How do I calculate motor torque from current?

Step 1: P = √3 × V × I × cos φ × η (shaft power in W). Step 2: T = P × 9549 / n (in N·m), where n is speed in RPM. The constant 9549 converts kW and RPM to N·m.

What is the 9549 constant?

T = P × 9549 / n comes from T = P / ω, where ω = 2πn/60. So T = P × 60/(2π×n) = P × 9549/n when P is in kW and T in N·m. For P in watts: T = P × 9.549/n.

What is rated torque vs starting torque?

Rated torque is the continuous torque at full load and rated speed. Starting torque is the torque at zero speed (locked rotor). For standard induction motors, starting torque is typically 150-300% of rated torque.

How does speed affect torque?

For constant power: T = P/ω, so torque is inversely proportional to speed. Below base speed with VFD (constant V/f): torque stays roughly constant. Above base speed (field weakening): torque drops as 1/n.

What is the difference between shaft torque and air-gap torque?

Air-gap torque is the electromagnetic torque produced in the motor. Shaft torque is lower due to friction and windage losses. For practical purposes, T_shaft = P_output × 9549 / n.

Free Engineering Tool

Motor Torque from Current Calculator

Chain calculation: Current → Power → Torque. T = P × 9549 / n (N·m). Enter current or power, voltage, speed.

T = 9549 × P/nI → P → TN·m / lb·ft

Results

Shaft Torque

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Shaft Power (Mechanical)

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Electrical Input Power

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Angular Velocity ω

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Current → Power → Torque Chain

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Nikolai Shelkovenko

Nikolai Shelkovenko is a vibration analysis engineer and the founder and CEO of Vibromera. For more than 15 years he has balanced rotating equipment in the field rather than on a test bench: mulchers, industrial fans, crushers, centrifuges, shafts and spindles. That work is what the Balanset instruments grew out of — they were designed as a tool a specialist can carry to the machine and use alone, on site, not as laboratory equipment. Vibromera was founded in 2017 and has been based in Porto, Portugal, since 2023. Development, assembly and support of the Balanset line all happen here. The flagship instrument is the Balanset-1A, a portable analyser for single- and two-plane balancing and for vibration diagnostics. Nikolai is personally involved in customer support, in working through difficult balancing cases and in the development of the software. He works with customers worldwide, in any language.

Vibromera

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Balanset-1A · €1975 Ask engineer