DC machines are electric machines which produce a DC voltage when acting as a generator, or are fed from a DC voltage source when acting as a motor.
The terminal voltage and current in DC machines are not alternating; however, the internal voltages and currents are alternating. A mechanism called a commutator is responsible is responsible for the AC-to-DC conversion in generating mode, and DC-to-AC conversion in motoring mode.
DC motors offer excellent speed controllability, but have the disadvantages of low reliability, high maintenance requirements, and high sensitivity to the operating environment due to the use of commutator mechanism