On Sun, 02 Apr 2017 22:42:36 -0400, clare wrote:

On Sun, 2 Apr 2017 19:27:38 -0700, MOP CAP wrote:

Many of such motors with wound fields required 90 volts for the field.
Why 90 volts and how was it derived?
thanks,
CP
Some had field rheostats for speed control. As for the 90 volts, likely
the output voltage of a rectified line voltage using the rectifier
technology and line voltage of the time.

90V probably became standard at some time in the dim past for reasons
that haven't applied since 1920, but has been carried forward because
it's a standard.

Just a guess, but you stumble across that sort of thing all the time.

--
Tim Wescott
Control systems, embedded software and circuit design
I'm looking for work! See my website if you're interested
http://www.wescottdesign.com