Star-delta starter using timers in PLC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Purpose of Timers in Star-Delta Starter

In a star-delta starter circuit using timers, what is the main purpose of the timer?

A To delay the transition from star to delta connection to reduce starting current

B To immediately switch from star to delta without delay

C To control the motor speed during operation

D To stop the motor after a fixed running time

💡 Hint

Think about why the motor needs a delay before switching connections.

The timer delays the switch from star to delta to reduce the high starting current and mechanical stress on the motor.

💻 Command Output

intermediate

Timer Output Behavior in Star-Delta PLC Code

Given a PLC timer TON with preset 5 seconds used in a star-delta starter, what will be the timer's output (Q) after 3 seconds of motor start?

PLC Programming

TON Timer1(IN:=Start, PT:=T#5s, Q=>Q1, ET=>ET1);

A Q1 = FALSE

B Q1 = TRUE

C Q1 = ERROR

D Q1 = UNDEFINED

💡 Hint

The timer output Q becomes TRUE only after the preset time elapses.

The TON timer output Q remains FALSE until the elapsed time ET reaches the preset PT of 5 seconds.

📝 Syntax

advanced

Correct Timer Usage in Star-Delta PLC Ladder Logic

Which of the following ladder logic snippets correctly implements a timer to switch from star to delta after 10 seconds?

A TON T1(IN:=Start, PT:=T#10s); IF TimerDone THEN DeltaCoil:=TRUE;

B TON T1(IN:=Start, PT:=10, Q=>TimerDone); IF TimerDone THEN DeltaCoil:=TRUE;

C TON T1(IN:=Start, PT:=T#10s, Q=>TimerDone); IF TimerDone = FALSE THEN DeltaCoil:=TRUE;

D TON T1(IN:=Start, PT:=T#10s, Q=>TimerDone); IF TimerDone THEN DeltaCoil:=TRUE;

💡 Hint

Check the timer preset format and the condition to activate delta coil.

Option D correctly uses the TON timer with a preset time of 10 seconds and activates the delta coil when the timer output Q is TRUE.

🔧 Debug

advanced

Identify the Error in Timer Logic for Star-Delta Starter

The following PLC code snippet is intended to switch from star to delta after 8 seconds, but the delta coil never energizes. What is the error?

PLC Programming

TON TimerStar(IN:=Start, PT:=T#8s, Q=>StarDone);
IF StarDone = FALSE THEN DeltaCoil := TRUE;

A The timer preset time is too short

B The timer input IN should be FALSE to start timing

C The condition should check if StarDone is TRUE, not FALSE, to energize DeltaCoil

D DeltaCoil should be energized when StarDone is FALSE

💡 Hint

Think about when the delta coil should be energized relative to the timer output.

Delta coil should energize only after the timer output Q (StarDone) becomes TRUE, indicating the star period is complete.

🚀 Application

expert

Sequence Timing in Star-Delta Starter PLC Program

In a star-delta starter PLC program, the star coil energizes immediately when the motor starts. After 6 seconds, the star coil de-energizes and the delta coil energizes. Which timer and logic sequence correctly implements this behavior?

A Use TON TimerStar with PT=6s; StarCoil = TimerStar.Q; DeltaCoil = Start AND NOT TimerStar.Q;

B Use TON TimerStar with PT=6s; StarCoil = Start AND NOT TimerStar.Q; DeltaCoil = TimerStar.Q;

C Use TOF TimerStar with PT=6s; StarCoil = Start AND TimerStar.Q; DeltaCoil = NOT TimerStar.Q;

D Use TON TimerStar with PT=6s; StarCoil = TimerStar.Q; DeltaCoil = TimerStar.Q;

💡 Hint

Star coil should be ON before timer finishes; delta coil ON after timer finishes.

TON timer output Q becomes TRUE after 6 seconds. Star coil is ON while timer Q is FALSE (first 6 seconds). Delta coil energizes when timer Q is TRUE (after 6 seconds).

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