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AC Drives - Basic Principles, Terminology and Theory

Mar 1st 2023

AC Drives - Basic Properties, Terminology and Theory

An AC Drive is an electronic device that converts a fixed frequency and voltage to an adjustable frequency and AC voltage source. It controls the speed, torque, horsepower and direction of an AC motor. AC Drive is also a term used for an AC inverter and is sometimes used to describe a particular section of an AC drive. The section uses the DC voltage from a previous circuit stage (DC Bus) to produce an AC current or voltage having the desired frequency. AC Drives are also referred to as Variable Frequency Drives (VFD's) or Adjustable Speed Drives (ASD's). These drives are gaining in popularity due to the energy savings that can be obtained related to the AC technology. In addition, AC motors are simpler than DC and usually an "off-the-shelf" item compared to DC motors. Advances in technology have made the size, cost, reliability and performance of AC drives very appealing in industrial variable speed applications.

Operator Motor Control

The operator control allows the operator to command the motor to function as desired through the use of motor control inputs and outputs. The motor control section controls the motor's speed by converting utility power into adjustable frequency power. The AC motor drives the device (fan, pump, etc.) by converting the electrical power to mechanical power.


Voltage and Frequency

AC Drives convert a fixed, 3 phase voltage and 60Hz frequency source into a variable voltage and frequency source. In order to control the speed of the motor, the frequency applied to the motor must also be controlled. The formula for this is:

N=120 x F/P. N=Speed in RPM, F=Frequency in Hz, and P=Number of pole pairs. 

Since the number of poles is constant, the only convenient factor to vary is the frequency. Frequency determines motor speed. However, the motor needs to supply rated torque, no matter what the speed is.

In order for rated torque to occur, it is necessary to keep the voltage and frequency in a constant relationship. This is called the Volts per Hertz curve.  The inverter supplies current to the motor, current is in direct relationship with the torque.

There is a specific voltage to frequency relationship that exists in an AC motor. With this relationship supplied to the motor plus current, the motor will be able to develop rated torque at all speeds. 

AC Drive Advantages

  • Versatile - more than motor control, this device has self-diagnostics, current status, multi-use programming ability, and precise speed control
  • Energy Savings - reduces peak energy demands by ramping up power drawn from the motor & maximizes energy put through the unit
  • Performance - simple operation, fast motor load change response (small set-point for precise operation)
  • Reliability - solid-state unit, no external controls
  • Size - lighter and smaller than most other methods of speed control

AC Drive Applications

  • Conveyors, belts, chains, screws, bulk/packaged material handlers
  • Fans, blowers, compressors, pumps
  • Machine tools, grinders, lathes, stamping presses
  • Custom machines, label makers, bottle washers, wire drawing, textiles, etc.
  • Extruders, process machinery, kilns, grinders, blenders, agitators

AC Drive Availability

AC Drives are available 110, 240, 480 and 575 voltages, and .25-2500 HP in continuous or variable torque styles. Select from a variety of NEMA types to fit your application.

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