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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
FIG. 17 is a simplified diagram of a printing press, using a DC motor and drive system.
FIG. 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.
FIG. 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.
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 6 18 indicates a simplified version of a ski lift system.
FIG. 18. Ski lift system (chair lift)
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. FIG. 19 indicates a diagram of hand/auto control.
FIG. 19. Hand/auto control
Another DC system application is found in material handling. FIG. 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.
FIG. 20. Barge unloader application
As seen in FIG. 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 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 joy stick 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. There fore 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 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.
FIG. 21. Common DC bus configuration
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. FIG. 21 indicates a common DC bus configuration.
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 non water-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.
FIG. 22. Coal classifier system
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. FIG. 22 indicates this 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, drop ping 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 out put possible. The powder is then forced into the combustion chamber, where it is used to fire the boiler and create steam for the turbine genera tor.
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
FIG. 23 indicates an energy-use comparison of variable-speed AC drive use versus outlet damper control.
FIG. 24 shows fan efficiency improvement using variable speed com pared with outlet damper control.
FIG. 25 shows the annual savings that a variable-speed fan can have compared with outlet damper control.
FIG. 23. Fan energy use (ABB Inc.)
FIG. 24. Fan efficiency improvement (ABB Inc.)
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 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 FIG. 26.
FIG. 25. Annual savings for variable-speed fan (ABB Inc.)
FIG. 26. PI control using a VFD (ABB Inc.)
FIG. 27. Cooling tower application (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. FIG. 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.
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 FIG. 28.
FIG. 28. Building automation system with VFD (ABB Inc.)
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 per form 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 *
• 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 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 sup plied 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.
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 temperature sensing devices, space heaters, friction brakes, and other items. Some con figurations 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.
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.
QUIZ
1. Name three or more devices that are used in closed-loop systems for set point or feedback conditions.
2. Why is proper tension control important in a web-fed system?
3. What is a dancer control?
4. What are jumpers or DIP switches used for when connected to analog input signals?
5. What is the difference between sinking and sourcing control logic?
6. What is meant by serial communications? How is it used with drives?
7. What are termination resistors and why are they used?
8. What is the advantage of using fiber optics instead of serial communications?
9. What is the benefit of using DC drives in applications such as printing and ski lifts (chair lifts)?
10. What is the common DC bus configuration and why is it used?
11. How is a VFD used to save energy in an outlet damper application?
12. What are the most significant items to be reviewed when evaluating AC or DC drive systems?
ANSWERS-- Section 6
1. Pressure, humidity, temperature transducers, potentiometers, a variety of electro-mechanical devices that change voltage output given the input signal.
2. Tension control is required or the product quality will suffer. Too little tension on the web will cause bunching and poor quality of the product.
Too much tension could stretch the material, causing a thinner product, or in the worst case, cause the web to break.
3. A dancer is a device that changes resistance per the amount of force being applied. This device is typically used to monitor and regulate the amount of tension placed on web-fed material.
4. Jumpers or DIP switches match the analog input signal with the drive input setting. Typical input signals are 4(0) to 20 mA and 0 to 10 VDC.
5. Sinking logic is where the control voltage is tied directly to circuit common. A command is done when a contact closure is between the terminal and ground. Sourcing logic is where the control voltage is directly applied to the appropriate terminal. All circuit commons and ground are tied together.
6. Serial communications is the transmission in sequence of bits of data to or from a controller device. Communications speeds range from 4800 to 19,200 baud. Drives are wired in parallel (daisy chained) to the control device. The controller could be a computer or any device that talks the same language as the drive. The drive may need a language "interpreter" if the language is not pre-loaded in the drive.
7. Termination resistors reduce the introduction of electrical noise into a communication system. They are placed (terminated) on the first and last drive of a communication network. The in-between drives are left non-terminated. These resistors also "close" the communication circuit.
8. Fiber optics are extremely immune to electrical noise. Since they use light as the transmission medium, little to no low-frequency radiation can affect the quality of the transmission (i.e., 60 Hz).
9. DC motors offer very high torque at low speeds. Printing and ski lifts are demanding applications. The system may need to move a fully rated load from zero speed, up to set point. Standard AC PWM, voltage-regulated drives cause the motor to slip to develop torque. However, the newer flux vector, or direct torque-control drives, have very similar operation, com pared with a DC drive system.
10. Common DC bus arrangements are used when the likelihood exists for an overhauling load. The extra energy generated by the motor is fed back to the common bus and then transferred to another inverter unit connected to the bus. This increases the overall efficiency of the system.
11. The VFD is used to vary the speed per the feedback from the pressure transducer in the duct (if PID control is used). The outlet dampers are fixed in the open position, with the drive varying the speed of the fan per the feedback received from the transducer. Lower fan speed and pressure in the duct is the result. The fan only supplies the airflow required to meet the application needs.
12. Most significant factors are motor, its options and accessories, transformer and other power equipment, PF and harmonic correction, wire, cable, and conduit costs, installation and commissioning expenses, and overall operating expenses.