Start-stop motor control in PLC Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main purpose of a start-stop motor control circuit?

To safely start and stop a motor using control inputs, ensuring the motor runs only when desired.

beginner

In a start-stop motor control, what role does the 'start' button play?

It energizes the motor starter coil, which closes the motor power contacts to start the motor.

intermediate

Why is a 'seal-in' or 'holding' contact used in start-stop motor control circuits?

To keep the motor running after the start button is released by maintaining the coil energized through a parallel contact.

beginner

What happens when the 'stop' button is pressed in a start-stop motor control circuit?

The circuit to the motor starter coil is broken, de-energizing the coil and stopping the motor.

intermediate

How does a normally closed (NC) stop button differ from a normally open (NO) start button in motor control?

The stop button is NC to break the circuit when pressed, stopping the motor; the start button is NO to complete the circuit only when pressed.

What type of contact is typically used for the stop button in a start-stop motor control?

A Latching

B Normally Open (NO)

C Momentary Open

D Normally Closed (NC)

The stop button is normally closed so pressing it opens the circuit and stops the motor.

What is the purpose of the seal-in contact in a start-stop motor control circuit?

A To stop the motor immediately

B To keep the motor running after releasing the start button

C To reset the circuit

D To provide overload protection

The seal-in contact holds the coil energized so the motor keeps running without holding the start button.

Which component directly controls the motor power in a start-stop motor control?

A Stop button

B Start button

C Motor starter coil

D Seal-in contact

The motor starter coil controls the power contacts that supply electricity to the motor.

What happens when the stop button is pressed in a start-stop motor control circuit?

A The motor stops

B The motor starts

C The motor speed increases

D Nothing happens

Pressing the stop button breaks the coil circuit, stopping the motor.

Why is the start button normally open (NO) in a start-stop motor control?

A To only complete the circuit when pressed

B To keep the circuit always closed

C To stop the motor

D To provide overload protection

The start button is NO so it only energizes the coil when pressed.

Explain how a start-stop motor control circuit works to start and stop a motor.

Think about what happens when you press start and then release it, and what pressing stop does.

Describe the difference between normally open and normally closed buttons in the context of start-stop motor control.

Consider how pressing each button changes the circuit.

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