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• Place wire numbers on a schematic diagram.
• Place corresponding numbers on control components.
• Draw a wiring diagram from a schematic diagram.
• Connect a control circuit using two stop and two start push buttons.
There may be times when it's desirable to have more than one start-stop push-button station to control a motor. In this experiment the basic start-stop push-button control circuit discussed previously will be modified to include a second stop and start push button.
When a component is used to perform the function of stop in a control circuit, it will generally be a normally closed component and be connected in series with the motor starter coil.
In this example a second stop push button is to be added to an existing start-stop control circuit. The second push button will be added to the control circuit by connecting it in series with the existing stop push button ( Ill. 1). When a component is used to perform the function of start, it's generally normally open and connected in parallel with the existing start button ( Ill. 2). If either start button is pressed, a circuit will be completed to M coil. When M coil energizes, all M contacts change position. The three load contacts connected between the three-phase power line and the motor close to connect the motor to the line. The normally open auxiliary contact connected in parallel with the two start buttons close to maintain the circuit to M coil when the start button is released.
Ill. 1 Adding a stop button to the circuit.
Ill. 2 A second start button is added to the circuit.
Developing the Wiring Diagram
Now that the circuit logic has been developed in the form of a schematic diagram, a wiring diagram will be drawn from the schematic. The components needed to connect this circuit are shown in Ill. 3. Following the same procedure discussed in Experiment 1, wire numbers will be placed on the schematic diagram ( Ill. 4). After wire numbers are placed on the schematic, corresponding numbers will be placed on the control components ( Ill. 5).
LABORATORY EXERCISE Name ___ Date __
Three-phase power supply Three-phase squirrel cage induction motor or simulated load 4 double-acting push buttons (N.O./N.C. on same button) Three-phase motor starter or contactor with overload relay containing three load contacts and at least one normally open auxiliary contact Control transformer
Ill. 3 Components needed to produce a wiring diagram.
Ill. 4 Numbering the schematic diagram.
Ill. 5 Numbering the components.
Ill. 6 The stop buttons are connected in parallel.
Ill. 7 The start buttons are connected in series.
Connecting the Circuit:
1. Using the schematic in Ill. 4 or the diagram with numbered components in Ill. 5, connect the circuit in the laboratory by connecting all like numbers together.
2. After the circuit has been connected, check with your instructor before turning on the power.
3. Turn on the power and test the circuit for proper operation.
4. Turn off the power and disconnect the circuit. Return all components to their proper place.
1. When a component is to be used for the function of start, is the component generally normally open or normally closed?
2. When a component is to be used for the function of stop, is the component generally normally open or normally closed?
3. The two stop push buttons in Ill. 2 are connected in series with each other. What would be the action of the circuit if they were to be connected in parallel as shown in Ill. 6?
4. What would be the action of the circuit if both start buttons were to be connected in series as shown in Ill. 7?
5. Following the procedure discussed previously, place wire numbers on the schematic in Ill. 7. Place corresponding wire numbers on the components shown in Ill. 8.
Ill. 8 Add wire numbers to these components.
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