What is a DC Motor?

A Direct Current (DC) Motor runs from a DC (Direct Current) source of electricity and the electricity runs at a constant voltage where as an AC (Alternating Current) Motor which runs on an oscillating supply with a fixed cycle between negative and positive. Batteries or a DC Drive produce DC current and power stations supply AC. The UK domestic supply is typically single phase 240V 50Hz AC and industrial supply is three phase 415V 50Hz AC. Both of these can be converted to DC using a suitable DC Drive.


The armature comprises a cylindrical iron core made of thin sheet stampings with a number of slots arranged along the outer periphery. The coils comprising the winding are fitted in these slots, being suitably insulated from each other and from the iron core. At one end of the armature winding is placed the commutator which consists of a number of copper segments built up into the form of a cylinder. These segments are insulated from each other and from the shaft. The current is fed into the armature by means of a series of carbon brushes pressing on the commutator and suitably connected to the machine terminals, the various commutator segments being connected to the corresponding armature conductors.

Mechanical Construction

D.C. electrical motors consist essentially of an armature on which the winding is mounted, a field system which sets up the necessary field the interaction between the current in the armature and the flux of the field producing the driving torque and some means of conveying current to the rotating pad.

The Field System

The field system is stationary and consists essentially of steel electromagnets excited by coils placed around it. There is a circular rolled steel or square laminated yoke with field mainpoles fitted at intervals on the internal surface. The mainpoles have a shaped surface to distribute the lines of magnetic flux so that they can enter the armature over a wide area. Also they are so shaped as to fit the cylindrical armature which they enclose, leaving only a very small gap for the rotation of the armature. Also interposed between the mainpoles are interpoles which are there to create a field which will counteract the effect of armature reaction. The main active parts in a DC motor are: Mainpoles, Interpoles, Armature and Commutator.