Open-loop vs closed-loop control in Power Electronics - Visual Side-by-Side Comparison

Concept Flow - Open-loop vs closed-loop control

Start Control Process

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Open-loop Control

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Send Control Signal

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System Responds

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No Feedback Used

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Closed-loop Control

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Send Control Signal

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System Responds

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Measure Output

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Compare Output to Desired

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Adjust Control Signal

↩Back to Send Control Signal

Open-loop control sends commands without checking results; closed-loop control measures output and adjusts commands to reach the goal.

Execution Sample

Power Electronics

Desired Speed = 100 RPM
Open-loop: Set motor voltage to fixed value
Closed-loop: Measure speed, adjust voltage to keep 100 RPM

Shows how open-loop sets a fixed input, while closed-loop adjusts input based on measured output.

Analysis Table

Step	Control Type	Action	Measured Output	Adjustment Made	Result
1	Open-loop	Set voltage to 5V	Speed unknown	No adjustment	Motor runs at approx 90 RPM
2	Open-loop	No feedback	Speed unknown	No adjustment	Speed may vary with load
3	Closed-loop	Set initial voltage 5V	Measure speed = 85 RPM	Increase voltage	Speed moves closer to 100 RPM
4	Closed-loop	Adjust voltage to 5.5V	Measure speed = 98 RPM	Small increase	Speed near target
5	Closed-loop	Adjust voltage to 5.6V	Measure speed = 100 RPM	Fine tune	Speed stabilized at 100 RPM
6	Closed-loop	Maintain voltage	Measure speed = 100 RPM	No adjustment	Speed steady despite load changes
7	End	Process complete	-	-	-

💡 Open-loop stops after setting input; closed-loop continues adjusting until output matches desired value.

State Tracker

Variable	Start	After Step 1	After Step 3	After Step 4	After Step 5	Final
Voltage (V)	0	5	5	5.5	5.6	5.6
Speed (RPM)	0	Approx 90	85	98	100	100
Adjustment	None	None	Increase	Increase	Fine tune	None

Key Insights - 3 Insights

Why does open-loop control not adjust the voltage after the first step?

Because open-loop control does not use feedback, it sends a fixed input and does not measure or adjust based on output, as shown in execution_table rows 1 and 2.

How does closed-loop control know when to stop adjusting the voltage?

Closed-loop control measures the output and compares it to the desired value; it stops adjusting when the output matches the target, as seen in execution_table rows 5 and 6.

Why might the motor speed vary in open-loop control under different loads?

Because open-loop control does not measure output, it cannot compensate for changes like load variations, so speed can drift, shown in execution_table row 2.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what action does closed-loop control take after measuring speed?

A Keep voltage the same

B Increase voltage to raise speed

C Decrease voltage to lower speed

D Stop the motor

💡 Hint

Refer to execution_table row 3 under 'Adjustment Made' column.

At which step does the closed-loop control first reach the desired speed of 100 RPM?

A Step 3

B Step 4

C Step 5

D Step 6

💡 Hint

Check execution_table 'Measured Output' column for speed closest to 100 RPM.

If the open-loop control voltage was set higher initially, how would the motor speed change in the execution_table?

A Speed would likely increase but still not adjust

B Speed would decrease automatically

C Voltage would adjust to maintain speed

D Motor would stop

💡 Hint

Open-loop control does not adjust voltage after setting it, see variable_tracker Voltage and Adjustment rows.

Concept Snapshot

Open-loop control sends fixed commands without feedback.
Closed-loop control measures output and adjusts commands.
Closed-loop is more accurate and adapts to changes.
Open-loop is simpler but less precise.
Used in systems where feedback is or isn't possible.

Full Transcript

Open-loop control works by sending a fixed control signal to a system without checking the result. For example, setting a motor voltage and not measuring speed means the system cannot adjust if conditions change. Closed-loop control measures the output, compares it to the desired value, and adjusts the input to reach the target. This feedback loop allows the system to maintain performance despite disturbances. The execution table shows open-loop setting voltage once and not adjusting, while closed-loop measures speed and changes voltage until the speed matches the desired 100 RPM. Variables like voltage and speed change step-by-step in closed-loop but remain fixed in open-loop. Beginners often wonder why open-loop doesn't adjust or how closed-loop knows when to stop; the key is feedback and comparison to the goal. Quizzes test understanding of these steps and differences.

Practice

1. Which of the following best describes an open-loop control system?

easy

A. It constantly monitors output and adjusts input accordingly.

B. It is always more complex and expensive than closed-loop control.

C. It uses sensors to correct errors automatically.

D. It operates without feedback and does not adjust based on output.

Solution

1. Step 1: Understand open-loop control basics

   Open-loop control systems work without feedback, meaning they do not check or adjust their output.

2. Step 2: Compare with closed-loop control

   Closed-loop systems use feedback to adjust and correct output, unlike open-loop systems.

3. Final Answer:

   It operates without feedback and does not adjust based on output. -> Option D

4. Quick Check:

   Open-loop = No feedback [OK]

Hint: Open-loop means no feedback or adjustment [OK]

Common Mistakes:

• Confusing open-loop with closed-loop control
• Assuming open-loop always uses sensors
• Thinking open-loop adjusts output automatically

2. Which of the following is the correct statement about closed-loop control?

easy

A. Closed-loop control adjusts output based on feedback from the system.

B. Closed-loop control does not use sensors or feedback.

C. Closed-loop control is simpler than open-loop control.

D. Closed-loop control ignores the output when making decisions.

Solution

1. Step 1: Identify closed-loop control features

   Closed-loop control uses feedback from sensors to monitor output and adjust input accordingly.

2. Step 2: Eliminate incorrect options

   Options B, C, and D contradict the feedback principle of closed-loop control.

3. Final Answer:

   Closed-loop control adjusts output based on feedback from the system. -> Option A

4. Quick Check:

   Closed-loop = Feedback adjustment [OK]

Hint: Closed-loop always uses feedback to adjust output [OK]

Common Mistakes:

• Thinking closed-loop control is simpler than open-loop
• Believing closed-loop ignores output
• Confusing sensor use in open-loop and closed-loop

3. Consider a temperature control system that turns a heater on for 10 minutes regardless of room temperature. What type of control is this?

medium

A. Closed-loop control because it uses a timer.

B. Open-loop control because it does not adjust based on temperature.

C. Closed-loop control because it controls temperature.

D. Neither open-loop nor closed-loop control.

Solution

1. Step 1: Analyze system behavior

   The heater runs for a fixed time without checking the actual room temperature.

2. Step 2: Determine control type

   Since there is no feedback or adjustment based on temperature, this is open-loop control.

3. Final Answer:

   Open-loop control because it does not adjust based on temperature. -> Option B

4. Quick Check:

   Fixed time, no feedback = Open-loop [OK]

Hint: No feedback means open-loop, even if controlling temperature [OK]

Common Mistakes:

• Assuming any temperature control is closed-loop
• Confusing timer use with feedback
• Thinking fixed timing means closed-loop

4. A closed-loop motor speed control system is not maintaining the desired speed. Which of the following is the most likely cause?

medium

A. The input voltage is too high.

B. The system is using open-loop control.

C. The feedback sensor is faulty or disconnected.

D. The motor is running without any load.

Solution

1. Step 1: Understand closed-loop failure causes

   If the system cannot maintain speed, a common cause is a problem with the feedback sensor.

2. Step 2: Evaluate other options

   The system is using open-loop control. ('The system is using open-loop control.') is incorrect because the system is stated as closed-loop. Options A and C do not directly explain loss of speed control.

3. Final Answer:

   The feedback sensor is faulty or disconnected. -> Option C

4. Quick Check:

   Faulty feedback = control failure [OK]

Hint: Check feedback sensor first if closed-loop fails [OK]

Common Mistakes:

• Ignoring sensor issues in closed-loop systems
• Confusing open-loop with closed-loop failure
• Blaming load or voltage without checking feedback

5. In a power electronics system, why might an engineer choose an open-loop control instead of a closed-loop control despite lower accuracy?

hard

A. Because open-loop control is simpler and cheaper when precision is not critical.

B. Because open-loop control always provides better accuracy.

C. Because closed-loop control cannot be used in power electronics.

D. Because open-loop control uses feedback to improve performance.

Solution

1. Step 1: Consider trade-offs in control system choice

   Open-loop control is simpler and less expensive but less precise than closed-loop control.

2. Step 2: Understand practical reasons for choosing open-loop

   If the system does not require high accuracy, open-loop control can reduce cost and complexity.

3. Final Answer:

   Because open-loop control is simpler and cheaper when precision is not critical. -> Option A

4. Quick Check:

   Simple, cheap, less precise = Open-loop choice [OK]

Hint: Choose open-loop for cost and simplicity when precision is low [OK]

Common Mistakes:

• Thinking open-loop is more accurate
• Believing closed-loop cannot be used in power electronics
• Confusing feedback use in open-loop