List two common types of PLC programming devices.
Answer the following with reference to the unit process control relay ladder diagram of
Figure ‘ ·9 on page 11 :
a. When do the pressure switch contacts dose?
b. When do the temperature switch contacts dose?
c. How are the pressure and temperature switches connected with respect to each other?
d. Describe the two conditions under which the motor starter coil will become energized.
e. What is the approximate value of the voltage drop across each of the following when their
contacts are open?
(I) Pressure switch
121 Temperature switch
(l) Manual pushbutton
The correct answer and explanation is:
Common Types of PLC Programming Devices:
- Hand-Held Programmer (HHP): This portable device allows users to program PLCs without needing a computer. It can be used to input and edit control logic, monitor status, and configure settings on the PLC.
- Programming Software (PC-based): This is installed on a personal computer and connected to the PLC via communication cables or networks. The software allows for the creation and modification of control programs, simulation, debugging, and monitoring of PLC operations. Examples include RSLogix 5000, TIA Portal, and Step 7.
Unit Process Control Relay Ladder Diagram (Figure ‘ ·9 on page 11):
a. When do the pressure switch contacts close?
Pressure switch contacts close when the pressure in the system reaches or exceeds the set point defined in the switch. For example, in a system with a pressure switch, the contacts might close when the pressure rises above a predefined value, indicating that the system has reached the desired pressure level.
b. When do the temperature switch contacts close?
Temperature switch contacts close when the temperature in the system exceeds the setpoint specified for the switch. This could be when the temperature rises above a certain threshold, triggering the contact closure to send a signal to the control system.
c. How are the pressure and temperature switches connected with respect to each other?
In a typical ladder diagram, pressure and temperature switches are often connected in series or parallel, depending on the desired control logic. In a series connection, both switches must be activated (contacts closed) for the circuit to be complete and trigger the output (e.g., energizing the motor starter). In a parallel connection, either switch can trigger the action, meaning the contacts from either the pressure or temperature switch can close independently to complete the circuit.
d. Describe the two conditions under which the motor starter coil will become energized:
- Condition 1: The motor starter coil will be energized when both the pressure switch and temperature switch contacts are closed (if wired in series). This could indicate that both the system’s pressure and temperature have reached acceptable levels, triggering the system to start.
- Condition 2: The motor starter coil could also be energized if a manual pushbutton (such as a start button) is pressed, providing a direct signal to the motor starter coil to initiate motor operation, independent of the switches.
e. What is the approximate value of the voltage drop across each of the following when their contacts are open?
- Pressure Switch (open contacts): When the pressure switch contacts are open, the voltage drop across the contacts is very small—typically in the range of 0 to 1 volt. This is because an open contact in the circuit essentially creates a high resistance, and very little current flows.
- Temperature Switch (open contacts): Similarly, when the temperature switch contacts are open, there will also be a very small voltage drop across the switch, typically around 0 to 1 volt, for the same reason of high resistance and minimal current flow in the open state.
- Manual Pushbutton (open contacts): If the manual pushbutton is in the open position, the voltage drop is essentially negligible, as the open circuit prevents any current from flowing, resulting in a voltage drop close to 0 volts.
In summary, PLC programming devices come in forms like hand-held programmers and PC-based software. The ladder diagram for the system ensures that the pressure and temperature switches control the motor starter, either through series or parallel connections, with minimal voltage drops when contacts are open.