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VFD: Active Front End

diodemoduleActive front end VFDs. A little overview..

The most common VFD technology uses diode rectifiers to convert the incoming line AC to DC, which is then used as a DC Bus for the inverter sections to generate a synthesized variable frequency, variable voltage AC waveform.

Active front end VFDs use IGBTs in lieu of diodes. and this provides several advantages:

  1. Harmonic Mitigation. Many Active Front End (AFE) VFDs advertise there is no need for additional power line harmonic filtering.
  2. Input Unity Power Factor.. the voltage and current are in phase.
  3. Regulated DC Bus voltages. In passive front end (diode) VFDs, the DC Bus varies with the line.
  4. Allows line regeneration. This is, in fact, why most people purchase an active front end VFD.

The disadvantage of an active front end VFD is cost. Active front end VFDs are considerably more expensive. There are circumstances however, in which it can be justified.

As I mentioned earlier, if you need a fully line regenerative VFD, then you need an active front end. There are less expensive methods if you just need a quick stop at a relatively low duty cycle. For that, you can use a switched resistor across the VFD bus. This adds little cost. This is normally referred to as dynamic braking.. or in VFD tech parlance… a Bus burner.

But otherwise, if you need rapid reversing, high duty cycle braking, or need to run overhauling loads, then you need a fully line regen VFD, and an active front end is your best choice.

The cost of the AFE VFDs will undoubtedly drop as more are produced. It may well be now, in some circumstances, where eliminating external harmonic filtering, or power factor problems justify the added expense of an active front end VFD. But as a general rule, we’ve not found that to be the case.

At our company, EMA Inc., we’ve been seeing more active front end VFDs in higher horsepower VFDs. We recently did a dynometer project that required a high horsepower, 480 volt, fully regenerative VFD.

We’ve also seen an increase in inquiries for medium voltage fully regenerative VFDs; something that was unheard of just a few years ago.

If you’d like to talk further about this, then hit any of the contact methods on the right of this page, or tweet us @emadrives , or do it the old fashioned way and call us at 770-448-4644770-448-4644.

We sell and service low and medium voltage VFDs, both regen and non-regen, do preventive maintenance on them, and offer free online support via twitter or our online chat.

No One, Anywhere, is Better at Drives than we are. Try us and find out for yourself!


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