Variable Frequency Drives (VFD) are an electronic control device used to provide adjustable speed control for AC and DC electric motors. The most common AC motor type is an AC induction or Asynchronous motor. 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.
What size motors to VFD’s operate:
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 United States is 480V. Other common voltages are 230, 2300 and 4160V.
A motor is designed to run at a constant speed based on the number of poles of the motor. The frequency of the voltage supplied to the motor determines the speed of the motor. That speed is a function of the applied frequency times 120 divided by the number of poles per phase.
How does the VFD change the speed of a motor:
The VFD controls the motor by varying the frequency from 0 to 60 cycles per second or Hertz. They can also vary frequencies outside of the standard 0-60 HZ band and go up to 600 Hz. Once you get 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 sinewave to the motor. To accomplish this the VFD has four main sections:
- The Diode rectifier or Convertor section converts 60 Hz AC line voltage to DC voltage through the Diode rectifier.
- The DC Bus Section stores the converted AC voltage in capacitors. For a 480 V AC input, the peak voltage of the DC bus can be 648 V DC (= 480 V x Sq. Rt. √1.82) for three phase.
- The Invertor Section or IGBTs (Insulated Gate Bipolar Transistors) convert the stored DC voltage to a simulated alternating sinusoidal waveform called Pulse Width Modulation to simulate an AC waveform. The Pulse Width Modulation waveform has an Amplitude (Height), which is the maximum incoming voltage, typically 480 Volts and a Duty Cycle (Width). The number of Duty Cycles per second is the frequency in HZ. The rate at which the IGBT’s turn on and off is the carrier frequency. The carrier frequency can range for 2,000 Hz to 15,000 Hz. A higher Carrier Frequency will yield a better waveform. Think of a light switch on the wall allows the voltage and current to power the bulb. The voltage and current is flowing to the bulb at a rate of 60 cycles per second. Any light flicker is too fast to be detected by eye. The IGBT acts the same way for a motor and allows the voltage and current to flow from the capacitor storage.
- The VFD control section determines voltage and frequency or number of duty cycles that is sent to the motor. The motor uses pulse width modulated waveforms to control the speed of the motor.
VFD’s have evolved over time however the basic functionality remains. They are used in many applications and have helped industry grow. With the current surge of IIoT and smart manufacturing VFD’s will have an integral part of process improvement and provide meaningful data that will help manufacturers become leaders in their industries.