For most full-wave bridge rectifiers with holes in them, the device should
be mounted to a metal panel (minimally) or a heatsink with a proper heatsink
compound to transfer heat.

Many datasheets will indicate a notation that recommends heatsinking of a
device, when used in excess of a nominal power (watts) rating.

As Ned commented, speed regulation will be fairly poor with a variac. That
means the motor will slow down considerably from a set speed (no load),
during the actual metal cutting operation.
A variac will provide variable speed, which is a lot "better than" (it get's
a gold star) single speed.

Variacs (autotransformers) do not provide line voltage isolation, so the
output voltage is hazardous. Proper insulating practices need to be followed
to protect the equipment users from potential electrical shock hazards.

Most commercially-built 180VDC motor controllers are typically 240VAC input
modules, and the DC motor controllers for 90VDC motors are typically 120VAC
input modules.
So, the existing 180VDC motor will be slightly under-powered from a DC
supply derived from 120VAC.. the results may be similar to the performance
from the previous attempt with a treadmill controller.
Finding a commercially-built 180VDC motor controller would likely provide
the best results.. variable speed with excellent speed regulation, plus
other features.

Looking up the manufacturer's specs for the existing motor would be
worthwhile, before buying a controller that may not be suitable for your
application.

Another consideration could be the need to cool the variac to prevent
overheating, which could be accomplished with a small fan.

--
WB
..........


"Terry" wrote in message
...
My 13" SB metal lathe has been retrofitted with a PM DC motor. It's
not a treadmill motor, it's continuous duty according to the tag and
can handle up to 180 V. I assembled a speed control from a
treadmill-type board. Unfortunately it doesn't appear that the
controller provides enough oomph.

What I'd like to try instead is a 120v 20amp variable transformer,
output to a bridge rectifier, then filter the DC and send it to the
motor. By happy coincidence I have all the parts, total cost will be
simply a bit of time.

The rectifier I have is a GBPC1210W; data sheet says 12 amp, 1000 v.
It is rectangular with a hole through the center. It looks like it
ought to be mounted to a heat sink, but the data sheet I saw doesn't
say anything about doing so.

The question is: should a bridge rectifier be heat-sunk (sinked?) in
such an application? If so, how big (roughly) should the heat sink
be? Would it be sufficient to bolt the rectifier to the 4" square
metal enclosure? Thanks for your input.
--
Best -- Terry