Why this topic is important for ECET?
Inverters are a core part of Industrial Electronics. They convert DC power into AC power and are used in UPS systems, renewable energy (solar/wind), motor drives, and automation systems.
For ECET 2026 ECE, questions on inverters often test:
- Concepts (working, waveforms, applications)
- Formulas (output voltage, frequency, efficiency, THD)
- Numerical problems (harmonics, RMS values, modulation index, etc.)
Mastering this topic ensures easy scoring and builds confidence for advanced power electronics.
📘 Concept Notes
1. What is an Inverter?
- An inverter is a device that converts DC (direct current) to AC (alternating current).
- Used in industrial drives, UPS, renewable energy, HVDC transmission, induction heating, etc.
2. Types of Inverters
- Based on Output Waveform
- Square wave inverter
- Modified sine wave inverter
- Pure sine wave inverter
- Based on Circuit Topology
- Voltage Source Inverter (VSI)
- Current Source Inverter (CSI)
- Based on Number of Phases
- Single-phase inverter
- Three-phase inverter
3. Voltage Source Inverter (VSI)
- Input = DC voltage source.
- Produces AC output by switching devices (IGBTs, MOSFETs, SCRs, etc.).
- Controlled by PWM (Pulse Width Modulation).
Example: A single-phase VSI with 200V DC input → generates AC output with amplitude ≈ 200V.
4. Current Source Inverter (CSI)
- Input = constant DC current source.
- Output = AC current.
- Used in induction motor drives.
5. Pulse Width Modulation (PWM) Technique
- Controls output voltage and frequency by varying pulse width.
- Advantages:
- Low harmonics
- High efficiency
- Smooth motor control
6. Performance Parameters
- Output Voltage (Fundamental RMS) depends on modulation index (M).
- Harmonics → higher-order harmonics reduce efficiency.
- Total Harmonic Distortion (THD) is an important measure of waveform quality.
⚙️ Formulas
- RMS Output Voltage (single-phase inverter):
For n-pulse inverter, output frequency:
Total Harmonic Distortion (THD):
Modulation Index (PWM):
Fundamental RMS voltage in PWM inverter:
Efficiency of inverter:
🔟 10 MCQs
Q1. An inverter converts:
a) AC → DC
b) DC → AC
c) AC → AC
d) DC → DC
Q2. In a single-phase half-bridge inverter, the maximum RMS output voltage is:
a) VdcV_{dc}Vdc
b) Vdc/2V_{dc}/2Vdc/2
c) Vdc/2V_{dc}/\sqrt{2}Vdc/2
d) Vdc/4V_{dc}/4Vdc/4
Q3. Which device is mostly used in modern inverters?
a) BJT
b) IGBT
c) UJT
d) SCR only
Q4. A 220 V DC supply is given to a full-bridge inverter. Find fundamental RMS output voltage (square wave).
Q5. The output frequency of an inverter depends on:
a) DC input voltage
b) Switching frequency
c) Load resistance
d) Temperature
Q6. Which inverter is commonly used in induction motor drives?
a) VSI
b) CSI
c) Both
d) None
Q7. PWM inverters are preferred because:
a) Cheaper
b) Lower harmonics
c) Higher losses
d) No switching devices
Q8. If modulation index M>1M > 1M>1, the inverter is in:
a) Linear region
b) Overmodulation
c) Square wave region
d) Harmonic suppression
Q9. THD is minimum in:
a) Square wave inverter
b) PWM inverter
c) Six-step inverter
d) Half-wave rectifier
Q10. A three-phase inverter with switching frequency 600 Hz will give output frequency (fundamental):
a) 100 Hz
b) 200 Hz
c) 300 Hz
d) 600 Hz
✅ Answer Key
| Q No | Answer |
|---|---|
| Q1 | b |
| Q2 | c |
| Q3 | b |
| Q4 | 155.6 V |
| Q5 | b |
| Q6 | b |
| Q7 | b |
| Q8 | b |
| Q9 | b |
| Q10 | c |
🧠 Explanations
- Q1: Inverter = DC to AC → (b).
- Q2: For half-bridge inverter, RMS output = Vdc/2V_{dc}/\sqrt{2}Vdc/2 → (c).
- Q3: Modern inverters use IGBTs (fast switching, high efficiency) → (b).
- Q4: RMS = Vdc/2=220/2=155.6VV_{dc}/\sqrt{2} = 220/\sqrt{2} = 155.6 VVdc/2=220/2=155.6V.
- Q5: Output frequency depends on switching frequency, not on input → (b).
- Q6: Induction motors need current control → CSI is used → (b).
- Q7: PWM reduces harmonics → (b).
- Q8: If M>1M > 1M>1, inverter operates in overmodulation → (b).
- Q9: PWM inverter has lowest THD → (b).
- Q10: For 3-phase inverter: f=n2fsf = \frac{n}{2} f_sf=2nfs. If n=1n = 1n=1, f=300Hzf = 300 Hzf=300Hz → (c).
🎯 Motivation / Why Practice Matters
In ECET 2026 ECE, Industrial Electronics is a high-weightage subject.
- Inverters are commonly tested in numerical problems (RMS, modulation index, THD, harmonics).
- Practicing improves calculation speed and accuracy.
- Strong knowledge here also helps in GATE/PSU exams since inverter concepts are universal in power electronics.
Remember → One mark saved in ECET can decide your branch/college allocation! 🚀
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