Robbie remarked in 030113 MMDigest:

>[ Mike Ames mentions that the DC output polarity of the converter
>[ can change from one starting to the next.  A diode bridge could
>[ be added if needed, to ensure consistent polarity.

Note, this might be a "feature".  DC commutators that arc with one
polarity gradually pit the commutator bars on one side and build up
a deposit on the other.  Letting the polarity change could even that
out a little.

Also, the diodes needed are probably over 10 kV (based on 5 kV someone
mentioned, if you have much arcing) and are fairly high current, so
they would be expensive and failure prone.

Will Herzog

PS: I have to reverse the diode in a household gadget every
few years to keep the relay contacts from having to be serviced.