18.2 PRACTICE PROBLEMS

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1. TRUE / FALSE -- A PLC input can switch a relay coil to control a motor.

(ans. false - a plc OUTPUT can switch a relay)

2. In the figure below, properly connect the input devices to the contacts shown.

3. For the circuit shown in the figure below, list the input and output addresses for the PLC. If switch A controls the light, switch B the motor, and C the solenoid, write a simple ladder logic program.

4. The main PLC rack refers to an enclosure that contains:

a) several local racks

b) one local and one remote enclosure

c) the processor

d) input and output modules

(ans. c)

5. The digital I/O cards provide the interface between:

a) The PLC and the power supply

b) The CPU and a drop

c) Input modules and CPU

d) External equipment and the PLC

(ans. d)

6. We have a PLC (rack 1) with a 24 VDC input card (slot 3), and a 120VAC output card (slot 2). The inputs are to be connected to 4 push buttons. The outputs are to drive a 120VAC lightbulb, a 240VAC motor, and a 24VDC operated hydraulic valve.

a) Draw the electrical connections for the inputs and outputs. Show all other power supplies and other equipment/components required.

b) One of the switches will turn the motor on, another switch turns the motor off. When the motor is on, the hydraulic solenoid will turn on. The light is on when the motor is off. Draw a Petri net of the control program.

c) Develop ladder logic for one of the states and one of the transitions.

7. What is the difference between a motor starter and a contactor?

(ans. a motor starter typically has three phases)

8. Is AC or DC easier to interrupt?

(ans. AC is easier, it has a zero crossing)

9. What can happen if the rated voltage on a device is exceeded?

(ans. it will lead to premature failure)

10. What are the benefits of input/output modules?

11. Explain the operation of AC input and output conditioning circuits.

12. What is the wiring difference between a sourcing and sourcing output?

13. What will happen if a DC output is switched by an AC output.

14. You are planning a project that will be controlled by a PLC. Before ordering parts you decide to plan the basic wiring and select appropriate input and output cards. The devices that we will use for inputs are 2 contact switches, a push button and a thermal switch. The output will be for a 24Vdc solenoid valve, a 110Vac light bulb, and a 220Vac 50HP motor. Sketch the basic wiring below including PLC cards.