Electrical Theory

[briofoz]

I need some help on theory. Heres the background. Bro in law has bought a
Lamyai orchard at Lumphun. Well established with dam. It is 2 rows wide
and about 25 m wide, is 4 rai of land, which means its very long and
skinny.



He has put in irrigation pipes to a sprinkler on each tree. He runs a
3Hp electric motor to run the pump from the dam. The electrical problem
is that its 200m from the supply pole.



He is using 2.5mm single core copper to run the distance. It needs to be wound up after each use because it will go missing.



I have suggesting doubling the cable (parallel) to cut down losses.



Anyway, its 40 years since I did the theory. 200m to supply a 3HP Motor....



3HP is about 2200Watts, so motor will be drawing 10 amps. Loss of copper is 18mV/m from a chart I found. So...



18mV x 200 x 10 is a 36V loss in perfect conditions. But where I am
lost, copper is a resistive load, motor inductive load, so are my calcs
correct?

[David48]

Some issues there surely.

And, as you said, in perfect conditions ... presuming that there was 220V available at the take off point for the pump line.

Motors certainly can burn out.

Have a read of this

The Mod crossy is your man for an answer.

[Crossy]

That 3hp motor will be pulling 30A+ on start. Doing the design properly, as a minimum he would be needing 6mm² cable in copper (that would be under-volting the motor by 20% on start), a magnet for metal thieves

That run of 2.5mm² is probably dropping nearly 50% of the supply voltage when the pump starts, pretty amazing that it starts at all Once it's running you're looking at a 16% volt drop which is just about acceptable if the supply is 220V or a bit above, so if he's only doing one or two starts a day the pump will probably be fine.

A solution would be to use 10mm² aluminium cable, it's MUCH cheaper than copper and the thieves will only nick it once (or put a nice big sign on it). String it up on bamboo poles and the jobs good to go

[timaloy]

6mm twin and earth will do job.......or double up on your 2.5mm. This will do......if any doubt triple your 2.5mm. Just make sure all cable lengths are the same

[briofoz]

Thanks all for the input. I like the idea of 6mm Al on poles. Harder to steal and not as attractive to steal.