Page 283 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Example: Calculation of HP for domestic pump set.
A pump driven by a single phase AC motor of 240V, 50 Hz has to deliver 1000 litre to a height of 30 metre within
15 minutes. Find the HP of the motor if the efficiency of the motor is 80%.
Given
Working voltage - 240V, 50 Hz
Quanity of water to be delivered - 1000 litre

Height of the water delivered - 30 m
Efficiency of motor - 80%
Time of delivery - 15 minute
Solution
Work done by the pump / minute =

weight of the water x Height = 1000 x 30 kgm/min.
Time 15

since 1 litre of water = 1 kg. of water
and 4500 kgm/minute = 1HP

Next nearest HP of the motor recommended is 0.75 HP.
Pumps : Pumps can be classified mainly into two categories. They are
– Reciprocating pumps
– Rotary pumps.

Reciprocating pumps : In this type of pump, the main moving part has reciprocating motion only and hence the
name. Fig 1 shows the main parts of a reciprocating pump.
When the piston moves towards left, a partial vacuum is created inside the cylinder. The check valve 1 in Fig 1
closes due to the suction effect of the vacuum, spring action and head of water in the discharge tube 4 but valve
2 Fig 1 opens and allows the water to fill the cylinder through the suction pipe 3 due to atmospheric pressure
outside. This stroke of the piston is called suction stroke.
Fig 1

On the other hand when the piston moves towards right ie discharge or delivery stroke the liquid inside the
cylinder is pushed out through check valve 1 and delivery pipe 4. During the delivery stroke valve 2 remains
closed by the action of spring and the water pressure inside the cylinder.

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CITS : Power - Electrician & Wireman - Lesson 101-103