Using a 120V DC treadmill motor to replace ac motor in an old wood lathe. Need the variable speed control. Cheap 120V PWM speed controller, fed to rectifier bridge, does not give a smooth DC output. Is this ok to power a motor? Should I add a smoothing cap? Asking cos the motor seems to be sparking more than it should.

https://www.instructables.com/id/Replacing-a-Wood-Lathe-Motor-With-a-Treadmill-Moto/

 

What is the No. of the controller? MC60?
You cannot use a capacitor on this controller, do you have the series choke for the motor, this helps somewhat.
The PWM controllers are better, MC2100 etc.
Also are you using the flywheel?
Max.

 

Using the flywheel. The controller is the one described below. Tried posting the link, but not allowed. Amazon.The cap would be on the rectifier bridge output.

uniquegoods AC 220V 10000W High Power SCR Motor Speed Controller Voltage Regulator Dimming Attemperation Thermoregulation Board

 

OK that is a simple Triac with a bridge follower, quite a bit cruder than the MC60 or the KB style SCR bridge type.
Not a very good application for a DC spindle motor.
Max.