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,

You need the MOV's. The coils in the DC motors are wonderful inductors
and can put out all kinds of HV spikes. If you have measured the input
voltage correctly than the bridge diodes will have 106 volts applied
across them even without considering mains voltage fluctuations etc.
This makes selecting a suitable MOV impossible (even for the manufacturer)

MOV's are specified with an AC operating voltage which, in your case,
should be a little higher than 75 volts and a clamping voltage which
should be much lower than the PIV of the diodes (400 volts in your new
case). You also need to calculate the amount of stored power in joules -
with a moving mass of the motor this could be very high.

I suspect that something is not right with the circuit as you have
described it.