Tech Tips August 2005
August 30, 2005
Low-voltage fuse application guidance
Fuse voltage ratings are related to their capacity to open a circuit during overcurrent conditions. Voltage rating determines the fuse’s ability to suppress internal arcing produced after a fuse link melts.
Select a voltage rating higher, but never lower, than system or circuit voltage. With a lower voltage rating, arc suppression will be impaired and—under some fault-current conditions—the fuse may not safely clear the overcurrent.
Low-voltage fuses of a given (labeled) voltage rating can be safely applied on lower service voltages. That is, a 600-V class fuse will work as well at 230 or 480 V.
Lower ampere rating of a fuse does not always mean better protection. Consult manufacturers’ fuse “time-current characteristics” to find the fuse suitable to your application. Nominal rating of a fuse refers to the asymptotic current value to which the minimum-melting threshold “converges.” However, an overcurrent condition could be sustained indefinitely just below the minimum-melting threshold without converging, thus causing the fuse not to open.
Operating characteristics of a current-limiting fuse restrict actual current flow through the circuit to a value substantially lower than the prospective maximum. Let-through I
Holding let-through current to substantially under available fault current level will greatly reduce magnetic stresses and thus reduce fault damage in protected equipment. Low peak let-through current and I
Additional fuse application guidance can be found in the reference below.
Source: “Electric Power System Protection and Coordination,” by Michael A. Anthony, ISBN 0-07-002671-8, McGraw-Hill Inc. (1995), pp 88-92.
TECH TIP OF THE WEEK:
10 reasons to use adjustable-speed ac drives
Modern ac adjustable-speed drives (ASDs) have come a long way in their own right and as an alternative solution to dc drives. There are many reasons for using ASDs. Here's a summary of their benefits from ' Drives at Work—Do You Know These 10 Benefits? ' by Mark Kenyon, ABB Inc. product manager, presented at ABB Automation World Conference & Exhibition, in Houston, TX (April 2005).
Since more than 65% of industrial electric energy is consumed by electric motors, it makes sense to adjust motor operating speed to demands of the load. Varying load applications like centrifugal pumps and fans in particular benefit from ASDs. For example, when pump speed can be cut in half, resulting power consumption is reduced by a factor of eight! However, you need to know the load's duty cycle to get most out of energy savings.
Controlled starting current
High starting currents of ac motors (6-10 times full-load amps) stress windings, generate heat, and shorten motor life. ASDs start at zero frequency and voltage, extending motor life.
Reduced power line disturbances
Adjustable-speed ac drives virtually eliminate voltage sags caused by the staring of large or numerous motors. This minimizes tripping of voltage-sensitive equipment, reducing down time.
Lower power demand at start
Reducing 'demand charges'—or highest average demand recorded during any one time period within a billing period—also can reduce energy costs. Less power used during motor start means lower demand charges.
This ASD feature reduces stress on the motor, as well as on upstream power system components (transformers, switchgear, cabling, etc.). Customer equipment and sensitive products also are protected.
Infinitely adjustable operating speed
Adjustable-speed ac drives provide the right speed for the 'job,' allowing a production process to be optimized. Ability to easily make process changes widens applications.
Adjustable torque limit
An ASD can limit torque (current) supplied to the ac motor to protect against machinery damage or jamming. It also protects the product being manufactured, which can be fragile.
By changing the firing order of its output devices, the ac drive can electronically swap two output phases to reverse an ac motor's rotation. Eliminating separate contactors decreases panel space needed and lowers maintenance costs.
Elimination of components
ASDs can eliminate external components—mechanical (belts, transmissions, gear motors), electrical (PLCs, contactors, motor starters), and other process controllers.
Control of motor deceleration is as important as acceleration. Stopping time must suit the application, which ASDs provide either internal or external to the drive, without the need for a mechanical brake. Benefits are improved productivity and less scrap produced.
TECH TIP OF THE WEEK:
A touchscreen is a computer input device that enables users to make a selection by touching the screen, rather than typing on a keyboard or pointing with a mouse. Computers with touchscreens have a smaller footprint, can be mounted in smaller spaces, have fewer movable parts, and can be sealed. Touching a screen is more intuitive than using a keyboard or mouse, which translates into lower training costs.
Three common components
All touchscreen systems have three components. To process a user's selection, a sensor unit and a controller sense the touch and its location, and a software device driver transmits the touch coordinates to the computer's operating system. Touchscreen sensors use one of five technologies: resistive, capacitive, infrared, acoustic wave, or near field imaging.
Resistive touchscreens typically include a flexible top sheet and a glass base separated by insulating dots. Each layer is coated with a transparent metal oxide on its inside surface. Voltage applied to the layers produces a gradient across each. Pressing the top sheet creates electric contact between resistive layers, essentially closing a switch in the circuit.
Capacitive touchscreens are also coated with a transparent metal oxide, but the coating is bonded to the surface of a single sheet of glass. Unlike resistive touchscreens, where any object can create a touch, capacitive touchscreens require contact with a bare finger or conductive stylus. The finger's capacitance, or ability to store an electric charge, draws some current from each corner of the touchscreen, where voltage has been applied.
Infrared touchscreens are based on light-beam interruption technology. Instead of placing a layer on the display surface, a frame surrounds it. The frame has light sources, or light-emitting diodes (LEDs), on one side, and light detectors, or photosensors, on the opposite side, creating an optical grid across the screen. When any object touches the screen, the invisible light beam is interrupted, causing a drop in the signal received by the photosensors.
Acoustic wave touchscreens use transducers mounted at the edge of a glass screen to emit ultrasonic sound waves along two sides. The ultrasonic waves are reflected across the screen and received by sensors. When a finger or other soft-tipped stylus touches the screen, the sound energy is absorbed, causing the wave signal to weaken. In surface acoustic wave (SAW) technology, waves travel across surface of the glass, while in guided acoustic wave (GAW) technology, waves also travel through the glass.
Near field imaging (NFI) touchscreens consist of two laminated glass sheets with a patterned coating of transparent metal oxide in between. An ac signal is applied to the patterned conductive coating, creating an electrostatic field on the surface of the screen. When a finger—gloved or ungloved—or other conductive stylus comes into contact with the sensor, the electrostatic field is disturbed.
Elizabeth Morse, communication coordinator
Dynapro, Vancouver, BC, Canada)
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Source: LMorse, Elizabeth, 'How touchscreens work,' Back to Basics, Control Engineering , Sept. '98, p. 238.
TECH TIP OF THE WEEK:
Picking motion control amplifiers.
Selecting the right amplifier for a motion control application can make a big difference in both the cost and performance of the final system. Generally, two choices exist: buy an off-the-shelf amplifier, or design your own. The amplifier-one of three basic elements of a motion system along with the controller and motor-converts output signals from the controller into higher power motor coil voltages that drive various motor types. Controller and amplifier (sometimes even the motor) can be combined into one unit.
Servo motor amplifiers accept aired current (measured by a Hall-sensor or a dropping resistor). Other functions performed by the amplifier can include short-circuit and over-temperature protection, torque control, and commutation.
Off-the-shelf or design your own
An amplifier from stock is the most common design choice. Off-the-shelf units are available from numerous domestic and international sources, offering a wide variety of features and ranging from a cell-phone-sized block to an entire 19-in. rack.
Power output also varies from just a few watts to multiple kilowatts. Most off-the-shelf servo and step-motor amplifiers use switching drive methods to lower heat output. Some specialized units provide linear (transconductance) amplification necessary in applications that cannot tolerate electrical noise generated by switching power transistors.
It is common to purchase the amplifier and motor from the same company, but sometimes 'mixing and matching' from different vendors can offer better performance at lower cost.
The most popular choice when designing an amplifier is to buy an integrated H-bridge amplifier chip. These provide many functions found in box amplifiers, including switching logic, shoot-through protection, over-current and over-temperature sensing, etc. However, typical power output is limited, ranging from 1 to 3 A continuous (2-8 A peak).
This solution allows the amplifier to be designed into the same card that holds the motion controller. It saves space, but also makes machine service easier without interconnecting wires from controller to amplifier. In addition, this option is less expensive than an off-the-shelf amplifier-saving $100 or more per axis. Integrated H-bridges are available from several vendors, including Alegro, National Semiconductor, Siemens, STMicroelectronics, and Unitrode
As an alternative, amplifier design can begin by using discrete components. With this approach, MOSFETs (metal-oxide semiconductor field-effect transistors) or other switching chips are connected into an H-bridge or half-bridge configuration. However, the designer is responsible for all the 'higher order' functions such as shoot-through protection, charge pump, and over-current sensing. Designing in these extra functions can add substantial complexity to the project, but this option has the advantage of offering power output levels as high as needed (as determined by the switching ICs chosen and heat sinking). MOSFETs and other discrete power chips are made by International Rectifier, IXYS, Motorola, Texas Instruments, Siemens, STMicroelectronics, Unitrode, and others.
While most amplifiers designed today use digital switching methods (due to their power efficiencies), some applications require a linear power amplifier. It is important to remember, that for the same amount of applied power, a linear amp is much less efficient than a MOSFET switching amp. Typical efficiencies are in the 60-80% range, compared to over 90% for switching chips. This is why heat sinking becomes an important consideration with linear amplifiers.
When designing your own chip-based amplifier, H-bridge chips can simplify the task. However, to run at higher power levels using discrete MOSFET drivers requires substantial knowledge; it can still be worth the effort, if the application's volume and price sensitivity warrant it.
Chuck Lewin, president
Performance Motion Devices Inc., Lexington, MA
Source: Lewin, Chuck, “Motion Control Amplifier Choices,” Back to Basics, Control Engineering , Sept. ’00, p. 156.
TECH TIP OF THE WEEK:
Fluid chemical property terminology in hydraulics.
Various terms are commonly used to describe fluid chemical properties in hydraulic applications. These terms don’t have quantitative definitions, but their qualitative meanings are useful.
Emulsivity. This quality describes a fluid’s ability to form emulsions. An emulsion is a fluid that forms by suspending oily liquid in another liquid, often by means of a gummy substance.
Lubricity. This quality of a fluid describes the “oiliness” of the fluid, and refers to its adequacy when used as a lubricant. Many oils naturally contain some molecular species with boundary lubricating properties. Some vegetable oils, such as castor oil and rapeseed oil, contain more natural boundary lubricants than mineral oils. Therefore, additives are usually incorporated into mineral oils for the purpose of improving the lubricity. Lack of adequate lubrication properties promotes wear and shortens the life of hydraulic components.
Thermal stability. This quality of a fluid describes its ability to resist chemical reactions and decomposition at high temperatures. Fluids react more vigorously as temperature is increased, and may form solid reaction products.
Oxidative stability. This quality of a fluid describes its ability to resist reactions with oxygen-containing materials, especially air. Again, these reactions may form solid by-products within the fluid.
Hydrostatic stability. This quality of a fluid describes its ability to resist reactions with water. Undesirable formation of solids may result, or a stable water-in-oil emulsion may be formed that degrades lubricating ability and promotes rusting and corrosion. Demulsifier additives are often used to inhibit emulsion formations.
Compatibility. This quality of a fluid is a catch all that describes the fluid’s ability to resist chemical reactions with any material that may be used in the system to which it’s exposed. For instance, some fluids tend to soften seals and gaskets, which may cause them to be incompatible. Water is incompatible with steel because it causes steel to corrode.
Foaming. This term is used to describe a fluid’s ability to combine with gases, principally air, and to form emulsions. Entrained air reduces the lubricating ability and bulk modulus of a liquid. A reduction in the bulk modulus can severely limit dynamic performance, and, for this reason, fluids should have the ability to release air without forming emulsions. Antifoamant additives are used to encourage this ability.
Flash point. This is the lowest temperature at which the vapor of a volatile oil will ignite with a flash.
Pour point. This is the lowest temperature at which the fluid will flow.
Handling properties. These properties refer to the toxicity, odor, color, and storage characteristics of a fluid. These characteristics can be dangerous or annoying, thereby making the handling or use of fluid somewhat undesirable.
Source: Manring, Noah, “Hydraulic Control System,” John Wiley & Sons Inc., Hoboken, NJ, 2005, p. 38-39.