Shaft Power
P = T × ωLast updated:
Motor Sizing Calculator
Calculate motor horsepower, torque, and RPM requirements for load applications. Size electric motors for pumps, fans & conveyors. Free electrical engineering...
Motor Sizing Calculator
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Calculate motor power, torque, and current requirements for industrial applications. Includes starting analysis, efficiency calculations, and VFD considerations for AC induction motors.
Details
Mechanical Load
N·m
RPM
kg·m²
N·m
Electrical Supply
%
Motor Characteristics
×
×
×
Enter values above to see results.
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📚 In-Depth Guide
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Industrial Engineering
Motor Sizing Calculator: Power, Torque & Efficiency Guide
Select the right motor power, torque rating, and drive configuration for industrial, HVAC, and process applications.
Power Formula
P = T × ω
Torque
T = 9550P/n
Efficiency
85–97% IE class
Conversion
1 HP = 0.746 kW
Reviewed by: CalculatorApp Mechanical & Electrical Engineering Team
What Is Motor Sizing?
Motor sizing matches the mechanical power and torque requirements of a driven load to an electric motor that delivers reliable performance at optimal efficiency. Under-sized motors overheat and fail; over-sized motors waste energy and carry excess capital cost. Proper sizing accounts for duty cycle, starting conditions, ambient temperature, and future load growth.
Torque (rpm)
T = 9550 × P/nFull Load Current
I = P / (√3 V PF η)HP to kW
1 HP = 0.746 kW| Motor Type | Efficiency | Best Application |
|---|---|---|
| AC Induction (SCIM) | IE2-IE4 | Pumps, fans, conveyors |
| Permanent Magnet Synchronous | IE4-IE5 | High efficiency variable speed |
| BLDC / EC Motor | IE4+ | HVAC, appliances, robotics |
| Servo Motor | High (dynamic) | Precision positioning, CNC |
History Timeline
1821: Faraday demonstrates electromagnetic rotation — the principle behind electric motors.
1888: Tesla patents the polyphase AC induction motor, enabling scalable industrial power.
1891: 3-phase power transmission demonstrated at Frankfurt International Exhibition.
1960s: NEMA standardizes motor frame sizes for interchangeability across manufacturers.
1987: IEC 34-1 introduces efficiency classification (IE1-IE4) for global standardization.
2000s+: VFDs become standard for energy-efficient variable-speed industrial motor control.
IEA Motor Systems
Global motor energy efficiency policy and analysis.
NEMA Motor Standards
North American motor standards and specifications.
IEC Motor Standards
International electrotechnical motor efficiency standards.
DOE Motor Program
US Dept of Energy industrial motor efficiency resources.
Myth: Bigger motor always means better reliability.
Fact: Oversized motors run at low efficiency and poor power factor, increasing operating costs.
Myth: Nameplate kW is the power consumed at all loads.
Fact: Actual consumption depends on load; lightly loaded motors draw much less than nameplate.
Myth: VFDs are only for energy saving.
Fact: VFDs also improve process control, extend equipment life, and reduce mechanical stress.
Myth: Motor efficiency is the same at all speeds.
Fact: Efficiency peaks near rated load (75-100%); it drops significantly at light loads.
FAQ (12)
What is motor sizing?
Motor sizing determines the correct power, torque, and speed ratings for a motor to handle a given mechanical load reliably and efficiently.
Why is oversizing a motor wasteful?
Oversized motors run at low load factors, reducing efficiency and power factor, increasing energy costs and reactive power demand.
How do I calculate required torque?
T = P / ω — divide power in watts by angular velocity in rad/s. For rpm: T = 9550 × P(kW) / n(rpm).
What is service factor?
A multiplier (typically 1.0 to 1.5) applied to nameplate rating to allow for temporary overloads without damage.
When should I use a VFD?
Variable frequency drives are ideal for variable torque loads like fans and pumps where speed control saves significant energy.
What is efficiency class IE3 vs IE2?
IE3 is premium efficiency (IEC standard); IE2 is high efficiency. IE3 reduces energy losses by 15–20% vs IE2 at same rating.
How do starting current affect sizing?
DOL starting draws 6-8x full load current. Oversizing protection and cable is required for inrush.
What determines motor frame size?
NEMA or IEC frames define mounting dimensions and shaft sizes; frame is selected based on power and speed rating.
How do I convert HP to kW?
1 HP = 0.746 kW. Multiply horsepower by 0.746 to get kilowatts.
What is full load current (FLC)?
FLC = P / (1.732 × V × PF × η) for three-phase motors; used to size cables and protection devices.
What is the duty cycle impact?
Continuous duty (S1) requires full thermal rating; intermittent or periodic duty cycles allow smaller motors.
Why is ambient temperature important?
Motors are typically rated for 40°C ambient. Higher temperatures derate the motor; use a de-rating factor.
References
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