DC Gear Motor Buying Guide: RPM, Torque & Voltage

A gear motor is a DC motor paired with a gearbox that reduces the output shaft speed while multiplying torque. Choosing the wrong gear motor is one of the most common mistakes in robotics and automation projects — either the motor stalls under load or spins too fast to be useful. This guide breaks down everything you need to know about gear motors: gear ratios, RPM vs torque tradeoffs, motor types, BO motors for robotics, encoder motors, and practical wiring with motor drivers. By the end, you’ll be able to select the perfect gear motor for any project.

What is a Gear Motor?

A gear motor combines an electric DC motor with a mechanical gearbox in a single compact assembly. The motor itself spins at high RPM (typically 3000–15000 RPM for small DC motors) but with very little torque. The gearbox uses a series of interlocking gear stages to reduce the output shaft speed while proportionally increasing the torque delivered. The result is a motor that spins slowly but can lift, push, or pull significant loads — exactly what most real-world mechanical applications require.

Gear motors are used everywhere: from the windscreen wiper motor in your car to the tiny motor driving a surveillance camera pan-tilt mechanism, from the conveyor in a factory to the robot wheels on a school project. For Indian makers and engineers, gear motors are available in a wide range of sizes and specifications at accessible prices.

Gear Ratio Explained

The gear ratio is written as X:1 (e.g., 100:1) and tells you how many times the motor shaft rotates for every single rotation of the output shaft. A 100:1 gear ratio means the motor spins 100 times to produce one output shaft revolution.

The relationship is straightforward:

• Output RPM = Motor RPM ÷ Gear Ratio
• Output Torque = Motor Torque × Gear Ratio × Efficiency

For example: a motor spinning at 3000 RPM with a 100:1 gearbox produces 30 RPM output. If the motor produces 1 kg·cm of torque and the gearbox is 80% efficient, the output is 80 kg·cm of torque — 80 times more than without the gearbox.

Common gear ratios and their typical uses:

RPM vs Torque Relationship

For any given motor and power supply, speed and torque follow an inverse relationship. This is the motor characteristic curve: at zero load (free run), the motor spins at maximum RPM but produces almost no usable torque. At stall (zero RPM), the motor produces maximum torque but draws maximum current (stall current) and generates excessive heat.

The practical operating point is somewhere in the middle — ideally at around 50–70% of the stall torque where efficiency is highest. Always select a gear motor where your expected load torque is comfortably below the stall torque, ideally at 30–50% of stall. This provides headroom for acceleration, varying loads, and extended life.

Types of Gear Motors

1. Spur Gear Motors

The most common type, using straight-cut gears in a simple parallel arrangement. They are inexpensive, efficient (up to 90% per stage), and easy to manufacture. They can be noisy at high speeds and have lower load capacity per stage compared to helical gears. The widely available 25GA-370 motors at Zbotic are spur gear motors — reliable and affordable for most robotics and automation applications.

2. Planetary Gear Motors

Planetary gearboxes arrange gears in a concentric ring pattern, distributing load across multiple gear contacts simultaneously. This gives extremely high torque density in a compact cylindrical form factor, excellent gear ratio range (up to 1000:1 in a single stage), and better load distribution. They are ideal for high-torque, compact applications like robotic joints, servo replacements, and electric screwdrivers.

3. Worm Gear Motors

A worm gear uses a helical screw (worm) meshing with a wheel gear. The key advantage is self-locking: the mechanism cannot be back-driven, meaning the output shaft stays locked in position when power is removed. This is extremely useful for applications like motorised vents, valve actuators, and lifting mechanisms where you need the position to hold without constant power. Trade-off: worm gears are typically 40–60% efficient (much lower than spur or planetary).

BO Motors for Robotics

BO (Battery Operated) motors are the most popular DC gear motors for entry-level robotics in India and internationally. The term refers to small yellow plastic-bodied DC gear motors with a dual-shaft output — one extending forward for the wheel and one extending to the rear for wheel alignment on a chassis.

Key characteristics of BO motors:

• Operating voltage: 3V – 9V (typically used at 5V or 6V)
• Speed: 100 – 500 RPM depending on variant
• Torque: low to moderate (suitable for lightweight robots up to ~500g per motor)
• Shaft: D-shape or round, usually 3–4mm diameter
• Popular wheel sizes: 65mm – 80mm rubber wheels with traction patterns
• Current draw: 100–300mA running, 500–800mA stall

BO motors work well for 2-wheel and 4-wheel differential drive robots for school and college projects. For heavier robots (>1kg) or rough terrain, step up to metal-gearbox motors like the 25GA-370 series for better durability and torque.

Selecting a Gear Motor by Specifications

Use this framework to choose the right gear motor for your project:

1. Voltage: Match your power supply. 12V motors are most versatile (work with 3S LiPo, 12V SMPS, car battery). 6V motors suit USB/Li-ion powered projects.
2. Required output RPM: Calculate the wheel/shaft speed you need. For a robot wheel with 65mm diameter needing 0.5 m/s: RPM = (0.5 m/s × 60) / (π × 0.065m) ≈ 147 RPM.
3. Required torque: Estimate the load (weight × wheel radius) and add a 3x safety factor for acceleration and friction.
4. Shaft size and type: Ensure your wheels/couplings fit. D-shaft vs round shaft vs hex shaft.
5. Body diameter: Common sizes: 12mm, 25mm, 37mm, 63mm. Larger = more powerful but heavier.
6. With or without encoder: If you need closed-loop position or speed control, choose an encoder variant.

Wiring with Motor Driver and PWM Speed Control

A DC gear motor requires a motor driver IC or module to interface with a microcontroller. Never connect a motor directly to Arduino pins — the current draw (even at light load) exceeds the safe output current of the microcontroller.

Recommended motor drivers:

• L298N: Dual H-bridge, up to 2A per channel, 5–35V, popular and inexpensive. Good for BO motors and small gear motors.
• L293D: Dual H-bridge, 600mA per channel, suitable for BO motors only.
• BTS7960: High-current H-bridge, up to 43A, ideal for large gear motors (37mm, 63mm body).
• TB6612FNG: Compact dual H-bridge, 1.2A per channel, low heat, good replacement for L298N.

// Arduino PWM speed control with L298N motor driver
// Connections: IN1=pin7, IN2=pin8, ENA=pin9 (PWM)

const int IN1 = 7;
const int IN2 = 8;
const int ENA = 9;  // Connect to PWM-capable pin

void setup() {
  pinMode(IN1, OUTPUT);
  pinMode(IN2, OUTPUT);
  pinMode(ENA, OUTPUT);
  Serial.begin(9600);
}

void setMotor(int direction, int speed) {
  // direction: 1=forward, -1=reverse, 0=stop
  // speed: 0-255
  if (direction == 1) {
    digitalWrite(IN1, HIGH);
    digitalWrite(IN2, LOW);
  } else if (direction == -1) {
    digitalWrite(IN1, LOW);
    digitalWrite(IN2, HIGH);
  } else {
    digitalWrite(IN1, LOW);
    digitalWrite(IN2, LOW);
  }
  analogWrite(ENA, speed);
}

void loop() {
  // Ramp up speed
  for (int s = 0; s = 0; s -= 5) {
    setMotor(1, s);
    delay(50);
  }
  setMotor(0, 0);
  delay(1000);
}

Stall Current Considerations

The stall current of a DC gear motor is the current drawn when the shaft is held stationary while voltage is applied. It is typically 5–10 times the rated running current. For example, a gear motor rated at 300mA running current may draw 2–3A at stall. This has several important implications:

• Motor driver sizing: Your motor driver must handle the stall current, not just the running current. Use a driver rated for at least 2x the stall current.
• Power supply: Your power supply must provide enough peak current for all motors simultaneously, especially during startup or load spikes.
• Motor protection: Add a resettable fuse (polyfuse) or current-limiting motor driver to protect against prolonged stall conditions that can burn windings.
• Arduino pin current: Motor driver logic pins only carry milliamps — the high current flows through the driver’s separate power supply path, not through the Arduino.

Encoder Motors for Precision Control

Encoder gear motors add a magnetic or optical position sensor (encoder) to the rear shaft of the motor. This encoder generates pulses as the motor shaft rotates — typically 7 to 64 pulses per motor revolution. Counting these pulses lets the microcontroller calculate:

• Actual rotation speed (for closed-loop PID speed control)
• Cumulative rotation (for odometry in mobile robots)
• Absolute position (for positioning applications)

Encoder motors are essential for differential drive robots that need to travel in straight lines, for conveyor systems requiring precise travel distances, and for any application where you need reliable repeatable positioning without adding external sensors.

Applications of DC Gear Motors in India

Gear motors are the workhorses of mechanical design across many domains:

• Educational robotics: Line followers, obstacle avoiders, maze solvers, and competitive robots at IIT Techfest, Robocon, and school competitions.
• Industrial automation: Conveyor belts, packaging machines, mixing equipment, and small CNC machines.
• Smart home: Automated curtain and blind systems, automatic door bolts, cabinet locks, and pet feeders.
• Agriculture: Seed dispensers, drip valve actuators, and small soil sampling drills.
• Medical devices: Infusion pumps, patient lifts, and rehabilitation equipment use precisely controlled gear motors.
• Retail and display: Motorised product display turntables, sign rotators, and vending machine dispensing mechanisms.

Frequently Asked Questions

Q: What is a BO motor and why is it popular in India?

BO motor stands for Battery Operated motor. These are small plastic-body DC gear motors originally designed for inexpensive battery-operated toys. They became ubiquitous in Indian electronics hobbyist culture because they are extremely cheap (under ₹50–100 each), readily available everywhere from local electronics markets to online stores, have a convenient dual-shaft design, and are easy to mount on simple chassis. For student robotics projects on tight budgets, BO motors are the standard starting point.

Q: How do I calculate the gear motor size needed for my robot?

Estimate total robot weight, then calculate the torque needed to drive the wheels on the worst-case terrain. For a flat surface: Torque = (Total weight ÷ number of driven wheels) × wheel radius × friction coefficient. Add 3x safety factor. Then choose RPM based on desired speed: RPM = (Speed in m/s × 60) ÷ (Wheel circumference in meters). Select a motor with higher rated specs than your calculations show.

Q: What is the difference between 12V gear motors and 6V gear motors?

12V motors run on a higher voltage, meaning they can deliver more power (P = V × I) for the same physical motor size. They are more efficient over longer cable runs (less voltage drop). 6V motors are better for battery-powered projects running on 4 AA batteries or a single-cell LiPo with boost converter. For robot drive systems powered by 2S LiPo (7.4V) or 3S LiPo (11.1V), 12V motors are the natural choice.

Q: Can I use PWM to control the speed of any DC gear motor?

Yes, all DC brush motors respond to PWM speed control through a motor driver. However, very low PWM duty cycles (below 20–30%) may not provide enough voltage to overcome motor and gear friction, so the motor appears to not move even though the driver is active. There is always a minimum effective PWM threshold that depends on the motor, load, and gearbox friction.

Q: What does the 25GA in 25GA-370 mean?

The nomenclature refers to the gearbox body diameter (25mm) and the base motor type (GA-370). The 370 refers to the motor can size class — a very common small DC motor form factor widely manufactured in China. The 25GA-370 combination is one of the most popular gear motor series for hobbyists worldwide because of its compact size, metal gearbox construction, reliable performance, and availability in dozens of gear ratio options.