On Mon, 6 Jul 2009 21:45:02 -0700, DaveC wrote:

Stud-mount diodes (four in a bridge configuration) are protected by MOVs: one
across the AC connections, one across the DC connections.

These MOVs are antique-looking finned affairs (a la selenium rectifiers).
Both sport PN S01AAA?AAA (?=illegible) with Westinghouse trademark. The
wiring diagram symbol for these looks like a zener diode with an extra arrow
head (both arrow points touch, with the familiar "z" shaped line between
them).

The AC supply voltage to the bridge is 75 rms. The bridge diodes are
originally 100V/1A devices; I'm replacing with 400V/35A ones.

This rectifier drives a DC motor via 2 contactors: one for direction (by
reversing polarity) and the other for "GO/STOP". The MOVs are to limit the
kick from opening contacts, I presume.

Someone suggested that with such stout diodes available today that maybe the
MOVs are not required.

If you think I need to keep the MOVs, how do I select new ones?

What say y'all?

Thanks,

I'll bet that they are not MOVs. They are what is referred to as
"Tranzorbs".

MOVs would only be able to handle a certain LOW number of transient
events before they undergo operating characteristic changes. A Tranzorb,
on the other hand, will ALWAYS kill a transient, unless you breach the
tranzorb itself. With an MOV, each event changes the device's
characteristics of operation until it finally functions as a completely
different circuit element.

Try NOT to follow the lead of other idiots, and refrain from
cross-posting your queries.