Electrical Induction Motors - Torque vs. Speed

Calculate torque developed by asynchronous induction motors. Includes locked rotor torque, pull-up torque, break-down torque, and full-load torque calculations for electrical motors in both metric (kW) and imperial (hp) units.

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Inputs

Results

Full-load Torque (Metric)-- Nm
Full-load Torque (Imperial)-- lb ft
Torque TypeDescriptionUnitFormula / Reference
Locked Rotor TorqueStarting torque at zero speedNm / lb ftMotor dependent
Pull-up TorqueMinimum torque from start to full-loadNm / lb ftMotor dependent
Break-down TorqueMaximum torque before speed increaseNm / lb ftMotor dependent
Full-load TorqueTorque at rated power and speedNmT = 9550 × PkW / nr
Full-load TorqueTorque at rated power and speedlb ftT = 5252 × Php / nr
Accelerating TorqueAvailable Motor Torque - Load TorqueNm / lb ftMotor dependent

Key Concepts

Locked Rotor or Starting Torque: The torque an electrical motor develops when starting at zero speed. High starting torque is important for hard-to-start applications like positive displacement pumps and cranes.

Pull-up Torque: The minimum torque developed when the motor runs from zero to full-load speed, before reaching the break-down torque point. This may be critical for applications requiring power to overcome temporary barriers.

Break-down Torque: The highest torque available before it decreases as the machine continues to accelerate to working conditions.

Full-load (Rated) Torque: The torque required to produce the rated power at full-load speed.

Formulas

Metric Units: T = 9550 × PkW / nr (Nm)

Imperial Units: T = 5252 × Php / nr (lb ft)

Where:

  • T = rated torque
  • PkW = rated power (kW) or Php = rated horsepower
  • nr = rated rotational speed (rpm)