Rated Torque Basics
Rated torque is the turning effort a motor can deliver at its rated output speed. It links mechanical power with shaft speed. A higher power motor may still have low torque when it runs fast. A slower motor can deliver more torque from the same power rating. This is why speed must always be checked with the nameplate power.
Why It Matters
Induction motors are used on pumps, conveyors, fans, mixers, hoists, and machine tools. Each load needs enough torque to start, accelerate, and run. Rated torque is mainly a running value. It should not be confused with starting torque, breakdown torque, or locked rotor torque. Those values describe short conditions. Rated torque describes continuous output under normal cooling and supply conditions.
Speed and Slip
The synchronous speed depends on frequency and poles. Real induction motors run slightly slower. This difference is called slip. At rated load, slip creates rotor current and useful torque. Too much slip can mean overload, low voltage, poor sizing, or mechanical binding. Too little slip may happen at light load. The calculator can use measured speed or estimate speed from slip.
Using Electrical Input
When nameplate output power is unknown, input voltage, current, power factor, and efficiency can estimate output power. Three phase power uses the square root of three. Single phase power uses voltage times current. Efficiency converts input watts to shaft watts. This estimate depends on accurate field readings.
Design Practice
Select a motor with rated torque above the required running torque. Add service factor only when the motor nameplate allows it. Use overload allowance carefully. Heat rises quickly when motors run above rating. Gearboxes change torque and speed together. A reduction gearbox increases output torque, but losses reduce the final value.
Final Check
Torque calculation is simple, yet the decision is practical. Verify duty cycle, ambient temperature, enclosure, altitude, supply balance, and starting method. Also check belt tension and coupling alignment. Good motor selection lowers trips, noise, heat, and maintenance cost.
Record Keeping
Keep every calculation with the job notes. Record assumptions, units, and measured readings. This makes audits easier. It also helps another technician repeat the result before replacing a motor or changing a drive in service safely.