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Re: review of diode defence,6 pulse rectifier



Original poster: FIFTYGUY@xxxxxxx In a message dated 10/5/06 2:26:04 P.M. Eastern Daylight Time, tesla@xxxxxxxxxx writes:

>The 7th diode stack I was referring to is a freewheeling diode stack. To
>be honest, this is normally reserved for controlled rectifiers with
>SCRs, where continuous current is not assured over the full operating
>range.

Not only because of discontinuous current, but many SCR rectifiers drive inductive loads (DC motors). In addition to the multitude of protective features built into such motor drives, it's fairly common to have an accidental connection failure and thus instantaneously open the current loop. The resulting voltage spike from the inductive kickback is shunted by the "freewheeling" diode stack, the same way diodes quench transients across DC relay and solenoid coils. BTW, in my experience, I've seen plenty of fractional and small (~5 HP) SCR drives fail. But these tend to be cheap, physically small, single phase, and built with integrated rectifier "bricks". The larger SCR drives I've worked with (up to 150 HP) have nigh-indestructible power circuits. The only one I ever saw the power side fail on was a 600A 400V 3-phase half-wave drive, where a loose heatsink mounting bolt fell against the mounting plate... I've seen plenty of large DC motors fail (again, up to 150 HP), but they never damaged their drives. The old SCR drives were incredibly robust, and the new ones still use the same very mature technology.

-Phil LaBudde