100 PROBLEMS IN ELECTRICAL ENGINEERING - PART – 04 – FIVE PROBLEMS ON MOTOR RATING

DIFFERENCE BETWEEN CRANE, LIFT AND HOIST

The major difference between crane and hoist is that crane can move an object in many directions including vertical and horizontal, hoist can lift object vertical only.

There is a significant difference between a ‘Lift’ and a ‘Hoist’ – a Lift is for public access and a Hoist is an industrial/commercial workhorse also suitable for many residential applications.

PROBLEM – 01

A crane motor lifts 4000 kg through a 20 m in 50 sec. Find the current and energy when supply voltage is 400 d.c and three phase 400 a.c. Assume motor efficiency is 60 percent and induction motor power factor is 0.8. 

PROBLEM – 02

An lift is required to raise a load of 10 tons through a height of 60 m. 25 percent of energy is lost in the motor and gearing. Calculate the energy is kWh supplied. If time required to raise the load is 54 minute. Find the kW rating of the motor and current taken by the motor. Supply voltage is 220 V d.c. Assume the efficiency of the motor is 80 percent.                           

PROBLEM – 03

An electric hoist makes 10 upward and 10 downward journeys per hour. During upward journey it raises a load of 10000 kg in 5 minutes to a height of 100 m. It returns empty during downward journey in 2 minutes. The weight of the cage is 1 ton and balance weight is 3000 kg. The efficiency of the hoist and motor is 0.8 and 0.88. Find (a) electrical energy input to the system in one upward and downward journey (b) energy consumed in one hour (c) horse power of the motor.

PROBLEM – 04

What must be the horse power of the engine to drive by means of a belt a generator for supplying 5000 lamps each lamp taking 0.5 A at 230 V. The line drop is 5 V and efficiency of the generator is 95 percent. There is a 2.5 percent lost in the belt drive.

PROBLEM – 05

A 220 V d.c. drives a pump lifting 1.5 m^3 of water per minute to a height of 15 m. If efficiency of the motor and pump is 60 percent determine (a) electrical input (b) the current taken from the supply. Assume 1 m^3 of water has a mass of 1000 kg.