# Workings Of D.C. Motor

Show/Hide Sub-topics (Magnetism & Electromagnetism | O Level)

In order to understand how a D.C. motor works, we will need to learn how a current-carrying coil generates turning effect in a magnetic field.

Turning effect of a current-carrying coil in a magnetic field

The coil is placed horizontally between two magnets as shown in the figure above. The magnetic field points from the N to S. Using Fleming’s left hand rule, the force on the left-side of the coil is upwards (magnetic field points left, current into the page), while the force on the right-side of the coil is downwards (magnetic field points left, current out of the page). The magnitudes of the forces on the left-side and right-side of the coil are equal to each other.

The moment of force can be calculated by:

$\text{Moment} = F \times d$

, where

• F is the magnitude of the force on one side of the coil
• d is the horizontal distance between the two side. (the length of the wire connecting the left-side and right-side of the coil)

Note: The wire segments connecting the two side of the coil do not experience any force as the current and magnetic field are in the same direction. Hence, according to Fleming’s left hand rule, that will generate no force.

Factors affecting the strength of the moment of force:

• Number of turns in the coil
• Current in coil
• Strength of magnetic field

D.C. Motor

The figure above shows a d.c. motor in action. The coil is connected to a split-ring commutator (circular ring) via carbon brushes (the brown blocks in the figure). The split-ring commutator is vital to the operation of the d.c. motor. There is a gap in the split ring commutator which causes current flow to stop when the coil is vertical (Reference position $90^{\circ}$ and $270^{\circ}$).

Steps in the operation:

• Coil starts in reference posiiton $0^{\circ}$: Upward force on left-side, downward force on right-side. Coil rotates clockwise to position $90^{\circ}$
• Coil in reference position $90^{\circ}$: The split-ring commutator cuts off the current to the coil. No electromagnetic force is acting on the coil. The momentum of the coil from the previous turning motion causes it to rotate slightly beyond this vertical position.
• Coil in reference position $90^{\circ}$ + a slight tilt: Current passes through the coil again. Due to the turning, the originally labelled left-side of the coil is now right side and vice versa. This causes the current direction in the two sides to swap. The previously-labelled left-side of the coil (now: right-side) now have current coming out of the paper and experiences a downward force. The opposite is true for the other side. The coil will now rotate clockwise to position $180^{\circ}$.
• Coil in reference position $180^{\circ}$: This is the same as the starting position and the whole cycle repeats itself.

The split-ring commutator reverses the current direction in the coil every half a turn and allows the coil to always turn in the clockwise direction.

Factors affecting the speed of rotation of the d.c. motor: (larger turning effect = higher speed)

• Same as the factors affecting the strength of turning effect
• Inclusion of a soft iron cylinder: Soft iron is highly permeable to magnetic field. This allows the magnetic field to be concentrated at the coil which increases the magnetic field strength experienced by the coil $\rightarrow$ larger turning effect $\rightarrow$ higher speed.