DC Motor

What is a DC motor?

A DC (direct current) brushed motor is an electromechanical transducer that converts electrical power into rotational mechanical power. It consists of:

• Stator — permanent magnets that create a fixed magnetic field
• Rotor (armature) — a wound coil of wire that rotates inside the magnetic field
• Commutator — a segmented copper ring that switches current direction in the rotor coils as they rotate
• Brushes — stationary carbon contacts that maintain electrical contact with the spinning commutator

Why it matters for AI builders

DC motors are the most common actuator in mobile robotics. Every wheeled robot from a ₹300 toy car to a Boston Dynamics Spot uses DC motors (or their derivative, the brushless motor) for locomotion. Key roles:

• Differential drive — two independently controlled DC motors drive a wheeled robot. Speed difference produces turning.
• Conveyor and linear slides — translation stages in laboratory automation and pick-and-place systems
• Winch and cable actuation — lift mechanisms in climbing robots
• Cooling and ventilation — fan motors in embedded systems

Working principle

Back-EMF. When a DC motor spins, its rotating armature generates a voltage opposing the supply voltage, called back-EMF (electromotive force). Back-EMF increases with speed. At steady state:

V_supply = I × R_armature + V_back_EMF

This means current (and therefore torque) is highest when the motor is stalled (V_back_EMF = 0) and lowest at free-run speed. This is why stall current is typically 5–15× the no-load current.

PWM speed control. Since speed is proportional to average voltage, rapidly switching the voltage on and off (PWM) at 10–20 kHz produces an effective average voltage proportional to duty cycle. At 50% duty cycle with a 12V supply, the effective voltage is 6V.

Key parameters

Pinout

No polarity requirement. Unlike LEDs or capacitors, a DC motor has no strict polarity — swapping M+ and M- simply reverses the direction of rotation. The H-bridge driver handles polarity reversal electronically.

Wiring

Critical: Never connect a DC motor directly to a GPIO pin. Motors draw 150 mA no-load and 1–3 A stall — far beyond the 40 mA GPIO limit. Always use a motor driver (L298N, DRV8833, TB6612FNG).

Code

Basic forward/reverse control (with L298N)

// L298N connected to Arduino
const int ENA = 9;   // PWM speed control
const int IN1 = 7;   // direction bit 1
const int IN2 = 8;   // direction bit 2

void motorForward(int speed) {
  // speed: 0–255
  digitalWrite(IN1, HIGH);
  digitalWrite(IN2, LOW);
  analogWrite(ENA, speed);
}

void motorReverse(int speed) {
  digitalWrite(IN1, LOW);
  digitalWrite(IN2, HIGH);
  analogWrite(ENA, speed);
}

void motorStop() {
  digitalWrite(IN1, LOW);
  digitalWrite(IN2, LOW);
  analogWrite(ENA, 0);
}

void setup() {
  pinMode(ENA, OUTPUT);
  pinMode(IN1, OUTPUT);
  pinMode(IN2, OUTPUT);
}

void loop() {
  motorForward(200);  // ~78% speed
  delay(2000);
  motorStop();
  delay(500);
  motorReverse(150);  // ~59% speed
  delay(2000);
  motorStop();
  delay(500);
}

Differential drive (two-motor wheeled robot)

const int ENA = 9, IN1 = 7, IN2 = 8;   // Left motor
const int ENB = 10, IN3 = 5, IN4 = 6;  // Right motor

void drive(int left, int right) {
  // left, right: -255 to +255 (negative = reverse)
  // Left motor
  if (left >= 0) { digitalWrite(IN1, HIGH); digitalWrite(IN2, LOW); }
  else           { digitalWrite(IN1, LOW);  digitalWrite(IN2, HIGH); }
  analogWrite(ENA, abs(left));

  // Right motor
  if (right >= 0) { digitalWrite(IN3, HIGH); digitalWrite(IN4, LOW); }
  else            { digitalWrite(IN3, LOW);  digitalWrite(IN4, HIGH); }
  analogWrite(ENB, abs(right));
}

void setup() {
  pinMode(ENA, OUTPUT); pinMode(IN1, OUTPUT); pinMode(IN2, OUTPUT);
  pinMode(ENB, OUTPUT); pinMode(IN3, OUTPUT); pinMode(IN4, OUTPUT);
}

void loop() {
  drive(200, 200);   // Forward
  delay(1000);
  drive(200, -200);  // Spin right
  delay(500);
  drive(200, 150);   // Gentle right curve
  delay(1000);
  drive(0, 0);       // Stop
  delay(1000);
}

Test procedure

Common mistakes

Robotics and AI use cases

Beginner project — Variable speed fan

Wire a DC fan motor to an L298N and control its speed with a potentiometer. Turn the pot — fan speed changes proportionally.

Key learning: PWM as voltage control, the map() function, motor driver enable pin.

Advanced project — Differential drive robot with encoder odometry

Build a two-wheeled robot. Attach hall-effect encoders to each motor (many TT motor packs include them). Count pulses in interrupt service routines. At each control loop iteration, compute wheel velocity from pulse delta and time delta. Use a PID controller to maintain equal wheel speeds for straight-line driving.

Key learning: Hardware interrupts, encoder pulse counting, PID velocity control, differential drive kinematics.