Overview
This lecture explains the working principle of a simple DC motor, highlights the role of the split ring commutator, and describes how to control the motor’s rotation and speed.
Structure and Operation of a DC Motor
- A DC motor converts electrical energy into kinetic energy.
- The motor consists of a coil of wire free to rotate in a uniform magnetic field.
- Current through the coil creates a magnetic field that interacts with the permanent magnet’s field, generating force.
- Fleming's left hand rule predicts the direction of force on each side of the coil.
- The coil rotates as opposing upward and downward forces act on its sides.
- At vertical position, forces cancel out, but the coil's momentum keeps it turning slightly further.
- Without intervention, the coil would oscillate instead of rotating continuously.
Role of the Split Ring Commutator
- A split ring commutator reverses the current in the coil each half turn.
- Carbon brushes maintain sliding contact with the split ring as the coil rotates.
- At the vertical position, split ring gaps align with brushes, temporarily stopping current and force, but momentum keeps the coil moving.
- As rotation continues, the split ring reconnects with the brushes, reversing current and force direction, allowing continuous rotation in one direction.
Controlling Speed and Direction
- Increasing current, using stronger magnets, or increasing coil turns can increase rotation speed.
- The direction of rotation can be changed by reversing the current flow or the magnetic field (switch magnet poles).
Key Terms & Definitions
-
DC Motor — Device converting electrical energy into kinetic energy.
-
Split Ring Commutator — Component that reverses current direction in the coil every half turn to ensure continuous rotation.
-
Carbon Brushes — Conductive contacts that transfer current to the rotating split ring.
-
Fleming's Left Hand Rule — Rule to predict the direction of force on a current-carrying wire in a magnetic field.
FLEMING’S LEFT HAND RULE
-
Fleming's Left Hand Rule is a simple way to determine the direction of force experienced by a current-carrying conductor in a magnetic field, which is essential for understanding how a DC motor works.
Here’s how to use it:
- Thumb: Points in the direction of the Force (motion) on the conductor.
- First Finger (Index finger): Points in the direction of the Magnetic Field (from North to South).
- Second Finger (Middle finger): Points in the direction of the Current (conventional current flow, from positive to negative).
When you arrange your left hand so that the first finger points along the magnetic field and the second finger points along the current, your thumb will point in the direction of the force acting on the conductor.
Action Items / Next Steps
- Review details and diagrams in the syllabus figure referenced in the lecture.
- Practice applying Fleming’s left hand rule to predict force directions in motor problems.