Inverters

The single-phase full-bridge inverter produces a square (or quasi-square) output whose fundamental rms is (2√2/π)V_dc ≈ 0.9 V_dc. The square wave is rich in odd harmonics (3rd, 5th, 7th), which cause motor heating and torque pulsation.

Sinusoidal PWM compares a sine reference with a high-frequency triangular carrier; the modulation index m controls the fundamental amplitude linearly up to m = 1, and the dominant harmonics cluster around the carrier frequency.

(fundamental)

Relation 1 —

(fundamental)

Write this relation with symbols exactly as in Power Electronics — PS Bimbhra before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Relation 2 —

Write this relation with symbols exactly as in Power Electronics — PS Bimbhra before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Relation 3 —

Write this relation with symbols exactly as in Power Electronics — PS Bimbhra before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

The amplitude modulation index m_a = V_ref/V_carrier sets the fundamental output: for m_a ≤ 1 the peak fundamental phase voltage is m_a × V_dc/2. Overmodulation (m_a > 1) increases output but reintroduces low-order harmonics.

Dead time (a short blanking interval) is inserted between turning off one device and turning on the other in a leg to prevent shoot-through (both on = DC short). Excess dead time distorts the output near zero crossings.

In a three-phase 180°-conduction inverter each device conducts for half a cycle; the line voltage is √3 times the phase voltage and contains no triplen harmonics.

State assumptions explicitly before using any relation for inverters — steady state, uniform properties, linear elastic material, ideal gas, incompressible flow, etc., as applicable.

Wrong assumptions invalidate the entire solution even when the formula is correct. In Power Electronics viva and GATE descriptive questions, listing valid assumptions often earns separate marks.

1. Read the question and list given data with SI units (common in Power Electronics papers).
2. Draw a neat labelled diagram where applicable — examiners in Indian universities award diagram marks even when arithmetic slips.
3. Identify which relation from this topic applies to inverters.
4. Use equation 1: .
5. Use equation 2: .
6. Substitute values, compute, and verify units and sign (direction).
7. State conclusion in one line — e.g. safe/unsafe, stable/unstable, feasible/infeasible.

Inverters appears in drives, UPS, and grid interfaces. In Indian electrical curricula this topic is tested because it connects theory to controlled power conversion.

GATE and semester exams often combine inverters with earlier units — revise prerequisites before attempting mixed problems.

Industry interview panels sometimes ask: "Where did you use inverters?" — answer with a lab, mini-project, or plant visit example if possible.

• Using peak V_dc as the fundamental rms of a square wave (miss the 2√2/π factor)
• Confusing modulation index with duty ratio
• Forgetting dead time is essential to avoid shoot-through
• Ignoring that triplen harmonics cancel in the line voltage of a three-phase inverter

Before attempting inverters problems, confirm you can:

1. Harmonics reduced by PWM at switching frequency
2. Dead time prevents shoot-through in bridge legs
3. VSI vs CSI — voltage source vs current source inverter

Revise the solved examples in Power Electronics — PS Bimbhra and one previous-year GATE or university paper for this unit.