Backyard Aquaponics
A place to discuss aquaponics

ok, it is easy to see why as well. If you slow it down, there are less cycles and so less current.
And a motor is kinda the same principle but rotating
But some draw more power when slowed down....

Yup, same principle

Now... where is Charlie?

yeah where is charlie?
did he prove himself wrong?

When you first start an AC motor you get an inrush current. This is because the only major opposition to current flow is the resistance of the winding. As the motor starts to spin it starts generating a back EMF which then reduces the current being drawn by the motor. So when charlie stalled his fan with a screw driver he stopped any back EMF being generated, hence higher current drawn.

When an AC motor is spinning it will always have some amount of slip. Slip is when the motor is spinning slower than the rotating magnetic field. As you add more load the motor will draw more current because it has slowed down, reducing the back EMF being generated, and is trying to accelerate to the same speed as the rotating magnetic field.

My theory is that when you block the output of the centrifugal pump it is actually accelerating close to the speed of the rotating magnetic field. A hydraulic pump will be a positive displacement pump and would actually slow down/stall the motor, increasing current draw. With the types of pumps that are mostly being used in AP I don't think blocking the pump will do too much damage, but for a pump directly coupled to the motor this will very quickly kill the pump due to heat build up killing bearings etc.

Starting a centrifugal pump with the discharge blocked is actually a way of reducing the starting current.

So you are all right in at least some respects.

*Note* it has been a long time since it did AC motor theory and the above might be totally wrong.

PS the only way to control the speed of an AC motor is by changing the frequency. Loading it up will slow it down, but this will significantly reduce its life through heat and is a totally retarded way to try and control the speed of it.

Sent from my GT-I9300 using Tapatalk 2

That is exactly the way i remember it in mech engineering school, would scan the bookwork to back it up if i could be bothered finding it in our combined library of uni stuff thats right behind me.

Since I harped on about making use of eductors I thought that I had better include a pic of one of mine in use. It is a 0.5" model that I have connected to 19mm low density (LD) poly pipe stepped down from 25mm pipe which runs off a 2" irrigation main.
When I top up my tank with poorly oxygenated ground water I take the opportunity to blast it into the tank with as much air, froth and bubble that an eductor can muster. In this way the fish get additional oxygen, rather than less, and I generate some movement in the tank water in a way that my filtered, recirculating water can't achieve.
The floating plant is the native aquatic fern Azolla (Azolla pinnata).

Hey, you're right - Azolla is a native plant

I pulled a pile of it out in 2010 'cause Faye told me it was a noxious weed !

The BSF loved it though.

Yet another good use for Azolla - fly food!
Azolla spp. are native to several continents, apparently.

And helps fight global warming!

http://en.wikipedia.org/wiki/Azolla_event

As long as it is sequestered.

Interstingly there are two species of Azolla concidered native in Western Australia but they also occur elsewhere in the world.

http://florabase.dec.wa.gov.au/search/quick?q=azolla

Any reason that it can't be both native and a noxious weed?

Absolutely Simo. If it was introduced to somewhere it doesn't grow naturally in the right conditions it could become a weed.

One definition of a weed is 'any plant growing where you don't want it to grow'. Noxious weed, on the other hand, means the (state) government has 'declared' the species to be a pest. I have never heard of a native species being declared a noxious weed - it is kind of counter-intuitive - but maybe not out of the realms of possibility, especially with our changing environment.

So is it native to the southwest of WA, it maybe native to the tropical areas of the state and someone introduced it down here and now it is a noxious weed becaue it chokes the life out of any waterway it comes into contact with.

Like how the Kookaburra was introduced to WA and is now a pest out competing out native kingfisher species to near extiction.


EDIT looks like it is a southwest native http://florabase.dec.wa.gov.au/browse/profile/17737