Why Your VFD Output Amps Can Surpass Input (Despite Equal Voltage)

When both input and output voltages are 380V, why does input current sometimes exceed output current, and vice versa? The answer lies in power factor differences and operating conditions, not voltage equality.

Fundamental Principle
A VFD converts AC-DC-AC, following energy conservation:
Input Power ≈ Output Power + Losses
√3 × U × I_in × cosφ_in ≈ √3 × U × I_out × cosφ_out × η
Since voltage U is equal (380V), the current relationship depends entirely on power factor (cosφ) and efficiency (η).

Scenario 1: Low Speed, Light Load → Input Current > Output Current
At 25Hz with minimal load:

• Input side: High power factor (0.95–0.98), small active power 
• Output side: Very low power factor (0.1–0.3) due to magnetizing current

The motor needs large reactive current to maintain magnetic flux at low frequency, but this current does no real work. The VFD draws clean power from the grid (high power factor) while outputting mostly reactive current. Adding 3–5% VFD losses, input current exceeds output current.

Scenario 2: High Speed, Heavy Load → Output Current > Input Current
At 50Hz full load:

• Input side: Power factor ~0.95
• Output side: Power factor recovers to 0.85–0.90

Now both sides transfer active power efficiently. With motor power factor near rated values, the power balance yields I_out ≈ 1.06 × I_in—output current slightly exceeds input.

Key Insight
Current magnitude reflects power factor effectiveness, not just load size. Low frequency operation creates a "reactive current penalty" that distorts simple current comparisons.
Practical Implications

1. 1. Size input breakers by maximum input current, not output ratings
2. 2. Monitor output cable heat at low speeds—magnetizing current can reach 30–50% rated even when unloaded
3. 3. Don't correct input power factor—VFDs already achieve 0.95+; low motor-side power factor at low speeds is normal

Conclusion
The apparent paradox resolves through power factor analysis: input current dominates when output power factor is poor (low speed), while output current dominates when motor power factor is healthy (rated speed). Voltage equality is irrelevant—current follows the path of energy transfer efficiency.

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