Answer the following with reference to the process control relay ladder diagram

Question

Answer the following with reference to the process control relay ladder diagram of Figure 1-17 of this chapter When do the pressure switch contacts close? When do the temperature switch contacts close? How are the pressure and temperature switches connected with respect to each other? Describe the two conditions under winch the motor starter coil will become energized. What is the approximate value of the voltage drop across each of the following when their contacts are open? Pressure switch Temperature switch Manual pushbutton

Explanation / Answer

Since no idea about the external system has been given, these switches in ladder logic are to be considered in a process system.

a. Pressure switch contacts close when pressure at a particular point in the process system is more than the nominal value. The pressure is sensed using pneumatic sensors.

b. Similar to pressure switch, temperature switch operates when temperature is more than a nominal value. Thermal sensors are used to sense temperature.

c. As seen in the diagram, both the switches are connected in series.

d. The motor operates when either manual pushbutton is made ON or both pressure and temperature switches turn ON.

e. When switches are open, voltage across motor = 0V. So, voltage drop is across switches only. Total voltage is 120V. Since both pressure and temperature switches are in series, they share equal voltage across them. So voltage across pressure switch=voltage across temperature switch=60V.

Now since, manual pushbutton is connected directly across 120V, voltage across manual pushbutton is 120V.

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