Unit 1-2: Single-Phase vs. Three-Phase Power: Voltage & Current Comparison

Single-Phase vs. Three-Phase Power: Voltage & Current Comparison

1. Single-Phase Power (1Φ)

• Common Applications:
  • Household appliances (fans, lights, refrigerators)
  • Small motors (<5 HP)
  • Residential and light commercial buildings
• Voltage Standards:
  • 120V (North America, Japan)
  • 230V (Europe, Asia, Africa, Australia)
• Current Calculation:I=PV×cos⁡ϕ×ηI=V×cosϕ×ηP​
  • Example: A 3 kW (3000W) motor at 230V, PF = 0.85, η = 90%:
  I=3000230×0.85×0.9≈∗∗17A∗∗I=230×0.85×0.93000​≈∗∗17A∗∗
• Characteristics:
  • One live wire + neutral (+ground)
  • Pulsating power delivery (not constant)
  • Lower efficiency for heavy industrial loads

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2. Three-Phase Power (3Φ)

• Common Applications:
  • Industrial motors, pumps, compressors
  • Large HVAC systems
  • Factories, data centers, heavy machinery
• Voltage Standards:
  • 208V, 400V, 480V (common globally)
  • 230V/400V (Europe)
  • 277V/480V (US industrial)
• Current Calculation:I=P3×VL−L×cos⁡ϕ×ηI=3​×VL−L​×cosϕ×ηP​
  • Example: A 3 kW (3000W) motor at 400V, PF = 0.85, η = 90%:
  I=30001.732×400×0.85×0.9≈∗∗5.7A∗∗I=1.732×400×0.85×0.93000​≈∗∗5.7A∗∗
• Characteristics:
  • Three live wires (+neutral/ground optional)
  • Smooth, constant power delivery (120° phase shift)
  • Higher efficiency (lower current for same power)

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Key Differences: Single-Phase vs. Three-Phase

Parameter	Single-Phase (1Φ)	Three-Phase (3Φ)
Voltage (V)	120V, 230V	208V, 400V, 480V
Current (A)	Higher for same power	Lower for same power
Power Delivery	Pulsating	Continuous & balanced
Efficiency	Lower for heavy loads	Higher for industrial use
Wiring	2-wire (L+N) or 3-wire (L+N+G)	3-wire (L1+L2+L3) or 4-wire (L1+L2+L3+N)
Applications	Homes, small appliances	Factories, large motors

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Why Three-Phase is Preferred for Motors?

✔ Lower current → Thinner wires, less heat loss.
✔ Self-starting → No need for capacitors (unlike single-phase).
✔ Smoother torque → No power pulsations.

When to Use Single-Phase?

✔ Low-power devices (<3 kW).
✔ Where 3Φ supply is unavailable.

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Summary

• Single-phase: Simpler, used in homes, higher current for same power.
• Three-phase: More efficient, used in industries, lower current, balanced power.

For motors >5 HP (3.7 kW), three-phase is almost always better. Always check nameplate ratings and local voltage standards before installation