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page 226

Rated Current - The maximum current before contact damage occurs (welding or melting).

DC relays require special arc suppression. AC relays have a zero crossing to reduce relay arc problems.

AC relays require a shading pole to maintain contact. If a DC relay is used with AC power on the coil it clicks on-and-off at the frequency of the AC (also known as chattering).

18.2 PRACTICE PROBLEMS

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.

N L1 L2 L3

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

com

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

com

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

3

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

com

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

4

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

com

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

5

 

 

 

 

 

 

 

 

+

DC

-

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

com

 

 

 

 

 

 

 

 

power

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

supply

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

page 227

ANS.

N L1 L2 L3

1

com

2

com

3

com

4

com

5

DC

+power - com supply

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.

page 228

200

 

 

 

201

 

 

A

202

 

100

 

 

 

 

203

 

101

 

 

 

B

 

 

102

204

 

 

 

 

 

205

 

103

 

 

 

C

 

 

104

206

 

 

+

 

 

 

105

 

207

 

 

24VDC

 

 

 

106

+

com

 

 

 

 

210

solenoid

107

12VDC

 

 

valve

 

 

 

com

 

com

 

 

 

 

 

211

120VAC

 

 

 

 

 

com

 

 

 

ANS.

 

 

 

outputs:

100

 

200

200 - light

 

 

 

202 - motor

 

 

 

210 - solenoid

 

 

 

inputs:

102

 

202

 

 

 

100 - switch A

 

 

 

102 - switch B

 

 

 

104 - switch C

104

 

210

 

 

page 229

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.

page 230

ANS.

0

 

1

 

2

 

3

 

4

 

5

 

6

 

7

+

 

24VDC

com

-

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)

0

1

2

3

4

5

6

7

com

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

page 231

a 24Vdc solenoid valve, a 110Vac light bulb, and a 220Vac 50HP motor. Sketch the basic wiring below including PLC cards.

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