How to Calculate Rated Torque

Calculate motor torque based on power and rotational speed.

function calculateTorque() { // Get Input Values var powerInput = document.getElementById('motorPower').value; var unitInput = document.getElementById('powerUnit').value; var speedInput = document.getElementById('motorSpeed').value; var resultBox = document.getElementById('resultBox'); var resultDisplay = document.getElementById('torqueResult'); var unitDisplay = document.getElementById('torqueUnit'); var errorDisplay = document.getElementById('errorDisplay'); // Reset display errorDisplay.style.display = 'none'; resultBox.style.display = 'none'; // Validation if (powerInput === " || speedInput === ") { errorDisplay.innerHTML = "Please enter both power and speed values."; errorDisplay.style.display = 'block'; return; } var power = parseFloat(powerInput); var speed = parseFloat(speedInput); if (isNaN(power) || isNaN(speed)) { errorDisplay.innerHTML = "Please enter valid numbers."; errorDisplay.style.display = 'block'; return; } if (speed <= 0) { errorDisplay.innerHTML = "Speed must be greater than 0 RPM."; errorDisplay.style.display = 'block'; return; } // Calculation Logic var torque = 0; var resultUnitLabel = ""; if (unitInput === 'kW') { // Metric Formula: T (Nm) = (9550 * Power_kW) / Speed_RPM torque = (9550 * power) / speed; resultUnitLabel = "Newton Meters (Nm)"; } else { // Imperial Formula: T (lb-ft) = (5252 * Power_HP) / Speed_RPM torque = (5252 * power) / speed; resultUnitLabel = "Pound-Feet (lb-ft)"; } // Display Result resultDisplay.innerHTML = torque.toFixed(2); unitDisplay.innerHTML = resultUnitLabel; resultBox.style.display = 'block'; }

How to Calculate Rated Torque for Electric Motors

Rated torque is a critical specification for engineers and mechanics working with electric motors, gearboxes, and industrial machinery. It represents the rotational force a motor produces continuously at its rated speed and power without overheating. Understanding how to calculate rated torque ensures that you select the correct motor for your application, preventing mechanical failures or underperformance.

The Rated Torque Formula

The calculation of torque depends on the unit of power measurement (Kilowatts vs. Horsepower) and the rotational speed (RPM). There are two primary formulas used in the industry:

1. Metric System (kW and Nm)

T = (9550 × P) / n

Where:

• T = Rated Torque in Newton Meters (Nm)
• P = Power in Kilowatts (kW)
• n = Rotational Speed in Revolutions Per Minute (RPM)
• 9550 = A constant derived from unit conversions (60 / 2π × 1000)

2. Imperial System (HP and lb-ft)

T = (5252 × P) / n

Where:

• T = Rated Torque in Pound-Feet (lb-ft)
• P = Power in Horsepower (HP)
• n = Rotational Speed in RPM
• 5252 = A constant derived from the definition that 1 HP = 33,000 ft-lb/min

Calculation Example

Let's say you have a standard 3-phase induction motor with the following nameplate details:

• Power: 15 kW
• Speed: 1460 RPM

To find the rated torque in Newton Meters:

T = (9550 × 15) / 1460

T = 143,250 / 1460

T ≈ 98.12 Nm

Why Rated Torque Matters

Knowing the rated torque is often more important than knowing the horsepower. For applications involving heavy starting loads (like conveyors or crushers), the torque determines if the motor can physically move the load. If the required load torque exceeds the motor's rated torque, the motor may stall, trip the overload protection, or burn out.

Torque vs. Speed Relationship

There is an inverse relationship between speed and torque for a constant power output. As the speed of the motor decreases (through a gearbox, for example), the available torque increases proportionally. This is why gear reducers are used to drive slow-moving, heavy machinery using relatively small, high-speed motors.