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How a VFD Converts AC to DC | VFD Working Principle Explained | AC to DC Conversion in Variable Frequency Drives | Power Electronics Basics
Need for AC to DC conversion in VFD:
By doing this, the VFD drive kills two birds with one stone, i.e. frequency control and voltage control.
Induction motor speed control requires two major things…
- Frequency control
- Voltage control
AC is converted to DC. This can be done by a single-phase or a three-phase AC input supply. E.g. arrangement of a single-phase inverter.
Frequency control:
DC is inverted to variable frequency three-phase AC. To reduce harmonics, the PWM technique is used to generate a sinusoidal current through the induction motor.
Voltage control:
Alone, changing frequency to control the speed of the induction motor is not enough because when the speed changes, back emf also changes. At low speed, back emf is reduced, and the motor draws more current, which heats up the motor. Voltage needs to be reduced.
At high speed, back emf becomes dominant, so the current drawn by the motor is reduced. This impacts the torque of the motor. Voltage needs to be increased.
This can again be achieved with the help of PWM control.
Make pulses thin to reduce voltage.
Make pulses fat to increase voltage.
Voltage control is done automatically as frequency is varied by the drive using basic PWM control.
Note: Voltage can go above the input supply as the filtered DC capacitor charges to the peak of the AC supply after DC conversion.
FAQ SET:
1. What converts AC to DC inside a VFD?
A VFD uses a rectifier made of diodes or IGBTs to convert incoming AC power into DC.
2. What is the role of the rectifier in a VFD?
The rectifier changes the alternating AC wave into a unidirectional DC wave so the VFD can control motor speed.
3. What is the DC bus in a VFD?
The DC bus (or DC link) is a capacitor bank that filters, smooths, and stores the rectified DC power.
4. Why does a VFD need to convert AC to DC first?
A motor’s speed is controlled by frequency. Converting AC to DC allows the VFD’s inverter to reconstruct a new AC output with adjustable frequency.
5. What components smooth the DC inside a VFD?
Large electrolytic capacitors and sometimes inductors smooth the pulsating DC into stable DC.
6. Does the rectifier affect power factor?
Yes. Diode rectifiers lower the power factor slightly, while active rectifiers improve it.
7. Can a VFD work without DC bus capacitors?
No. Without capacitors, the DC would be too unstable to generate a clean, controlled AC output.
8. What happens after AC is converted to DC in a VFD?
The inverter stage uses IGBTs to chop the DC into a Pulse Width Modulated (PWM) AC waveform that drives the motor.
9. Does AC to DC conversion cause heat in the VFD?
Yes. Rectifiers and capacitors generate heat during conversion, so cooling systems are essential.
10. Is the AC to DC conversion loss significant?
Losses are minimal (typically 2–5%), but depend on rectifier type and load conditions.
11. Why not control AC directly?
The power from the grid is fixed:
Voltage: e.g., 230V or 415V
Frequency: 50 Hz
But motor speed depends on frequency.
Speed (RPM) ≈ (120 × Frequency) / Poles
So to control speed, we must change the frequency, and the grid cannot provide variable frequency.
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