So I have a 90V DC motor for a CNC machine. I need a controller for it. I'm not sure of the motor's power (no datasheet that I can find)
If I run it from my PSU which goes up to 30V, it draws about 0.3A

Assuming that I don't need it to go in both directions, is there any good reason why I shouldn't do this: use a stepdown transformer and bridge rectifier to give me 90V DC, and switch it on and off with a relay. Use a hefty rheostat as a voltage divider to vary the speed. Add a rectifier to guard against back EMF.

If I wanted to go a step further and control the speed with an input voltage (0-10V) could I do this with a power transistor?

 

maybe something like this???
https://www.ebay.com.au/itm/373755787507?hash=item570594b8f3:g:Tp8AAOSwWaVhZ9kI
the speed controller may quite well run on a 90 or 100V transformer output.
And, limit the PWM to maybe 50% to protect the motor if the volts are a bit high?
There will be others with lower voltage outputs, but I did not look further.

 

You don't want to control the Motor's speed by controlling the Voltage,
this will result in an un-predictable, and sluggish, response to any Input changes.

PWM is recommended,
a lower base Frequency, ( ~100hz or less ), is better for Motor responsiveness at low RPMs.

If You don't go with a PWM-Controller for some reason,
then the next best method is an Adjustable-Current-Controller,
which are also extremely easy to build.

If You want to build a Controller yourself, and You want suggestions, or a Schematic,
we will need more information about the Motor,
and a description of its exact role in the system,
how it needs to be controlled,
what the Motor is expected to do,
DC-Resistance of the Motor, ( or "Locked-Rotor-Amps" ),
type of Motor, ( is it a Permanent-Magnet-Brushed-Motor ) ?, ( does it have a Gear-Box ) ?, etc.
And any information printed on the outside Case of the Motor.
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For a CNC machine, obviously you need some kind of fairly precise control.
Pick up one of the MC-60 or the better PWM MC2100 ex TM controller boards.
The MC2100 requires a simple PWM controller, the MC-60, just a 10k pot.
You can often find them in scrapped TM,s or one's advertised for free pick up, not working.

 

Thanks. The motor is a Sherline 4345 model number 970-506. I can't find anything about it on the Sherline website
My measurement shows 7ohm 8.5mH on the single winding (there are other wires but I think they are for thermal protection)

The Ebay controller looks potentially OK and probably less trouble than building something myself

 

Be careful ........
90V / 7-Ohms = ~13-Amps, 90V X 13A = 1200-Watts,
so make sure that the Controller You buy is rated for at least ~10,000-Watts,
( because they calculate Power-Ratings differently in China ).
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The nice thing about the TM MC versions, they have current monitoring on board so it is hard to exceed the rating of the controller.
With a (semi) intelligent controller, you do not have to worry a great deal about full voltage when stationary. The MC versions have built in ramp up.
The TM motors I have on hand measure around the same resistance.
Some have gone the cheap route and used one of the Triac power tool controllers followed by a bridge rectifier, but this method does not offer any of the nice features you get on the TM variety.

 

Incidentally what is the dia & length of the motor you have, many of the Sherline motors fitted to the mini- mill & lathe were not that high in HP?