Start-stop motor control in PLC Programming - Step-by-Step Execution

Concept Flow - Start-stop motor control

Press Start Button

↓

Set Motor ON Coil

↓

Motor Runs

↓

Press Stop Button

↓

Reset Motor ON Coil

↓

Motor Stops

This flow shows how pressing start sets the motor ON coil, running the motor, and pressing stop resets the coil to stop the motor.

Execution Sample

PLC Programming

Start_Button = False
Stop_Button = False
Motor_ON = False

# When Start pressed
if Start_Button and not Motor_ON:
    Motor_ON = True

# When Stop pressed
if Stop_Button:
    Motor_ON = False

This simple PLC logic turns the motor ON when start is pressed and turns it OFF when stop is pressed.

Execution Table

Step	Start_Button	Stop_Button	Motor_ON Before	Condition Checked	Motor_ON After	Action Taken
1	False	False	False	Start_Button and not Motor_ON = False	False	No change
2	True	False	False	Start_Button and not Motor_ON = True	True	Motor_ON set to True (Motor starts)
3	True	False	True	Start_Button and not Motor_ON = False	True	No change
4	True	True	True	Stop_Button = True	False	Motor_ON reset to False (Motor stops)
5	False	True	False	Stop_Button = True	False	Motor_ON remains False
6	False	False	False	No conditions met	False	No change

💡 Execution stops as no further input changes occur; motor is OFF.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6
Start_Button	False	False	True	True	True	False	False
Stop_Button	False	False	False	False	True	True	False
Motor_ON	False	False	True	True	False	False	False

Key Moments - 2 Insights

Why does Motor_ON stay True at step 3 even though Start_Button is still True?

Because the condition 'Start_Button and not Motor_ON' is False at step 3 since Motor_ON is already True, so no change happens (see execution_table row 3).

Why does pressing Stop_Button reset Motor_ON immediately?

At step 4, Stop_Button is True, so the condition 'if Stop_Button' triggers and Motor_ON is set to False right away (see execution_table row 4).

Visual Quiz - 3 Questions

Look at the execution table, what is Motor_ON after step 2?

A True

B False

C Undefined

D Remains as before

💡 Hint

Check the 'Motor_ON After' column in row for step 2.

At which step does the motor stop running?

A Step 3

B Step 4

C Step 5

D Step 6

💡 Hint

Look for when Motor_ON changes from True to False in the execution table.

If Start_Button stayed False throughout, what would Motor_ON be at step 6?

A Depends on Stop_Button

B True

C False

D Cannot tell

💡 Hint

Refer to variable_tracker for Motor_ON when Start_Button is False.

Concept Snapshot

Start-stop motor control logic:
- Press Start to set Motor_ON coil True
- Motor runs while Motor_ON is True
- Press Stop to reset Motor_ON coil False
- Motor stops when Motor_ON is False
- Start only works if motor is OFF
- Stop works anytime to stop motor

Full Transcript

This example shows a simple start-stop motor control using PLC logic. When the start button is pressed and the motor is off, the motor coil turns on, starting the motor. The motor stays running until the stop button is pressed, which resets the motor coil and stops the motor. The execution table traces each step with button states and motor coil state changes. The variable tracker shows how inputs and motor state change over time. Key moments clarify why motor state changes or stays the same depending on conditions. The quiz tests understanding of motor state at different steps and effects of inputs. This visual step-by-step helps beginners see how start-stop control works in automation.

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