Tech Tips June 2005
JUNE 28, 2005
TECH TIP OF THE WEEK:
When to use RTDs or thermocouples.
The two most common ways of measuring industrial temperatures are with resistance temperature detectors (RTDs) and thermocouples. But when should control engineers use a thermocouple and when should they use an RTD? The answer is usually determined by four factors: temperature, time, size, and overall accuracy requirements.
What are the temperature requirements? If process temperatures fall from -328 to 932°F (-200 to 500°C), then an industrial RTD is an option. But for extremely high temperatures, a thermocouple may be the only choice.
What are the time-response requirements? If the process requires a very fast response to temperature changes—fractions of a second as opposed to seconds (i.e. 2.5 to 10 sec)—then a thermocouple is the best choice. Keep in mind that time response is measured by immersing the sensor in water moving at 3 ft/sec with a 63.2% step change.
What are the size requirements? A standard RTD sheath is 0.125 to 0.25 in. dia., while sheath diameters for thermocouples can be less than 0.062 in.
What are the overall requirements for accuracy? If the process only requires a tolerance of 2°C or greater, then a thermocouple is appropriate. If the process needs less than 2°C tolerance, then an RTD is the only choice. Keep in mind, unlike RTDs that can maintain stability for many years, thermocouples can drift within the first few hours of use.
Although not a technical point, price may be another consideration. An average thermocouple costs approximately $35, while an average RTD costs $55. Cost of extension wire must also be considered. Thermocouples require the same type of extension wire material as the thermocouple, which can cost up to $1 per ft. Standard nickel-plated, teflon-coated RTD wire averages pennies per ft.
Once parameters are defined, the type of RTD or thermocouple is chosen. RTDs provide a resistance vs. temperature output and are passive devices, needing no more than 1.0 mA to run. The most common RTD is a 100 ohm, platinum sensor, with an alpha coefficient of 0.00385 ohms/ohm/C. It can be ordered as DIN A or DIN B which specifies the initial accuracy at 0°C (ice point) and the interchangeability over the operating range. IEC 751 states that DIN A is 0.15°C
RTDs can also be constructed from nickel, copper, or nickel/iron. Each metal has a different alpha coefficient and operating range. An RTDs alpha coefficient must be matched to its instrumentation or an error of several degrees can occur.
Thermocouples can be made with any combination of two dissimilar materials. ISA recognizes twelve thermocouples. Eight of the 12 have letter designations including Type J, Type K, Type T and Type E.
The most common determining factor for chosing thermocouple type is the temperature range of its intended application. Type J is suitable for a temperature range of 32 to 1,400°F (0 to 759.99°C). Type K is appropriate for a temperature range of 32 to 2,300°F (0 to 1,259.99°C). Type T handles a temperature range of -300 to 700°F (-184.44 to 371°C). Type E fits a temperature range of 32 to 1,600°F (0 to 871.11°C).
Standard limits of error and special limits of error must also be considered. These values relate to the purity of the wire used to manufacture the thermocouple. For very little additional cost, thermocouple specifiers can often improve accuracies greatly (100% or greater).
Specifying the correct thermocouple or RTD for an unconventional application may be a difficult task. Many manufacturers of RTDs and thermocouples offer applications engineering support to help customers select the right combination of temperature measurement equipment.
Jim Sulciner, national sales manager
Burns Engineering, Minnetonka, MN
Source: Sulciner, Jim, 'Choosing RTDs and thermocouples,' Back to Basics, Control Engineering, Feb. '99, p. 152.
JUNE 21, 2005
TECH TIP OF THE WEEK:
AC induction motor anatomy
Often referred to as the 'workhorse' of industrial electric motors, ac induction motors offer users simple, rugged construction and easy maintenance. These factors have promoted standardized motor designs and development of a manufacturing infrastructure leading to a vast installed base worldwide. Cost-effective pricing is a further advantage.
An ac induction motor consists of two basic assemblies—stator and rotor—and is analogous to an ac transformer with a rotating secondary. The stator structure is composed of steel laminations (or stampings) shaped to form poles around which are wound copper wire coils. These primary windings connect to, and are energized by, the voltage source to produce a rotating magnetic field. Three-phase windings spaced 120 electrical degrees apart are popular in industry.
The rotor (or rotating secondary) is another assembly of laminations over a steel shaft core. Radial slots around the laminations' periphery house rotor bars-cast-aluminum or copper conductors shorted at one end and positioned parallel to the shaft (see photo). Arrangement of the rotor bars, viewed on end, looks like a 'squirrel cage,' hence the colloquial reference: squirrel-cage induction motor.
The motor's name comes from the alternating current (ac) 'induced' into the rotor by the rotating magnetic flux produced in the stator. Motor torque is developed from interaction of currents flowing in the rotor bars and the stator's rotating magnetic field.
Synchronous and actual speeds
The magnetic field rotates at synchronous speed, VS-the motor's theoretical top speed that would result in no torque output. In actual operation, rotor speed always lags the magnetic field's speed, allowing the rotor bars to cut magnetic lines of force and produce useful torque. This speed difference is called slip speed. Typical slip values range 2-5% of VS at running speed, but can be large at motor startup. Slip also increases with load, so for accurate control of speed, closed-loop control or feedback is needed.
In equation form, VS=120(f/p), where f is supply frequency and p is the number of poles. Thus, for a 60-Hz motor with 4 poles, and 3% slip, VS=1,800 rpm and actual speed, Va=1,746 rpm. Other common synchronous speeds (aka, base speeds) are 600, 900, 1,200, and 3,600 rpm. For 50-Hz motors, VS has proportionately lower values. The rotating magnetic field's direction and activation sequence of the phase voltages determine direction of motor rotation.
Induction motors are available in an extremely wide size range from very small units to hundreds of kW-even thousands of kW for custom designs. Some common input voltages are 230, 460, and up to 575 V for 60-Hz operation (up to 690 V for 50-Hz-rated units).
Design, construction varieties
Standardized motor designs have evolved based on such motor characteristics as current, torque, and slip. Examples are NEMA (National Electrical Manufacturers Association) design A, B, C, and D motor types. (See CE, July 1998, pp. 91-99 and OnLine Extra at www.controleng.com , for more details.) IEC-style induction motors made for European and international markets are also available.
Motor frames can be cast iron, rolled steel, cast aluminum, etc. Variable torque refers to motors intended for near zero to base speed operation. Constant torque motors provide full torque over a specific speed range, for example, 4:1 or 10:1, even 1,000:1 with special cooling. Enclosure varieties include ODP (open drip proof), TENV (totally enclosed, non-ventilated), and TEFC (totally enclosed, fan cooled).
Other design varieties include premium efficiency motors built with higher quality electrical materials and higher grades of magnet wire insulation.
Frank J. Bartos, executive editor
Source: Bartos, Frank, 'Anatomy of an ac induction motor,' Back to Basics , Control Engineering, Dec. '99, p. 68.
JUNE 14, 2005
TECH TIP OF THE WEEK:
Selecting the most appropriate wire and cable types.
Selecting cable used to be easy. Users picked wire for communications and control or to supply power. They added accessories, trays and tracks, and connectors to fit their layout and facilities, and specified environmental protections, such as chemical/oil resistance and shielding/interlocking armor.
Because standard hook-up wire or multiconductor twisted-pair is still by far the most used wiring, these basic criteria remain crucial. However, more sophisticated sensors, I/O devices, data acquisition, and communications, as well as varied power requirements, and multiplying robotics and machining center applications, are making wire and cable selection increasingly complex. Still, specifying the correct cable doesn't have to be a mystery.
There are eight main choices when it comes to non-stationary, communication, or more traditional wire and cable. These include continuous flex; torsional robotic; servo; control; data bus; European wire, cable, and cordage; European Unitronic; and crane and conveyor system cable. Variable frequency drive, fiber-optic, and more specialized cable types and combinations are also widely used depending on specific applications.
Continuous flex cable, for example, is designed for high-speed automated applications, such as industrial robots, pick-and-place machines, handling systems, machine tools, and conveyor systems. Standard control cables are also used in these applications, though they're unable to match continuous flex's usual minimum performance of 5-10 million cycles. Continuous flex cable should also be able to travel at 6.5-13 ft/sec and accelerate at 25 ft/sec.
Continuous flex cables are made of resilient materials, so they can immediately return to an unstressed state after each flexing cycle. Their copper conductors must be made of finely drawn (no more than 34 gauge) strands rather than coarse ones, which aids movement and reduces wear.
When selecting and installing continuous flexible cable, users must also be aware of its bend radius. This is an evaluation of how tight an angle (usually 12x diameter or less) that it can follow without loss of elastic memory or other damage. Users must also project the flex life their cable will need by calculating the approximate number of cycles it will need to travel.
Torsional robotic cable is designed to perform in twisting and bending applications without failure or fatigue, generally for more than 2 million cycles. It must also meet the flame, voltage, and aging requirements of standard cable.
Servo cable can combine up to three types in one cable for power, signal, and control of servo motors to achieve exact movement over large or small distances. Certain servo systems require high flex-cycle life cables, and may contain several different gauge conductors. Power conductors drive the motor, while control conductors direct desired movements. Servo motors often operate in environments that require them to be shielded from electromagnetic interference (EMI), which can distort control and feedback signals.
Control cable allows data transmission throughout applications and facilities. This cable traditionally operates at 115 V ac or 24 V dc over small and medium gauge, shielded cable. This type is sometimes known as 'electronic wire.'
Data bus cable transmits information in high-speed, high-bandwidth applications via device level networks that connect plant floor devices to control systems, without the need for input/output (I/O) interfaces. Fieldbus wiring also performs many of the same tasks, though it may also deliver low power to some applications.
European wire, also known as 'harmonized' cable, is used in equipment intended for export to European countries adhering to European Committee for Electrotechnical Standardization (CENELEC) guidelines, while complying to CE directives. Harmonization cable standards apply to power supply cable and hook-up wire.
European Unitronic electronic cables are multipair cables that likewise satisfy European standards such as VDE and DIN. They are used in electronics, communications, process control, and instrumentation applications.
Richard Buchicchio, marketing director, Olflex Wire & Cable, Fairfield, N.J.
Source: Richard Buchicchio, 'More tasks need more versatile wire, cable types,' Back to Basics, Control Engineering, June '00, p. 92.
JUNE 7, 2005
TECH TIP OF THE WEEK:
How to read P&IDs.
Instrumentation detail varies with the degree of design complexity. For example, simplified or conceptual designs, often called process flow diagrams, provide less detail than fully developed piping and instrumentation diagrams (P&IDs). Being able to understand instrumentation symbols appearing on diagrams means understanding ANSI/ISA's S5.1-1984 (R 1992) Instrumentation symbols and identification standard. S5.1 that defines how each symbol is constructed using graphical elements, alpha and numeric identification codes, abbreviations, function blocks, and connecting lines.
ISA S5.1 defines four graphical elements-discrete instruments, shared control/display, computer function, and programmable logic controller-and groups them into three location categories (primary location, auxiliary location, and field mounted).
Discrete instruments are indicated by circular elements. Shared control/display elements are circles surrounded by a square. Computer functions are indicted by a hexagon and programmable logic controller (PLC) functions are shown as a triangle inside a square.
Adding one horizontal bar across any of the four graphical elements indicates the function resides in the primary location category. A double line indicates an auxiliary location, and no line places the device or function in the field. Devices located behind a panel-board in some other inaccessible location are shown with a dashed horizontal line
Letter and number combinations appear inside each graphical element and letter combinations are defined by the ISA standard. Numbers are user assigned and schemes vary with some companies use of sequential numbering, others tie the instrument number to the process line number, and still others adopt unique and sometimes unusual numbering systems.
The first letter defines the measured or initiating variables such as Analysis (A), Flow (F), Temperature (T), etc. with succeeding letters defining readout, passive, or output functions such as Indicator (I), Record (R), Transmit (T), and so forth.
Example shows the story
Referring to the 'Example P&ID' diagram, FT 101 represents a field-mounted flow transmitter connected via electrical signals (dotted line) to flow indicating controller FIC 101 located in a shared control/display device. A square root extraction of the input signal is applied as part of FIC 101's functionality. The output of FIC 101 is an electrical signal to TY 101 located in an inaccessible or behind-the-panel-board location. The output signal from TY 101 is a pneumatic signal (line with double forward slash marks) making TY 101 an I/P (current to pneumatic transducer). TT 101 and TIC 101 are similar to FT 101 and FIC 101 but are measuring, indicating, and controlling temperature. TIC 101's output is connected via an internal software or data link (line with bubbles) to the setpoint (SP) of FIC 101 to form a cascade control strategy.
Often P&ID's include a cover page where common and typical terms, symbols, numbering systems, etc., are defined. On the example, Typical YIC would likely appear on the cover page and the simplified form of YIC would appear throughout the P&IDs.
Typical YIC indicates an on/off valve is controlled by a solenoid valve and is fitted with limit switches to indicate open (ZSH) and closed (ZSL) positions. All inputs and outputs are wired to a PLC that's accessible to the operator (diamond in a square with a solid horizontal line). The letter 'Y' indicates an event, state, or presence. The letter 'I' depicts indication is provided, and the letter 'C' means control takes place in this device.
Adherence to ISA's S5.1 Instrumentation Symbols and Identification standard ensures a consistent, system independent means of communicating instrumentation, control, and automation intent is developed for everyone to understand.
For more useful diagrams on 'General instrument or function symbols,' 'Identification letters,' and 'Common connecting lines,' visit www.manufacturing.net/ctl/article/CA152141?text=yic&spacedesc=news . For more information on ISA standards, visit www.isa.org .
Dave Harrold, senior editor
Source: Dave Harrold, 'How to read P&IDs,' Back to Basics, Control Engineering, Aug. '00, p. 116.