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• Describe a step-by-step procedure for designing a motor control circuit.
• Design a basic control circuit.
• Connect the completed circuit in the laboratory.
Ill. 1 First step in the circuit design.
Ill. 2 Completing the circuit logic.
Name ______ Date ______
Three-phase power supply Three-phase motor starter 8- or 11-pin on-delay relay with appropriate socket Three-phase motor or equivalent motor load Pilot light Buzzer or simulated load Control transformer 8-pin control relay and 8-pin socket In this experiment a circuit for a large printing press will be designed in a step-by-step procedure. The owner of a printing company has the following concern when starting a large printing press:
The printing press is very large and the surrounding noise level is high. There is a danger that when the press starts, a person unseen by the operator may have his or her hands in the press. To prevent an accident, I would like to install a circuit that sounds an alarm and flashes a light for 10 seconds before the press actually starts. This would give the person time to get clear of the machine before it starts.
To begin the design procedure, list the requirements of the circuit. List not only the concerns of the owner but also any electrical or safety requirements that the owner may not be aware of. Understand that the owner is probably not an electrical technician and does not know all the electrical requirements of a motor control circuit.
1. There must be a start and stop push-button control.
2. When the start button is pressed, a warning light and buzzer turn on.
3. After a delay of 10 seconds, the warning light and buzzer turn off and the press motor starts.
4. The press motor should be overload protected.
5. When the stop push button is pressed, the circuit will de-energize even if the motor has not started.
To begin design of the circuit, fulfill the first requirement of the logic: "When the start button is pressed, a warning light and buzzer turn on for a period of 10 seconds." This first part of the circuit can be satisfied with the circuit shown in Ill. 1. In this example a timer is used because the warning light and buzzer are to remain on for only 10 seconds. Since the warning light and buzzer are to turn on immediately when the start button is pressed, a normally closed timed contact is used. This circuit also assumes that the timer contains an instantaneous contact that's used to hold the circuit in after the start button is released.
The next part of the logic states that after a delay of 10 seconds the warning light and buzzer are to turn off and the press motor is to start. As the present circuit's shown in Ill. 1, when the start button is pressed, TR coil will energize. This causes the normally open instantaneous TR contacts to close and hold TR coil in the circuit when the start button is released. At the same time, timer TR starts its timing sequence. After a delay of 10 seconds, the normally closed TR timed contact connected in series with the warning light and buzzer will open and disconnect them from the circuit.
The only remaining circuit logic is to start the motor after the warning light and buzzer have turned off. This can be accomplished with a normally open timed contact controlled by timer TR ( Ill. 2). At the end of the timing sequence, the normally closed TR contact will open and disconnect the warning light and buzzer. At the same time, the normally open TR timed contact will close and energize the coil of M starter. The normally closed overload contact connected in series with the rest of the circuit will de-energize the entire circuit in the event of motor overload.
Now that the logic of the control circuit has been completed, the motor load can be added as shown in Ill. 3.
Ill. 3 The complete circuit.
Addressing a Potential Problem
The completed circuit shown in Ill. 3 assumes the use of a timer that contains both timed and instantaneous contacts. This contact arrangement is common for certain types of timers such as pneumatic and some clock timers, but most electronic timers don't contain instantaneous contacts. If this is the case, a control relay can be added to supply the needed instantaneous contact by connecting the coil of the control relay in parallel with the coil of TR timer ( Ill. 4).
Ill. 4 Adding a control relay.
Connecting the Circuit
1. It will be assumed that the timer in this circuit's the electronic type. Therefore, it will be assumed that a control relay will be used to provide the normally open holding contacts.
Assuming the use of an electronic on-delay timer and an 8-pin control relay, place pin numbers beside the components of the timer and control relay shown in Ill. 4. Circle the numbers to distinguish them from wire numbers.
2. Place wire numbers beside the components in Ill. 4.
3. Connect the control portion of the circuit. (Note: It may be necessary to use a pilot light for the buzzer if one isn't available.)
4. Turn on the power and test the control part of the circuit for proper operation.
5. Turn the power off.
6. If the control part of the circuit operated properly, connect the motor or simulated motor load to the circuit.
7. Turn on the power and test the entire circuit for proper operation.
8. Turn off the power and return the components to their proper location.
1. What should be the first step when beginning the design of a control circuit?
2. Why is it sometimes necessary to connect the coil of a control relay in parallel with the coil of a timer?
3. Refer to the circuit shown in Ill. 3. Assume that the on-delay timer is replaced with an off-delay timer. Describe the action of the circuit when the start button is pressed.
4. Describe the operation of the circuit when the stop button is pressed. Assume the circuit's running with an off-delay timer as described in question 3.
5. Refer to the circuit shown in Ill. 4. Assume the owner decides to change the logic of the circuit as follows:
When the operator presses the start button, a warning light and buzzer turn on for a period of 10 seconds. During this 10 seconds, the operator must continue to hold down the start button. If the start button should be released, the timing sequence will stop and the motor won't start. At the end of 10 seconds, provided the operator continues to hold the start button down, the warning light and buzzer will turn off and the motor will start. When the motor starts, the operator can release the start button and the press will continue to run.
Amend the circuit in Ill. 4 to meet the requirement.
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