3.7 PRACTICE PROBLEMS
(Note: Problem solutions are available at http://sites.google.com/site/automatedmanufacturingsystems/)
1. Can a PLC input switch a relay coil to control a motor?
2. How do input and output cards act as an interface between the PLC and external devices?
3. What is the difference between wiring a sourcing and sinking output?
4. What is the difference between a motor starter and a contactor?
5. Is AC or DC easier to interrupt?
6. What can happen if the rated voltage on a device is exceeded?
7. What are the benefits of input/output modules?
8. (for electrical engineers) Explain the operation of AC input and output conditioning circuits.
9. What will happen if a DC output is switched by an AC output.
10. Explain why a stop button must be normally closed and a start button must be normally open.
11. 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.
12. We have a PLC rack with a 24 VDC input card in slot 3, and a 120VAC output card in slot 2. The inputs are to be connected to 4 push buttons. The outputs are to drive a 120VAC light bulb, a 240VAC motor, and a 24VDC operated hydraulic valve. Draw the electrical connections for the inputs and outputs. Show all other power supplies and other equipment/components required.
13. 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 limit 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.
14. Add three push buttons as inputs to the figure below. You must also select a power supply, and show all necessary wiring.
15. Three 120Vac outputs are to be connected to the output card below. Show the 120Vac source, and all wiring.
16. Sketch the wiring for PLC outputs that are listed below.
- a double acting hydraulic solenoid valve (with two coils)
- a 120 Vac high current lamp
- a low current 12Vdc motor
17. Draw a ladder wiring diagram for a system that has 2 PNP inputs, and 2 solenoid outputs. All inputs and outputs are 24Vdc. Include ALL safety circuitry.
