I would assume that you are asking how the basic DC drive functions. Commonly a DC drive is a device that uses AC as poer source to power and control a DC motor. The cheap definition... The drive has two parts the power components and a controler. The power components convert AC power to DC by a Rectifier. The controller (among other things) is basicly a high speed timer that fires a solid state device (BPT, IGBT or some other gate) very rapidly (Pulse width modulation on and off). The controller gets a setpoint from a potentiometer or some other means perhaps a digital number via some protocol and uses a means of maintaing this setpoint by calculating the difference between the setpoint and the actual speed with a tach or some other means and adjust the output accordingly.
DC drives have been around for 100 years in one form or another.
Many early forms applied a fixed voltage to the armature and varied the motors field strength. By making the field stronger you increase the motors "back EMF" (Voltage generated by the motor that opposes the voltage causing it to spin). This in turn causes the motor to draw less current and slow down. Before solid state devices became available the main method of controlling the field strength was using some form of resistance, a rheostat or carbon pile (stack of carbon slices that change in resistance with applied pressure).
Another method used a variation on a transformer called Saturable Reactor (Magnetic Amplifier)
Then along came Thyratrons and Ignatrons, these were a vacuum tube type of device with similar characteristics to an SCR.
Transistors of course were also used to control the field strength.
When SCRs (silicone controlled rectifiers) came along about 45 years ago they revolutionized the control of AC and DC motors allowing the control of much larger currents. The SCR based drive (3 phase) usually had a rectifier made up from 6 SCRs. These were fired on earlier or later in the AC cycle to change the voltage across the DC motors armature. A typical SCR drive had a small tacho generator mounted on the end of the DC motor. The drive compared the tacho generator voltage to the speed setpoint voltage adjusting the SCR firing point to speed up/slow down the DC motor until the tacho voltage matched the speed setpoint voltage. There must be millions of this type still in service.
You also see a crude form of this drive in consumer electronics however here they use the "back EMF" voltage generated by the motor instead of a tacho generator to set the firing angle. A typical example would be the speed controller of an electric hand drill.
I/m sure William Warden is correct in his description of a modern drive.
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