Start-stop motor control in PLC Programming - Mini Project: Build & Apply

Start-stop motor control

📖 Scenario: You are programming a simple motor control system using a PLC (Programmable Logic Controller). The motor should start when the Start button is pressed and stop when the Stop button is pressed. This is a common control pattern in factories and machines.

🎯 Goal: Build a start-stop motor control logic using PLC ladder logic style scripting. You will create variables for the buttons and motor state, configure a latch to keep the motor running after the start button is released, and implement the logic to stop the motor when the stop button is pressed.

📋 What You'll Learn

Create variables for Start button, Stop button, and Motor state

Add a variable to hold the motor latch state

Write logic to set the motor state when Start is pressed and reset it when Stop is pressed

Output the motor state to show if the motor is running or stopped

💡 Why This Matters

🌍 Real World

Start-stop motor control is a fundamental automation task in factories, conveyor belts, and machines where operators need to control motors safely and reliably.

💼 Career

Understanding start-stop control logic is essential for PLC programmers, automation engineers, and technicians working with industrial control systems.

1

Define input and output variables

Create three boolean variables called Start, Stop, and Motor. Initialize Start and Stop to False, and Motor to False.

PLC Programming

# Define input and output variables
# Your code here

Hint

Use simple boolean variables to represent the buttons and motor state.

2

Add a latch variable for motor state memory

Add a boolean variable called Motor_Latch and initialize it to False. This variable will remember if the motor should stay running.

PLC Programming

Start = False
Stop = False
Motor = False
# Add a latch variable for motor state
# Your code here

Hint

The latch variable keeps the motor running even after the start button is released.

3

Write the start-stop motor control logic

Write logic to set Motor_Latch to True when Start is True. Set Motor_Latch to False when Stop is True. Then set Motor equal to Motor_Latch.

PLC Programming

Start = False
Stop = False
Motor = False
Motor_Latch = False
# Write start-stop logic
# Your code here

Hint

Use simple if statements to control the latch and motor state.

4

Display the motor state output

Write a print statement to display the text "Motor running" if Motor is True, otherwise display "Motor stopped".

PLC Programming

Start = False
Stop = False
Motor = False
Motor_Latch = False

if Start:
    Motor_Latch = True
if Stop:
    Motor_Latch = False
Motor = Motor_Latch
# Print motor state
# Your code here

Hint

Use an if-else statement to print the motor state message.

Practice

1. What is the main purpose of a start-stop motor control circuit in PLC programming?

easy

A. To reverse the motor direction when the Stop button is pressed

B. To run a motor only while the Start button is held down

C. To turn a motor on with a Start button and keep it running until a Stop button is pressed

D. To stop a motor automatically after a set time

Solution

1. Step 1: Understand start-stop control basics

   Start-stop control uses two buttons: Start to turn on the motor, Stop to turn it off.

2. Step 2: Identify motor running behavior

   The motor stays on after Start is pressed and released, until Stop is pressed.

3. Final Answer:

   To turn a motor on with a Start button and keep it running until a Stop button is pressed -> Option C

4. Quick Check:

   Start keeps motor running until Stop pressed = D [OK]

Hint: Motor runs after Start pressed, stops only when Stop pressed [OK]

Common Mistakes:

• Thinking motor runs only while Start is held
• Assuming motor stops automatically without Stop button
• Confusing Stop button with reversing motor

2. Which of the following is the correct ladder logic symbol for a normally open (NO) Start pushbutton in a start-stop motor control?

easy

A. A contact that closes when the button is pressed

B. A contact that stays closed all the time

C. A contact that opens when the button is pressed

D. A coil that energizes the motor

Solution

1. Step 1: Recall NO contact behavior

   A normally open (NO) contact is open by default and closes when pressed.

2. Step 2: Match to Start button function

   The Start button is NO so it closes circuit when pressed to start motor.

3. Final Answer:

   A contact that closes when the button is pressed -> Option A

4. Quick Check:

   NO contact closes on press = B [OK]

Hint: Start button uses NO contact that closes on press [OK]

Common Mistakes:

• Confusing NO with NC (normally closed)
• Thinking Start button contact is always closed
• Mixing coil symbol with contact symbol

3. Given this PLC ladder logic snippet for start-stop control:

Start NO ---[ ]---+---( ) Motor
Stop NC ----[ ]---|
Motor ---[ ]-------+

What happens when the Start button is pressed and then released, assuming the motor was off initially?

medium

A. Motor turns on and stays on until Stop is pressed

B. Motor turns on only while Start is pressed

C. Motor never turns on

D. Motor turns on and off repeatedly

Solution

1. Step 1: Analyze Start button effect

   Pressing Start closes NO contact, energizing Motor coil.

2. Step 2: Understand holding circuit

   Motor contact in parallel holds coil energized after Start released.

3. Final Answer:

   Motor turns on and stays on until Stop is pressed -> Option A

4. Quick Check:

   Holding circuit keeps motor on after Start release = A [OK]

Hint: Holding contact keeps motor on after Start release [OK]

Common Mistakes:

• Thinking motor turns off immediately after Start release
• Ignoring Stop button effect
• Misreading holding contact function

4. In a start-stop motor control program, the motor never turns off when the Stop button is pressed. What is the most likely error?

medium

A. Motor coil is missing from the circuit

B. Start button contact is wired as normally closed (NC)

C. Holding contact is missing from the circuit

D. Stop button contact is wired as normally open (NO) instead of normally closed (NC)

Solution

1. Step 1: Check Stop button wiring

   Stop button must be NC to break circuit when pressed.

2. Step 2: Identify effect of NO Stop button

   If Stop is NO, pressing it does not open circuit, so motor stays on.

3. Final Answer:

   Stop button contact is wired as normally open (NO) instead of normally closed (NC) -> Option D

4. Quick Check:

   Stop must be NC to stop motor = C [OK]

Hint: Stop button must be NC to break motor circuit [OK]

Common Mistakes:

• Wiring Stop as NO contact
• Confusing Start and Stop button contacts
• Forgetting holding contact role

5. You want to add an emergency stop (E-Stop) button to a start-stop motor control circuit that immediately stops the motor when pressed. Which is the best way to add it?

hard

A. Add the E-Stop as a normally open (NO) contact in parallel with the Start button

B. Add the E-Stop as a normally closed (NC) contact in series with the Stop button

C. Add the E-Stop as a coil that energizes the motor

D. Add the E-Stop as a normally open (NO) contact in series with the motor coil

Solution

1. Step 1: Understand emergency stop function

   E-Stop must break power immediately to stop motor safely.

2. Step 2: Choose wiring method

   Placing E-Stop as NC contact in series with Stop button breaks circuit when pressed.

3. Final Answer:

   Add the E-Stop as a normally closed (NC) contact in series with the Stop button -> Option B

4. Quick Check:

   E-Stop NC in series cuts motor power = A [OK]

Hint: E-Stop wired NC in series to cut motor power immediately [OK]

Common Mistakes:

• Wiring E-Stop as NO contact
• Placing E-Stop in parallel with Start button
• Using E-Stop as coil instead of contact