Star-delta starter using timers in PLC Programming - Step-by-Step Execution

Concept Flow - Star-delta starter using timers

Start Button Pressed

↓

Star Contactor ON

↓

Start Timer T1

↓

T1 Done?

No→Wait

Yes↓

Star Contactor OFF

↓

Delta Contactor ON

↓

Start Timer T2

↓

T2 Done?

No→Wait

Yes↓

Delta Contactor stays ON

↓

Stop Button Pressed?

No→Run

Yes↓

All Contactors OFF

↓

END

The flow starts with pressing the start button, energizes the star contactor and timer T1. After T1 finishes, star contactor turns off and delta contactor turns on with timer T2. After T2, delta contactor stays on until stop is pressed.

Execution Sample

PLC Programming

IF StartButton THEN
  StarContactor := TRUE;
  TimerT1(IN:=TRUE, PT:=T#5s);
  IF TimerT1.Q THEN
    StarContactor := FALSE;
    DeltaContactor := TRUE;
    TimerT2(IN:=TRUE, PT:=T#3s);
  END_IF;
END_IF;

// Note: Full code would include StopButton logic and latching for StartButton.
IF StopButton OR NOT StartButtonLatch THEN
  StarContactor := FALSE;
  DeltaContactor := FALSE;
  TimerT1(IN:=FALSE);
  TimerT2(IN:=FALSE);
END_IF;

This PLC code snippet starts the star contactor and timer T1 when start is pressed. When T1 finishes, it switches off star and switches on delta contactor and timer T2. Added note for stop logic.

Execution Table

Step	StartButton	StarContactor	TimerT1.Q	DeltaContactor	TimerT2.Q	Action
1	TRUE	TRUE	FALSE	FALSE	FALSE	Start star contactor and start TimerT1
2	TRUE	TRUE	FALSE	FALSE	FALSE	TimerT1 running, waiting for 5s
3	TRUE	TRUE	TRUE	FALSE	FALSE	TimerT1 done, turn off star, turn on delta, start TimerT2
4	TRUE	FALSE	TRUE	TRUE	FALSE	TimerT2 running, waiting for 3s
5	TRUE	FALSE	TRUE	TRUE	TRUE	TimerT2 done, delta contactor stays ON
6	FALSE	FALSE	TRUE	TRUE	TRUE	Stop pressed (transition state)
7	FALSE	FALSE	FALSE	FALSE	FALSE	All contactors OFF, timers reset, system stopped

💡 Stop button pressed, all contactors turned off, system stopped

Variable Tracker

Variable	Start	After Step 1	After Step 3	After Step 5	After Step 6	Final
StartButton	FALSE	TRUE	TRUE	TRUE	FALSE	FALSE
StarContactor	FALSE	TRUE	FALSE	FALSE	FALSE	FALSE
TimerT1.Q	FALSE	FALSE	TRUE	TRUE	TRUE	FALSE
DeltaContactor	FALSE	FALSE	TRUE	TRUE	TRUE	FALSE
TimerT2.Q	FALSE	FALSE	FALSE	TRUE	TRUE	FALSE

Key Moments - 3 Insights

Why does the star contactor turn off only after TimerT1.Q becomes TRUE?

Because TimerT1.Q indicates the timer has finished its preset time (5 seconds). The execution_table row 3 shows that only after TimerT1.Q is TRUE, the star contactor is turned off and delta contactor is turned on.

What happens if the stop button is pressed during the delta contactor ON phase?

As shown in execution_table row 6-7, pressing stop sets StartButton to FALSE, which causes all contactors to turn off and timers reset, stopping the motor safely.

Why do we need two timers in the star-delta starter?

TimerT1 controls how long the motor runs in star mode to reduce starting current. TimerT2 ensures the delta contactor is energized for a safe transition period. This is shown in the flow and execution_table steps 1-5.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of StarContactor at Step 4?

A TRUE

B FALSE

C UNKNOWN

D Depends on TimerT2

💡 Hint

Check the StarContactor column at Step 4 in the execution_table.

At which step does TimerT1.Q become TRUE?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look at the TimerT1.Q column in the execution_table to find when it changes to TRUE.

If the StartButton remains TRUE, what happens after TimerT2.Q becomes TRUE?

A Delta contactor stays ON

B Delta contactor turns off

C Star contactor turns on again

D System stops automatically

💡 Hint

Refer to execution_table Step 5 where TimerT2.Q is TRUE and DeltaContactor status.

Concept Snapshot

Star-delta starter uses two timers to switch motor from star to delta mode.
Start button energizes star contactor and TimerT1.
After TimerT1 finishes, star contactor turns off, delta contactor turns on, TimerT2 starts.
After TimerT2 finishes, delta contactor stays ON until stop pressed.
Stop button turns off all contactors.
This reduces motor starting current safely.

Full Transcript

This visual execution trace shows how a star-delta starter works using timers in a PLC program. When the start button is pressed, the star contactor energizes and TimerT1 starts counting 5 seconds. While TimerT1 is running, the star contactor stays ON. When TimerT1 finishes, the star contactor turns OFF and the delta contactor turns ON, starting TimerT2 for 3 seconds. After TimerT2 finishes, the delta contactor remains ON to run the motor normally. Pressing the stop button at any time turns off all contactors, stopping the motor. The execution table tracks each step, showing variable states and actions. Key moments clarify why timers control contactors and what happens on stop. The quiz tests understanding of contactor states and timer transitions. This method safely reduces motor starting current by switching from star to delta connection using timed delays.

Practice

1. What is the main purpose of using timers in a star-delta starter circuit?

easy

A. To control the time the motor stays connected in star and delta modes

B. To increase the motor speed instantly

C. To change the motor direction automatically

D. To measure the motor temperature

Solution

1. Step 1: Understand star-delta starter function

   The star-delta starter reduces starting current by initially connecting the motor in star mode, then switching to delta mode.

2. Step 2: Role of timers in this process

   Timers control how long the motor stays in star mode before switching to delta mode to ensure smooth transition and reduce electrical stress.

3. Final Answer:

   To control the time the motor stays connected in star and delta modes -> Option A

4. Quick Check:

   Timers control connection time = A [OK]

Hint: Timers manage star and delta connection duration [OK]

Common Mistakes:

• Thinking timers increase motor speed
• Confusing timers with direction control
• Assuming timers measure temperature

2. Which of the following is the correct way to declare a timer in a PLC program for star-delta starting?

easy

A. TIMER Timer1 (START=Start, TIME=5s, DONE=Done)

B. TON Timer1 (IN:=Start, PT:=T#5s, Q=>Done)

C. TON Timer1(IN:=Start, PT:=T#5s, Q=>Done);

D. Timer1 = TON(Start, 5)

Solution

1. Step 1: Identify correct timer syntax in PLC

   In IEC 61131-3 standard, TON timers are declared with input IN, preset time PT, and output Q using := and => operators.

2. Step 2: Compare options

   TON Timer1 (IN:=Start, PT:=T#5s, Q=>Done) uses correct syntax: TON Timer1 (IN:=Start, PT:=T#5s, Q=>Done). Others have syntax errors or incorrect format.

3. Final Answer:

   TON Timer1 (IN:=Start, PT:=T#5s, Q=>Done) -> Option B

4. Quick Check:

   Correct IEC timer syntax = B [OK]

Hint: Use TON with IN:=, PT:=, Q=> for timers [OK]

Common Mistakes:

• Using incorrect assignment operators
• Missing time format T#5s
• Adding semicolons inside declarations

3. Given the following PLC code snippet for a star-delta starter:

IF Start THEN
  TimerStar(IN:=TRUE, PT:=T#10s);
  IF TimerStar.Q THEN
    TimerDelta(IN:=TRUE, PT:=T#5s);
  END_IF
END_IF

What is the output state of TimerDelta.Q after 12 seconds of Start being TRUE?

medium

A. TRUE

B. TimerDelta.Q resets to FALSE

C. Undefined

D. FALSE

Solution

1. Step 1: Analyze TimerStar behavior

   TimerStar starts when Start is TRUE and runs for 10 seconds. After 10 seconds, TimerStar.Q becomes TRUE.

2. Step 2: Analyze TimerDelta behavior after TimerStar.Q

   Once TimerStar.Q is TRUE (after 10s), TimerDelta starts and runs for 5 seconds. At 12 seconds, TimerDelta has been running for 2 seconds, so TimerDelta.Q is still FALSE. But the code only sets TimerDelta.IN to TRUE when TimerStar.Q is TRUE, so TimerDelta is active.

3. Step 3: Re-examine timing

   At 12 seconds, TimerDelta has been running for 2 seconds (less than 5s), so TimerDelta.Q is FALSE.

4. Final Answer:

   FALSE -> Option D

5. Quick Check:

   TimerDelta.Q after 2s < 5s = FALSE [OK]

Hint: Timer output Q is TRUE only after preset time PT [OK]

Common Mistakes:

• Assuming TimerDelta.Q is TRUE immediately
• Ignoring timer preset times
• Confusing timer input IN with output Q

4. Identify the error in this star-delta timer PLC code snippet:

IF Start THEN
  TimerStar(IN:=Start, PT:=T#10s);
  IF TimerStar.Q THEN
    TimerDelta(IN:=Start, PT:=T#5s);
  END_IF
END_IF

What is the main problem?

medium

A. TimerDelta never starts because IN is tied to Start, not TimerStar.Q

B. TimerStar timer preset time is too short

C. Start input is not used correctly

D. Timers are missing reset logic

Solution

1. Step 1: Check TimerStar input

   TimerStar.IN is correctly tied to Start, so it runs when Start is TRUE.

2. Step 2: Check TimerDelta input

   TimerDelta.IN is tied to Start, not TimerStar.Q. This means TimerDelta starts immediately with Start, not after TimerStar.Q becomes TRUE.

3. Step 3: Identify the problem

   TimerDelta should start only after TimerStar.Q is TRUE to ensure star mode completes before delta starts. Current code starts TimerDelta too early.

4. Final Answer:

   TimerDelta never starts because IN is tied to Start, not TimerStar.Q -> Option A

5. Quick Check:

   TimerDelta.IN must depend on TimerStar.Q [OK]

Hint: TimerDelta.IN should depend on TimerStar.Q, not Start [OK]

Common Mistakes:

• Linking TimerDelta.IN directly to Start
• Ignoring timer sequence dependencies
• Not resetting timers properly

5. You want to design a star-delta starter PLC program where the motor starts in star mode for 8 seconds, then switches to delta mode for 10 seconds, and finally stops. Which timer logic sequence correctly achieves this?

hard

A. Start triggers both TimerStar and TimerDelta simultaneously, motor stops after 18s

B. Start triggers TimerDelta (10s), when TimerDelta.Q is TRUE start TimerStar (8s), then stop motor

C. Start triggers TimerStar (8s), when TimerStar.Q is TRUE start TimerDelta (10s), when TimerDelta.Q is TRUE stop motor

D. Start triggers TimerStar (8s), motor stops immediately after TimerStar.Q is TRUE

Solution

1. Step 1: Understand star-delta timing sequence

   The motor must start in star mode for 8 seconds, then switch to delta mode for 10 seconds, then stop.

2. Step 2: Analyze options

   Start triggers TimerStar (8s), when TimerStar.Q is TRUE start TimerDelta (10s), when TimerDelta.Q is TRUE stop motor correctly sequences timers: TimerStar runs first, then TimerDelta starts after TimerStar.Q is TRUE, then motor stops after TimerDelta.Q is TRUE.

3. Step 3: Eliminate incorrect options

   Start triggers TimerDelta (10s), when TimerDelta.Q is TRUE start TimerStar (8s), then stop motor reverses sequence, Start triggers both TimerStar and TimerDelta simultaneously, motor stops after 18s runs timers simultaneously (wrong), Start triggers TimerStar (8s), motor stops immediately after TimerStar.Q is TRUE stops motor too early.

4. Final Answer:

   Start triggers TimerStar (8s), when TimerStar.Q is TRUE start TimerDelta (10s), when TimerDelta.Q is TRUE stop motor -> Option C

5. Quick Check:

   Sequential timers for star then delta = C [OK]

Hint: Chain timers: star first, then delta, then stop [OK]

Common Mistakes:

• Starting timers simultaneously
• Reversing star and delta order
• Stopping motor too early