How does a variable frequency drive (VFD) control the speed of a motor?

A Variable Frequency Drive (VFD) controls the speed of a motor by varying the frequency of the electrical power supplied to the motor. The speed of an AC induction motor is directly proportional to the frequency of the applied voltage. A VFD takes incoming AC power at a fixed frequency (typically 50 or 60 Hz) and converts it into a variable frequency AC power supply. The VFD first rectifies the AC power to DC power using a rectifier circuit. Then, the DC power is filtered to smooth out any voltage variations. Finally, the DC power is converted back to AC power with a variable frequency and voltage using an inverter circuit. The inverter circuit uses electronic switches, such as insulated-gate bipolar transistors (IGBTs), to rapidly switch the DC voltage on and off, creating a pulsating waveform that approximates a sine wave. The frequency of the switching determines the frequency of the AC voltage supplied to the motor. By adjusting the frequency, the VFD can control the motor's speed precisely. The VFD also adjusts the voltage to maintain a constant voltage-to-frequency ratio, which ensures that the motor develops the required torque at different speeds. For example, if the frequency is reduced by 50%, the voltage is also reduced by 50% to maintain the same torque capability. This control method allows for efficient and precise speed control of AC motors, resulting in energy savings and improved process control. VFDs are commonly used to control the speed of fans, pumps, and other HVAC equipment.