Active Front End Variable Frequency Drives


Written by:
Rob Kelly, Emerson Control Techniques

Active Front End (AFE) drives use two power sections. An active rectifier is used to minimize the amount of harmonic distortion reflected to the power grid by drawing nearly sinusoidal current from the power grid. Standard 6-pulse rectifiers used in most drives create harmonics (e.g.5th, 7th, etc.) that can generate anywhere from 30 to 100 percent current distortion. The active rectifier used in the AFE drive creates cancelling harmonics that effectively eliminate those that would be created in a standard 6-pulse drive.

An AFE drive offers several benefits over an 18-pulse harmonic reduction drive: cost, energy savings and less harmonic distortion. While the printed performance specifications may show only a slight improvement using AFE over 18-pulse, the difference can be significant in the field.

For 18-pulse drives to achieve their stated reduction levels, the power entering the drive system must be balanced to within levels not practical in the real world. Eighteen-pulse systems provide the best results when the driven load is at full load conditions. This provides the least energy saving opportunity. This type of balanced power system is rarely the case due to the nature of the mixture of 3-phase and single-phase loads in a building, and because the loads are not always continuous. 

AFE drives offer enhanced performance not only under ideal conditions, but also under conditions that exist in most building installations. AFE drives employ active filtration to compensate for harmonics generated by nonlinear current of the drive at any given moment. This effectively reduces the harmonics to the specified levels regardless of typical imbalances in input voltage.

Power Considerations

AFE drives have a .99 power factor, which is an important consideration in areas where commercial and industrial electricity rates are based in part on power factor. Eighteen-pulse drives use filters that reduce power factor by about 5 percent. Many electric companies bill their commercial accounts not only on the basis of kilowatt hours used, but also power factor (kVA or kVAR) and peak demand. Reduced power factor increases the cost for utilities to supply power. Likewise, peak demand requires the utility to provide the capability to meet that peak demand even though the normal demand for power may be much less.

Reduced Cost

AFE drives cost less than 18-pulse systems. HVAC systems using AFE drives of less than 50 hp can save up to 20 percent of the cost of an equally sized 18-pulse system. When the drives have a horsepower above 50 hp, the AFE drives approach the cost of 18-pulse drive but yield better performance.

System Basics

Typical AFE drive systems consist of a dedicated HVAC drive with an active front end controller and EMC filtering, providing low harmonic distortion. Benefits include:

  • IEEE519-1992 harmonic compliance at the drive input terminals
  • Single-package design-no need for external filters or transformers
  • UL508A approved
  • 100kA SCCR ratings
  • NEMA 1 enclosure is standard, other enclosure ratings available
  • Optional easy-to-use electronic bypass control, with easy to use touchpad

The HVAC drive also provides benefits:

  • Integrated network connectivity-BACnet, Metasys N2 and Modbus
  • Wide range of I/O and communications option modules including Ethernet and LonWorks
  • Static and rotational auto tune
  • Eight preset speeds; eight sets of acceleration/deceleration rates
  • Catch spinning motor function
  • Built-in dual PID
  • Energy savings mode
  • Power metering
  • Motor pre-heat circuit
  • Plain text LCD keypad with on-board help
  • Fire Mode for building occupant safety
  • Two option slots for other communication options and/or I/O expansion
  • SmartCard for parameter storage and copying
  • Free software tools for estimating energy savings

Where there is need to mitigate harmonic distortion, AFE drive systems may be the solution for saving energy and lowering overall system costs.


See also:

Upstream Pumping Solutions

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