Video summary

【기본반】 전기기기 19강(p.218~236)_5장 정류기(회전 변류기, 수은 정류기, 전력용 반도체 소자, 다이오드 정류회로, SCR 사이리스터 위상 제어, 전력 변환)

Main summary

Key takeaways

Educational

Main Ideas / Lessons Conveyed

1) What Rectification Is (Core Concept)

  • Rectification: the process of converting AC (alternating current) into DC (direct current).
  • A rectifier is the device/system that performs this conversion.
  • The lesson is structured around:
    • Rotary rectification (rotary armature / rotary converter)
    • Mercury rectification (described via a “tilted tube”/arc idea)
    • Power semiconductor devices (diodes, SCR, etc.)
    • Diode rectifier circuits (half-wave/full-wave; single-phase/three-phase)
    • SCR phase control (how firing angle affects DC output)
    • Power conversion categories (converter/inverter/choppers, etc.)
    • AC commutator motor and related motor types

2) Rotary Rectifier / Rotary Converter (Key Principle)

The rotary converter combines the behavior of:

  • a synchronous motor (electrical input produces rotation)
  • and a DC generator (rotation produces DC via commutator/brushes)

Exam distinction (rotary armature type)

  • In rotary armature type rotary rectifiers, the rotating magnetic field rotates in the opposite direction to the rotor (this “opposite direction” emphasis is repeated).

Example practice logic: 4-pole, 60 Hz rotary current transformer

  • Synchronous speed:
    • ( n_s = \dfrac{120f}{p} )
    • With ( f = 60\,\text{Hz} ), ( p = 4 ):
      • ( n_s = 120 \times 60 / 4 = 1800\,\text{rpm} )
  • Direction note:
    • The answer is 1800 rpm, but it rotates in the opposite direction to the rotor.

3) Mercury Rectifier Principle (Valve Action + Arc Behavior)

Principle explanation (tilting experiment analogy)

  • Tilt a glass tube containing liquid:
    • It intermittently connects “positive” and “negative” regions so current flows.
  • Return it to the original position:
    • The connection breaks and an arc discharge/spark is produced.
  • Takeaway:
    • Current effectively flows in only one direction at the load (DC-like behavior).

“Valve action” definition

  • Valve action = enabling continuous current flow from positive electrode to negative electrode.
  • Because of this, the output becomes rectified (DC).

Abnormal phenomenon: “Yoho”

  • Yoho: the rectifier’s valve function is lost (current no longer properly behaves as one-direction-restricted).
  • Causes (listed):
    • Overload current or overvoltage
    • Increased pressure of remaining gas inside
    • Droplets / pure metal droplets from the positive electrode attaching
    • Impurities/droplets attaching to the anode surface
    • Simplified idea: vacuum level decreases
  • Solutions (listed):
    • Ensure proper cooling (avoid overheating/overcooling)
    • Prevent overload
    • Restore/maintain sufficiently high vacuum level

4) Power Semiconductor Devices (Diodes → SCR → Others)

PN junction diode (rectifying behavior)

  • Emphasis on terminals:
    • Anode (positive) and cathode (negative)
  • Conduction rule:
    • The diode conducts only when the anode is at higher potential than cathode (forward bias), and blocks in reverse bias.
  • The diode conducts in the arrow direction; otherwise it blocks the output.

SCR / Silicon Controlled Rectifier (key differences vs diode)

  • SCR = Silicon Controlled Rectifier
  • Crucial difference from a diode:
    • SCR has a Gate terminal (extra control lead).
  • Operation concept:
    • SCR turns ON when gate current is applied (while anode-to-cathode bias exists).
    • Once ON, it keeps conducting even after the gate is removed.
    • It cannot be turned off using gate alone; turn-off depends on circuit conditions.

Key characteristic ideas

  • One-direction behavior / reverse blocking
  • Turning off requires circuit conditions (simplified via current/holding logic in the lecture)
  • Parameters:
    • Turn-on / conduction delay (“open time”)
    • Latching current: minimum anode current to keep SCR from turning off after triggering
    • Holding current: minimum current required to maintain the ON state
      • (holding current < latching current)

“Car parking push” analogy for ON/OFF

  • Latching current ≈ force needed to get the car moving initially
  • Holding current ≈ force needed to keep it moving afterward
  • If force is too low after starting, it stops (SCR would turn OFF if current drops below holding current).

Other devices (terminal count + directionality for memorization)

The instructor emphasizes exam-style memorization using:

  • Unidirectional vs bidirectional
  • Number of terminals
  • Common devices mentioned:
    • SCR: reverse-blocking, 3 terminals
    • GTO: gate turn-off SCR-like (gate can turn it off)
    • Triac: bidirectional, 3-terminal thyristor
    • Diac: bidirectional, 2-terminal diode-like device
    • Others referenced for pattern memorization:
      • Zener diode (2 terminals; reverse-blocking)
      • LASCR / light-activated SCR (noted as “SC”, 3 terminals)
      • SCS (reverse-blocking, 4-terminal)
  • General rule repeated: check directionality + terminal count

5) Diode Rectifier Circuits (Method List + Computation Results)

Rectifier circuit types to remember

  • Single-phase
    • Half-wave rectifier (1 diode)
    • Full-wave rectifier (2 or 4 diodes depending on topology)
  • Three-phase
    • Half-wave rectifier
    • Full-wave rectifier

The lecture stresses memorizing these categories and their output voltage relationships.

Rectification: Detailed Exam-Relevant Bullet Points (Formulas + Results)

A) Single-Phase Half-Wave Rectifier

  • Circuit: 1 diode
  • Operation:
    • Only the positive half-cycle passes; the negative half-cycle is blocked.
  • Output waveform:
    • DC-like but pulsating (half-wave shape).

Average (DC) output voltage

  • Common relation used:
    • ( V_{DC} \approx 0.45 V_{AC} )
  • Example:
    • 100 V AC → ~45 V DC

PIV (Peak Inverse Voltage)

  • Treated conceptually as the maximum reverse voltage magnitude.
  • Emphasis: maximum reverse withstand, tied to AC peak value (with √2-type conversions).

Quality / efficiency metrics

  • Rectification efficiency (once stated): 40.6%
  • Pulsation rate (remaining AC component in DC output):
    • Lower pulsation rate → better rectification (closer to steady DC)

B) Single-Phase Full-Wave Rectifier

  • Circuit types:
    • one shown using 2 diodes
    • one bridge approach using 4 diodes
  • Operation:
    • Both AC polarities contribute to positive DC (full-wave).

Average DC voltage

  • Relation stated:
    • ( V_{DC} \approx 0.92 V_{AC} )
  • Example:
    • 100 V AC → ~90 V DC

PIV differences

  • For the 2-diode topology:
    • PIV increases (described as effectively 2× compared to the “normal” reference used in the lecture)
  • For the 4-diode bridge:
    • PIV is smaller than the 2-diode case (lecture gives √2-related comparison)
    • Bridge is used as the “bridge-type” memory hook

Ripple frequency

  • Full-wave ripple has higher frequency than half-wave:
    • single-phase full-wave → 2× fundamental frequency (for ripple/content comparison)

C) Three-Phase Rectifiers

Three-Phase Half-Wave

  • Simplified output multiplier:
    • DC ≈ 1.17 × V (with phase/line context handled)
  • Ripple frequency comparison:
    • 3×
  • Pulsation quality:
    • Pulsation rate stated as about 17% (values may be rounded based on lecture convention)

Three-Phase Full-Wave

  • Standard multiplier:
    • DC ≈ 1.35 × V
  • Ripple frequency comparison:
    • 6×
  • Pulsation quality:
    • Better (smaller pulsation rate than single-phase cases)

D) Final Memory Structure (Rectifier Outputs)

To memorize:

  • Half-wave (single-phase): 0.45 × V
  • Full-wave (single-phase): 0.92 × V
  • Three-phase half-wave: 1.17 × V
  • Three-phase full-wave: 1.35 × V

Also emphasized:

  • Pulse/ripple improves from single-phase half-wave → three-phase full-wave.

6) SCR Phase Control (Firing Angle ( \alpha ))

Key exam claim

  • SCR control = phase (angle) control.
  • With firing angle ( \alpha ), average DC output depends on the load type.

Lecture-highlighted load dependence

  • Pure resistive load (R only):
    • Multiply by ( (1 + \cos \alpha) )
  • RL load (lecture simplified using the steady/freewheeling-diode behavior):
    • treated similarly to the resistive rule in the simplified explanation
  • If ( L \to \infty ) (very large inductance):
    • Multiply by ( \cos \alpha ) (not ( 1+\cos\alpha ))

Memorization takeaway

The “what to multiply” depends on:

  1. Load composition (R vs RL)
  2. Whether L is effectively infinite (steady conduction behavior)

7) Power Conversion Vocabulary (Converter / Inverter / Chopper)

  • Categories by input/output:
    • AC → DC: rectifier
    • DC → AC: inverter
    • AC → AC: cycloconverter (frequency conversion)
    • DC → DC: DC chopper (chops into smaller pieces)

8) AC Commutator Motors and “Universal Motor”

AC commutator motor

  • Converts AC to DC internally using a commutator to produce DC-type motor action.
  • Back EMF (“speed EMF”) is adapted for AC commutator motor using speed rather than angular-frequency-style terms.

Four motor types emphasized

  1. single-phase series commutator motor
  2. single-phase repulsion motor
  3. three-phase series commutator motor
  4. three-phase shunt commutator motor

Single-phase series commutator motor → Universal motor

  • Can operate with both AC and DC → called universal motor
  • Tradeoffs under AC:
    • field/armature branch weakened due to impedance effects
    • then compensated to maintain torque/power factor
  • Countermeasure mentioned:
    • use compensation winding to address armature reaction/power factor

Sources / Speakers Featured

  • Nam Min-su — instructor at Dasan Haedi (Dasan Hae-di)

Original video