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Electronics Engineering QUESTION #11722
Question 1
A VFD (Variable Frequency Drive) is set to operate a 3-phase induction motor at 30 Hz instead of 50 Hz. To maintain constant motor flux (V/Hz ratio), the output voltage should be set to:
  • $380\,\text{V}$
  • $228\,\text{V}$✔️
  • $190\,\text{V}$
  • $480\,\text{V}$
Correct Answer Explanation
The V/Hz ratio for constant flux: $\dfrac{V}{f} = \dfrac{380\,\text{V}}{50\,\text{Hz}} = 7.6\,\text{V/Hz}$. At 30 Hz: $V = 7.6 \times 30 = \mathbf{228\,\text{V}}$. This constant V/f control maintains constant air-gap flux, preserving motor torque capability below rated speed. Above rated frequency, voltage is held constant (field-weakening region).