Motor Torque Calculator

Parameters

Five fields drive the calculation. Only the first two are used arithmetically. The other three select rows out of small internal tables of typical percentages, which is worth understanding before you read anything into the starting, breakdown or pull-up figures.

Power

Rated shaft power from the nameplate, in kilowatts. Form range 0.1 to 10000, default 7.5. Full load torque is directly proportional to it, so doubling the power doubles every torque figure on the page. Gotcha. This is mechanical output, not electrical input. Do not enter the kilovolt-ampere figure or the input kilowatts from a meter reading, or every torque result will be high by roughly one over the motor efficiency.

Speed

Rated speed in revolutions per minute. Form range 50 to 6000, default 1450. Torque is inversely proportional to it, so a slow-speed motor of the same kilowatt rating produces proportionally more torque. Values below 600 or above 3600 raise a warning. Gotcha. Use the nameplate full load speed, not the synchronous speed. For a 4-pole 50 Hz motor that is around 1450 revolutions per minute, not 1500. Using 1500 understates full load torque by about 3 per cent. Nothing on this page cross-checks the speed against a pole count or a supply frequency, so a typo here is silently accepted.

Motor type

Induction, synchronous or DC. Unitless selection. It does not touch the full load torque calculation. It selects three typical values: breakdown torque as a percentage of full load torque (250 for induction, 180 for synchronous, 300 for DC), pull-up torque (150, 160 and 180 respectively) and the power factor reported at load (0.87, 0.95 and 0.92 respectively). Gotcha. Because the synchronous breakdown value is 180 per cent and the tool's breakdown check demands at least 200 per cent, selecting synchronous will always fail that check regardless of your motor. See worked example 3.

Starting method

Direct on line, star-delta, soft starter or variable frequency drive. Unitless selection. It sets starting torque as a typical percentage of full load torque: 175 per cent for direct on line, 33 per cent for star-delta, 75 per cent for a soft starter and 125 per cent for a drive. Gotcha. These are single typical values, not ranges and not your motor. A real direct on line locked rotor torque runs anywhere from about 150 to 300 per cent depending on the IEC 60034-12 design class, and a soft starter or drive produces whatever its current limit setting allows. Selecting direct on line with a power above 7.5 kW raises an inrush warning.

Load type

Constant torque, variable torque or constant power. Unitless selection. It changes no torque output at all. Its only job is to set the minimum starting torque the compliance check demands: 100 per cent of full load torque for constant torque, 50 per cent for variable torque and 75 per cent for constant power. Gotcha. There is no model of the driven machine's torque-speed curve, no breakaway torque input and no inertia input. A loaded conveyor and an empty one are the same constant torque selection here, which is exactly the difference that decides whether a star-delta start will actually get the thing moving.

The underlying engine also accepts a pole count and a slip percentage, but neither is on this form. Pole count changes nothing. Slip changes nothing either, beyond suppressing a reminder that induction motors typically slip 2 to 5 per cent.

Assumptions and limits

What the tool assumes

• Only full load torque is actually calculated. It comes from power multiplied by 9549.3 and divided by speed, where 9549.3 is 60000 divided by 2 pi. Every other torque figure on the page is that number scaled by a fixed typical percentage from a lookup table.
• The percentage tables are indicative, not transcribed from a standard. They are representative values for general purpose machines. They are not taken from AS/NZS 1359 or from the IEC 60034-12 design class tables, and they are certainly not your motor's data. A manufacturer torque-speed curve overrides everything here.
• Power factor at load is a fixed number per motor type. It is not calculated from anything you enter and it does not vary with load. A motor running at part load will sit well below the value shown.
• Torque per ampere is a rough proportionality, not a real ratio. It is computed as 4.82 multiplied by the power factor and divided by speed in thousands of revolutions per minute. It never sees a voltage, a current or an efficiency, so it cannot be the full load torque divided by the full load current. Treat it as a relative indicator between two options at the same voltage and nothing more.
• No dynamics. There is no acceleration time, no load inertia, no motor inertia, no run-up curve, no thermal withstand check and no starts-per-hour limit. Whether the motor can actually accelerate the load within the thermal limits of the rotor is not answered anywhere on this page.
• Voltage is assumed to be nominal. Torque falls with the square of applied voltage, so a start at the end of a long submain with significant volt drop produces materially less torque than shown. No voltage field exists to capture that.

What the compliance checks actually check

Three checks run. The first compares the starting torque percentage for the chosen starting method against the minimum for the chosen load type, so it is a comparison of two table lookups rather than a test against your machine. The second requires breakdown torque of at least 200 per cent of full load torque, which the induction and DC selections always pass and the synchronous selection always fails. The third fails only one specific combination, constant torque load with star-delta starting. Passing all three means the typical values happen to line up, not that the motor and starter will work on your load.

What this must not be used for

• Selecting a starter or a drive. That needs the manufacturer torque-speed curve, the load torque-speed curve, the combined inertia and an acceleration time calculation.
• Verifying a motor will start a specific load. Breakaway torque and inertia are the deciding factors and neither is an input here.
• Motor thermal protection, overload relay settings or starts-per-hour assessment.
• Mechanical design of shafts, keys, couplings or gearboxes, all of which need peak and shock torque figures rather than typical percentages.
• Any conclusion about a synchronous machine drawn from the breakdown torque check, which is failed by construction.
• Evidence of compliance. This calculator is not validated or certified. Check every result against the manufacturer data and the current editions of AS/NZS 1359, AS/NZS 3000 and IEC 60034, and have it signed off by the person responsible for the installation.

Worked examples

Three examples covering the starting method, load type and motor type variants, including two that fail. Every figure comes from the formulas the calculator runs, so you can reproduce them on the form above.

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).