A brushed DC motor converts electrical energy from a direct current source into rotational motion. It is common in small robots because it is inexpensive, easy to control, and produces useful torque at low speeds. Inside the motor, permanent magnets create a magnetic field while current-carrying coils on the rotor experience forces that make the shaft turn.
Understanding its parts helps students connect circuits, magnetism, energy, and mechanical motion in one device.
The brushes and commutator act as a mechanical switch that reverses current in the armature windings at the right moments. This keeps the magnetic torque pushing the rotor in the same direction as it spins. Motor behavior is often described with a torque-speed curve, where stall torque occurs at zero speed and no-load speed occurs when torque is nearly zero.
Brushes wear because they slide against the rotating commutator, causing friction, heat, electrical arcing, and carbon dust.
Key Facts
- Magnetic force on a current-carrying wire is F = BIL sin(theta).
- Motor torque is approximately proportional to armature current: tau = k_t I.
- Back emf increases with speed: E_back = k_e omega.
- Armature current is limited by resistance and back emf: I = (V - E_back) / R.
- Mechanical power output is P = tau omega.
- For an ideal brushed DC motor at fixed voltage, torque decreases roughly linearly as speed increases.
Vocabulary
- Armature
- The rotating part of a brushed DC motor that contains windings carrying current and producing torque.
- Commutator
- A segmented rotating conductor that reverses current through the armature coils every half turn.
- Brush
- A stationary conductive contact, often made of carbon, that delivers current to the spinning commutator.
- Back emf
- The voltage generated by a spinning motor that opposes the applied voltage and reduces current.
- Stall torque
- The maximum torque a motor produces when the shaft is not rotating.
Common Mistakes to Avoid
- Ignoring back emf when calculating current is wrong because a spinning motor draws less current than a stopped motor at the same voltage.
- Assuming brushes only carry current is incomplete because they also experience sliding friction, heat, sparking, and gradual wear.
- Thinking maximum power occurs at stall is wrong because speed is zero at stall, so P = tau omega gives zero mechanical output power.
- Reversing battery polarity without considering direction is a mistake because it reverses armature current and usually reverses motor rotation.
Practice Questions
- 1 A motor has torque constant k_t = 0.040 N m/A and armature current I = 3.0 A. What torque does it produce?
- 2 A brushed DC motor has supply voltage V = 12 V, resistance R = 2.0 ohms, and back emf E_back = 8.0 V while spinning. Find the armature current.
- 3 Explain why a brushed DC motor draws a large current at startup and why that current decreases as the motor speeds up.