Re: 90v dc brake voltage

Don't know why you want isolation.
The Break winding is insulated from the metal
housing just like a Transformer is.

But if you insist, you can get a Transformer
with a 90 to 110 volt secondary (or so), in which case
you only need the one Transformer for isolation.

If you do use two back to back xformers, each xformer should
have a Wattage rating equal to or preferably greater than
the Wattage rating of your Break.

Example: if your Break is 0.5 Amps when 110 Volts is applied.
than that is 55 Watts. (0.5 * 110)
So... If your Transformers have 12 Volt Secondaries, then the
Secondary Amper rating must be at least (55 Watts / 12 Volts)
4.6 Amps.
Because of various losses I'd use a 6 Amp or better.

Don't forget to use a Fuse.

Tom M.