Start-stop motor control in PLC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of a simple start-stop motor ladder logic

Given the following ladder logic snippet for a start-stop motor control, what will be the state of the motor output (M) after pressing the start button once and then releasing it?

Inputs:
- Start button (I0.0) is momentary normally open
- Stop button (I0.1) is normally closed

Ladder logic:
--| I0.0 |----+----( M )
|
--| I0.1 |----| --| M |----+

A Motor output M toggles ON and OFF repeatedly

B Motor output M remains ON after releasing the start button

C Motor output M turns OFF immediately after releasing the start button

D Motor output M never turns ON

💡 Hint

Think about how the motor output is latched in the circuit.

The motor output M is latched by its own contact in parallel with the start button. Pressing start energizes M, which then keeps itself energized even after releasing the start button, as long as the stop button remains closed.

📝 Syntax

intermediate

Identify the correct rung for start-stop motor control

Which of the following ladder logic rungs correctly implements a start-stop motor control with a normally open start button (I0.0), normally closed stop button (I0.1), and motor output coil (M)?

A

--| I0.0 |----+----( M )<br>           |<br>--| I0.1 |----|
--| M   |----+

B

--| I0.1 |----+----( M )<br>           |<br>--| I0.0 |----|
--| M   |----+

C

--| I0.0 |----+----( M )<br>           |<br>--| M   |----|
--| I0.1 |----+

D

--| I0.1 |----+----( M )<br>           |<br>--| M   |----|
--| I0.0 |----+

💡 Hint

Remember the stop button is normally closed and should break the circuit when pressed.

Option A correctly places the normally open start button and normally closed stop button in series with the motor coil and latching contact. The stop button breaks the circuit when pressed, stopping the motor.

🔧 Debug

advanced

Find the error in this start-stop motor control ladder logic

The following ladder logic is intended to control a motor with start and stop buttons. However, the motor never stays ON after pressing start. What is the error?

Rung:
--| I0.0 |----+----( M )
|
--| I0.1 |----| --| I0.0 |----+

A The stop button I0.1 is normally open instead of normally closed

B The start button I0.0 is wired in series with the stop button incorrectly

C The motor coil M is not latched; the latching contact should be M, not I0.0

D The motor coil M is missing from the rung

💡 Hint

Check which contact is used for latching the motor coil.

The latching contact should be the motor coil M itself, not the start button I0.0. Using I0.0 as latching contact means the motor turns off as soon as the start button is released.

🚀 Application

advanced

Design a start-stop motor control with indicator lamp

You want to design a start-stop motor control circuit that also turns ON a green indicator lamp (L) when the motor is running. Which ladder logic rung correctly implements this?

A

--| I0.0 |----+----( L )<br>           |<br>--| I0.1 |----|
--| M   |----+
--| M   |--------( M )

B

--| I0.0 |----+----( M )<br>           |<br>--| I0.1 |----|
--| I0.0 |----+
--| M   |--------( L )

C

--| I0.0 |----+----( M )<br>           |<br>--| I0.1 |----|
--| L   |----+
--| M   |--------( M )

D

--| I0.0 |----+----( M )<br>           |<br>--| I0.1 |----|
--| M   |----+
--| M   |--------( L )

💡 Hint

The indicator lamp should be controlled by the motor coil contact.

Option D correctly latches the motor coil M and uses M's contact to turn ON the lamp L when the motor is running.

🧠 Conceptual

expert

Why is a normally closed stop button used in start-stop motor control?

In start-stop motor control circuits, the stop button is usually wired as a normally closed contact. What is the main reason for this design choice?

A To ensure the motor stops immediately when the stop button is pressed by breaking the circuit

B To keep the motor running only when the stop button is pressed

C To allow the motor to start automatically when power is applied

D To prevent the motor from starting if the start button is held down

💡 Hint

Think about what happens to the circuit when the stop button is pressed.

A normally closed stop button keeps the circuit closed during normal operation. Pressing it opens the circuit, immediately stopping the motor.

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