1. Programmable Logic Controllers (PLCs)
Introduction to Ladder Diagrams
Direct-On-Line Starters
Set/Reset Latch
Latch/Unlatch Output Instructions
ACOE347 (Spring2017)
Latching Instructions

2. Latching Instructions
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
ACOE347 (Spring2017)
Latching Instructions

3. Latching Instructions
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)
Relay
Contactor
Power Contacts
Relay Coil
AuxiliaryContacts
ACOE347 (Spring2017)
Latching Instructions

4. 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
 need to press “Start” button to start the motor after power is restored.
ACOE347 (Spring2017)
Latching Instructions

5. Ladder Implementation of Direct-on-Line Starter
Electrical Circuit
PLC Wiring Circuit
Ladder Diagram
for NC push button
ACOE347 (Spring2017)
Latching Instructions

6. Set/Reset Latch using a Hold-in Contact
Notes:
O1:00’ means that the output is unchanged
If both inputs are ON then normally the output is OFF.
Example:
ACOE347 (Spring2017)
Latching Instructions

7. Latching Instructions
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
ACOE347 (Spring2017)
Latching Instructions

8. Set/Reset Latch using Latch/Unlatch outputs
Notes:
O1:00’ 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.
ACOE347 (Spring2017)
Latching Instructions