they are called universal motors if i'm not mistaken
i have an old skill saw from WW2 that my grandfather passed down to me
she doesn't see much service anymore, but it does have an ac and dc rating
As mentioned the AC/DC motors are called universal motors. Universal motors have carbon brushes and both the field and the armature are wound with wire. Most (Not all) universal motors will be less efficient when running on DC and will wear out carbon brushes much faster. This reduced efficiency means less power available to do work and/or more heat generated for the same load.
You can still vary the speed whether it is running on AC or DC. Harbor Freight sells an inexpensive speed control that will let you run on AC and have speed control of a universal motor. They call it a Router Speed Control, most routers are universal motors.
They made everything so robust back in those days.
I can see where the option of AC or DC would be beneficial in certain industrial/international applications.
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