DC motor principle and role of commutator and brushes

A DC motor converts DC electrical energy into mechanical energy. Its operating principle is that a current-carrying conductor placed in a magnetic field experiences a force $F = B I l$, whose direction is given by Fleming's left-hand rule.

Operating principle

1. The field winding on the poles, fed with DC, sets up a steady magnetic flux.
2. DC supply is fed to the armature conductors through brushes and commutator.
3. Each armature conductor carrying current in the field experiences a force $F = BIl$. Conductors under the N pole carry current in one direction and those under the S pole in the opposite direction, so all forces act in the same rotational sense and produce a torque $T = \dfrac{P\phi Z I_a}{2\pi A}$.
4. As the armature turns, a conductor moving from N pole to S pole has its current reversed by the commutator, so the torque remains unidirectional and the armature keeps rotating.
5. The rotating conductors cut the flux and an emf is induced which opposes the supply: the back emf $E_b$, with $V = E_b + I_a R_a$. Back emf limits the current and makes the motor self-regulating with load.

          N
   +---------------+
   |  x  x  x  x   |  <- force (left)
   |   ( armature ) |     rotation
   |  .  .  .  .   |  -> force (right)
   +---------------+
          S
   x: current in, .: current out

Functions of commutator segments

The commutator is a cylinder of copper segments insulated by mica, each joined to coil ends.

• Mechanical inverter: the supply is DC, but the current in each conductor must reverse as it passes from one pole to the next to keep torque in one direction. The commutator reverses the current in each coil at the correct moment.
• Unidirectional torque: by this reversal, conductors under a given pole always carry current in the same direction.
• Connection of coils: it joins the coils in series to form the parallel paths (lap or wave).
• Sliding contact surface for brushes, transferring current to the rotating armature.

Functions of carbon brushes

Brushes are blocks of carbon or graphite held in brush holders pressed on the commutator by springs.

• Lead current from the stationary supply terminals to the rotating commutator.
• Short-circuit the coil under commutation briefly, helping current reversal.
• Good commutation: carbon has fairly high contact resistance, which helps current reversal (resistance commutation) and reduces sparking.
• Self-lubricating and soft: graphite lubricates the commutator, wears itself rather than the costly commutator, and is easily replaced.