Re: review of diode defence,6 pulse rectifier

["Tesla list" <tesla@xxxxxxxxxx>]

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