In one line
A DC motor consists of a stationary field system (yoke, poles, field winding) and a rotating armature (armature core, winding, commutator), connected to the supply through carbon brushes.
Explained simply
Think of a DC motor as two parts: the outer stationary part creates the magnetic field (like a fixed stage), and the inner rotating part carries current and spins (like a performer on the stage). The commutator and brushes are the sliding connections that feed current to the spinning part in the right direction.
Technically, the stator provides the main flux path and the field; the rotor (armature) houses the conductors in which torque is produced and back EMF is induced.
How it works
| Part | Material | Function |
|---|---|---|
| Yoke (frame) | cast iron (small), cast or rolled steel (large) | mechanical support; carries the magnetic flux between poles |
| Pole core and pole shoe | laminated steel | carries field winding; shoe spreads flux over the air gap and supports the coil |
| Field winding | copper | produces main magnetic flux when excited by DC |
| Armature core | laminated silicon steel, slotted | holds armature conductors; low-reluctance path; laminations reduce eddy-current loss |
| Armature winding | copper, lap or wave connected | conductors where torque is produced and back EMF is induced |
| Commutator | hard-drawn copper segments insulated by mica | reverses current in armature conductors so torque is unidirectional |
| Brushes | carbon or graphite | sliding contact between supply and commutator |
| Shaft, bearings, end covers | steel | transmit torque, allow smooth rotation, protect inside |
- Field winding carries DC and magnetises the poles.
- Flux crosses the air gap into the armature core and returns through the yoke.
- Current enters through a brush, flows through the armature conductors via the commutator, and leaves through the other brush.
Quick example
In a car's starter motor (a DC series motor), worn carbon brushes are a common reason for failure; replacing brushes restores contact with the commutator. Small DC motors in toys use permanent magnets in place of the field winding.
Common mistakes
- Saying the armature core is solid; it is laminated to reduce eddy-current loss.
- Writing that brushes are made of copper; they are carbon or graphite.
- Mixing up the roles of pole shoe and yoke.
How MU asks it
Numericals are not expected in this module (as per the MU syllabus), so questions are theory. Typical: "Explain the construction of a DC machine with a neat diagram" (5–8 marks). For full marks draw a labelled cross-section (yoke, poles, field coils, armature, commutator, brushes, shaft) and list each part with material and function, as in the table above.