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.
Understanding Robotics: Brushed DC Motor
The commutator does more than simply pass electricity into the spinning part. Its copper segments are connected to different armature coils. As the rotor turns, each brush touches a new segment and selects the coil that should receive current next.
A coil produces its strongest turning effect when its sides are placed well across the magnetic field. Near certain positions, its turning effect becomes small. Switching current at that time prevents the rotor from being pulled back toward its old position.
Motors often use several coils and many commutator segments. This makes the torque smoother and helps the rotor start from almost any angle.
Starting a motor is a demanding moment for the battery and the driver circuit. A stationary rotor produces no generator voltage of its own, so the supply can push a large current through the windings. That current creates strong starting torque, but it also creates heat.
If a wheel is jammed, the motor remains near this condition. The wire insulation, brushes, battery, and control transistor can then overheat. As speed rises, the motor generates an opposing voltage.
Less current enters the windings, so the available torque falls. This explains why a robot may move quickly on a smooth floor yet slow down sharply when it climbs a ramp or pushes an object.
A robot normally needs a motor driver between the controller board and the motor. A common driver circuit called an H bridge can reverse the connection to the motor, which reverses the direction of rotation. Pulse width modulation controls average power by switching the supply rapidly on and off.
A short on time gives less average current than a long on time. The pulses are still full battery voltage, so the driver must be chosen for the motor's possible starting or stall current, not just its usual running current. Gears are often added because wheels need more turning force than a small motor shaft can provide directly.
A gearbox trades shaft speed for greater wheel torque. It cannot create extra energy, and friction in the gears reduces some of the available power.
Brushes are a practical limit of this motor type. When a brush leaves one commutator segment and reaches the next, current may briefly jump through the air. This arcing wears the surfaces and produces electrical noise.
Small capacitors across the motor terminals can reduce noise that might disturb sensors or a microcontroller. Students should notice the difference between a motor that spins freely with no load and one that performs useful work. Measure battery voltage while the robot runs, listen for grinding gears, and feel for unusual heat only after power is removed.
A weak battery, loose wire, worn brush, stuck wheel, or overloaded gearbox can all make a motor seem faulty. Testing one part at a time makes the real cause easier to find.
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.