brains needed

[buddha]

hi,having a little difficulty with these questions,i got the anwsers but need to be shown the working out,here goes;

a pumping set delivers 0.6m cubed of water per minute from a well 5m deep.

the pump efficiency is 62%,that of the motor is 74%,and the terminal voltage is 234v.calculate

(a)the motor current

(b)the cost of pumping 100m cubed of water with energy at 5.18p per unit

©the cross-sectional area of the copper cable which will supply the set from a point 50m away with a voltage drop of not more than 6v.(the resistivity of copper is 1.78x10-8 ohmsmeters

 

[buddha]

and finally,

a pump which raises 0.12m cubed of water per minute through a vertical distance of 8.5m is driven by a 240v d.c motor.assuming that the efficiency of the pump is 72% and that of the motor is 78%,calculate the current taken by the motor.

i thought i knew this one,but according to the answers given in watkins and kitcher basic electrical calculations i didntheadbang

any help in the path taken to get the answers would be much appreciated:good luck:

 

[Apache]

hi,having a little difficulty with these questions,i got the anwsers but need to be shown the working out,here goes;a pumping set delivers 0.6m cubed of water per minute from a well 5m deep.

the pump efficiency is 62%,that of the motor is 74%,and the terminal voltage is 234v.calculate

(a)the motor current

(b)the cost of pumping 100m cubed of water with energy at 5.18p per unit

©the cross-sectional area of the copper cable which will supply the set from a point 50m away with a voltage drop of not more than 6v.(the resistivity of copper is 1.78x10-8 ohmsmeters

None of the sparks have had a go so let the Vet at it!

We know that 1 litre of water weighs 1kg.

We also know that 1 cubic meter of water contains 1000l and weighs 1 ton.

We also know that work done = force multiplied by distance.

Work done is measured in joules, force is measure in Newtons and is the mass (in Kilograms) multiplied by the acceleration due to gravity 9.81.

So we can see that 0.6 cubic meters of water weigh 600kg, the distance is 5 meters and we move that volume in 1 minute, at an efficiency of 62%.

So we need to pump 600kg divided by efficiency 0.62 = 968 kg

Work done = 968kg (mass of water) X 9.81 (to convert mass to force) X 5 (distance in meters)

Work done in 1 minute = 47,480 joules

1 watt is 1 joule per second, there are 60 seconds in a minute so 47,480/60 = 791 watts! But the motor is working harder than that as it is only 74% efficient, so we divide this by 0.74 and get 1069 watts

So we know that power (in watts) = volts multiplied by current.

Rearrange as we have the power and we have the voltage

current = power divided by volts

current = 1069 watts / 234

Current = 4.6 amps

We need to work out the cost of pumping 100 cubic meters of water. We know it required 1069 watts to pump 0.6 cubic meters of water.

so 1069 divided by 0.6 means it requires 1782 watts to pump 1 cubic meter of water so it requires 100 times this = 178,167 watts

= 178 killerwatts. A unit of energy costs 5.18p, =

 

[Apache]

and finally,a pump which raises 0.12m cubed of water per minute through a vertical distance of 8.5m is driven by a 240v d.c motor.assuming that the efficiency of the pump is 72% and that of the motor is 78%,calculate the current taken by the motor.

Same method

120kg water, pump 72% efficient = 167kg water

work done per minute = 167kg water * 9.81 * 8.5

work done per minute = 13,925

Work done per second = 232 watts @ 78% efficient

Work = 298 watts

So we know that power (in watts) = volts multiplied by current.

current = 298 watts / 240V

Current = 1.24 amps

Am I close?

 

[PC Electrics]

We need to work out the cost of pumping 100 cubic meters of water. We know it required 1069 watts to pump 0.6 cubic meters of water.

so 1069 divided by 0.6 means it requires 1782 watts to pump 1 cubic meter of water so it requires 100 times this = 178,167 watts

= 178 killerwatts. A unit of energy costs 5.18p, =

 

[ianmacd]

Wonder where they are getting their lekky so cheap.

 

[Andy™]

= 178 killerwatts. A unit of energy costs 5.18p, =

 

[buddha]

thanks everyone for all your help,i might be dragged through college yet!!!

here are the anwsers from the book.

the first question a)4.57

b)15.35p

c)1.36mm squared

the second question a)1.19A (i make it 1.2 something,so maybe im not rounding up or down or something,or maybe im just wrong)

 

[PC Electrics]

thanks everyone for all your help,i might be dragged through college yet!!!here are the anwsers from the book.

the first question a)4.57

b)15.35p

c)1.36mm squared

the second question a)1.19A (i make it 1.2 something,so maybe im not rounding up or down or something,or maybe im just wrong)

That'll be the result of rounding at some stage of the calculation. If you're calculating an answer and getting 8DP, but only writing down 3DP and inputing that into the next part, you will introduce rounding errors. The trick is to make use of the memory facility on your calculator to keep these errors to a minimum. If this was a mathematics O level then you'd lose a mark or two for rounding errors, because part of what you're being tested on is that you know how to use a calculator!