Programmable Logic Controllers PLCs Introduction to Ladder Diagrams
Programmable Logic Controllers (PLCs) Introduction to Ladder Diagrams • • • Direct-On-Line Starters Set/Reset Latch/Unlatch Output Instructions ACOE 347 (Spring 2017) Latching Instructions 1
Motor Starters • According to Electrical Installation regulations high power motors can not be connected directly (through an On/Off switch) on the electrical supply lines. – Why? . . Assume that a motor drives a conveyor belt and while the motor was On it went Off due to an electricity failure. A technician might assume that the problem is due to the conveyor mechanisms and attempt to fix it. If the electricity supply is restored, the motor will switch on automatically with a high possibility of harming the technician. – Thus some means of protection are required • Thus power motors must be connected to the electricity supply lines through a “Starter” • Simplest one called “Direct-on-Line” starter • Based on two push buttons – Start: Normally Open (NO) push button. When momentarily pressed it switches On the motor. The motor remains On even if the start button is released. – Stop: Normally Closed (NC) push button. If the stop push button is momentarily pressed the motor stops. The motor can only start again only if the start button is pressed. – If the motor has stopped due to a failure such as electricity supply failure or motor overload, it will start again only if the start button is pressed ACOE 347 (Spring 2017) Latching Instructions 2
Relays and Contactors • Contactor: A heavy duty relay – can drive loads with high power ratings such as power motors – Relays can drive DC or AC loads with lower power requirements such as in automotive applications and domestic appliances • A contactor consists of: – A relay coil – A number of power contacts – A pair of auxiliary (or Hold-On) contacts (low power contacts smaller) Contactor Relay Power Contacts Relay Coil ACOE 347 (Spring 2017) Latching Instructions Auxiliary Contacts 3
Direct-on-Line Starter • “Start” button pressed – Relay is energised auxiliary contact is closed – Relay is energised power contact is closed Motor Runs • “Start” button released – Relay is still energised through auxiliary contact power contact is closed Motor Runs • “Stop” button pressed – Relay is not energised power contact is open Motor Stops • Power failure while motor runs – Relay is not energised power contact is open Motor Stops need to press “Start” button to start the motor after power is restored. ACOE 347 (Spring 2017) Latching Instructions 4
Ladder Implementation of Direct-on-Line Starter Electrical Circuit PLC Wiring Circuit Ladder Diagram for NC push button ACOE 347 (Spring 2017) Latching Instructions 5
Set/Reset Latch using a Hold-in Contact Notes: O 1: 00’ means that the output is unchanged If both inputs are ON then normally the output is OFF. Example: ACOE 347 (Spring 2017) Latching Instructions 6
Output Instructions • Output Latch/Set and Output Unlatch/Reset (OTL), (OTU) If the condition of the left link of the OTL is momentary ON then the corresponding bit in the output data memory is set, and remains set even if the condition switches to the OFF state. The output will remain set until the condition of the left link of the OTU is momentary ON ACOE 347 (Spring 2017) Latching Instructions 7
Set/Reset Latch using Latch/Unlatch outputs Notes: – O 1: 00’ means that the output is unchanged – If both inputs are ON then normally the output is OFF, since the Unlatch rung appears last in the ladder diagram. ACOE 347 (Spring 2017) Latching Instructions 8
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