---
title: "Selecting lift motor size"
subject: "Industrial Power Distribution and Illumination"
chapter: 5
key: "semester_6/industrial_power_distribution_and_illumination:lift-motor-size"
source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/lift-motor-size
site: BCT Engineering
---

# Selecting lift motor size

Industrial Power Distribution and Illumination · Chapter 5: Electrification of Industrial Building

- Asked in: 2079 Chaitra, 8 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/lift-motor-size
- Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-5

## Question

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

## 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.

## Answer

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
```text
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).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How is the counterweight chosen?
   A. Car weight + 40-50 % of rated load
   B. Equal to the rated load
   C. Car weight + 100 % of rated load
   D. Twice the car weight
   Answer: A. This balances part of the passenger load so the motor lifts only the smaller unbalanced weight. (Key point: Counterweight)

2. In the example, rated load is 680 kg, car 700 kg, counterweight 1006 kg. What is the unbalanced mass?
   A. 680 kg
   B. 1380 kg
   C. 374 kg
   D. 700 kg
   Answer: C. (680 + 700) - 1006 = 374 kg. (Key point: Unbalanced mass)

3. How is lift motor power found?
   A. P = m x g x v / η
   B. P = m x g x v x η
   C. P = m x g / v
   D. P = m x v^2 / 2
   Answer: A. Power equals unbalanced weight times speed divided by efficiency: 374 x 9.81 x 1.5 / 0.5 = 11.01 kW. (Key point: Power formula)

4. What motor is selected for 11.01 kW and why?
   A. 11 kW, exactly the calculated value
   B. 7.5 kW, as the load is partly balanced
   C. 15 kW, the next standard size, giving margin for starting and frequent duty
   D. 30 kW, double the value
   Answer: C. The next higher standard rating (15 kW) is chosen to cover starting and intermittent duty. (Key point: Final selection)

## Related in Industrial Power Distribution and Illumination

- [Transformer size for given industry (figure)](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Atransformer-size-attached-figure) — Industrial Power Distribution and Illumination, chapter 4, asked 2×
- [Selecting size and number of lifts](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Alift-size-number-selection) — Industrial Power Distribution and Illumination, chapter 5
- [Selecting distribution transformer size using Ku and Ks](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Atransformer-size-ku-ks) — Industrial Power Distribution and Illumination, chapter 4, asked 3×
- [Emergency power supply system for industry](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aemergency-supply-system-industry) — Industrial Power Distribution and Illumination, chapter 9, asked 2×
- [Industry room 30 × 20 m LED lighting and DB](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aindustry-room-30x20-led) — Industrial Power Distribution and Illumination, chapter 7, asked 2×
- [Transformer size for four blocks](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Atransformer-size-four-blocks) — Industrial Power Distribution and Illumination, chapter 4

## Related in other subjects

- [Selecting distribution transformer size and location](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atransformer-size-and-location) — Transmission and Distribution System Design, chapter 7, asked 2×
- [Factors for selecting a motor](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Afactors-selecting-motor) — Utilization of Electrical Energy, chapter 2, asked 2×
- [Selecting HRC fuse rating for motor](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Ahrc-fuse-motor-rating) — Switchgear and Protection, chapter 3
- [Linked list file system; block size impact](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Afs-linked-list-block-size) — Operating System, chapter 5