(1) Misconceptions about Stepper Motors Explained – MachineDesign 2017feb18
machinedesign.com/…

(2) Step Motor Basics – GeckoDrive 2017nov03
geckodrive.com/support/…

(3) What is a chopper drive for a stepper motor? – Dantelle Collins, 2017aug11
linearmotiontips.com/…

(4) Optimized Current Control Chopper Modes – Trinamic(see full text)