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There may be times when it's desirable to have more than one start-stop push-button station to control a motor. In this section, the basic start-stop push-button control circuit 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 the existing start stop control circuit shown in Ill. 1. The second push button will be added to the control circuit by connecting it in series with the existing Stop push button.
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 closes to maintain the circuit to M coil when the Start button is released.
Developing a 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 section 22, wire numbers are placed on the schematic diagram (Ill. 4). After wire numbers are placed on the schematic, corresponding numbers are placed on the control components (Ill. 5).
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?
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