Start-stop motor control in PLC Programming

Introduction

Start-stop motor control lets you turn a motor on and off safely using simple buttons. It helps control machines easily and prevents accidents.

You want to start a motor with a button and stop it with another button.

You need to keep the motor running after pressing start until you press stop.

You want to make sure the motor does not start by itself after a power failure.

You want to control a motor in a factory or workshop with simple controls.

Syntax

PLC Programming

Start Button (I0.0) --| |-- + Motor Output (Q0.0)
Stop Button (I0.1) --|/|--
Motor Output (Q0.0) --| |-- (Seal-in contact)

The start button is a normally open contact that closes when pressed.

The stop button is a normally closed contact that opens when pressed.

The motor output coil energizes the motor and also holds itself on via a seal-in contact.

Examples

This ladder logic uses start (I0.0) and stop (I0.1) inputs to control motor output (Q0.0).

PLC Programming

LD I0.0
AND NOT I0.1
OR Q0.0
= Q0.0

Structured Text version using variables StartButton, StopButton, and Motor.

PLC Programming

(* Start-Stop Motor Control in Structured Text *)
IF StartButton AND NOT StopButton THEN
  Motor := TRUE;
ELSIF StopButton THEN
  Motor := FALSE;
END_IF;

Sample Program

This program shows how pressing start and stop buttons controls the motor state. Motor stays on after start until stop is pressed.

PLC Programming

(* Simple Start-Stop Motor Control in Structured Text *)
VAR
  StartButton : BOOL; (* Input: Start button *)
  StopButton : BOOL;  (* Input: Stop button *)
  Motor : BOOL;       (* Output: Motor control *)
END_VAR

(* Logic: Motor starts when StartButton pressed and StopButton not pressed. Motor stays on until StopButton pressed. *)
IF StartButton AND NOT StopButton THEN
  Motor := TRUE;
ELSIF StopButton THEN
  Motor := FALSE;
END_IF;

(* Simulate pressing StartButton and StopButton *)
StartButton := TRUE;
StopButton := FALSE;
(* Run logic *)
IF StartButton AND NOT StopButton THEN
  Motor := TRUE;
ELSIF StopButton THEN
  Motor := FALSE;
END_IF;

(* Output motor state *)
(* Print result *)
(* In real PLC, Motor output controls the motor coil *)
(* Here we just show the Motor variable value *)

Important Notes

The seal-in contact (motor output) keeps the motor running after releasing the start button.

Stop button is wired normally closed to break the circuit when pressed.

This control prevents the motor from restarting automatically after power loss.

Summary

Start-stop control uses two buttons: start (normally open) and stop (normally closed).

Motor output is latched on by a seal-in contact to keep running after start.

Pressing stop breaks the circuit and turns off the motor safely.

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