Thread: Is a motor start relay switch needed for a 3ph 2.8kw pump motor?

Originally Posted by Ben Dono

I'm glad I asked Eskimo!

It was a long shot as I could not find any chatter on the net about it (psi vs tank volume).

There is a big difference in force at the tap from a full tank to empty.. It's a 40 OOO gallon tank plumbed in with a 2" line. It was a guess that there must be a psi difference between a full and empty tank but high school physics was many yrs ago.... Sounds like it was one of those pie in the sky guesses! I thought a $40 psi gauge would be a very simple remote monitoring solution.
I was looking at avoiding electronics as mechanical gauges are easier to service.

The tank has a float gauge, pump and switching gear is down on the river and I'm in the middle. I was trying to set it up in one place as through winter we may only pump up once but the first month of summer I may pump every week until the rain starts.

That also might need some looking into! Thanks for the heads up!

1 ft head of water is 0.4332 psi
so an assumes tank at ground level, for 6ft head it will only read <3psi or so...if its 100 ft high on a stand then it will be higher if reading at ground level so a psi gauge might or could be somewhat indicative...but lets say the tank height is only 6ft, this still only gives <3 psi between full and empty...so you need a gauge that can distinguish 1, 2 and 3 psi...not easy on a 20psi gauge..unless it has very large dial ie 12 inches or so

but if tank is at ground level ...well i'd be thinking a float type..or if you want go electronic with wifi if available and send to receiver of computer....

"here is a big difference in force at the tap from a full tank to empty"
with a 2 inch pipe there will be!

Hi Ben Dono,
Since its a pump you can expect up to 6 x full load current when you start the pump as it is starting under load. Using an overload will give you a thermal cut-off if the motor gets hot, and using a D Type Circuit Breaker at the Board will give you thermal and magnetic protection on the cables to the motor and because its a D-Type breaker will give longer period to trip point. An other thought is to look at a soft-starter.
There are float switches that you can use to switch the pump or probe type switches that will give a high and low level output.

Regards
Keith_W

Originally Posted by Keith_W

An other thought is to look at a soft-starter.

nice, but an over kill in this application

Ask and ye shall receive! So it's the resolution of the psi gauge that's the issue. Your right Their would not be enough in a little cheap gauge to work with... A 12" gauge may give some difference but sounds like it's the wrong path.
Your right, head at the gauge does not make a difference. All in all it will still be 6ft of resolution between high and empty... Class it as 3-4ft as I don't let it go too low..

I'm glad I asked that question!
Thanks Eskimo! Much appreciated.

Originally Posted by Ben Dono

The tank has a float gauge, pump and switching gear is down on the river and I'm in the middle.
There is a big difference in force at the tap from a full tank to empty..

Just how much head have you got at the location(the middle I assume) you'd prefer to read the tank?
Not much I'm assuming. 10ft when the tank if full?
Run a piece of pipe up the wall.
Use clear pipe. Have a tap at the high you want your low point to be. Set up a float.

Stuart

p.s. a digital psi gauge might do the job.......... but would you trust it? and you just know when the batteries would go flat lol

Here you go. set that up right(which granted would be a pita) and you should be able to read the tank level to the nearest mm
http://www.ebay.com.au/itm/Digital-M...item19eac08c43

or this

http://www.jaycar.com.au/Outdoors-%2...ensor/p/XC0331

or

a pair of Swavroski binos', just a little pair, might set you back about 800-900 or so, , and one of these

http://www.ebay.com.au/itm/Water-Lev...-/201401288082

Originally Posted by Ben Dono

There is a big difference in force at the tap from a full tank to empty.. It's a 40 OOO gallon tank plumbed in with a 2" line. It was a guess that there must be a psi difference between a full and empty tank but high school physics was many yrs ago.... Sounds like it was one of those pie in the sky guesses! I thought a $40 psi gauge would be a very simple remote monitoring solution.
I was looking at avoiding electronics as mechanical gauges are easier to service.

I use a pressure transducer to monitor the level in a 90,000 litre tank very accurately - it's a 50kPa transducer, which will measure up to 5m of water - a 7psi gauge would do the same job (but I couldn't monitor & log that from anywhere in the world).

As for pumps, these days pretty much any pump, I'll stick a drive on it. I'm replacing a bore pump with a little 1.1 kW unit and even it will get an ABB ACS310 drive - just adds so much flexibility.

I was told that a pressure transducer was what was used on top loading washing machines to sense water level. Really, it's just a matter of picking your pressure range and you are away.
Do you really need a pressure switch though? A pair of float switches will turn a pump on and off if wired correctly and as Rusty says with a drive you can do all sorts of things. I dare say there are wireless level transmitters available so you could have a level readout in your shed/ house/ car/... , and just use it to check that the auto on the pump is working as it should.

Michael

Originally Posted by RustyArc

I use a pressure transducer to monitor the level in a 90,000 litre tank very accurately - it's a 50kPa transducer, which will measure up to 5m of water - a 7psi gauge would do the same job (but I couldn't monitor & log that from anywhere in the world).

yes that is a good way to measure it...much more accurate than a gauge that one could get off the shelf at a reasonable price

I prefer the KISS principle with stuff like that..

I am not a fan of spending a few hundred on something just for it to last a couple of years, then need to be replaced, with no direct replacement available as what you bought has been superseded by five models in that time and you need to spend another hundred dollars just to fit the new one.

A person I know went and installed DAB pumps because they were cheap over Davey pumps.. Of course now the local dealer in DAB is no more, the DAB pump is screamer like a screamer, and it is only bearings I would say, but no dealer to get a new seal from so the pump bearings can be replaced. While Davey, seals are an off the shelf item from anyone..