Design Of Input & Output Choke For VFD

What do you want to know? In general, modern VFD manufacturers test their VFDs and state in their manuals (and/or application data sheets) what output choke should be used (and cable length and PWM frequency interaction if you are lucky) to ensure best (taking economical and real world application considerations into account ) results.

Additionally there is quite a bit of information on the internet (and VFD manufacturer websites) regarding this topic in general (including EMI considerations).

A choke of a few mH, and short shielded motor cable is generally enough for most applications using modern VFDs.

_{Jack - Used to test, repair and sell VFDs.}

Thanks, jack of all trades, yes the so-called dV/dt filters are quite simple. It's the lower-frequency sine-wave filters that are challenging. I've known of a few cases where a motor made objectionable noise at the PWM frequency, with the simple filters supplied, and an external sine-wave filter was needed.

If the PWM frequency is modest, say 2.5kHz, and one sets out to design a high-power sine-wave filter, one quickly discovers that the inductors involved can get quite large and expensive. At least that's the case for what we might call a brute-force filter. [See pic at right. OK, maybe not that big, but pretty big!] Several of us on sci.electronics.design tossed about possible alternatives a few years back, but without success. Pay the big bucks, and get the manufacturer's recommended filter.