About the Electric Motor Torque Calculator
This electric motor torque calculator indicates the relationship between motor speed, power, and torque. Are you looking to determine how much torque you can get at a specific rotational speed? Then keep reading — we discuss how an electric motor works and indicate the formulas used to calculate its torque. We also define, exclusively for alternating current, what electric motor slip is.
💡 You might also like to check out our torque calculator and the torque to hp calculator after using this tool.
How does an electric motor work?
It is easy to determine whether an electric motor is working correctly, but few choose to learn how they work. In case you think you have never seen an electric motor, you have one in the water pump of your house or in your living-room fan. Electric motors are everywhere, converting electrical energy into mechanical energy by making a shaft rotate. There are two main types of electric motor:
- AC electric motor: These work with alternating current. Alternating current periodically reverses its direction and magnitude over time. This feature makes it perfect for creating alternating magnetic fields that rotate the motor.
- DC electric motor: This type works with direct current. The rotating part is called the armature and the stationary part the stator. The stator contains magnets that build a magnetic field covering the armature. The armature is energized by the DC power, and the current flowing through it creates an electromagnetic field. The interaction between the electromagnetic field and the stator's magnetic field makes the armature rotate.
What is the electric motor torque?
In both motor types, the shaft rotates with a certain angular speed, producing torque. We can use such torque to rotate driven equipment like impellers in water pumps. In other words, electric motor torque is the amount of rotational force the motor develops.
In both types of electric motor, torque depends on the intensity of the magnetic fluxes inside the motor; however, their speed–torque relationship is different. In a DC motor, a high-torque condition will rotate the shaft considerably slower than at other loads — that is why the phrase "high torque, low speed electric motor" is mostly related to DC motors. An AC electric motor, on the other hand, experiences its maximum torque at a specific rotating speed, called the breakdown torque.
How to calculate the electric motor torque?
Mechanical power, rotational speed, and torque are linked by a single relationship. Power equals torque multiplied by angular velocity:
T = P / ω = 60 × P / (2π × N)
| Symbol | Quantity | Metric unit | US unit |
|---|---|---|---|
| T | Torque | N·m (newton-metre) | lb·ft (pound-foot) |
| P | Power | kW (kilowatt) | HP (horsepower) |
| N | Rotational speed | RPM (revolutions per minute) | |
| ω | Angular velocity | rad/s = 2π × N / 60 | |
A handy shortcut for each unit system:
- Metric: T (N·m) = 9549 × P (kW) / N (RPM)
- US: T (lb·ft) = 5252 × P (HP) / N (RPM)
Three Ways to Use This Calculator
1. Find Torque (T)
Calculate torque when you know the motor power and rotational speed.
Formula: T = 60 × P / (2π × N)
Example: P = 7.5 kW, N = 1450 RPM → T ≈ 49.4 N·m
2. Find Power (P)
Calculate the mechanical power when you know torque and rotational speed.
Formula: P = T × 2π × N / 60
Example: T = 50 N·m, N = 1450 RPM → P ≈ 7.59 kW
3. Find Speed (N)
Calculate the rotational speed when you know power and torque.
Formula: N = 60 × P / (2π × T)
Example: P = 7.5 kW, T = 50 N·m → N ≈ 1432 RPM
What is slip in electrical motors?
For AC induction motors, the rotor never quite spins as fast as the rotating magnetic field — if it did, there would be no relative motion to induce current in the rotor, and therefore no torque. The difference between the magnetic field's synchronous speed (Nₛ) and the actual rotor speed (N) is called slip.
s = (Nₛ − N) / Nₛ × 100%
The synchronous speed depends on the supply frequency and the number of poles:
Nₛ = 120 × f / p (f = frequency in Hz, p = number of poles)
Typical induction motors run at 2–6 % slip under full load. Tick the "Calculate AC motor slip" box in the calculator and enter the synchronous speed to obtain the slip percentage and the slip speed.
Unit Systems Supported
Metric (SI)
- Power: kW (kilowatts)
- Torque: N·m (newton-metres)
- Speed: RPM / rad/s
US Customary
- Power: HP (horsepower)
- Torque: lb·ft (pound-feet)
- Speed: RPM / rad/s
Results always include automatic conversions between N·m, lb·ft, kgf·m, and N·cm for torque, plus kW/HP/W for power and RPM/rad/s/Hz for speed.
Key Conversion Factors
| From | To | Multiply by |
|---|---|---|
| HP | W | 745.699872 |
| kW | W | 1000 |
| N·m | lb·ft | 0.737562 |
| lb·ft | N·m | 1.355818 |
| N·m | kgf·m | 0.101972 |
| RPM | rad/s | 0.104720 (2π/60) |
Frequently Asked Questions
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How do I calculate motor torque from power and RPM?
Use T = 60 × P / (2π × N), with power in watts and speed in RPM to get torque in N·m. In practical units, T (N·m) = 9549 × P (kW) / N (RPM), or T (lb·ft) = 5252 × P (HP) / N (RPM).
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Why does torque go down as motor speed goes up?
For a fixed power output, torque and speed are inversely proportional: T = P / ω. Doubling the rotational speed halves the available torque. This is why high-torque applications often use a gearbox to trade speed for torque.
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What is slip and why is it never zero in an induction motor?
Slip is the relative difference between the synchronous speed of the stator's rotating field and the actual rotor speed: s = (Nₛ − N) / Nₛ. If slip were zero, there would be no relative motion to induce rotor current and the motor would produce no torque. A loaded induction motor therefore always runs slightly below synchronous speed.
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Is 1 N·m of torque the same as 1 joule of energy?
They share the same base units but are different physical quantities. Torque (N·m) describes a rotational tendency; energy (J) describes work done. Mechanical power, however, does combine them: power (W) = torque (N·m) × angular velocity (rad/s).
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Does this calculator work for both AC and DC motors?
Yes. The power–speed–torque relationship T = P/ω applies to any rotating shaft, AC or DC. The slip calculation is only meaningful for AC induction motors, so it is optional — leave the "Calculate AC motor slip" box unchecked for DC and synchronous machines.