Motor Rated Torque

Calculate rated motor torque from power and speed with multi-mode conversion.

Inputs

P

n

Speed

Formula Interpretation

Rated Torque Formula

T = 9550 \dfrac{P}{n}

Rated torque $T$ is proportional to power $P$ and inversely proportional to speed $n$ , which is fundamental for motor sizing.

Power Back-Calculation

P = \dfrac{T n}{9550}

When torque $T$ and speed $n$ are known, power $P$ can be obtained by inverse calculation.

Speed Back-Calculation

n = 9550 \dfrac{P}{T}

For a fixed torque $T$ , higher power means higher speed. This inverse form is used to determine rated speed.

Knowledge Points

Rated torque vs rated power

Rated power is the continuous mechanical output at rated condition, while rated torque is the shaft twisting moment under that condition. They are linked by speed.

Speed effect on torque

At constant power, lower speed gives higher torque, and higher speed gives lower torque. This trade-off is central in motor selection.

Unit consistency

The 9550 coefficient applies when power is in kW, torque in N·m, and speed in r/min. Convert units first if using imperial units.

Typical applications

Used in motor sizing for pumps, fans, conveyors, and machine tools, often combined with efficiency and service-factor checks.

Example

A motor has rated power $P = 7.5\,\text{kW}$ and rated speed $n = 1450\,\text{r/min}$ . Find its rated torque.

Step 1 — Substitute into rated torque formula

T = 9550 \dfrac{P}{n} = 9550 \dfrac{7.5}{1450}

Step 2 — Compute and arrange units

T \approx 49.4\,\text{N}{\cdot}\text{m}

The rated motor torque is approximately $T \approx 49.4\,\text{N}{\cdot}\text{m}$ .

Extended Knowledge

•Power-torque curve：In constant-power regions, torque decreases as speed increases; in constant-torque regions, power increases linearly with speed.
•Efficiency and safety factor：Practical sizing should include drivetrain efficiency, transmission losses, and safety factors, so required motor power is usually above the ideal value.
•Pole count and speed：For AC motors, synchronous speed depends on pole count. At 50 Hz, typical levels are 3000, 1500, 1000, and 750 r/min.
•Load-type matching：Constant-torque loads (conveyors) and quadratic-torque loads (fans/pumps) require different motor selection strategies.