A hoist lifts 50 kg at 0.2 m/s on a Ø200 mm drum, at 85% efficiency and a 1.1 safety factor. What preliminary motor power follows, before separate hoist-safety checks?
Four steps from application to power
The required force F follows from the application type: for hoisting, just the weight (F = m·g); for a roller or belt conveyor, just friction (F = μ·m·g); for an incline, gravity and friction together (F = m·g·(sinθ + μ·cosθ)). From there, speed follows from the surface speed and wheel diameter (n = v·60/(π·D)), and torque at the drum from T = F·D/2. Load power is F·v; required motor-shaft power after the entered drivetrain efficiency is P_motor = F·v/η. This is a simplified model for a first-pass sizing — not a full DIN 22101/FEM calculation for belt conveyors, which also accounts for acceleration and multiple partial resistances.
Worked example: hoisting, 50 kg at 0.2 m/s
The force is F = 50 × 9.81 = 490.5 N. With a Ø200 mm drum, n ≈ 19.1 rpm and T = 490.5 × 0.1 = 49.05 N·m follow at the drum. The load power is F·v ≈ 98.1 W; after the entered 85% drivetrain efficiency, the required motor-shaft power is 98.1/0.85 ≈ 115.4 W. With a 1.1 safety factor, the design power becomes ≈ 127.0 W — and the selected preferred series rounds that up to 0.18 kW. This is a preliminary power selection, not approval of a hoist drive.
Two margins, not double safety
The chosen power factor (here 1.1) is only an arithmetic allowance; it does not establish starting or acceleration capability. Rounding to 0.18 kW follows the selected preferred list, not a universal constraint: manufacturers may offer other ratings. Together that gives about 0.18/0.115 ≈ 57% margin above the required motor-shaft power before the 1.1 factor — don't count that twice as extra safety on top of that factor.
Assumptions and applicability limits
The model describes a steady-state mechanical operating point with a single, simplified resistance formula per application. Acceleration, inertia, electrical input power, motor and drive losses, and a specific manufacturer's power availability per pole count/frame fall outside it. For hoisting, separately check starting and acceleration torque, transmission ratio, braking and holding capacity, duty cycle and thermal loading; the 1.1 power factor does not cover these. For belt conveyors, check the full DIN 22101/FEM resistances, and always verify the final motor choice against the manufacturer's catalogue.