Formula used
- Rotational power: P = T · ω, where ω = 2π·rpm/60.
- Acceleration torque (optional): Tacc = J · α, with α = ω/t.
- Linear power: P = F · v.
- Fluid power: Phyd = Δp · Q, then divide by process efficiency.
- Required output sizing: Preq,out = Pout(1+margin)·service factor.
- Electrical input: Pin = Preq,out/(ηdrive·ηmotor).
- Apparent power: S = Pin/PF.
- Current: I = S/(√3·V) for three‑phase, or I = S/V for single‑phase.
How to use this calculator
- Select the calculation mode that matches your load.
- Enter torque and rpm, or force and velocity, or pressure and flow.
- Turn on acceleration torque if fast ramps matter.
- Set efficiencies, power factor, margin, and service factor.
- Enter supply voltage and choose system phase.
- Press calculate to view results above the form.
- Use the CSV and PDF buttons to export results.
Example data table
| Case | Inputs | Typical outputs |
|---|---|---|
| Rotational | Torque 12.5 N·m, Speed 1450 rpm, Margin 15%, ηdrive 95%, ηmotor 90%, PF 0.85, 400 V three‑phase | Input power ≈ 2.8 kW, Current ≈ 5.0 A |
| Linear | Force 420 N, Velocity 0.65 m/s, Margin 10%, ηdrive 92%, ηmotor 88%, PF 0.80, 230 V single‑phase | Input power ≈ 0.5 kW, Current ≈ 2.7 A |
| Fluid | Δp 250 kPa, Flow 12 L/min, Process η 70%, Margin 15%, ηdrive 95%, ηmotor 90%, PF 0.85, 400 V three‑phase | Input power ≈ 1.4 kW, Current ≈ 2.5 A |
Motor power requirement overview
1) Why power sizing matters
Motor sizing links physics to reliability. Undersized motors overheat, trip protections, and lose torque at low voltage. Oversizing raises cost and may reduce efficiency at light load. A practical target is stable operation near 60–90% of rated power for many industrial duty profiles.
2) Rotational loads and torque data
For shafts, power depends on torque and angular speed. Typical continuous-duty loads include conveyors, mixers, and fans. If torque varies, use the average running torque and add margin for peaks. This calculator converts common torque units and uses the entered rpm to compute angular speed.
3) Acceleration, inertia, and ramp time
Starting events can dominate required torque. Acceleration torque follows T = J·ω/t, so doubling ramp time halves the inertia torque. High-inertia systems include large drums, centrifuges, and flywheels. If starts are frequent, consider thermal limits and select an appropriate service factor.
4) Linear motion power in practice
Linear power uses P = F·v. Common inputs are thrust force from actuators, hoists, and belt drives. If the force includes friction plus load weight components, the result reflects real operating demand. Convert velocity carefully when mixing ft/s, km/h, or mph with force units.
5) Fluid power and hydraulic data
Pumps and blowers are often sized from differential pressure and flow. Hydraulic power is Δp·Q. Real systems also have pump or fan efficiency, commonly 50–80% depending on size and operating point. Enter process efficiency to translate hydraulic power into required shaft power before motor losses.
6) Losses, efficiencies, and margin
Drive losses from belts, gears, or couplings can range from about 90–98%. Motor efficiency varies with size and loading, often improving near rated load. A 10–25% safety margin is common when torque data is uncertain, while service factor addresses duty severity, shock loading, and start frequency.
7) Power factor and current estimation
Electrical current depends on apparent power, not only real power. Many induction motors operate around 0.70–0.95 power factor depending on load. This calculator estimates line current using voltage, phase selection, and power factor. For protection design, also account for starting current and local electrical codes.
8) Interpreting results for selection
Use the input power as a baseline for nameplate selection, then compare against available standard ratings. If acceleration torque is large, check that the motor can deliver peak torque without stalling. When operating in hot environments or with frequent starts, prioritize thermal capacity and consider a higher service factor.
FAQs
1) Should I size from output or input power?
Start from the mechanical or hydraulic output, then include drive and motor efficiencies. The calculator reports electrical input power, which aligns with nameplate selection and supply sizing.
2) What safety margin is reasonable?
Many designs use 10–25% when load data is uncertain. Use lower values for well-measured steady loads and higher values for wear, fouling, or variable process conditions.
3) How do I estimate inertia J?
Use manufacturer data or calculate equivalent inertia reflected to the motor shaft. Include rotating components, couplings, and driven elements. Reflect through gear ratios using the square of the ratio.
4) Why include process efficiency for fluid mode?
Pressure and flow give hydraulic power, but pumps and fans are not ideal. Process efficiency converts hydraulic power to shaft power, improving the realism of motor sizing for fluid systems.
5) Does the current result include starting current?
No. The current shown is an operating estimate based on power factor and voltage. Motor starting current can be several times higher and should be checked separately.
6) What power factor should I use?
If unknown, use 0.85 as a practical default for many induction motors at moderate load. If your load is very light, power factor may be lower.
7) Why add both margin and service factor?
Margin covers uncertainty in load estimates and future changes. Service factor addresses duty severity, shock, starts, and thermal stress. Using both can better match real operating risk.