5.12
Time Delay Relays J. FRYDMAN (1985)
B. G. LIPTÁK
(1995)
W. P. DURDEN
(2005)
TD
“On” delay (set at 10 min.)
TD
“Off ” delay (set at 3 min.)
Flow sheet symbol
1030 © 2006 by Béla Lipták
Type of Design:
A. Thermal devices, bimetallic strip springs B. Dashpots C. Pneumatic delays D. Inertial devices, weights, flywheels E. Solid state F. Motor-driven delays G. Software delay
Type of Delay:
On-delay, off-delay, interval, single-shot, pulse detection, and repeat cycle
Delay Settings:
A. 0.2 to 3 sec B. 1 to 120 sec C. 0.1 to 300 sec D. 0.05 to 0.2 sec F. Can be many hours
Timing Inaccuracy:
Generally 5 to10 %. Type A is 5 to20 %; for better accuracy use interval timers instead of time delays. Crystal-controlled oscillators can provide 0.1%
Reset Time:
About 100 ms
Life Expectancy:
Electrical types at full load usually operate a minimum of 100,000 times; mechanical types are good for 20 million operations or more
Load Switching:
Relay contacts, snap-action switches, SCRs, transistors, triacs
Environmental Limits:
14 to 140°F (−10 to 60°C); 5 to 95% relative humidity
Mounting Configurations:
Plug-in, surface-mount, panel-mount, in-line module
Costs:
From $20 to $200
Partial List of Suppliers:
ABB SSAC Inc. (www.ssac.com) Agastat (www.agastat.com) Allen-Bradley (www.ab.com) Amperite Co. (www.amperite.com) Artisan Controls Co. (www.artisancontrols.com) Automatic Timing & Control (ATC) (www.flw.com/atc/atc.htm) Bircher (www.bircher.com) Bright Toward Industrial Ltd. (www.relays.com.tw) Castel (www.castel.com) Comadan Producktion a/s (www.comadan.com) Crouzet (www.crouzet-usa.com) Dold (www.dold.com) Eaton-Durant Products (www.durant.com) Emerson Climate Technologies (www.emerersonclimate.com) Fiber (www.fiber.it) Fuji (www.automationdirect.com)
5.12 Time Delay Relays
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General Electric (www.geindustrial.com) Hiquel (www.hiquel.com) Idec (www.idec.com) Instrumentation & Control Systems (www.ics-timers.com) Kessler-Ellis Products (www.kep.com) KOYO (www.automationdirect.com) Kübler (www.kubler.com) Kuhnke (www.kuhnke.de) Lovatao Electric (www.lovatoelectric.com) Macromatic Controls (www.marcromatic.com) Omron (www.oeiweb.omron.com) Pilz (www.pilz.com) Phoenix Contact (www.phoenixcontact.com) Schleicher (www.schleicher-de.com) Siemens Energy & Automation (www.siemens.com) Square D Co. (www.squared.com) Tele Hause Steuergeräte (www.tele-power-net.com) Telemecanique (www.telemecanique.com) Texas Instruments (www.ti.com)
Time delay devices are used for delaying the start-up, shutdown, recycling, or continuation of processing operations until the desirable requirements have been satisfied or the required conditions have been obtained.
[hermetically sealed], or mercury [hermetically sealed]), or solid-state devices (e.g., transistors, SCRs, or triacs).
Timer Modes and Characteristics GENERAL CHARACTERISTICS Time delay relays are special-purpose relays or logic components that have some characteristics of both relays and timers. Time is one of the variables of any process, and it often need to be monitored or controlled. Some operating steps in a process might need to coincide or may need to be separated from some other steps by a specific interval of time. Time delay relays serve to satisfy such timing requirements. The two main components of time delay relays are the timing circuit mechanism that produces the required time interval and the load switching contacts that are actuated at the end of that time interval. For a time interval to be determined, the following prerequisites must be present: 1. 2. 3. 4.
A power source (if needed in addition to signal power) Signal power A change of state of the time-determining device An indication that the time-determining device has changed to the desired state
The power or signal power can be thermal, pneumatic, AC, or DC electric current. The change in the state of the timer can be mechanical (e.g., caused by the rotation of a motor, the motion of a plunger in a restricting fluid, or the bending of a bimetal strip caused by a change in temperature), electrical (e.g., caused by an accumulation of charge of a capacitor or a count of oscillations or pulses), or software-based (e.g., based on the number of program scans or interrupt-driven). The load switching can occur via snap-action switches, snap-action valves, relay contacts (e.g., SPDT, DPDT, reed
© 2006 by Béla Lipták
The mode of the time delay determines the relationship between the time when the signal power is applied to the time of load switching. This relationship may also depend on the continuous presence of the power source. The four most prevalent modes are on-delay, off-delay, interval, and single-shot. A special combination of the on-delay and the off-delay is called a repeat cycle timer. In addition to the aforementioned combinations, one must consider the following characteristics when choosing time delay relays from among the vast number available: 1. Timing range 2. Fixed versus adjustable timing 3. Accuracy a. Dial setting accuracy b. Tolerance c. Repeat accuracy d. Time between operations 4. Environmental factors a. Temperature range b. Vibration, shock 5. Loading switching a. Duty cycle b. Type of load: resistive, inductive, lamp c. Life expectancy with load 6. Mounting considerations a. Size limitations b. Mounting style: surface-mount, panel-mount, plugin, in-line module c. Terminals: plug-in, quick connect, solder lug, screw terminal
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PLCs and Other Logic Devices
7. Housing a. Open chassis b. Dustproof c. Weatherproof d. Explosionproof e. Totally encapsulated
Power source
On On Off
Signal power
Delay
On Off
On-Delay An on-delay is alternatively called delay on make, delay on operate, delay on pull-in, delay on energize, slowacting, or slow-operating. The load switching occurs a certain time after the application of the signal power (Figure 5.12a). If a power source is normally required and the power is interrupted before load switching takes place, the timing cycle has to be repeated from 0 to effect a delayed load switching (except in certain synchronous motor-driven delay relays). Off-Delay An off-delay is alternatively called delay on break, delay on de-energize, delay on drop-out, delay on release, slow release, drop-out delay, or delayed drop-out. The load switching occurs a certain time after the removal of the signal power (Figure 5.12a). If the power source is normally required and the power is interrupted before the minimum amount of time or after removal of the signal power prior to load switching, the amount of time delay may be inaccurate. Interval An interval timer is alternatively called interval on, on interval, pulse shaping, bypass timing, interval delay, and delay on energization with instantaneous transfer. After application of the signal power and while the signal power is maintained, the load switching occurs for a certain time only, and then the load switching is de-energized (Figure 5.12a). If the signal power is interrupted or the power source (if normally required) is removed before the completion of the time delay, the load switching is de-energized instantaneously. Single-Shot A single-shot is alternatively called latched interval, latching off delay and latching delay on de-energization, momentary actuation, or one-shot. Load switching occurs for a certain time only, and then the load switching de-energizes after a momentary application of the signal power (Figure 5.12a). The signal power may be applied longer without altering the load switching interval. If the power source is normally required and it is interrupted while the load switching is energized, it deenergizes instantaneously. Pulse Detection A pulse detection relay monitors a contact or control input to activate an interval time period. The output
© 2006 by Béla Lipták
On Off
Load switching On Delay
Types of Time Delays Recovery Time Recovery time is the minimum amount of time between removal of the signal power and its reapplication, which is necessary so that the subsequent operation will have the desired repeatability. The analog or digital solidstate timing circuits have the shortest recovery time.
Power interruption Off On Off
On
Power source
On Off
Signal power
Delay On Off
Load switching
Power interruption Off On Off Delay On Off
Off Delay Power interruption Off
On
Power source
On Off
Signal power
On Off
Load switching
On Off
Delay On Off
Interval On
Power Removing signal power source after timing begins has no effect on timing
On Off Delay On Off
Power interruption Off
Signal power
On Off
Load switching
On Off
Single shot
FIG. 5.12a Timing charts of the various types of time delays.
contact may already be activated or on with the application of power or it may wait until the control input is active. Once the control input is on the control input must cycle within the interval time period to either maintain the output or to drop the output as selected. Repeat Cycle A repeat cycle relay is alternatively called dual delay; combination delays; on-delay, off-delay; or slow acting-slow releases; or asymmetric pulser. The load switching occurs a certain time delay after the application of the signal power, and remains energized for another time delay after the removal of the signal power. If the power source is normally required and it is interrupted while the load switching is energized, it may be de-energized prematurely. The asymmetric pulser has the added feature of two time periods, one for the on period and one for the off period.
5.12 Time Delay Relays
Moving contact
Stationary contact
OL 1 Coil
Iron core
OL 2
Armature Typical relay
Spring
Spring
Armature
Oil or fluid Dashpot
A
A
Piston
FIG. 5.12b Dashpot time delay.
Types of Designs Dashpot or Pneumatic In a dashpot-type time delay the armature of a relay is augmented with a piston that travels in an oil or fluid. The piston contains several holes through which the fluid passes. The current of the coil must be applied continuously to exert a pull on the armature. The armature travels slowly because of the resistance of the fluid, if it has the required viscosity to produce the necessary delay. The size of the aperture holes may be modified to vary the time delay. Once the armature travels the required distance, the moving contacts engage the stationary contacts. A dashpot time delay is shown in Figure 5.12b. In a pneumatic time delay, the time it takes for a certain volume of air to pass through a preset size of opening is used for timing. This design is made dustproof by a diaphragm and a cap, which encase the head in which the air is recirculated. Thermostatic In this version of a time delay design, one of the contacts is a bimetallic element that is wrapped with an insulated heating coil. The signal power is applied to the heating coil. The delay time is the time required to raise the temperature of the contact high enough to produce warping, which will finally cause the contact to transfer. The timing tolerance (accuracy) of standard thermostatic time delay relays is typically only ±20% but special designs are available with better tolerances. These units are also only Stationary contact
repeatable to approximately ±5%, but on the other hand, they are of low cost and inherently resistant to transients. A thermostatic time delay along with two forms of packaging is depicted in Figure 5.12c. Motor-Driven This type of relay is powered by a synchronous motor. A tripping arm is driven through a simple train of machine-cut spur gears. The arm actuates a snap-action switch, and the motor speed and the gear reduction determine the time delay. Resetting is instantaneous by means of an electromagnetic coil that clutches and declutches the trip locking gear as required. The clutch coil is often equipped with a set of auxiliary contacts. The power source is normally applied to the motor circuit, while the signal power is normally applied to the clutch coil. In this manner, there are instantaneous contacts actuated by the clutch coil and time-delayed contacts actuated by the tripping arm. When the power to the clutch is maintained, the time delay is increased by the delay in power interruption. Viewed otherwise, the relay accumulates all of the time that the power source has applied to the motor until it equals the time that is preset on the time delay relay. The use of a synchronous motor ensures great accuracy and, with appropriate gearing, allows for very long delays, up to and including hundreds of hours. Sometimes these relays are provided with a progress pointer. A motor-driven delay is illustrated in Figure 5.12d. Analog Solid State Analog time delays are based on the RC circuit, in which a capacitor is charged via a resistor until it reaches a certain voltage. When the predetermined voltage is reached, the load switching electromechanical relay or a solidstate switching device turns on or off, depending on the function. This type of timing is much simpler, more reliable, and less expensive than the mechanically complex synchronous motor-driven delay relay. However, there are some shortcomings, which include the fact that the time-constant curve for the capacitor charging is exponential, making it difficult to set a potentiometer (the variable R) accurately. Another limitation is that the RC circuit is sensitive to variations in supply voltage. Also, the setability and repeat accuracy of this design are affected by variations in the resistance because of variations in temperature, and in addition the
Bi-metallic moving contact Heating coil
Base
FIG. 5.12c Components of thermostatic time delay relay in hermetically sealed, glass tube and plastic case. (Courtesy of Amperite Co.)
© 2006 by Béla Lipták
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PLCs and Other Logic Devices
Power source to motor
Signal power clutch
Reset spring
Load switching relay
Load switching Preset
Free running oscillator
FIG. 5.12d Motor-driven delay.
Digital Solid State The digital method for creating a time delay uses a frequency counting or dividing circuit. This approach, although more expensive than the analog RC circuit, provides better accuracy and repeatability and lends itself to medium- or large-scale (chip) integration. In this type of circuit, pulses from an oscillator or from the frequency of the power line are applied to a counter or a divider chain, which is preset to output a pulse at a specific count. This output pulse turns on or off the load switching electromechanical relay or solid-state switching device. The power line frequency, 50 or 60 Hz, is used because of its stability. Using a higher frequency free-running oscillator provides finer time increments. For ultimate accuracy, a crystal-controlled oscillator is used, providing absolute accu-
Load switching relay R Voltage detector
Relay driver
C
Input voltage
Trigger voltage
FIG. 5.12f Digital time delay.
racy and repeatability, setable to the fourth significant figure. This type of delay relay would most often be supplied with a thumbwheel switch or dip switches to program the dividing counter. A digital time delay is shown in Figure 5.12f. Software-Based Time Delays Modern control systems make increasing use of programmable controllers or microprocessors for all the logic, sequencing, and timing functions. All these functions are emulated in software by a program stored in memory. The program is merely a series of binary instructions written by the applications designer, upon which the central processing unit interprets and acts. The software-based delays use the digital method of generating delays. Internal clock pulses are accumulated in a memory register or in a counter, which subsequently transfers its count to a memory register. The program regularly compares the accumulated count with a preset value that is stored in another memory register. When the accumulated count equals or exceeds the preset value, the program branches to another set of instructions that require the delay for further logic sequence. An error may be present in the aforementioned method, affecting accuracy and repeatability. Because the software program takes varying amounts of time for acting on a set of instructions, successive scans through the whole program involve differing amounts of time. Therefore, when the software program returns each scan to compare the actual count with the preset count, the actual may exceed the preset by an unacceptable amount for the accuracy desired. Some microprocessor-based devices have a modified digital method that corrects this problem. The actual count takes 0 1 2 3
Time constant curve
Control Timer designation Enable
Analog time delay
FIG. 5.12e Analog time delay.
© 2006 by Béla Lipták
Relay driver
Counter
capacitor shifts its value as it ages. Steps can be taken to offset or lessen these adverse effects of voltage or temperature variations, but these steps add to the cost of the analog time delay relay. An analog time delay is shown in Figure 5.12e.
Input voltage
Comparator
T
0.1
4 5 6 7
Preset values At preset Time increments 0.1 seconds Not at preset Current value storage location
Time
FIG. 5.12g Software-based time delay. Timer block general form as viewed on a CRT of a typical programming panel of a PLC.
5.12 Time Delay Relays
place in an integrated circuit, which creates an interrupt when it reaches the preset count. This interrupts the main software program immediately, and load switching can be executed without incurring additional time delay. A software-based delay is depicted in Figure 5.12g.
Bibliography “All About Circuits,” http://www.allaboutcircuits.com/vol_4/chpt_5/3 .html, December 2003. Asfahl, C. R., Robots and Manufacturing Automation, New York: Wiley, 1992. Auger, R. W., The Relay Guide, New York: Reinhold Publishing Co. Benoit, C. N., “Specifying Solid-State Timing Devices,” Automation, November 1972. Collin, M., “TDRs Are Alive and Well,” Instruments and Control Systems, December 1977. DeVries, W. R., Microcomputer Applications in Manufacturing, New York: Harper and Row, 1984. FAST Applications Handbook 1987, South Portland, ME: National Semiconductor Corporation, 1988.
© 2006 by Béla Lipták
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Global Spec, http://electronic-components.globalspec.com/LearnMore/ Electrical_Electronic_Components/Relays_Timers/Solid_State_Timer s_Time_Delay_Relays, December 2003. Kalpakjian, S., Manufacturing Engineering and Technology, Reading, MA: Addison-Wesley, 1992. Kaufman, M. and Seidman, A. H., Handbook of Electronic Calculations for Engineers and Technicians, New York: McGraw-Hill, 1988. Kaufman, M. and Seidman, A. H., Handbook for Electronic Engineering Technicians, New York: McGraw-Hill, 1984. Kosow, I. L., Control of Electric Machines, Englewood Cliffs, NJ: Prentice Hall, 1974. Margolin, R., “Time Delay Relays,” Electronics Products Magazine, May 1979. “Meter Relays, Analog Controllers,” Measurements and Control, February 1993. Mitra, S. K., Introduction to Digital and Analog Integrated Circuits and Applications, New York: Harper and Row, 1984. Schwartz, J. C., “Understanding Timers for Better Control,” Instruments and Control Systems, December 1975. Slater, N., “Solid-State Timing Shows Gains,” Design Engineering, July 1980. “Selecting and Specifying Time Delay Relays,” Instruments and Control Systems, March 1979. Strong, J. A., Basic Digital Electronics, London: Chapman and Hall, 1991.
