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Thread: VFD Line-side Snubbers

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    VFD Line-side Snubbers

    For a typical PWM VFD, having snubbers on the IGBT to protect them from dV/dt problems is normal. But, what about the diode bridge on the line-side input? Are snubbers used to reduce power line noise and transients and prevent damage to the AC-to-DC diode bridge?

    I ask because I was told by a reliable source that under-sizing of the line-side snubbers (filtering?) can lead to drive failure. I did a literature search but came up empty handed.
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    I'm not familiar with the application of snubbers on a VFD (however definitely on MV transformer primaries), but are you referring to line reactors? If you are, maybe your literature search will turn up more using line reactor instead of snubber.
    Ron

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    Quote Originally Posted by beanland View Post
    For a typical PWM VFD, having snubbers on the IGBT to protect them from dV/dt problems is normal. But, what about the diode bridge on the line-side input? Are snubbers used to reduce power line noise and transients and prevent damage to the AC-to-DC diode bridge?

    I ask because I was told by a reliable source that under-sizing of the line-side snubbers (filtering?) can lead to drive failure. I did a literature search but came up empty handed.
    Depends on the topology of the front end. Some use an SCR based front end in order to ramp the voltage into the DC bus caps on initial power up. Those designs will use RC snubbers around the SCRs, but those snubbers are not really there for protection from line transients, they are necessary to avoid false commutation (random firing) from line noise. Most of the time, line transient spike protection for VFDs will be MOVs, or on cheap ones, nothing. but if you have a Delta power supply, you must disconnect the MOVs from their ground reference, otherwise they attempt to create a Wye for the entire grid system if there is a ground fault, so they are toast on the first instance. That may be the reason some people perceive it to be a sizing issue. It isn't though.
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