Motor

• An AC motor operates on the forces produced between magnetic fields. The oscillating nature of AC current produces magnetic fields that rotate around the motor's axis. These forces cause the motor's axis to spin at a speed given by the following formula:
  RPM = 120 x F / P
  Where RPM is the motor speed in revolutions per minute, F is the line frequency, and P is how many magnetic windings, or "poles" the motor has.
  
  While you can run a standard AC motor from a VFD power unit, manufacturers have developed "inverter duty" motors with different cooling and electrical insulation to handle the demands of variable-speed operation.
  
  

Rectifier

• To drive a motor at different speeds, you need to change the frequency of its electric power. The first step in this process is to change the incoming AC to DC. A rectifier accomplishes this, eliminating the current reversals in AC so the current will flow in only one direction. A set of high-current diodes make up the rectifier.
  
  

DC Storage

• The DC current from the rectifier flows into a set of capacitors. These serve as a buffer, a place to store energy temporarily, in order to make the motor run efficiently as its energy demands change with changing loads and speeds.
  
  

Inverter

• The capacitors feed DC current to the inverter. The inverter drives the motor. This high-power circuit generates a frequency you can change from a knob or other control. You can vary this frequency from zero to 90 or more cycles per second. Because of the circuits that produce the new frequency, its output is not the smooth ripple that the power company provides, but has small sharp edges. To better handle the different power signal, engineers developed the inverter-duty motors described above.
  
  

Benefits

• Commercial and industrial heating, ventilation, and air conditioning (HVAC) systems depend on AC motors to run compressors, pumps, and fans. These systems can work more effectively if the operator has a range of motor speeds to choose from. Fans and pumps that can run at lower speeds consume less energy and make less noise. Motors that can run continuously at higher and lower speeds can avoid the extra wear and tear from repeated start-stop cycles.