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Time delay relays can be divided into two general classifications: the on-delay relay, and the off-delay re lay. The on-delay relay is often referred to as DOE, which stands for "Delay On Energize." The off-delay relay is often referred to as DODE, which stands for "Delay on De-Energize." Timer relays are similar to other control relays in that they use a coil to control the operation of some number of contacts. The difference between a control relay and a timer relay is that the contacts of the timer relay delay changing their position when the coil is energized or de-energized. When power is connected to the coil of an on-delay timer, the contacts delay changing position for some period of time. For this example, assume that the timer has been set for a delay of 10 seconds. Also assume that the contact is normally open.
When voltage is connected to the coil of the on-delay timer, the contacts will remain in the open position for 10 seconds and then close. When voltage is removed and the coil is de-energized, the contact will immediately change back to its normally open position. The contact symbols for an on-delay relay are shown.
The operation of the off-delay timer is the opposite of the operation of the on-delay timer. For this example, again assume that the timer has been set for a delay of 10 seconds, and also assume that the contact is normally open. When voltage is applied to the coil of the off-delay timer, the contact will change immediately from open to closed. When the coil is de-energized, however, the contact will remain in the closed position for 10 seconds before it reopens. The contact symbols for an off-delay relay are shown in Fgr. 2. Time delay relays can have normally open, normally closed, or a combination of normally open and normally closed contacts.
Although the contact symbols shown in Fgrs 1 and 2 are standard NEMA symbols for on-delay and off-delay contacts, some control schematics may use a different method of indicating timed contacts. The abbreviations TO and TC are used with some control schematics to indicate a time-operated contact. TO stands for time opening, and TC stands for time closing. If these abbreviations are used with standard contact symbols, their meaning can be confusing.
Fgr. 3 shows a standard normally open contact symbol with the abbreviation TC written beneath it.
This contact must be connected to an on-delay relay if it's to be time delayed when closing. ___ shows the same contact with the abbreviation TO beneath it. If this contact is to be time delayed when opening, it must be operated by an off-delay timer. These abbreviations can also be used with standard NEMA symbols.
Pneumatic, or air timers, operate by restricting the flow of air through an orifice to a rubber bellows or diaphragm. --- the principle of operation of a simple bellows timer. If rod "A" pushes against the end of the bellows, air is forced out of the bellows through the check valve as the bellows con tracts. When the bellows is moved back, contact TR changes from an open to a closed contact. When rod "A" is pulled away from the bellows, the spring tries to return the bellows to its original position. Before the bellows can be returned to its original position, however, air must enter the bellows through the air in let port. The rate at which the air is permitted to enter the bellows is controlled by the needle valve. When the bellows returns to its original position, contact TR re turns to its normally open position.
Pneumatic timers are popular throughout industry because they have the following characteristics:
A. They are unaffected by variations in ambient temperature or atmospheric pressure.
B. They are adjustable over a wide range of time periods.
C. They have good repeat accuracy.
D. They are available with a variety of contact and timing arrangements.
Some pneumatic timers are designed to permit the timer to be changed from on-delay to off-delay, and the con tact arrangement to be changed to normally opened or normally closed . This type of flexibility is another reason for the popularity of pneumatic timers.
Many timers are made with contacts that operate with the coil as well as time delayed contacts. When these contacts are used, they are generally referred to as instantaneous contacts and indicated on a schematic diagram by the abbreviation INST. printed below the contact. These instantaneous contacts change their positions immediately when the coil is energized and change back to their normal positions immediately when the coil is de-energized.
Another timer frequently used is the clock timer. Clock timers use a small AC synchronous motor similar to the motor found in a wall clock to pro vide the time measurement for the timer. The length of time of one clock timer may vary greatly from the length of time of another. E.g., one timer may have a full range of 0 to 5 seconds and another timer may have a full range of 0 to 5 hours. The same type of timer motor could be used with both timers. The gear ratio connected to the motor would determine the full range of time for the timer. Some advantages of clock timers are:
A. They have extremely high repeat accuracy.
B. Readjustment of the time setting is simple and can be done quickly. Clock timers are generally used when the machine operator must make adjustments to the time length.
Fgr. 9 Clock driven timer.
Min. Time Delay: 0.05 second
Fgr. 10 (above) Typical specifications.
When a process has a definite on and off operation, or a sequence of successive operations, a motor-driven timer is generally used.
A typical application of a motor-driven timer is to control laundry washers where the loaded motor is run for a given period in one direction, reversed, and then run in the opposite direction.
Generally, this type of timer consists of a small, synchronous motor driving a cam-dial assembly on a common shaft. A motor-driven timer successively closes and opens switch contacts, which are wired in circuits to energize control relays or contactors to achieve desired operations.
+Capacitor Time Limit Relay
Assume that a capacitor is charged by connecting it momentarily across a DC line, and then the capacitor direct current is discharged through a relay coil. The current induced in the coil will decay slowly, depending on the relative values of capacitance, inductance, and resistance in the discharge circuit.
If a relay coil and a, capacitor are connected parallel to a DC line, the capacitor is charged to the value of the line voltage and a current appears in the coil. If the coil and capacitor combination is now removed from the line, the current in the coil will start to decrease along the curve shown in Fgr. 12.
If the relay is adjusted so that the armature is released at current i1, a time delay of t1 is obtained. The time delay can be increased to a value of t2 by adjusting the relay so that the armature won't be released until the current is reduced to a value of i2. --- shows a relay used for this type of time control.
A potentiometer is used as an adjustable resistor to vary the time. This resistance-capacitance (RC) theory is used in industrial electronic and solid-state controls also. This timer is highly accurate and is used in motor acceleration control and in many industrial processes.
Electronic timers use solid-state components to provide the time delay desired. Some of these timers use an RC time constant to obtain the time base and others use quartz clocks as the time base. RC time constants are inexpensive and have good repeat times.
The quartz timers, however, are extremely accurate and can often be set for 0.1 second times. These timers are generally housed in a plastic case and are designed to be plugged into some type of socket. An electronic timer that's designed to be plugged into a standard eight-pin tube socket. The length of the time delay can be set by adjusting the control knob shown on top of the timer.
Eight-pin electron timers are intended to be used as on-delay timers only. Many electronic timers are designed to plug into an eleven-pin tube socket and are more flexible. Two such timers are shown. Either of these timers can be used as an on-delay timer, an off-delay timer, a pulse timer, or as a one-shot timer. Pulse timers continually turn on and off at regular intervals, timing period chart for a pulse timer set for a delay of 1 second. A one-shot timer will operate for one time period only. A timing period chart for a one shot timer set for 2 seconds.
Most electronic timers can be set for a wide range of times. The timer uses a thumbwheel switch to enter the timer setting. The top selector switch can be used to set the full range value from 9.99 seconds to 999 minutes. This timer has a range from0.01 second to 999 minutes (16 hrs. 39min.).
The timer can be set for a range of 0.01 second to 100 hours by adjusting the range and units settings on the front of the timer. Most electronic timers have similar capabilities.
Connecting Eleven-Pin Timers
Connecting eleven-pin timers into a circuit's generally a little more involved than simply connecting the coil to power. The manufacturer's instructions should always be consulted before trying to connect one of these timers. Although most electronic timers are similar in how they are connected, there are differences.
The pin connection diagram for the timer is shown. Notice that a normally open push-button switch is shown across terminals 5 and 6. This switch is used to start the action of the timer when it's set to function as an off-delay timer or as a one-shot timer. The reason for this is that when the timer is to function as an off-delay timer, power must be applied to the timer at all times to permit the internal timing circuit to operate. If power is removed, the internal timer cannot function. The start switch is actually used to initiate the operation of the timer when it's set to function in the off-delay mode. Recall the logic of an off-delay timer: When the coil is energized, the contacts change position immediately. When the coil is de-energized, the contacts delay returning to their normal position. According to the pin chart shown in Fgr. 20, pins 2 and 10 connect to the coil of the timer. To use this timer in the off-delay mode, power must be connected to pins 2 and 10 at all times. Shorting pins 5 and 6 together causes the timed contacts to change position immediately. When the short circuit between pins 5 and 6 is removed, the time sequence begins. At the end of the preset time period, the contacts will return to their normal position.
If electronic off-delay timers are to replace pneumatic off-delay timers in a control circuit, it's generally necessary to modify the circuit. E.g., in the circuit shown in Fgr. 21, it's assumed that starters 1M and 2M control the operation of two motors, and timer TR is a pneumatic off-delay timer. When the start button is pressed, two motors start at the same time. The motors will continue to operate until the stop button is pressed, which causes motor #1 to stop running immediately. Motor #2, however, will continue to run for a period of 5 seconds before stopping.
Now assume that the pneumatic off-delay timer is to be replaced with an electronic off-delay timer. In this circuit, notice that the coil of the timer is connected directly across the incoming power, which permits it to remain energized at all times. In the circuit the timer actually operates with starter 1M. When coil 1M energizes, timer TR energizes at the same time. When coil 1M de-energizes, timer TR de-energizes also. For this reason, a normally open auxiliary contact on starter 1M will be used to control the operation of the electronic off-delay timer.
In the circuit a set of normally open 1M contacts is connected to pins 5 and 6 of the timer. When coil 1M energizes, contact 1M closes and shorts pins 5 and 6, causing the normally open TR contacts to close and energize starter coil 2M. When coil 1M is de-energized, the contacts reopen and timer TR begins timing. After 5 seconds, contacts TR reopen and de-energize starter coil 2M.
All electronic timers are similar, but there are generally differences in how they are to be connected.
The connection diagram for the timer shown in Fgr. 17B is shown in Fgr. 23. Notice that this timer contains RESET, START, and GATE pins. Connecting pin 2 to pin 5 activates the GATE function, which interrupts or suspends the operation of the internal clock. Connecting pin 2 to pin 6 activates the START function, which operates in the same manner as the timer shown in Fgr. 17A. Connecting pin 2 to pin 5 activates the RESET function, which resets the internal clock to zero. If this timer were to be used in the circuit shown in Fgr. 22, it would have to be modified as shown in Fgr. 24 by connecting the 1M normally open contact to pins 2 and 6 instead of pins 5 and 6.
Construction of a Simple Electronic Timer
The schematic for a simple on-delay timer is shown in Fgr. 25. The timer operates as follows: When switch S1 is closed, current flows through resistor RT and begins charging capacitor C1. When capacitor C1 has been charged to the trigger value of the unijunction transistor, the UJT turns on and discharges capacitor C1 through resistor R2 to ground. The sudden discharge of capacitor C1 causes a spike voltage to appear across resistor R2. This voltage spike travels through capacitor C2 and ?res the gate of the silicon-controlled rectifier (SCR). When the SCR turns on, current is pro vided to the coil of relay K1.
Resistor R1 limits the current flow through the UJT. Resistor R3 is used to keep the SCR turned off until the UJT provides the pulse to ?re the gate. Diode D1 is used to protect the circuit from the spike voltage produced by the collapsing magnetic field around coil K1 when the current is turned off.
By adjusting resistor RT, capacitor C1 can be charged at different rates. In this manner, the relay can be adjusted for time. Once the SCR has turned on, it will remain on until switch S1 is opened.
Programmable controllers, which will be discussed in sections 53 through 55, contain "internal" electronic timers. Most programmable controllers (PLCs) use a quartz-operated clock as the time base. When the controller is programmed, the timers can be set in time increments of 0.1 second. This, of course, provides very accurate time delays for the controller.
1. What are the two basic classifications of timers?
2. Explain the operation of an on-delay relay.
3. Explain the operation of an off-delay relay.
4. What are instantaneous contacts?
5. How are pneumatic timers adjusted?
6. Name two methods used by electronic timers to obtain their time base.
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