Motor Torque Calculator

Motor Torque Calculation Guide for AS/NZS 1359 and IEC 60034

Motor torque is the rotational force an electric motor produces at its shaft, measured in Newton-metres (Nm). Every motor installation requires a torque analysis to confirm the motor can accelerate the driven load from standstill to full speed, sustain the load at rated speed, and handle momentary overloads without stalling. This calculator determines full load torque, starting torque, breakdown torque, and pull-up torque based on motor nameplate data, starting method, and load characteristics. It is used during motor selection, starter specification, and mechanical coupling design in accordance with AS/NZS 1359 and IEC 60034.

Key concepts

• Full load torque (FLT). The continuous torque at rated speed and rated power. Calculated as T = (P x 1000) / (2 x pi x n / 60), where P is the shaft power in kW and n is the rated speed in RPM. This is the baseline for all other torque comparisons.
• Starting torque. Also called locked rotor torque, this is the torque the motor produces at zero speed. For standard induction motors it is typically 150% to 300% of FLT. The starting torque must exceed the load breakaway torque, or the motor will not begin to rotate.
• Breakdown torque. The maximum torque the motor can deliver before it stalls. If the driven load exceeds this value at any point during operation, the motor speed collapses and the motor draws locked rotor current until the protection device trips. IEC 60034-12 defines minimum breakdown torque ratios by motor design class.
• Voltage and torque relationship. Motor torque is proportional to the square of the applied voltage. A 10% voltage reduction results in a 19% torque reduction. This relationship is critical when evaluating reduced-voltage starting methods and the impact of supply voltage drop on motor performance.

Common scenarios

• Selecting a motor for a conveyor system. Conveyors are constant-torque loads that require high breakaway torque when starting fully loaded. The electrician calculates the required torque from the belt load, friction, and incline angle, then selects a motor with a starting torque that exceeds the breakaway requirement. If star-delta starting is specified, the reduced starting torque (approximately 33% of DOL) must still be verified against the loaded conveyor torque.
• Verifying a pump motor will not stall under transient conditions. Centrifugal pumps are variable-torque loads, but sudden pressure changes (such as a valve closing) can spike the torque demand. The electrician checks that the motor breakdown torque provides sufficient margin above the worst-case transient load to prevent stalling and nuisance tripping.
• Sizing a VFD for a fan application. Fans follow a cubic power curve, so torque demand rises with the square of speed. At low speeds the torque is minimal, but at full speed the fan may require close to full load torque. The electrician uses the torque curve to verify the VFD can deliver the required torque across the entire speed range, paying particular attention to the constant-torque region below base speed.

Motor types

• Induction: Most common. Asynchronous with slip. Robust, low cost, suits constant load.
• Synchronous: Runs at exactly line frequency. PFC and constant-speed apps.
• DC: Precise speed control; high starting torque. Specialised use.

Starting methods

• DOL: Maximum starting torque; high inrush current.
• Star-Delta: ~1/3 starting current of DOL; lower starting torque.
• Soft starter: Electronic current limiting; smooth ramp-up.
• VFD: Full speed control; optimised starting profile.

Load types

• Constant torque: Independent of speed (compressors). More power at higher speeds.
• Variable torque: Varies with speed squared (centrifugal pumps, fans).
• Constant power: Power independent of speed (hoisting, winding).