Motor Efficiency Calculator
Calculate motor efficiency from electrical input power and useful mechanical shaft output power. The calculator supports complete voltage/current/torque/RPM operating points, direct input/output power, inverse solves, power loss breakdown, operating-point comparison, and no-load versus loaded references.
MOT-005 is boundary-aware: motor-only, drive-system, and gearmotor efficiency can be different numbers depending on where electrical input and mechanical output are measured. It does not replace detailed copper-loss, thermal-rise, PWM, gearbox, or full efficiency-map models.
Engineering tool
Motor Efficiency Calculator
Calculate motor electrical input power, mechanical shaft output power, efficiency, loss, loss breakdown, and operating-point comparisons.
Calculation mode
Parameter panel
Result console
- Motor Efficiency
- 87.26646%
- Electrical Input Power
- 120W
- Mechanical Shaft Output
- 104.7198W
- Total Power Loss
- 15.28024W
- Loss Percent
- 12.73354%
- Status
- Valid Motoring Operating Point
- Angular Velocity
- 104.7198rad/s
- Speed
- 1000RPM
Reference table
| Nominal efficiency | 87.26646 | % |
|---|---|---|
| Minimum reference | 83.84458 | % |
| Maximum reference | 90.828 | % |
Motor Efficiency Formula Audit
| Efficiency Boundary | Motoring positive-power mode: electrical input compared with useful mechanical shaft output. |
|---|---|
| Motoring Efficiency Definition | η = Pmechanical,out / Pelectrical,in. |
| Electrical Input Power Definition | For DC terminal-power measurements, Pin = VI. |
| Mechanical Output Power Definition | Shaft output power is torque times angular velocity. |
| Torque / Speed Formula Reuse | MOT-005 reuses MOT-001 torque, RPM and angular-velocity utilities. |
| Efficiency Formula | η = Pout / Pin. |
| Efficiency Percent | η% = η × 100%. |
| Loss Formula | Ploss = Pin - Pout. |
| Loss Percent | Loss% = Ploss / Pin × 100%. |
| Solve Output Formula | Pout = η Pin. |
| Solve Input Formula | Pin = Pout / η with η > 0. |
| Loss Decomposition Model | Known copper, driver and other losses are subtracted from total loss to find unaccounted loss. |
| Physical Efficiency Range | Values above 100% are flagged as inconsistent; they are not clamped. |
| Stall Boundary | At zero RPM, shaft output is zero and efficiency is zero while electrical loss can be high. |
| No-Load Boundary | No-load useful output is near zero; no-load input power mainly reflects internal loss. |
| Controller Boundary | If Pin is measured at the DC bus, controller loss may be included. |
| Gearbox Boundary | Gearbox output torque/speed with motor electrical input gives motor plus gearbox efficiency. |
| BLDC / Inverter Boundary | Bus VI may represent motor-plus-controller input, not winding electrical input. |
| Regenerative Boundary | V1 does not redefine efficiency for generating or braking energy flow. |
| MOT-006 Scope Boundary | Detailed copper-loss decomposition remains in MOT-006. |
Formula
Formula reference
Motor efficiency formulas
Mechanical shaft power uses the same torque/RPM convention as MOT-001. Efficiency above 100% is reported as inconsistent rather than clamped.
Pin = V Iω = 2πRPM / 60Pout = τωη = Pout / Pinη% = η × 100%Ploss = Pin - PoutLoss% = Ploss / Pin × 100%Pout = ηPinPin = Pout / ηVariable definitions
- Pin
- electrical input power
- Pout
- mechanical shaft output power
- V
- motor terminal or selected boundary voltage
- I
- input current at the same boundary
- τ
- shaft torque
- ω
- angular velocity in rad/s
- η
- efficiency as a fraction
- Ploss
- total power loss
Motor Efficiency Formula Audit
| Efficiency Boundary | Motoring positive-power mode only. |
|---|---|
| Motoring Efficiency Definition | η = Pmechanical,out / Pelectrical,in. |
| Electrical Input Power Definition | For DC terminal measurements, Pin = VI. |
| Mechanical Output Power Definition | Pout is useful shaft power. |
| Torque / Speed Formula Reuse | MOT-001 utility calculates Pout = τω and ω = 2πRPM/60. |
| Efficiency Formula | η = Pout / Pin. |
| Efficiency Percent | η% = η × 100%. |
| Loss Formula | Ploss = Pin - Pout. |
| Loss Percent | Loss% = Ploss / Pin × 100%. |
| Solve Output Formula | Pout = ηPin. |
| Solve Input Formula | Pin = Pout / η. |
| Loss Decomposition Model | Known copper, driver and other losses are compared with total loss; unaccounted loss is the remainder. |
| Physical Efficiency Range | 0% to 100% is normal for motoring; values above 100% are flagged and not clamped. |
| Stall Boundary | At RPM = 0, shaft power is zero and efficiency is zero. |
| No-Load Boundary | No-load useful output is near zero; no-load input power is an internal loss reference. |
| Controller Boundary | Bus-side input includes controller losses; motor-terminal input may not. |
| Gearbox Boundary | Gearbox output shaft data changes the efficiency boundary. |
| BLDC / Inverter Boundary | Vbus × Ibus may describe motor plus inverter system efficiency. |
| Regenerative Boundary | Regenerative braking needs a different signed-power definition and is outside V1. |
| MOT-006 Scope Boundary | Detailed copper-loss modeling remains separate. |
Worked Examples
| Example | Calculation | Result |
|---|---|---|
| 24 V, 5 A, 1 N·m, 1000 RPM | Pin = 120 W; Pout = 1 × 104.719755 | η ≈ 87.2665%, loss ≈ 15.2802 W |
| Pin = 100 W, Pout = 80 W | η = 80 / 100 | η = 80%, loss = 20 W |
| Pin = 500 W, η = 90% | Pout = 0.9 × 500 | Pout = 450 W |
| Pout = 450 W, η = 90% | Pin = 450 / 0.9 | Pin = 500 W |
| η = 0 and Pout is nonzero | Pin = Pout / 0 | Rejected |
| Pin = 100 W, Pout = 120 W | η = 120% | Flagged as inconsistent |
| Pin = 100 W, Pout = 100 W | η = 100 / 100 | η = 100% |
| Pin = 100 W, Pout = 0 W | η = 0 / 100 | η = 0% |
| Stall: 12 V, 10 A, 0 RPM | Pin = 120 W; Pout = 0 | η = 0%, loss = 120 W |
| No-load: 12 V, 1 A, useful torque near 0 | Pin = 12 W; Pout ≈ 0 | η ≈ 0%, internal loss reference |
| 2 N·m at 3000 RPM | Pout = 2 × 314.159265 | Pout ≈ 628.3185 W |
| 48 V, 15 A, same Pout | Pin = 720 W | η ≈ 87.2665% |
| Efficiency fraction 0.8 | 0.8 × 100 | 80% |
| Efficiency 80% | 80 / 100 | 0.8 fraction |
| Total loss 20 W with 12 W copper and 3 W driver | 20 - 15 | 5 W unaccounted |
| Known losses greater than total | 21 W known loss with 20 W total loss | Rejected |
| Point B higher efficiency than A | Compare complete points | Positive efficiency difference |
| 1000 mN·m | Unit conversion | 1 N·m |
| 1 kW | Unit conversion | 1000 W |
| Torque/RPM power | Uses MOT-001 P = τω | Matches mechanical power formula |
| Input → efficiency → output | Pin × η | Round-trip output recovered |
| Output → efficiency → input | Pout / η | Round-trip input recovered |
Engineering Notes
Motor Efficiency
Efficiency compares useful shaft output power with electrical input power at the same operating point.
Electrical Input Power
For DC terminal measurements, Pin = VI. For BLDC systems, bus-side power may include controller loss.
Mechanical Output Power
Shaft power should come from torque and speed measured at the same shaft.
Shaft Power
Torque and RPM must be converted to Pout = τω; RPM is not directly multiplied by torque.
Power Loss
Loss is the difference between input and output power and eventually becomes heat or unrecovered energy.
Copper Loss
Winding I²R loss grows strongly with current and is handled in detail by MOT-006.
Iron Loss
Core loss depends on speed, flux, material and waveform; V1 does not model it separately.
Friction
Bearings, brushes and seals consume power even with little useful load.
Windage
Air drag and rotor motion losses increase with speed.
Rated Efficiency
Rated efficiency applies near specified voltage, load, speed and temperature.
Peak Efficiency
Peak efficiency is usually a best operating point, not the full operating range.
Stall
At stall the useful shaft output is zero while input power can become destructive heat.
No-Load
No-load input power is useful for estimating internal losses, not useful output.
Gearmotor
Using gearbox output torque and speed includes gearbox loss in the boundary.
Controller Efficiency
Controller and inverter losses may or may not be included depending on where electrical input is measured.
Common Mistakes
- Writing η = Pin / Pout instead of Pout / Pin.
- Multiplying torque directly by RPM without converting RPM to rad/s.
- Calling VI shaft power.
- Mixing voltage/current from one operating point with torque/RPM from another.
- Clamping efficiency above 100% instead of investigating the inputs.
- Treating stall mechanical power as VI.
- Reporting nonzero stall efficiency at zero RPM.
- Assuming no-load 0% efficiency means the motor is defective.
- Mixing gearbox output and motor shaft boundaries.
- Failing to state whether controller loss is included.
- Accepting known losses that exceed total loss.
- Confusing efficiency fraction with percent.
- Using rated efficiency as a fixed value at all speeds and loads.
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Open CalculatorMotor Kv & Kt Calculator
AvailableUse MOT-004 for motor speed constant, torque constant and back-EMF constant conversions.
Open CalculatorMotor Copper Loss Calculator
AvailableUse MOT-006 for detailed motor winding I²R copper-loss, phase-loss, stepper-loss, and hot-resistance analysis.
Open CalculatorMotor Gear Ratio & Output Calculator
AvailableUse MOT-009 for gearbox output speed, output torque, mechanical efficiency and gearbox loss.
Open CalculatorPower Calculator
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Open CalculatorEfficiency Calculator
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Open CalculatorFAQ
Support reference
FAQ
How do I calculate motor efficiency?
In motoring mode, divide useful mechanical shaft output power by electrical input power: efficiency = Pmechanical / Pelectrical.
How do I calculate motor electrical input power?
For a DC terminal-power measurement, electrical input power is voltage times current: Pin = V × I.
How do I calculate motor mechanical output power?
Mechanical shaft output power is torque times angular velocity: Pout = τω.
How do I calculate shaft power from torque and RPM?
Convert RPM to angular velocity with ω = 2πRPM/60, then multiply by torque.
How do I calculate motor power loss?
Total power loss is electrical input power minus mechanical shaft output power: Ploss = Pin - Pout.
What does 90% motor efficiency mean?
It means 90% of the input electrical power is converted into useful mechanical output at that operating point, while 10% is lost as heat or other losses.
Why can a calculated motor efficiency exceed 100%?
Efficiency above 100% indicates inconsistent inputs, rounded datasheet values, mixed operating points, or incompatible measurement boundaries.
Why is motor efficiency zero at stall?
At stall, RPM is zero, so shaft mechanical output power is zero even though the motor may draw high current and dissipate heat.
Can a motor overheat at zero mechanical output power?
Yes. A stalled motor can have zero useful output while converting substantial electrical input into winding, driver, and other losses.
Why is no-load motor efficiency low?
With little useful shaft load, mechanical output is near zero. No-load input power mainly supplies internal losses such as friction, windage, iron loss, and brush loss.
What losses reduce motor efficiency?
Common contributors include copper loss, iron or core loss, friction, windage, brush loss, driver loss, stray loss, and temperature-related effects.
Does motor efficiency include controller losses?
It depends on the measurement boundary. DC bus input power includes controller or inverter loss; motor-terminal power usually excludes it.
Does motor efficiency include gearbox losses?
If torque and speed are measured at the gearbox output while electrical input is measured at the motor or bus, the result is combined motor plus gearbox efficiency.
What is the difference between rated and peak efficiency?
Rated efficiency is specified near a defined rated operating point. Peak efficiency is the maximum efficiency over a range and may occur at a different load and speed.
How does motor temperature affect efficiency?
Winding resistance, magnet behavior, lubricant friction, and electronics losses can change with temperature, so efficiency can shift as the motor warms.
Why does motor efficiency change with load?
Fixed losses dominate at light load, while copper loss grows with current. Efficiency therefore depends strongly on torque, speed, voltage, current, and temperature.
How do I compare two motor operating points?
Use complete voltage, current, torque, and speed data for each point. Do not mix current from one point with torque or RPM from another.
Engineering Disclaimer
This calculator provides first-pass motor efficiency estimates. Production motor selection should use measured operating points, manufacturer efficiency maps, thermal limits, controller loss data, gearbox data, and application duty cycle.