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• Discuss the operation of an on-delay timer.
• Draw the NEMA contact symbols used to represent both normally open and normally closed on-delay contacts.
• Discuss the difference in operation between pneumatic and electronic timers.
• Connect a circuit in the laboratory employing an on-delay timer.
Timers can be divided into two basic types: on-delay and off-delay. Although there are other types such as one shot and interval, they are basically an on- or off-delay timer. In this unit the operation of on-delay timers is discussed. The operating sequence of an on-delay timer is as follows:
When the coil is energized, the timed contacts will delay changing position for some period of time. When the coil is de-energized, the timed contacts will return to their normal position immediately. In this explanation, the word "timed contacts" is used. The reason is that some timers contain both timed and instantaneous contacts. When using a timer of this type, care must be taken to connect to the proper set of contacts.
>>>Helpful Hint: When the coil is energized, the timed contacts will delay changing position for some period of time. When the coil is de-energized, the timed contacts will return to their normal position immediately.
The timed contacts are controlled by the action of the timer, while the instantaneous contacts operate like any standard set of contacts on a control relay; when the coil energizes, the contacts change position immediately, and when the coil de-energizes they change back to their normal position immediately.
The standard NEMA symbols used to represent on-delay contacts are shown in Ill. 1.
The arrow points in the direction the contact will move after the delay period. The normally open contact, for example, will close after the time delay period, and the normally closed con tact will open after the time delay period.
Ill. 1 NEMA standard symbols for on-delay contacts.
Ill. 2 Instantaneous contact symbols.
Ill. 3 The motor starts after the start button is pressed.
Ill. 4 A control relay furnishes the instantaneous contact.
Instantaneous contacts are drawn in the same manner as standard relay contacts. Ill. 2 illustrates a set of instantaneous contacts controlled by timer TR. The instantaneous contacts are often used as holding or sealing contacts in a control circuit. The control circuit shown in Ill. 3 illustrates an on-delay timer used to delay the starting of a motor.
When the start push button is pressed, TR coil energizes and the normally open instantaneous TR contacts close immediately to hold the circuit. After the preset time period, the normally open TR timed contacts will close and energize the coil of M starter, which connects the motor to the line.
When the stop button is pressed and TR coil de-energizes, both TR contacts return to their normal position immediately. This de-energizes M coil and disconnects the motor from the line.
Control Relays Used with Timers
Not all timers contain instantaneous contacts. Most electronic timers, for example, don't. When an instantaneous contact is needed and the timer does not have one available, it's common practice to connect the coil of a control relay in parallel with the coil of the timer (Ill. 4). In this way the electronic timer will operate with the timer. In the circuit shown in Ill. 4, both coils TR and CR will energize when the start button is pressed.
This causes CR contact to close and seal the circuit.
Time Delay Methods
Although there are two basic types of timers, there are different methods employed to obtain a time delay. One of the oldest methods still in general use is the pneumatic timer. Pneumatic timers use a bellows or diaphragm and operate on the principle of air displacement. Some type of needle valve is generally used to regulate the airflow and thereby regulate the time delay.
Pneumatic timers are simple in that they contain a coil, contacts, and some method of adjusting the amount of time delay. Because of their simplicity of operation, when control circuits are in the design stage, the circuit logic is generally developed with the assumption that pneumatic timers will be used. After the circuit logic has been developed, it may be necessary to make changes that will accommodate a particular type of timer.
Another very common method of providing a time delay is with an electric clock similar to a wall clock. These timers contain a small single-phase synchronous motor. As a general rule, most clock timers can be set for different full-scale values by changing the gear ratio.
Electronic timers are becoming very popular for several reasons:
1. They are much less expensive than pneumatic or clock timers.
2. They have better repeat accuracy than pneumatic or clock timers.
3. Most can be set for 0.1-second delays and many can be set to an accuracy of 0.01 second.
4. Many electronic timers are intended to be plugged into an 8- or 11-pin tube socket. This makes replacing the timer much simpler and takes less time.
LABORATORY EXERCISE Name _________ Date __
Three-phase power supply Control transformer 2 double-acting push buttons (N.O./N.C. on each button) 2 three-phase motor starters with at least one normally open auxiliary contact Dayton Solid-State Timer-model 6A855 or equivalent and 11-pin socket 8-pin control relay and 8-pin socket 2 three-phase motors or equivalent motor loads
The First Circuit:
The first circuit to be connected is shown in Ill. 4. In this circuit it will be assumed that an 11-pin timer is being used and that the coil is connected to pins 2 and 10, and a set of normally open timed contacts is connected to pins 1 and 3. The coil of the 8-pin control relay is connected to pins 2 and 7 and a normally open contact is connected to pins 1 and 3. When using control devices that are connected with 8- and 11-pin sockets, it's generally helpful to place pin numbers beside the component. To prevent pin numbers from being confused with wire numbers, a circle will be drawn around the pin numbers (Ill. 5).
Ill. 5 Placing pin numbers beside the components.
Connecting Circuit #1
1. Using the circuit shown in Ill. 5, place wire numbers beside the components.
2. Connect the control part of the circuit by following the wire numbers placed beside the components. Note the pin numbers beside the coils and contacts of the timer and control relay.
3. Plug the timer and control relay into their appropriate sockets. Set the timer to operate as an on-delay timer and set the time period for 5 seconds.
4. After checking with the instructor, turn on the power and test the operation of the circuit.
5. Turn off the power.
6. If the control part of the circuit operated correctly, connect the motor or equivalent motor load.
7. Turn on the power and test the total circuit for proper operation.
8. Turn off the power and disconnect the circuit.
Discussing Circuit #2
In the next circuit, two motors are to be started with a 5-second time delay between the starting of the first motor and the second motor. In this circuit a normally open auxiliary contact on starter 1M is used as the holding contact, making the use of the control relay unnecessary.
When the start button is pressed, coils 1M and TR energize immediately. This causes motor #1 to start operating and timer TR to begin timing. After 5 seconds, TR contacts close and connect motor #2 to the line. When the stop button is pressed, or if an overload on either motor should occur, all coils will be de-energized and both motors will stop.
Connecting Circuit #2
1. Using the circuit shown in Ill. 6, place pin numbers beside the timer coil and normally open contact.
2. Place wire numbers on the circuit in Ill. 6.
3. Connect the control part of the circuit.
4. Turn on the power and test the circuit for proper operation.
5. Turn off the power.
6. If the control part of the circuit operated properly, connect the motors or equivalent motor loads.
7. Turn on the power and test the circuit for proper operation.
8. Turn off the power and disconnect the circuit.
Ill. 6 Motor 2 starts after motor 1.
1. Explain the operation of an on-delay timer.
2. Explain the difference between timed contacts and instantaneous contacts.
3. Refer to the circuit shown in Ill. 3. If the timer has been set for a delay of 10 seconds, explain the operation of the circuit when the start button is pressed.
4. In the circuit shown in Ill. 3, is it necessary to hold the start button closed for a period of at least 10 seconds to ensure that the circuit will remain energized? Explain your answer.
5. Assume that the timer in Ill. 3 is set for a delay of 10 seconds. Now assume that the start button is pressed, and after a delay of 8 seconds the stop button is pressed.
Will the motor start 2 seconds after the stop button is pressed?
6. What is generally done to compensate when a set of instantaneous timer contacts is needed and the timer does not contain them?
7. Refer to the circuit shown in Ill. 6. Assume that it's necessary to stop the operation of both motors after the second motor has been operating for a period of 10 seconds.
Using the space provided in Ill. 7, redraw the circuit to turn off both motors after the second motor has been in operation for 10 seconds. (Note: It will be necessary to use a second timer.)
8. After your instructor has approved the design change, connect the new circuit in the laboratory and test it for proper operation.
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