PWM control for inverters in Power Electronics - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define the basic PWM frequency variable.

Power Electronics

pwm_frequency = [1]  # in Hertz

Drag options to blanks, or click blank then click option'

A1000

B50

C0.5

D50000

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing too low frequency like 50 Hz causes poor output waveform.

Using extremely high frequency like 50000 Hz increases switching losses.

The PWM frequency for inverter control is typically set around 1000 Hz to balance switching losses and output quality.

2fill in blank

medium

Complete the code to calculate the duty cycle for a given modulation index.

Power Electronics

duty_cycle = [1] * modulation_index

Drag options to blanks, or click blank then click option'

A0.5

Bpwm_frequency

Cmodulation_index

Dswitching_period

Attempts:

3 left

💡 Hint

Common Mistakes

Using frequency instead of a scaling factor.

Multiplying by modulation_index twice.

The duty cycle is calculated by multiplying the modulation index by 0.5 to scale it between 0 and 1 for PWM signals.

3fill in blank

hard

Fix the error in the code that generates the PWM signal based on the carrier and reference signals.

Power Electronics

pwm_signal = 1 if reference_signal > [1] else 0

Drag options to blanks, or click blank then click option'

Aswitching_period

Bmodulation_index

Cpwm_frequency

Dcarrier_signal

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing reference signal with modulation index.

Using frequency value in comparison.

The PWM signal is generated by comparing the reference signal with the carrier signal, not the modulation index or frequency.

4fill in blank

hard

Fill both blanks to create a dictionary mapping inverter phases to their PWM duty cycles.

Power Electronics

phase_duty_cycles = {'A': [1], 'B': [2]

Drag options to blanks, or click blank then click option'

Aduty_cycle_A

Bduty_cycle_B

Cmodulation_index

Dpwm_frequency

Attempts:

3 left

💡 Hint

Common Mistakes

Using modulation_index or pwm_frequency instead of duty cycle variables.

Each inverter phase has its own duty cycle variable, which are assigned in the dictionary for control.

5fill in blank

hard

Fill all three blanks to create a filtered output voltage dictionary for the inverter phases.

Power Electronics

filtered_voltage = { 'A': [1], 'B': [2], 'C': [3] }

Drag options to blanks, or click blank then click option'

Avoltage_A_filtered

Bvoltage_B_filtered

Cvoltage_C_filtered

Dmodulation_index

Attempts:

3 left

💡 Hint

Common Mistakes

Using modulation_index instead of filtered voltage variables.

The filtered voltages for each phase are stored in the dictionary to represent the inverter output after PWM filtering.

Practice

1. What does PWM stand for in the context of inverter control?

easy

A. Pulse Width Modulation

B. Power Wave Management

C. Phase Width Measurement

D. Pulse Wave Multiplication

Solution

1. Step 1: Understand the abbreviation PWM

   PWM is a common term in electronics that stands for Pulse Width Modulation, which means controlling the width of pulses to regulate power.

2. Step 2: Match the correct full form

   Among the options, only 'Pulse Width Modulation' correctly describes PWM used in inverter control.

3. Final Answer:

   Pulse Width Modulation -> Option A

4. Quick Check:

   PWM = Pulse Width Modulation [OK]

Hint: Remember PWM controls pulse widths to regulate power [OK]

Common Mistakes:

• Confusing PWM with power wave terms
• Mixing phase and pulse terms
• Assuming PWM means multiplication

2. Which of the following is the correct way to increase the output voltage of an inverter using PWM?

easy

A. Increase the pulse width (duty cycle)

B. Decrease the pulse frequency

C. Reduce the pulse amplitude

D. Shorten the pulse period

Solution

1. Step 1: Understand duty cycle effect on output voltage

   In PWM, increasing the duty cycle means the pulse is ON longer, which raises the average output voltage.

2. Step 2: Identify the correct action to increase voltage

   Increasing pulse width (duty cycle) directly increases output voltage, unlike changing frequency or amplitude.

3. Final Answer:

   Increase the pulse width (duty cycle) -> Option A

4. Quick Check:

   Higher duty cycle = Higher output voltage [OK]

Hint: More ON time means higher voltage output [OK]

Common Mistakes:

• Confusing frequency with voltage control
• Thinking amplitude changes output voltage
• Assuming shorter periods increase voltage

3. Consider a PWM inverter with a fixed frequency of 10 kHz. If the duty cycle is set to 40%, what is the average output voltage relative to the maximum voltage?

medium

A. 60% of maximum voltage

B. 40% of maximum voltage

C. 10% of maximum voltage

D. 100% of maximum voltage

Solution

1. Step 1: Understand duty cycle and average voltage relation

   The average output voltage in PWM is proportional to the duty cycle percentage of the maximum voltage.

2. Step 2: Calculate average voltage for 40% duty cycle

   40% duty cycle means the output voltage is 40% of the maximum voltage.

3. Final Answer:

   40% of maximum voltage -> Option B

4. Quick Check:

   Duty cycle 40% = 40% output voltage [OK]

Hint: Average voltage equals duty cycle percentage times max voltage [OK]

Common Mistakes:

• Mixing duty cycle with frequency
• Assuming output voltage is inverse of duty cycle
• Confusing average with peak voltage

4. A PWM inverter is not producing the expected output voltage even though the duty cycle is set correctly. Which of the following could be a likely cause?

medium

A. Setting pulse amplitude to zero volts

B. Using a higher duty cycle than 100%

C. Incorrect switching frequency causing distortion

D. Using a PWM signal with constant pulse width

Solution

1. Step 1: Analyze the effect of switching frequency on output

   Incorrect switching frequency can cause waveform distortion, reducing effective output voltage.

2. Step 2: Evaluate other options for feasibility

   Duty cycle cannot exceed 100%, pulse amplitude zero means no output, constant pulse width means no PWM control; only frequency mismatch explains distortion despite correct duty cycle.

3. Final Answer:

   Incorrect switching frequency causing distortion -> Option C

4. Quick Check:

   Wrong frequency = distorted output voltage [OK]

Hint: Check switching frequency if voltage is off despite duty cycle [OK]

Common Mistakes:

• Thinking duty cycle can be >100%
• Ignoring frequency effects on output quality
• Assuming constant pulse width is PWM

5. In a three-phase inverter using PWM control, how can the output voltage be balanced across all phases?

hard

A. By using the same duty cycle for all phases without phase shift

B. By setting all PWM signals to zero duty cycle

C. By increasing the switching frequency for one phase only

D. By synchronizing the PWM signals with 120° phase shift

Solution

1. Step 1: Understand three-phase inverter PWM requirements

   To balance output voltage in three-phase systems, PWM signals must be phase-shifted by 120° to match the three-phase power nature.

2. Step 2: Evaluate options for balancing voltage

   Using synchronized PWM signals with 120° phase shift ensures balanced voltages; same duty cycle without phase shift causes imbalance; changing frequency or zero duty cycle disrupts output.

3. Final Answer:

   By synchronizing the PWM signals with 120° phase shift -> Option D

4. Quick Check:

   120° phase shift = balanced three-phase voltage [OK]

Hint: Use 120° phase shift in PWM for balanced three-phase output [OK]

Common Mistakes:

• Using identical PWM signals for all phases
• Changing frequency for only one phase
• Setting duty cycle to zero disables output