. Also be sure that the cable plugs are completely seated into the receptacle by listening for the “snap” action. This allows for the maximum transfer of “light” data into the receptacle. DC Systems The basics of DC drives have already been covered. At this point, it would be helpful to review several DC applications and summarize the characteristics of each system. Printing Press Figure 6-17 is a simplified diagram of a printing press, using a DC motor and drive system. Motors&Drives.book Page 247 Monday, January 10, 2005 1:54 PM Chapter 6: Drive System Control Methods — DC Systems 247 Figure 6-16. Transmit and receive fiber-optic connections (Courtesy of ABB Inc.) Color 1 Inkwell Unwind Stand Color 2 Inkwell Color 3 Inkwell Color 4 Inkwell To Additional Process Accumulator Figure 6-17. DC drive system in a printing press DC drives have been traditionally used in applications that require high starting torque. Printing presses have long been a prime candidate for DC drives and motors. Figure 6-17 is a very simplified drawing of a four-color offset press. In an actual press, more rollers would be seen, as well as a variety of sensors, limit switches, and transducers. In some cases, the press may be connected to a common line shaft (i.e., meaning only one motor, connected to a long shaft; each ink station would have a drive shaft connected to the common shaft by a gear box). Each of the color stations impress a specific color onto the paper web. The key to the success of the press is the tension control and coordination between all of the stations. Each inkwell station and roller set is operated by an individual DC drive and motor. In many cases, the drive is located inside the machine, in a clean and dry cubicle with a constant stream of filtered air. Motors&Drives.book Page 248 Monday, January 10, 2005 1:54 PM 248 Motors and Drives The lower pinch roll is operated by the drive, which is similar to how a surface-driven winder would be controlled. The upper roll carries the ink plate with the specific color. The accumulator is available to take up any slack in the web, before it moves to the next process (i.e., coating, folding, cutting, bundling, etc.). This is a prime example of where high-speed fiber-optic communications is essential. Each drive needs to operate at a slightly faster speed than the previous drive. If that ratio of speed is done by each drive, the proper tension will be maintained on the web. If too much tension occurs, the web breaks and the machine must be stopped and re-threaded, which may take up to 1/2 hour to accomplish. If the web has too little tension, the web starts to bunch with the result being uneven ink transfer and wrinkled paper sent to the cutting process. In addition, the register could be off. (The synchronization of all colors means colors printing exactly where they are suppose to print. Off register printing leaves the “shadow” effect, with a blurry image.) If the printing system is “tuned” properly, all the operator has to do is operate the system speed, to increase or decrease production. The entire coordination is in the automated electronics, not in manual manipulation of torque, speed, tension, and ink coloration. As time goes on, some of the DC printing systems are being retrofitted with flux vector AC drive systems. As the technology of torque control with AC drives improves, this trend will continue for many years to come. Ski Lifts Another traditional DC drive system is that seen in ski lift units. These types of applications are also found at state fairs, theme parks, and any other location where above ground “people movers” are found. Figure 618 indicates a simplified version of a ski lift system. Motor D Figure 6-18. Ski lift system (chair lift) Motors&Drives.book Page 249 Monday, January 10, 2005 1:54 PM Chapter 6: Drive System Control Methods — DC Systems 249 Additional components are found in ski lift systems. Sensors, current limit devices, safety limit switches, and monitors are just a few of the additional items found in modern lift systems. Typically the drive is located inside a clean, dry, heated room, which may or may not be in the loading building. Quite often, the drive system is asked to start up at full torque, with rated capacity of people on board the chairs. In some cases, the entire system is manually controlled by an operator located in the loading building or a control station. The operator’s job is to observe the system and change speed if necessary. In other cases, the system is operated automatically from the control station (auto), or by remote, from another location—possibly at the drop-off point (hand). This is only one instance where hand/ auto control is possible. There are a variety of other applications where hand/auto functions are required for convenience of the operators. Figure 6-19 indicates a diagram of hand/auto control. Process Equip Variable Speed Proc Out 3 - Phases DC Dr DC Motor Voltage Frequency Hand Before Auto Before Hand Auto Start Fwd/Rew Start Fwd/Rew Process Processing and logic Internal to Drive Figure 6-19. Hand/auto control Another DC system application is found in material handling. Figure 6-20 shows a barge unloader application with coal being the material. Coal is delivered by ship and brought to the utility by barge, which can maneuver into tighter locations compared with a freighter. This application would be found at coal-fired power utility plants or at any factory where material is delivered by ship or barge. As seen in Figure 6-20, the barge, connected by cable to a DC motor, is pulled into position by a DC drive. Another cable, connected to the other end of the barge, would also be operated by a DC motor and drive (not shown in the figure). The function of that drive is to stabilize the barge and to control back tension. As the barge moves in the direction indicated, the other DC drive slowly moves the motor in the opposite direction, maintaining tension at all times. As the barge is slowly moving, the coal Motors&Drives.book Page 250 Monday, January 10, 2005 1:54 PM 250 Motors and Drives Coal Unload Movement Operator Coal Unloader Feed Conveyer Barge Movement Coal Barge Figure 6-20. Barge unloader application unloader rotates like a Ferris wheel, in the direction indicated. Using swivel-type scoops, coal is deposited onto the feed conveyor to be transported to the coal yard for storage or directly to the utility boiler system. The operator controls the process, manually moving the coal unloader up and down and the gantry back and forth, to achieve maximum removal. In this application, DC drives and motors work well for tension control of the coal barge. In this case, 50- to 100-HP DC motors can operate the application satisfactorily. The gantry motor and drive can also be DC or even AC and as low as 50 HP. Since precise torque or tension control is not required in the gantry system, a standard AC PWM drive will perform the functions required. Limit switches and joystick control are standard manual operator devices. (Note: Joysticks have a center off position. When pushed away from the operator, the drive speed is forward. When the joystick is pulled toward the operator, the drive speed is reversed.) This automated control comes where the two DC drives and motors have to slightly “fight” each other to provide adequate tension control. Therefore the DC drives have to coordinate in speed and torque reference to maintain the tension required. Too little torque could mean an unstable barge and coal not removed by the semi-automated process. Extra time and effort would be required to manually unload any remaining coal. There are many more DC applications that use torque, tension, and precise speed control throughout the process. The above are only a few, but they Motors&Drives.book Page 251 Monday, January 10, 2005 1:54 PM Chapter 6: Drive System Control Methods — AC Systems 251 do highlight the torque and tension capability of DC systems, as well as low-speed control. AC Systems The AC drive and motor system has gained acceptance in the coordinated system environment because of the improvements in power semiconductor technology. In addition, high-speed process control, microprocessor, and communications improvements make the AC drive look like another node on the process network. Steel and aluminum processing, converting lines, and paper machines all require precise speed and torque control. The basics of AC drives have already been covered. At this point, it would be helpful to review several AC applications and summarize the characteristics of each system. Many drive applications use multiple motors to provide coordinated control. In certain applications, one section of the machine may operate faster than commanded speed. In cases like these, and where overhauling loads are possible, a common DC bus configuration is able to regenerate energy back to the DC bus. That energy is then used by another inverter section to power a different section of the system. Figure 6-21 indicates a common DC bus configuration. Supply section Diode or Thyristor Drive sections Common DC BUS Braking sections Control Inverter Supply Unit Inverter Braking chopper PLC 230/115 VAC Transformer Resistor Drives Tools Monitoring Figure 6-21. Common DC bus configuration Motors&Drives.book Page 252 Monday, January 10, 2005 1:54 PM 252 Motors and Drives The supply section converts AC to DC through a fixed diode bridge rectifier. When horsepower is in the 1500- to 2000-HP range, the input converter section may be SCRs, to handle the high current. Reverse connected IGBT bridges may be used for full, four-quadrant regenerative braking. This scheme also has the capability of operating in a very low harmonic mode. Several manufacturers offer water-cooled units, which allow for increased sizes of drives, using diode bridge rectifiers, in smaller sizes than nonwater-cooled. The braking chopper is part of a DB, IGBT sensing circuit, that closes when a fast deceleration or stop is required. As in many cases with large horsepower, the controller unit is typically PLC mounted in a separate cabinet, along with other software and control devices. Coal Classifier As an AC coordinated system component, a coal classifier uses devices found in many control systems. This system is found in coal-fired power utility plants, but the principles are the same whether filtering coal, cement, sand, or any other medium. Figure 6-22 indicates this system. Figure 6-22. Coal classifier system The classifier is part of a much larger, coordinated system. The output of the boiler /steam generator is dependent on the quality and purity of the coal powder that is burned as fuel. Raw coal is loaded by conveyor into the hopper of the classifier. The classifier is operated by an AC motor and sifts through the coal particles, dropping the small pieces into the coal pulverizer. The output of the pulverizer is a fine coal dust that burns cleanly and evenly, with the highest BTU output possible. The powder is then forced into the combustion chamber, Motors&Drives.book Page 253 Monday, January 10, 2005 1:54 PM Chapter 6: Drive System Control Methods — AC Systems 253 where it is used to fire the boiler and create steam for the turbine generator. The classifier is operated as a closed-loop system. The plant operator controls the ultimate speed of the entire system. However, the classifier is part of that coordinated effort. A desired set point speed is entered at the operator console. The drive accepts that set point, and through PI control, looks at the actual speed feedback of the feed conveyor. The drive then makes speed corrections to power the classifier motor at the optimum speed. Too high of a speed would allow too many coal particles to enter the classifier, overloading the system. Too low of a speed would mean that few coal particles would enter the classifier and pulverizer. A lean burn would result in the combustion chamber, and BTU output would be reduced. By means of PI control, this section of the system would operate at peak efficiency. The pulverizer unit would have a similar coordinated scheme, set up in the controller software. HVAC Systems AC variable-frequency drives (VFDs) are well known for their energy-saving capabilities. The savings can be quite substantial, as indicated in the next figures. Assumptions: • Full rated flow = 178,000 CFM @ 3” of H2O • Fan/blower efficiency = 85% • Motor efficiency = 94% • Drive efficiency = 98% • Rated shaft power = 100 HP • Cost per KWH = $0.10 Figure 6-23 indicates an energy-use comparison of variable-speed AC drive use versus outlet damper control. Figure 6-24 shows fan efficiency improvement using variable speed compared with outlet damper control. Figure 6-25 shows the annual savings that a variable-speed fan can have compared with outlet damper control. It is clear that the amount of energy savings is substantial. The greatest savings are available when the fan is operated at 40–70% flow for the majority of the operating time. Savings can be also realized with VFDs versus inlet guide vanes. However, the highest savings will be realized using VFDs with a flow rate of only 30% for the majority of time. Even at that Motors&Drives.book Page 254 Monday, January 10, 2005 1:54 PM 254 Motors and Drives 800 700 600 Energy Usage (Mwh) 500 400 300 200 100 0 30% 40% 50% Variable Speed Damper 60% Flow 70% 80% 90% 100% Figure 6-23. Fan energy use (Courtesy of ABB Inc.) 70% 60% 50% Efficiency Improvement 40% 30% 20% 10% 0% 30% 40% 50% 60% 70% 80% 90% 100% (10%) Flow Figure 6-24. Fan efficiency improvement (Courtesy of ABB Inc.) flow rate, a savings of less than $25,000 annually is seen, about the same as an outlet-damper system and at the same flow rate. The system that enables the energy savings above is shown in Figure 6-26. Motors&Drives.book Page 255 Monday, January 10, 2005 1:54 PM Chapter 6: Drive System Control Methods — AC Systems 255 $35,000 $30,000 $25,000 $20,000 Savings $15,000 $10,000 $5,000 $0 30% ($5,000) Flow 40% 50% 60% 70% 80% 90% 100% Figure 6-25. Annual savings for variable-speed fan (Courtesy of ABB Inc.) Transducer Signal Feed (Transducer Signal Fed (Or directly drive) directly intointo drive) Speed Feedback Static Pressure Sensor VFD Motor Supply Fan Figure 6-26. PI control using a VFD (Courtesy of ABB Inc.) In this example, the energy savings would come as a result of completely opening the outlet damper. The drive then operates as a closed-loop controller, responding to the static pressure feedback. Cooling Towers Another system that can realize substantial energy savings is a cooling tower. Figure 6-27 indicates how a cooling-tower system operates. The speed of the cooling tower fan(s) is controlled by the VFD. In traditional systems, the fan would operate at full speed 24 hours per day, unless cooling water was not required. With the VFD, operating in PI control, the set point temperature is converted to a voltage set point. The feedback from a temperature transducer allows the drive to calculate temperature error and respond with increased or decreased fan speed, or zero speed. Motors&Drives.book Page 256 Monday, January 10, 2005 1:54 PM 256 Motors and Drives Fan motor speed based on Sump Temperature VFD (Return) CWR (Supply) CWS Sump temperature sensor Figure 6-27. Cooling tower application (Courtesy of ABB Inc.) HVAC is a systems environment for AC drives. Seldom, if ever, are AC drives manually operated in office buildings, schools, or any other location where temperature or humidity is critical for daily operation. A typical VFD system connected to a building automation system is shown in figure 6-28. DDC panel Drive Information VFD start / stop signal Fan or pump speed signal Fan or pump KW consumption Misc. Fault signals K Additional information as Fan Figure 6-28. Building automation system with VFD (Courtesy of ABB Inc.) Motors&Drives.book Page 257 Monday, January 10, 2005 1:54 PM Chapter 6: Drive System Control Methods — AC versus DC Drive Systems 257 Distributed digital control (DDC) provides the automated set points to the drives. The drive’s on-board PI control provides motor speed appropriate to control the medium required. In return, the DDC systems can poll the drive for important operating information, such as start/stop, faults, KW consumption, and more. Using the systems that are on the market today, it is possible to connect to a variety of building automation systems, using almost any manufacturer’s drive. However, comparisons should be made between vendors, to verify how much and what type of information can be obtained by the DDC system. Some drives only allow start/stop and speed reference signals to be transmitted. A very few manufacturers will allow up to 60 parameters (points) to be viewed by the building automation system. In this age of timely information, the easier it is to acquire operating information, the more efficient building operators can be. AC versus DC Drive Systems An adequate economic comparison between two types of drives requires an analysis of all of the costs incurred over the entire life cycle of the equipment. In addition to the purchase price of the drives and related equipment, this includes all of the material and labor costs required to obtain and install the equipment. It also requires an analysis of the costs to put the drive into operation, plus all of the costs to operate and maintain the equipment during the entire time it is expected to be in service. The best way to determine which is the most economical system is to perform a detailed analysis. There are no rules of thumb that will consistently and accurately predict the outcome of an analysis. Since the introduction of AFDs (Adjustable Frequency Drives) in the late 1960s, these drives have been slowly proving to be the most economical choice in an increasing variety of applications, but individual application details can often tip the balance either way. The following is an outline of elements typically included in total life cycle cost. The items marked with an asterisk are the most significant items. • Procurement expenses • Project engineering expenses of selecting and specifying the equipment • Purchasing department expenses • Freight and receiving expenses • Cost of equipment and installation materials • Controller options and accessories • Motor, options, and accessories * Motors&Drives.book Page 258 Monday, January 10, 2005 1:54 PM 258 Motors and Drives • Operator interface equipment • Supervisory control equipment • Machine interface equipment • Transformer and other power distribution equipment* • Power factor and harmonic correction equipment* • Wire, cable, conduit, etc.* • Installation and commissioning expenses* • Operating expenses* • Electric power • Periodic maintenance • Planned downtime • Unplanned downtime • Cost of routine or major anticipated repairs • Spare and/or replacement parts and equipment In addition to the above, the following factors should be considered when analyzing AC versus DC systems. Technology Because it is relatively easy and economical to control the speed and torque of a DC motor, DC drives have long been the adjustable-speed drive of choice. However most drive users prefer to use VFDs wherever possible because AC motors are much more rugged and reliable than DC motors and they require less maintenance. For many years, drive manufacturers have been working to develop adjustable-frequency drives that will allow AC motors to be controlled as effectively and economically as DC motors. It is evident that this development effort would result in an overall shift in drive use from DC drives toward AC drives. The motor is the controlling element of a DC drive system, while the electronic controller is the controlling element of an AC drive system. Since the emphasis on technology advancement is primarily electronic rather than electromechanical, the overall progress in technology has a greater impact on AC drives. Performance Capabilities With the introduction of flux vector drives, there are virtually no fundamental performance limitations that would prevent a VFD from being used in any application where DC drives are used. Using the latest control Motors&Drives.book Page 259 Monday, January 10, 2005 1:54 PM Chapter 6: Drive System Control Methods — AC versus DC Drive Systems 259 techniques, the performance available from AC motors equals or exceeds the performance available from DC motors. In areas such as high-speed operation, the inherent capability of AC motors exceeds the capability of DC motors. Several manufacturers now offer inverter duty motors that are specifically designed for use with VFDs. Inverter-duty motors have speed-range capabilities that are equal to or above the capabilities of DC motors. In addition, DC motors usually require cooling air forced through the interior of the motor to operate over wide speed ranges. Totally enclosed AC motors are also available with wide speed range capabilities. The only question should be one of availability of models in the required horsepower range or implementation of certain optional capabilities or special functions. Motor Purchase Price The price of the motor must be evaluated along with the cost of all of the other drive system equipment. Although DC motors are usually significantly more expensive than AC motors, the motor-drive package price for an VFD is often comparable to the price of a DC drive package. However, if spare motors are required, the package price tends to favor the VFD. Since AC motors are more reliable in a variety of situations and have a longer average life, the DC drive alternative may require a spare motor while the AC drive may not. Since DC motors tend to be less efficient than AC motors, they generally require more elaborate cooling arrangements. Most AC motors are supplied in totally enclosed housings that are cooled by blowing air over the exterior surface which is in intimate contact with the stator core, the source of the majority of the losses. Since cooling air does not enter the interior of the motor, dirt and contaminants in the air do not usually cause problems. Totally enclosed DC motors are usually very expensive because they must be over-sized to adequately dissipate heat because of losses in the armature. DC motors are usually cooled by blowing air through the interior of the motor. At a minimum, this means that the motor will be equipped with a blower and filter box. If the atmosphere is particularly dirty or corrosive, clean air must be ducted in from a centralized cooling system. In evaluating the price of the motor, it is important to consider the cost of a cooling arrangement that is adequate for the application. Cost of Motor Options AC motors are available with a wide range of optional electrical and mechanical configurations and accessories. DC motors are generally less flexible and the optional features are generally more expensive. Optional mechanical configurations include various types of enclosures, special shafts, optional conduit box locations, special bearings, and other options. Motors&Drives.book Page 260 Monday, January 10, 2005 1:54 PM 260 Motors and Drives Mounting options include vertical mounting and several types of flanged end brackets. Motor accessories include separately powered blowers or fans, tachometer generators or encoders, various types of temperaturesensing devices, space heaters, friction brakes, and other items. Some configurations such as explosion-proof enclosures are very expensive options for DC motors compared with AC motors. A number of AC motor manufacturers have developed motors specifically designed for use with adjustable-frequency drives. As a result, AC motors are readily available with special cooling arrangements or enhanced thermal capacity for wide-speed ranges. Motor-mounted tachometer generators or encoders are also readily available. A VFD without tach feedback can be used in some applications where DC drives typically require tach feedback. Since a tach generator or encoder adds significantly to the price of the motor, it is important to carefully consider whether or not it is required. Additional System Component Costs As mentioned earlier, a valid comparison of equipment prices must include the cost of all of the components of the drive system. Most DC drive installations require a drive-isolation transformer or input-line chokes. The transformer or chokes provide impedance, which reduces power line notching caused by the SCRs in the DC controller. Since PWM drives have a diode bridge input section, they do not cause line notching and therefore have less need for added input impedance. Large DC drives, with motors rated 1000 HP or higher usually require rather costly armature circuit chokes to provide sufficient commutating reactance to ensure spark-free commutation and acceptable brush life. Large AC drives require no equivalent expenditure. The cost of power factor and harmonic-correction equipment must also be considered as part of the total drive package price. As mentioned earlier, DC drives sometimes require a centralized cooling system that provides clean cooling air through ducts to the motor. Although a centralized cooling system is used to supply air to multiple motors, it is a drive-system component that must be considered in any cost comparison. Summary Expenses incurred in selecting and specifying drive systems tend to be lowest for the type of equipment that is the most familiar to the specifier. The equipment supplier can help to reduce this expense by providing application information and assistance. Ultimately, it is the user that must be satisfied with the purchase of the drive system. The most efficient use of system capabilities will be obtained if up-front time is taken to review the application, and match the system with the requirements. Motors&Drives.book Page 261 Monday, January 10, 2005 1:54 PM Chapter 6: Drive System Control Methods — Chapter Review 261 Chapter Review Drive systems operate in a coordinated fashion, with control between the controller (PLC) and the drive unit. A variety of sensors, switches, and transducers are a part of the overall scheme of automation. Proportional integral derivative control is used when automatic control of some quantity is required. Temperature, pressure, and humidity levels are just a few of the items that can be conveniently controlled by PID. Tension control is a major part of any coordinated system that processes web material. Dancer control is similar to tension control, in that a separate regulator signal is fed back to the drive for correction to take place. Proportional gain and integration time play a part in the tuning of a web system. A variety of remote operator devices are available for interfacing signals to the drive unit. Remote-operator stations are the simplest form of remote control. Standard I/O would include start/stop, speed reference, digital inputs, analog outputs, and relay outputs. Both sinking and sourcing control are used in industry today. Serial communications is the simplest form of communication link to a drive. Typically, multiple drives are controlled by one system controller, which could be a computer that is set up to talk to the protocol that is installed in the drive. Fiber-optic communications has the highest immunity to noise compared with other forms of drive communications. Optical fibers are connected in a ring structure and can be connected with plastic or glass fiber. Building automation systems or Ethernet systems are able to talk to many drives on the market today. DC systems have traditionally been associated with printing press, ski lift, and material handling applications. A major benefit of DC is the high starting torque at zero speed. AC systems have their roots in energy-saving applications. Substantial energy savings can be realized using VFDs instead of fixed-speed outlet damper control. Additional applications for AC drives include conveyors, overhead gantry units, overhauling loads, etc. A multitude of questions should be asked when comparing AC systems with DC systems. Initial procurement, operating, and maintenance costs need to be analyzed over the life of the equipment to be installed. Check Your Knowledge 1. 2. 3. Name three or more devices that are used in closed-loop systems for set point or feedback conditions. Why is proper tension control important in a web-fed system? What is a dancer control? Motors&Drives.book Page 262 Monday, January 10, 2005 1:54 PM 262 Motors and Drives 4. 5. 6. 7. 8. 9. 10. 11. 12. What are jumpers or DIP switches used for when connected to analog input signals? What is the difference between sinking and sourcing control logic? What is meant by serial communications? How is it used with drives? What are termination resistors and why are they used? What is the advantage of using fiber optics instead of serial communications? What is the benefit of using DC drives in applications such as printing and ski lifts (chair lifts)? What is the common DC bus configuration and why is it used? How is a VFD used to save energy in an outlet damper application? What are the most significant items to be reviewed when evaluating AC or DC drive systems? ...
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