Table of Contents
What Is Motor Torque?
Motor torque is the rotational force produced by an electric motor's shaft. It determines the motor's ability to perform mechanical work, such as turning a fan blade, driving a conveyor belt, or spinning a drill bit. Torque is measured in Newton-meters (N·m) in SI units or pound-feet (lb-ft) in imperial units.
The relationship between power, torque, and speed is fundamental in motor selection. A motor with high torque at low speed is ideal for heavy loads like cranes, while a high-speed, lower-torque motor suits applications like centrifugal pumps and fans.
Torque Formula
Where T is torque in N·m, P is power in watts, ω is angular velocity in rad/s, and n is rotational speed in RPM. When efficiency is considered, the output power is P × efficiency.
Motor Types & Torque Characteristics
| Motor Type | Starting Torque | Speed Control | Common Use |
|---|---|---|---|
| AC Induction | Low-Medium | VFD Required | Pumps, fans, conveyors |
| DC Brushed | High | Voltage control | Power tools, toys |
| BLDC | High | Electronic | Drones, EVs, robotics |
| Stepper | High (holding) | Pulse control | 3D printers, CNC |
| Servo | Very High | Feedback loop | Robotics, automation |
Applications
- Electric vehicles: EV motors deliver instant torque from zero RPM, providing rapid acceleration.
- Industrial machinery: Conveyors and mixers require high starting torque to overcome static friction.
- HVAC systems: Fans and compressors are sized based on required torque at operating speed.
- Robotics: Precise torque control enables smooth, accurate joint movement.
Frequently Asked Questions
How does torque relate to horsepower?
Horsepower and torque are directly related through speed: HP = Torque (lb-ft) × RPM / 5,252. At 5,252 RPM, torque and horsepower are always equal. Below this speed, torque is numerically larger; above it, horsepower is larger.
Why do electric motors have higher starting torque than gasoline engines?
Electric motors produce maximum torque at zero RPM because the electromagnetic force is strongest when there is maximum slip between the stator field and the rotor. Gasoline engines require a minimum RPM to generate useful torque due to their reliance on combustion dynamics.
What is the difference between stall torque and rated torque?
Stall torque is the maximum torque when the motor shaft is prevented from rotating (RPM = 0). Rated torque is the continuous torque the motor can deliver at its rated speed without overheating. Operating at stall torque for extended periods can damage the motor due to excessive current draw and heat generation.