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Retrofit 18 Pulse VFDs

There are less expensive ways to retrofit Multi-Pulse VFDs

current pulseIn many high horsepower applications, manufacturers utilize phase shifting transformers to allow multi-pulse drives (12 pulse, 18 pulse VFD s) as a way to significantly reduce harmonics. For a better understanding of harmonics see “Harmonics, how do they affect your equipment?”

AC variable frequency drives utilize DC capacitors as part of the main power supply; on the front end, AC is rectified through a series of diodes in order to charge the DC Bus capacitors. Capacitors do not pull current smoothly from the line like a typical, linear load, so they create “notches” in the line, leading to harmonics. One of the best analogies I have heard on this is comparing a drive to Pac-Man in the way it eats off of the line, taking big bites at a time.

18 pulse input

To try and smooth out the notches in the line, extra diode bridges are used to give more pulses to the DC Buss. A standard 3 phase VFD utilizes 6 diodes (3 phases, + and -) to rectify the AC to DC, thus we refer to it as a 6 pulse VFD. As you increase the number of rectifier diodes (in multiples of 6), you are adding more pulses to the DC Buss, smoothing out the notches. That’s why you’ve likely heard of 12, 18, 24, and 36 pules drives. In order for this to work properly, a phase-shifting transformer is also required, which adds significant cost and size to your cabinet.

A common problem multi-pulse VFD users run into is: what to do when they break? Retrofitting with a brand new, multi-pulse drive adds extreme cost and labor.

At EMA, we can offer alternatives to this expensive problem. If you have an existing application where the phase-shifting transformer is already in place, you can retrofit the failed drive with a standard, less expensive, readily available, 6 pulse VFD and add additional diode bridges externally.

Diodes within themselves are common, inexpensive devices with multiple brands available. As long as all the pulses eventually end up on the VFD’s DC buss, the VFD does not discriminate where they come from.

EMA engineers have performed many retrofits like this all over the country, and we’d be happy to do it for you. Don’t pay twice or sometimes 3 times the cost for a multi-pulse drive when you already have many of the elements involved already.

Let EMA engineer a much more cost-effective retrofit option for you. Contact us by any of the methods on the right of this page, or call us at 800-848-2504.

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