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Selecting lift motor size

Short answer

Calculate unbalanced mass (rated load + car - counterweight) and power P=m g v/η. For a 10-person lift: rated 680 kg, car 700 kg, counterweight 1006 kg, unbalanced 374 kg, at 1.5 m/s and η=0.5 gives 11.01 kW. Select 15 kW motor with VVVF drive.

Explain with an example how will you select the size of the motor of the Lift System for an industry.

The lift motor must raise the unbalanced load (car + passengers − counterweight) at rated speed, allowing for the efficiency of the gear, ropes and motor.

Method

  1. Rated load: number of persons × 68 kg (standard value per person, IS 14665 / EN 81).
  2. Car weight: from the manufacturer (depends on size and finish).
  3. Counterweight = car weight + 40–50 % of rated load, so the motor works against the smaller unbalanced weight in both directions.
  4. Unbalanced mass (worst case, full car going up): m = (rated load + car weight) − counterweight
  5. Power: P = m × g × v / η, where v = rated speed (m/s), η = overall efficiency (about 0.45–0.6 for geared, 0.7–0.85 for gearless drives).
  6. Select the next higher standard motor rating; check starting duty (number of starts per hour, about 120–180), and use VVVF drive for smooth starting.

Example: 10-person lift in an industry office block

Rated load      = 10 × 68 = 680 kg
Car weight      = 700 kg (assumed)
Counterweight   = 700 + 0.45 × 680 = 1006 kg
Unbalanced mass = (680 + 700) − 1006 = 374 kg
Speed v         = 1.5 m/s
Overall η       = 0.5 (geared machine)

P = m × g × v / η
  = 374 × 9.81 × 1.5 / 0.5
  = 11 007 W ≈ 11.01 kW

Selected motor: 15 kW (next standard size above 11.01 kW, giving margin for starting and frequent duty), 3-phase, 400 V, with VVVF drive.

Other points in selection

  • Motor duty is intermittent (S4/S5), so a lift-duty motor with high starting torque is used.
  • Cable and MCCB are sized on the drive input current; standby (DG) supply is required for at least one lift for evacuation.
  • For goods lifts in industry, rated load is the heaviest material load plus trolley, and speed is lower (0.5–1 m/s).

✓ Answer checked by Claude Opus 5.5 (high effort)

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