PWM Motor Speed Control

Unless you don't care about possibly burning up the motor, I don't recommend using PWM, VFD, etc on any older motor, they most often do work, however because of the way these systems work a newer motor design that's better able to deal with the inductive heat is a better deal.

PWM - Pulse Width Modulation is a fancy way of averaging a lower voltage to control a DC motor while still getting the higher torque of using the higher voltage, it's just on average the voltage is lower. This is the simple answer, there's more to it than this, but most simple implmentations will have a pulse rate (usually a few khz) and a duty cycle of whatever the voltage is. So 12V w/60% duty cycle is an average voltage of 8V. The sometimes encountered problem if you're running it quite a bit, is the PWM freq is constantly turning the motor on and off, which can lead to inductive heating in the coils armature, etc.

VFD - Variable Frequency Drive is much more complicated and is kinda the go-to for most AC motors, the VFD system can change the pulse frequency (so from 60hz AC line to 30hz) it can change the voltage, etc. Unlike a PWM system that operates in open loop control, most VFDs are closed loop. meaning they can "read" motor state, either by measuring back EMF through the feedlines, and a lot of other complex means. I've never seen a VFD burn up a motor, however I won't say it can't happen.
 
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