control circuit design

[Scott K]

I am trying to design a control circuit to start a generator to power an irrigation pump in the following sequence.

1. start generator on water level drop. allow generator to run 3-4 mins to warm up.
2. start irrigation pump.
3. stop pump on water level rise.
4. allow generator to run 3-4 mins to cool down then shut off.
5. reset for next sequence.

The irrigation pump is existing and has a magnetic starter with a 120 volt coil controlled by an existing float switch to maintain water level.
The start circuit for the generator is 12v DC.

I think this can be done with a couple of time delay relays. I have a "True off" TD relay with 12v input to stop the generator and a "on delay" TD relay with 120v input to start the pump. Am I on the right track?

Any advice or suggestions would be greatly appreciated.

Thanks in advance!

 

[ptonsparky]

The trick will be to release the "starter"of the gen when it starts. . Looks you dont have a cold weather problem so that should help. Do you have to choke it?

 

[steve66]

I'd say you are on the right track. I'm assuming you have a commercial generator that will take care of the starter and things like the choke automatically. So I'm assuming it will run when you give it a signal and stop when you take that signal away.

Once your water level drops, you probably want to latch a relay so waves don't cause the generator to start and stop several times. Use that relay to start the generator, and start a time delay for the pump to run.

That should get you started if you are familiar with ladder logic at all.

 

[cadpoint]

What is the source of power that is going run the sensor to signal tank level?

Or even if mechanical tank sensor, what's going to be the source to signal out?

Don't they have some of these diagrams in the back of "UGLY" Book?

:dunce: Don't mind me a box of rocks here!!!

 

[Jraef]

Actually, you will have several challenges here.
1) You need to be able to sense the water level when there is no power from the generator yet.
2) You must somehow know that the generator is running and putting out full voltage before attempting to start the pump.
3) Your Engine Start signal must be momentary, but also repeatable in case it doesn't start immediately, then you must decide on how many attempts to make.
4) What will your response strategy be if the engine fails to start after those attempts are exhausted?

Since your only source of power is the 12VDC battery that will start the engine, I suggest starting off with a 12VDC powered "Smart Relay". That can give you the input from the float switch to tell the engine to start, the ability to do a "one shot" output as the engine start signal (with an adjustable time frame for the Start), the ability to see if it DID start, attempt a set number of re-starts (based on battery capacity), get an input from a voltage relay that tells you when the generator is at full power, time the engine warm-up period, close an output to turn on and off the pump starter, and time an engine cool-down period. Plus, if you want to, you can have it turn on a light or auto-dialer or something that tells you it failed.

Or... use an engine powered pump and forgo the generator altogether.

 

[Scott K]

control circuit design

I'd say you are on the right track. I'm assuming you have a commercial generator that will take care of the starter and things like the choke automatically. So I'm assuming it will run when you give it a signal and stop when you take that signal away.

Once your water level drops, you probably want to latch a relay so waves don't cause the generator to start and stop several times. Use that relay to start the generator, and start a time delay for the pump to run.

That should get you started if you are familiar with ladder logic at all.

Yes. You are correct about the generator, it has a hand-off auto selector switch and I have identified the auto start circuit. There are no waves to worry about as the float switch is positioned in what the farmers call a rim ditch which is a huge ditch that runs around the entire farm. The water level drops and rises very slowly.

Unfortunately I am not familiar with ladder logic...I think I'm in over my head. :ashamed1:

 

[Smart $]

...

Unfortunately I am not familiar with ladder logic...I think I'm in over my head. :ashamed1:

Don't let the terminology scare you. Can you interpret a typical control schematic?

 

[under8ed]

You seem to have it mostly sorted out. I am giving you the benefit of the doubt, you seem to have collected parts that I can see working as you planned. The original float can complete the 12 volt relay circuit, (starting wired though closed contacts on the low voltage relay). Once running, the 120 output will power up the "on delay" relay to complete the magnetic starter circuit to the pump as timed. Your 12 volt relay also needs to have a set of closed contacts at this state to complete the 120 pump starter circuit. When the float opens the 12 volt portion, the "off delay" relay starts timing. Depending on the relay, you may need one more low voltage relay to drop your 120 pump control voltage path at this point, but allow the generator to run until it times out. I would say you are on the right track if you have the proper relays with the correct contacts available. All of the replies you have had have been thoughtful, and in your best interest. These are professional electricians and engineers that have been trained to consider all angles, (it will not surprise me if fault is found in my logic here, as I didn't even draw a diagram). If others concur this will work, I feel you will not be over your head if you understand how the relays operate.

 

[steve66]

Yes. You are correct about the generator, it has a hand-off auto selector switch and I have identified the auto start circuit. There are no waves to worry about as the float switch is positioned in what the farmers call a rim ditch which is a huge ditch that runs around the entire farm. The water level drops and rises very slowly.

Unfortunately I am not familiar with ladder logic...I think I'm in over my head. :ashamed1:

Look up "ladder logic" on Wikipedia, or other web sites, and they will give you a primer.

Most generator transfers switches have a generator cool-down delay. But I'm assuming you are somewhere without utility power, and you don't have a transfer switch.

 

[just the cowboy]

Also try PLCTALK.net

Also try PLCTALK.net

If you make an antempt to figure out the program and post it on PLCTALK.NET allot of them guys willhelp you write it as long as you try. Smart relays are not as hard as you think

 

[Jraef]

Look up "ladder logic" on Wikipedia, or other web sites, and they will give you a primer.

Most generator transfers switches have a generator cool-down delay. But I'm assuming you are somewhere without utility power, and you don't have a transfer switch.

Is there a transfer switch? I didn't see that.

 

[Smart $]

Is there a transfer switch? I didn't see that.

I don't think so.

I believe he was just saying if there there was one, such as in a BUG (backup generator) scenario, he wouldn't need the off delay relay/function.

 

[Smart $]

Here's a basic circuit diagram that I think will work using discrete components.

 

[steve66]

Here's a basic circuit diagram that I think will work using discrete components.

That would be OK if the float switch has some deadband in it.

But otherwise, the pump will just cycle on and off as the water level bounces just above and below the on/off level.

It really needs a latch in the first rung. And that would require and another time delay to turn off the latch, or a higher level float switch to turn it off.

 

[Smart $]

That would be OK if the float switch has some deadband in it.

But otherwise, the pump will just cycle on and off as the water level bounces just above and below the on/off level.

It really needs a latch in the first rung. And that would require and another time delay to turn off the latch, or a higher level float switch to turn it off.

I considered that, and you are correct about having a dead band (FWIW, I've never run across one that don't)... but you also have to remember the pump is only started by an on-delay relay. The float switch has to stay closed long enough for the on delay to kick in. The pump will not cycle if the float switch is closed for a period less than the on delay. Every time the float switch opens, the on-delay timing is reset.

 

[Cow]

Here's a basic circuit diagram that I think will work using discrete components.

What program did you use to make that diagram Smart?

 

[Smart $]

What program did you use to make that diagram Smart?

http://www.canvasx.com/en/products/canvas-15

It's not a circuit diagramming program... basically an illustration program, sort of like Photoshop and Illustrator, and a few other non-Adobe programs, rolled into one app'.

 

[Cow]

http://www.canvasx.com/en/products/canvas-15

It's not a circuit diagramming program... basically an illustration program, sort of like Photoshop and Illustrator, and a few other non-Adobe programs, rolled into one app'.

Oh, looks good.

 

[petersonra]

Here's a basic circuit diagram that I think will work using discrete components.

I would look a lot closer.

No feedback to tell if the genset is on before turning on the pump.

no restart try if the genset does not start.

I am not sure what is meant by a "true" off delay, but I am suspicious that it won't work as desired.

 

[GoldDigger]

I am not sure what is meant by a "true" off delay, but I am suspicious that it won't work as desired.

To me "true" off delay means that the mechanism restarts the off time count each time the coil is energized even momentarily.
One example of a relay that does NOT meet this criterion is a bimetal with heater.
If the coil is on and is then turned off, the contacts will open after the delay. If the coil is energized, but not for a long enough time to heat the bimetal to full temperature, it will then open in less than the specified delay when the coil is de-energized for the second time.
A dashpot relay could satisfy this requirement if it had a valve so that the coil energizing would reset the dashpot position immediately (or close enough for practical purposes.)