A geared DC motor combines a small direct current motor with a reduction gearbox to make a compact source of controlled rotation. The motor spins fast but usually has low torque, while the gearbox trades speed for turning force at the output shaft. This makes gearmotors useful in robots, wheels, arms, grippers, conveyors, and small mechanisms.
Understanding the relationship between voltage, speed, torque, and gear ratio helps students choose the right motor for a task.
Key Facts
- Gear ratio = motor speed / output speed
- Output speed = motor speed / gear ratio
- Ideal output torque = motor torque × gear ratio
- Real output torque = motor torque × gear ratio × gearbox efficiency
- Electrical power input = V × I
- Mechanical power output = torque × angular speed, P = τω
Vocabulary
- DC motor
- A DC motor is an electric motor that converts direct current electrical energy into rotational mechanical motion.
- Gearbox
- A gearbox is a set of gears that changes rotational speed and torque between an input shaft and an output shaft.
- Gear ratio
- Gear ratio is the factor by which a gearbox reduces speed and multiplies torque, often written as input rotations to output rotations.
- Stall torque
- Stall torque is the maximum torque a motor can produce when its shaft is prevented from rotating.
- Free-run speed
- Free-run speed is the motor speed when it runs with almost no load on the shaft.
Common Mistakes to Avoid
- Ignoring gearbox efficiency, because real gears lose energy to friction and heat so output torque is less than the ideal gear ratio calculation.
- Using stall torque as a normal operating torque, because a stalled motor draws very high current and can overheat or damage the driver.
- Confusing motor speed with output shaft speed, because the gearbox reduces the speed before motion reaches the robot wheel or mechanism.
- Choosing a gear ratio without checking both torque and speed, because a high ratio may give enough torque but make the robot too slow.
Practice Questions
- 1 A DC motor spins at 6000 rpm and is connected to a 50:1 gearbox. What is the ideal output speed in rpm?
- 2 A motor produces 0.03 N m of torque before the gearbox. If the gearbox ratio is 100:1 and efficiency is 70 percent, what is the real output torque?
- 3 A robot wheel gearmotor moves too slowly but has more than enough torque. Should the designer choose a higher gear ratio or a lower gear ratio for the next version? Explain the reasoning.