What is a Variable Frequency Drive?
A variable frequency drive (VFD) is an electronic control device used to provide adjustable speed control for AC and DC motors. The most common types of AC motors are AC induction motors or asynchronous motors. The benefits of using a VFD are energy savings for HVAC fan and pump applications, improved motor torque and speed control in industrial applications, and improved motor protection.
Motor sizes for VFD operation:
Variable frequency drives can operate motors ranging from ¼ horsepower at 120 volts single phase to 20,000 kilowatts at 11,000 volts three phase. Industrial motors are typically three-phase, and the most common voltage in the U.S. is 480 V. Other common voltages are 230, 2300, and 4160 V. The motor is designed to operate on a variety of voltages depending on the poles of the motor.
Motors are designed to run at a constant speed depending on the number of poles in the motor. The frequency of the voltage supplied to the motor determines the speed of the motor. This speed is a function of the applied frequency multiplied by 120 divided by the number of poles per phase.
How a VFD changes the speed of a motor:
VFDs control motors by varying the frequency between 0 and 60 cycles or Hertz per second. They can also change frequencies outside of the standard 0-60 Hz band up to 600 Hz. Once above 600 Hz, the application is typically a high-speed spindle motor. Once above 600 Hz, the application is typically a high speed spindle motor. the VFD varies the input frequency by adjusting the output frequency of the AC sine wave to the motor. To accomplish this, the VFD has four main components:
- The diode rectifier or converter section converts the 60 Hz AC line voltage to DC voltage via a diode rectifier.
- The DC bus section stores the converted AC voltage in a capacitor. For a 480 V AC input, the peak voltage at the three-phase DC bus can be 648 V DC (= 480 V x Sq. Rt. √1.82).
- The inverter section or IGBT (Insulated Gate Bipolar Transistor) converts the stored DC voltage into an analog AC sine waveform called pulse width modulation to simulate an AC waveform. The pulse width modulated waveform has an amplitude (height), which is the maximum input voltage, usually 480 volts, and a duty cycle (width). The number of duty cycles per second is the frequency in Hertz, and the rate at which the IGBT turns on and off is the carrier frequency. Carrier frequencies range from 2,000 Hz to 15,000 Hz. Higher carrier frequencies produce better waveforms. Imagine a light switch on the wall that allows voltage and current to power a light bulb. Voltage and current flow to the bulb at a rate of 60 cycles per second. Any light flicker is too fast to be detected by the naked eye.
- The VFD control section determines the voltage and frequency or duty cycle number sent to the motor. The motor uses a pulse width modulated waveform to control the speed of the motor.
The Evolution of Variable Frequency Drives
VFDs have evolved over time, but the basic functionality remains. They are used in many applications and help industries evolve. With the current surge in IIoT and smart manufacturing, VFDs will become integral to process improvement and provide meaningful data to help manufacturers become leaders in their industries.