As a critical component of electrical drive systems, VFDs often exhibit differences between input and output currents. This phenomenon is primarily influenced by factors such as operating frequency, harmonic effects, power factor, efficiency, and other technical characteristics.

1. Impact of Operating Frequency

Output Voltage Varies with Frequency: The output voltage of a VFD is proportional to its operating frequency. When the frequency is low, the output voltage decreases, leading to an increase in current to maintain constant power.

Input Voltage Remains Constant: The input voltage to a VFD is typically fixed at standard values like 380V and does not vary with frequency.

Example:
At an operating frequency of 30Hz, assuming a V/F curve of 1/2 linear relationship:

Output voltage ≈ 228V

Input current ≈ 45.58A

Output current ≈ 75.97A
This demonstrates a significant current difference.

2. Harmonic Effects

Source of Harmonics: VFDs generate numerous harmonics, especially high-frequency ones, during rectification and inversion processes.

Impact of Harmonics: These harmonics distort current waveforms and reduce measurement accuracy, affecting both input and output current values.

Mitigation Measures: Installing harmonic filters or suppression devices can minimize these effects, ensuring accurate current measurements.

3. Combined Effects of Power Factor and Efficiency

Efficiency: The overall efficiency of a VFD system is the product of the VFD's efficiency and the motor's efficiency. At a load above 75% and a frequency over 40Hz, VFD efficiency exceeds 95%, and system efficiency can reach over 85%. For high-voltage, high-power VFDs, system efficiency can exceed 96%.

Power Factor: The power factor of a VFD depends on the phase difference between voltage and current and the harmonic content.

At base frequency and full load, the VFD's power factor is usually not lower than that of the motor running directly on mains power.

Power factor increases with load and speed, being lower at light loads and low speeds.

Improvements: Adding DC reactors specifically designed for VFDs can reduce input current distortion, lower harmonic reactive power, and improve the overall system power factor.

4. Other Factors Influencing Input and Output Current Differences

Inverter Output Characteristics: The inverter stage in a VFD uses pulse-width modulation (PWM) to convert DC to an AC waveform. To ensure the motor receives adequate power, the output current often exceeds the input current by around 10%, a design feature.

Harmonic Components: Both input and output currents contain harmonic components that complicate waveform shapes, making accurate phase angle measurement challenging. Traditional measurement methods may yield significant errors when harmonics are present.

Energy Conservation Principle: The VFD's input power factor is typically high (around 0.95), while the motor's power factor is relatively lower (approximately 0.85). To balance power equations, the VFD must output more current than it inputs to meet the motor's energy demands and maintain power factor stability.

Conclusion

The phenomenon of VFD input current being smaller than output current results from a combination of factors, including operating frequency, harmonics, power factor, and efficiency. Understanding these factors is crucial for optimizing VFD performance, improving system stability and efficiency, and ensuring safe operation. When using VFDs, it is essential to account for these differences to maintain reliable and efficient system functionality.