Desalinator [moved from MV Catapult build]

The watery and smelly sides of life.
he b gb
Posts: 34
Joined: Wed Mar 20, 2013 11:05 am

Re: MV catapult

Post by he b gb » Thu Jan 21, 2016 10:34 am

Thanks Groper, I'll check out ebay and let you know how I go. cheers, Gerald.

mahnamahna
Posts: 580
Joined: Wed Aug 02, 2006 4:48 pm
Location: Gosford NSW

Re: MV catapult

Post by mahnamahna » Thu Jan 21, 2016 2:31 pm

5000 w/10000w peak
http://www.ebay.com.au/itm/AU-Stock-100 ... Sw-W5Uyu~I
3000w/ 12000w peak
http://www.ebay.com.au/itm/3000W-12000W ... SwDk5T1imt

Both around $400

There is an 8000w/16000 peak at $2400, I think you want a decent amount of AH not to drain your entire system faster than you can say start the gennie.

http://www.ebay.com.au/itm/High-Power-p ... SwcBhWZ617

he b gb
Posts: 34
Joined: Wed Mar 20, 2013 11:05 am

Re: MV catapult

Post by he b gb » Thu Jan 21, 2016 6:31 pm

Thanks Mahnamahna, now I,m getting really confused ! Would the 850w, 240 v, motor flatten my 150 amp agm battery even only after running it for an hour every few days? I realize that they use heaps on start up but I thought that was only for a few seconds until it got up to speed. Does any body know if you can somehow reduce the pressure of the high pressure pump manually during start up to reduce the draw on the battery. Maybe I might be better of buying the 12volt, 20ltr/hr model and running it for 3 hours using 45amps. Anybody have any ideas if this would be a better way to go? Thanks, Gerald.

mahnamahna
Posts: 580
Joined: Wed Aug 02, 2006 4:48 pm
Location: Gosford NSW

Re: MV catapult

Post by mahnamahna » Thu Jan 21, 2016 7:44 pm

Sorry Gerald, didnt mean to confuse you. My understanding of electrics is not great so I may have been way off in that rather flippant comment.

Whilst I think you are right that if the draw is much less after start up then you wont be draining the battery at the rate that say the full 8000w would drain it (8000w at 12v = 660 amps, so you would drain a 600 ah bank in less than an hour, 150ah in 15 mins).

You would imagine it would be impossible for a watermaker to work at 12v if a 240v pump would drain at those sorts of rates so I would say the reality is that you will use more power because the larger inverter needs more just to run, and of course a bigger pump is going to draw more too.

What I am told though is you should have the minimum inverter that your highest rated appliance requires because the higher the ability of the inverter the more power it uses even at lower ends of its capability. (I know people that have 2 inverters, one low spec say 800w and one high spec to handle the greater loads and use the smaller inverter wherever they can).

So say you need 800w and have a 800/1600 inverter and a 3000/6000 then you are going to use less power powering that 800w need with the smaller inverter.

Perhaps one of the techies could explain it for all our benefit.

he b gb
Posts: 34
Joined: Wed Mar 20, 2013 11:05 am

Re: MV catapult

Post by he b gb » Thu Jan 21, 2016 8:27 pm

Thanks for your reply. Yes, if I go for the 240v watermaker the large inverter would be just for it as I already own a smaller inverter for lower draw items. I think I'll have to talk to an expert before I commit to buying the 240v unit. Cheers, Gerald.

groper
Posts: 239
Joined: Sun Nov 18, 2012 11:23 am
Location: cairns

Re: MV catapult

Post by groper » Thu Jan 21, 2016 11:35 pm

Gerald - if you only have 1 battery - and it is 12v 150ah - then you can only realistically use half of that or you will kill your battery in no time - even 50% depth of discharge is pushing it, a 30% DoD is a typical design goal for a lead acid battery. So you have at best 12v * 75ah = 0.9kwh to burn. So you can run your watermaker for just over an hour before your battery will be pretty drained, and at this rate is will probably get a bit hot too. I would say you should seriously think about expanding your battery bank to do this... Otherwise - definately consider the smaller 20L version...

Inverters are 90% efficient - so if your running a 900W load - then the power consumption will be 990W - 10% disappears in the form of heat.
So this 800W watermaker will actually use 880W via an inverter. A larger inverter wont use more power than a smaller one - well not significantly anyway, its proportional to the loads your running, not the rated maximum the inverter can deliver. The more power your draw through it, the more heat your generate - its proportional. The standby current of my 3000W inverter is 1A (at 12V) with no load... smaller units may be a little better on this but its a drop in the ocean compared to the other loads your running and also compared to the 10% efficiency losses...

The line of thinking about the energy requirements on my boat hasnt changed from day one- rather than run super expensive but efficient equipment, its cheaper overall and also more flexible to spend the dollars saved on bigger solar power... The money ive saved just by using 240v fridge and chest freezer compared to 12v boat or camping refrigeration - has more than paid for the entire solar system on my boat. Then i also have enough energy for watermaking and running my dive compressor, laptop, or whatever else i want without running into a brick wall on energy budgets like gerald is now contemplating... the tables have really turned now days with solar power being so cheap compared to only a few years ago...

groper
Posts: 239
Joined: Sun Nov 18, 2012 11:23 am
Location: cairns

Re: MV catapult

Post by groper » Thu Jan 21, 2016 11:49 pm

And one more thing about big motors...

Induction motors typically use between 5 and 8 times their full load current on startup - its called inrush current, and its so large because the rotor is not turning and their is no inductive reactance to provide any other impedance to current flow. Only the resistance of the stator coil is there until the rotor gets moving, and typically a small induction motor like this will have a stator winding resistance of a few ohms - like 2-4ohms or similar. If you apply DC theory to that youll know that current I, = v/R. If you assume a winding resistance of 3 ohms (about typical) then you should see 240v/3ohms = 80amps @ 240VAC - woah! But yes its true if only for a split second... So why does the running current of the motor look more like 4 amps? As the rotor builds speed, it interacts with the magnetic field of the stator and you have a conductor moving in a magnetic field - so the rotor generates a back EMF which opposes the current flow - so even tho the winding resistance is still 3 ohms - the inductive reactance is also at play and adds to the resistance to give a total impedance. Impedance refers to circuits which are generally AC in nature and can be not just in the form of copper resistance like in DC theory, but also inductive reactance, and capacitive reactance which occur in AC circuits but have the same effect - more impedance mean less current flows...

The problem with motors runn from inverters is tripping the overload protection of the inverter. If you try to start an electric motor and it pulls in excess of the burst current rating of the inverter - it will trip out and you loose all power until it resets and repeats. Net result is you cant get your motor moving and your SOL... So you need a big rating on the inverter so the inrush current wont trip it before it gets moving. Once the motor is turning, your all good provided the motors electrical power input (remember you also need to account for power factor on an AC motor - typically 0.8) doesnt exceed the maximum continous current rating of the inverter. So my 3000w -6000w burst inverter will have an instant trip at 25amps (6000/240) and it will be able to sustain a current of 12.5amps - which is only 3000w at power factor = 1. Its less than 3000W as the power factor drops - which it will do with induction motors running off it - 2400W if you assume PF = 0.8 as the current is still 12.5A...

This gives you the idea as to why generators are rated in KVA and not KW - they dont know what the power factor will be as it depends on the loads the consumer connects to it!

Matt L.
Posts: 127
Joined: Sun Nov 01, 2015 5:03 pm

Re: MV catapult

Post by Matt L. » Fri Jan 22, 2016 5:47 am

Great posts Groper. I'll have to come back to it one day when my brain has room to try to understand it all? Only enough matter in there to focus on one prize at a time. Great reference of info. Matt.
Feel free to season my comments with IMO's and IMHO's to taste. ;)

44c
Posts: 1148
Joined: Tue Aug 08, 2006 5:08 am
Location: Hervey Bay, Qld

Re: MV catapult

Post by 44c » Fri Jan 22, 2016 6:27 am

It's a pity there's such a big jump in production between the 12 volt version and the smallest 240 volt version - while prices are more similar.

The 12 volt one still looks like good value when compared to other brands though.

groper
Posts: 239
Joined: Sun Nov 18, 2012 11:23 am
Location: cairns

Re: MV catapult

Post by groper » Fri Jan 22, 2016 7:59 am

I have found other motors which you could use... they are available here in OZ, 240VAC motors which will fit on that pump down to 370W. And also 12V motors which go upto 375w etc... choice is yours... If you cant find something that works for you - we can make it work or just build your own like we are...

There are 2 sizes of smaller cat pumps than the one we are using. So you could use those and use even less current / make less water if 500W is still too much power for your liking... Let me know ASAP as im just about to order the high pressure housings and membranes from my australian supplier... $800 for a 2540 size housing and membrane...

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