DC and geared motors

Learn

What you’ll learn

• How a DC motor turns electricity into rotation
• What voltage, speed, torque and stall current mean for a motor
• Why robots use gear motors

How it works

Inside a brushed DC motor, a coil of wire sits between magnets. Current through the coil makes it a magnet too, which pushes and pulls against the fixed magnets and makes it spin. Brushes and a commutator keep switching the current so it keeps turning.

• More voltage → faster (up to the motor’s rating).
• Swap the two wires → it spins the other way.

Speed versus torque

A motor spins fastest with no load. As the load increases it slows and draws more current. Held completely still (stalled) it draws the most current of all, the stall current, and heats up fast.

Situation	Speed	Current
No load	Fastest	Lowest
Normal driving	Medium	Medium
Stalled (blocked)	Zero	Highest: can overheat

Gear motors

Bare hobby motors spin at thousands of rpm with little torque, which is useless for wheels. A gear motor adds a gearbox. The yellow “TT” motors in many kits have about a 1:48 gearbox and turn at roughly 100–250 rpm on 3–6 V.

Never from a pin

A motor needs far more current than an Arduino pin can give and makes electrical noise. Always drive motors through a motor driver (next lesson), powered from the battery.

Examples

Example: choosing a motor

Need	Choose
Small 2WD car	TT gear motors, 3–6 V
Heavier robot, more precise	Metal gear motors (e.g. 12 V, 100–300 rpm) with encoders
Fast spinning (fan, weapon)	Bare DC motor, no gearbox

Example: why the robot drifts

Two “identical” motors never spin at exactly the same speed. Give the faster side a slightly smaller value, for example motors(60, 57), or use encoders to correct it automatically.

Practice

Try it

In the simulator, each motor value from −100 to 100 is like the voltage you give that motor.

Motor speeds

▶ Run↺ Reset

Your code (JavaScript)function loop() { if (millis() < 2000) motors(30, 30); // slow else if (millis() < 4000) motors(80, 80); // fast else if (millis() < 5000) motors(-50, -50); // reverse else motors(0, 0); }Load starter code

Robot commands

• motors(left, right): wheel power, −100 to 100
• lineLeft(), lineRight(): true over the black line
• distance(): cm to the nearest thing ahead (max 200)
• distanceLeft(), distanceRight(): cm to the sides
• linePosition(): where the line is, −1 (left) to 1 (right), or null
• millis(): milliseconds since Run
• print(value): show a message below
• setup() runs once; loop() runs 50 times a second

Faster and slowerEasy

Change the speeds and watch how far the robot travels in each 2-second phase.

Compensate a driftMedium

Pretend the right motor is 10% stronger: write motors(60, 66) and watch it curve. Then fix it in code so it drives straight. Lower the right side by the same amount: motors(60, 60) here, or in general scale the stronger motor down, e.g. motors(60, 66 * 0.91).

Stall dangerHard

A motor’s stall current is 1.5 A, and your motor driver can supply 1 A per channel. What happens if the robot gets stuck against a wall, and how could code protect it? The driver overheats or shuts down. Protect it by stopping when the robot hasn’t moved for a while (e.g. distance unchanged for 1 s, or encoders not counting), and by ramping speed up gradually.

Quick check

1. How do you reverse a brushed DC motor?

Lower the voltageSwap its two wires (reverse the polarity)Add a resistorYou can’t

2. When does a motor draw the most current?

Spinning with no loadWhen stalledWhen switched offAt half speed

3. Why do robots use gear motors?

More torque at a useful speedThey are silentThey need no powerThey spin faster

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