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Looking for pointers re; control box build


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#21 klangers

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Posted 24 May 2017 - 11:59 AM

There's no point using the relays I mentioned if your PIDs are running on mains power. The whole point of a control voltage is to minimise the number of components that use mains voltage. So NOTHING should be 240V apart from:

  • Main isolating switch
  • Elements and their SSR
  • Pumps
  • And the load-side of the relays that power the above

Everything else should be done in low voltage. You need a transformer to make this low voltage. It's kind of all or nothing - as I said, it may be easier to just keep it simple and not worry about relays. There is absolutely no point in using mains power as a control voltage for relays (unless your load voltage is >20kV).

 

This is really really hard to describe with words. A diagram would be better.

 

Good that you have access to an electrical engineer, although switchboard design is a far cry from robotics. I'm a manufacturing engineer; I have some formal uni training in automation/robotics and electrical engineering so I get by, but it's not my forte.


Edited by klangers, 24 May 2017 - 12:00 PM.


#22 Zorco

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Posted 24 May 2017 - 12:25 PM

Only a tiny addition I think.

'Everything else should be done in Extra Low Voltage'.

PELV or SELV,

http://www.snowyhydr...ow Voltage (ELV).PDF

#23 philistine

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Posted 24 May 2017 - 02:42 PM

Ok, so that clears some of it up. I think.
But now im back at a previous point of confusion - that being The amp rating of the main on/off switch.

Its frustrating because i dont even really know how to ask the right questions..... im getting this overwhelming sense of having bitten off more than i can chew......

Im a horticulturist. I understand plants.

Anyway, im sort of using a few different sources as references for my own build.
The main one being "the electric brewery" site followed by a build documented as an "instructable" by a guy who wanted to do a dumbed down cheaper version of the electric brewery build.
Both of those builds use 10a rated switch components (for all of the switching requirements) and yet the amperage those builds are sucking down is huge. Way more than mine!

So, Going back to just the main on/off switch, the only difference i can see in my build vs. theirs is that they include relays.
Does the use of a relay somehow "buffer" or in a sense "bypass" the switch component itself from the excessive power being drawn by the system? From what i can tell, they're using 10a switch components but theyre running 15-30a loads.
So i guess in a way, im trying to figure out - when set up with a relay, is the main current load that passes through the system when in use basicaly going straight through the relay with the "on/off" switch being used solely to trigger the relay itself?
In other words, the switch itself is not actually handling the load..... is that right?

Im sorry if we're going round in circles! I just dont really know who else to ask these things to be honest!

I just really want to understand and would hate to "throw in the towell" because i cant wrap my head around some basic circuitry!

#24 klangers

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Posted 24 May 2017 - 03:04 PM

It helps knowing your background, as I can try and draw comparisons to something you find intuitive.

 

A fairly good analogy of electricity is water flowing through pipes. The water is the electricity, and the pipes (and valves etc) are the wires and components in the circuit. The analogy degrades fairly quickly, but works very well for basic stuff. I suggest you try and draw up a circuit diagram and post up. Don't even worry about active vs neutral, just do a single line diagram.

 

Voltage is then analogous to water pressure; current to flow rate. A switch is analogous to a valve.

 

If you have a valve (switch) that's not rated to the water (electrical) pressure (voltage), it'll burst spectacularly (arc, catch fire and burn your house down). If you have a valve that's not big enough for the flow rate, when it closes it'll hammer like crazy and physically damage itself. Switches that attempt to close an excessive current can arc and burn out, or even weld the contacts together.

 

Similarly, if you have lots of sprinklers branching off a main, then the main needs to be sized to the worst-case but still possible scenario. The largest load that's possible to occur, and it depends on how you operate the system:

  • All sprinklers on at once - add all the loads together (big main pipeline; max running load = connected load)
  • Only one sprinkler at once - the largest single sprinkler (littler main pipeline; max running load << connected load)

Electrically, the proportion of the "connected load" compared to the actual load is known as the "diversity factor". If we all designed every electrical switchboard to be rated to the connected load, we'd have no money and no copper left on earth. We design to the worst-case that's actually possible.

 

I can't speak for the electric brewery, but it's possible that they have determined that only a proportion of their connected load is on at any one time, and thus at any one time they cannot draw more than 10A.

 

If, however, they are using 10A switches with a 30A load then that's bad bad bad. 

 

I think you've got the idea of the relays - the relay does the heavy switching, and the switch turns on the relay with a little control voltage.


Edited by klangers, 24 May 2017 - 03:05 PM.


#25 philistine

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Posted 24 May 2017 - 03:33 PM

Ok awesome- its all falling into place, thank you!

Well, in that case, it makes sense that i either need to install at least 1 relay (that being at the very start, the mains on/off) OR i need to replace the switch component itself to something rated for 15A.
I'll get round to attempting a diagram of sorts and see how it looks to experienced eyes.
Then try and work out which is the better and more cost effective way to get over this final hurdle.

Thanks HEAPS dude!

#26 Camo6

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Posted 24 May 2017 - 09:04 PM

I'd recommend you make use of relays or preferably contactors (generally higher current ratings) for a few reasons.

 

One reason is for safety - if you run an ELV (ie 24v) control circuit you reduce the hazard of having LV touch potential in a switch likely to be handled in a wet area. Even using contactors with 230V coils will eliminate the presence of high currents in the control circuit.

 

Another reason is to reduce cost - this is more of a cost saving exercise practised in industry. Relays/contactors allow a load to be switched by a much lighter load therefore allowing the use of lighter gauge copper and switch gear at the control location. Reduces cost, space and demand.

 

Then there is functionality and reliability to consider. Controlling a heavy load by switching the active conductor will create an arc which in turn results in a carbon build up on switching contacts. This can quickly lead to component failure. While a contactor isn't immune to these effects, it is specifically designed to switch loads. As the coil on a contactor is of negligible load, control components can comfortably handle repeated switching without suffering the effects of arcing.

 

Lastly, there is a certain satisfaction in hearing the 'thunk' of contactors kicking in when you throw the switch.



#27 klangers

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Posted 25 May 2017 - 07:20 AM

I'd recommend you make use of relays or preferably contactors (generally higher current ratings) for a few reasons.

 

Lastly, there is a certain satisfaction in hearing the 'thunk' of contactors kicking in when you throw the switch.

 

Yeah my terminology is a little "flexible"... probably should say contactors cause that's what I mean rather than a "relay" as such. 

 

And indeed that thunk is very satisfying.



#28 mofox1

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Posted 25 May 2017 - 09:24 AM

And indeed that thunk is very satisfying.


Easily the best part of my brewery.

#29 philistine

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Posted 27 May 2017 - 08:07 PM

Thanks again guys, it's starting to make much more sense now.
So I guess my next questions are:

Are either of these two options suitable?
https://www.jaycar.c...-relay/p/SY4044
 

Link from another thread - so I know its suitable


What's the difference between the two? Is one better suited than the other?
I found the first one by searching for "240v relay' on the jaycar website, the other I found refferred to as a contactor in a similar discussion.



#30 lmullan

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Posted 27 May 2017 - 09:40 PM

That jaycar one looks reasonable. The important parts are that the coil can be switched using 240VAC which is handy. If you read through that thread that you linked to, they discuss using 24VDC to operate the coil, while this makes it safer (lower voltage flowing through the buttons and switches that the user touches) it does mean you need a power supply with a 24VDC output which adds complexity!

In short as far as I can see reading the data sheet from jaycar, that one should be fine. Often the term relay and contactor are interchangeable, a contactor being a sub category of relay.

#31 philistine

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Posted 27 May 2017 - 10:01 PM

cheers.
In hindsight I probably would have designed it to use a 24v control voltage if I knew what I know now, but unfortunately I've already bought 240v powered PID's (and one of them from auber $$$)