AC Voltage Controllers

Phase control fires the SCR pair partway through each half cycle, so only the latter part of each half-wave reaches the load. The rms output falls as the firing angle α increases, from full voltage at α = 0 to zero at α = 180° (resistive load).

These controllers are used for lamp dimmers, fan regulators and furnace control. They generate harmonics and draw a lagging power factor even with a resistive load because the current is delayed.

Relation 1 —

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.

For a resistive load, the output rms is V_o = V_s√[(π − α + (sin2α)/2)/π], where V_s is the source rms and α is in radians. Load power is then V_o²/R.

Integral-cycle (burst) control switches whole cycles on and off instead of chopping within a cycle; it avoids switching harmonics and suits slow thermal loads like furnaces, but is unsuitable for lighting (flicker).

An RC snubber across each SCR limits dv/dt to prevent false turn-on, and a series inductor limits di/dt at turn-on.

State assumptions explicitly before using any relation for ac voltage controllers — 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 ac voltage controllers.
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.

AC Voltage Controllers 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 ac voltage controllers with earlier units — revise prerequisites before attempting mixed problems.

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

• Using α in degrees inside the rms formula (it must be in radians)
• Assuming unity power factor with a resistive load (current is delayed, so pf < 1)
• Forgetting to square the rms voltage when finding load power
• Applying the resistive-load formula to an inductive load (extension angle differs)

Before attempting ac voltage controllers problems, confirm you can:

1. Used for speed control of fan motors, furnaces, lighting dimmer
2. Triac pair for AC switching
3. RC snubber for dv/dt protection of SCR in AC controller

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