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#181092 - 09/22/08 05:27 PM Help with irrigation control circuit design  
LoneGunman  Offline
Joined: Oct 2005
Posts: 141
I've spent about 8 hours of my own time trying to figure this out and for some reason I'm not getting it.

We have a customer that has several very large orange groves. The groves irrigation is supplied by a system of canals and pumps. We recently just got this account.

I do most of the control work for the company I work for so I got sent out to figure out why the auto setting on their motor control is not working. The system consists of 2 75hp motors driving two pumps, the system can be controlled manually or automatically. The way it WAS controlled automatically is in my opinion extremly hazardous so I will not put it back that way. The way it was controlled is as follows:

Four wires run from the control panel to the irrigation canal, they are only using three. Two of the wires carry 120 volts and one wire is the control wire going back to the control panel, these wires are placed in a 6" conduit placed vertically in the canal. The control wire has a large split bolt attatched to it, the two other wires are connected to these electrode looking things, they are using the water within the conduit to make the connection from the two hot wires to the signal wire. Obviously all of this stuff was home made, it's been this way for 30 years. The 120v starter is not energized unless both 120v leads are in the water, this threw me off so I put 120v to one wire and tested what I had coming back, it was 57v, due to voltage drop and the resistance of the water, this is why there are two 120v wires out there.

While the system is very hazardous in my opinion I also thought it was pretty ingenius

My problem is I need a way to energize the 120V coil on the starter when the water is low, deenergize when the water hits a certain level and not reenergize until the water is at a level between the low and high water sensors.

It's done this way so when the pump kicks off from hitting the high water it doesn't cycle on and off due to water turbulence ETC. I need to keep this as low tech as possable. I have found small float switches from Grainger but I am having major problems trying to figure out how not to kick the pumps back on until the water reaches a level lower than the high water level.

I also need to do the same thing on the feeder canal but this is to shut the pumps off in case of a low water situation so the pumps don't run dry and burn up.

I keep drawing it out with three floats and three relays but I'm just not getting it.

Tools for Electricians:

#181100 - 09/22/08 07:26 PM Re: Help with irrigation control circuit design [Re: LoneGunman]  
LarryC  Offline
Joined: Jul 2004
Posts: 783
Winchester, NH, US
Try a single float switch and a Time Delay Relay set for however much time that is needed for the water to settle down.

What is the difference in height between low level and high level?

Mount the TDR in the control cabinet.

Larry C

#181101 - 09/22/08 07:36 PM Re: Help with irrigation control circuit design [Re: LarryC]  
LoneGunman  Offline
Joined: Oct 2005
Posts: 141
Thanks for the reply Larry. I thought of the timed relay but being it's a canal, I have no way of knowing what the downstream usage is at that particular time or the near future. The low water is a few feet off the bottom of the canal, high water is four feet or so above that and I need the pumps to come back on again between the low and high, two feet or so lower than the high water.

#181104 - 09/22/08 09:04 PM Re: Help with irrigation control circuit design [Re: LoneGunman]  
gfretwell  Offline

Joined: Jul 2004
Posts: 9,069
If these are the float switches I am thinking about they have quite a bit of hysteresis. Once tripped on the water level has to drop several inches to get them to turn off or vice versa depending on whether you get the "over" or "under" switch.
I have 3 in my aerator tank on my well water system. You can change the hysteresis by making the cord a little longer or shorter from the pivot point.

Greg Fretwell

#181105 - 09/22/08 09:18 PM Re: Help with irrigation control circuit design [Re: gfretwell]  
LoneGunman  Offline
Joined: Oct 2005
Posts: 141
Greg, Thanks for the reply, I'm familar with the float switches you are talking about, the problem is as I said above, I only have a 6" conduit so corded float switches are a no go, not enough room for them.

I think I may be able to use latching relays to accomplish what I want, trying to draw it out now.

#181108 - 09/22/08 11:39 PM Re: Help with irrigation control circuit design [Re: LoneGunman]  
gfretwell  Offline

Joined: Jul 2004
Posts: 9,069
You could hold the relay through a N/O point that is in series with an intermediate level switch and pick it with the high level switch

Greg Fretwell

#181109 - 09/23/08 12:21 AM Re: Help with irrigation control circuit design [Re: gfretwell]  
LoneGunman  Offline
Joined: Oct 2005
Posts: 141
Totally lost me on that one, please explain. Are you talking about using a latching relay or regular relay?

#181110 - 09/23/08 12:42 AM Re: Help with irrigation control circuit design [Re: LoneGunman]  
Scott35  Offline

Broom Pusher and
Joined: Oct 2000
Posts: 2,707
Anaheim, CA. USA
Lone Gunman;

The most simplest and "Low Tech" approach to your Control logic is to set up Two (2) Float Switches as a typical "Start / Stop" Motor Control Circuit.

The "Start" Switch would be the Low Water Float Switch.
The "Stop" Switch would be the High Water Float Switch.

These floats would drive the coil of the Motor Starter directly, or indirectly through a Control Relay if needed.

This approach will eliminate the Pump from surging on/off, due to instable high Water levels.

Brief Details of the Control Logic:

The "Low Water" Float Switch is a Normally Closed switch.
When the Water level is at the "Low" set point, this float's contacts will close, and therefore energize the Motor Starter's Coil.

The "Low Water" switch is shunted by the latching contact (as would be found on a typical 3 wire Start / Stop control circuit), so the Coil remains energized as the Water level rises above the "Low" limit point.
This, in turn, opens the contacts on the "Low Water" Float, when the Water rises.

The "High Water" Float Switch is also a Normally Closed Contact switch.
This contact is in series with the "Low Water" contacts + the Latching Contact - as would be normal for the "Stop" Switch on a 3 Wire control circuit.

Until the Water reaches the preset "High Water" level, the float's contacts remain closed.

When the float is activated by the High Water Limit point, the conatcts open, resulting in the Motor Starter's Coil dropping out + opening the Latching Contact; thereby effectively de-energizing the Motor until the Water once again reaches the "Low" level point.

Only Two float switches are required. Each Float only needs one N.C. contact.

If you need a drawing, let me know.


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

#181114 - 09/23/08 01:49 AM Re: Help with irrigation control circuit design [Re: Scott35]  
gfretwell  Offline

Joined: Jul 2004
Posts: 9,069
Assume you want the pump to come on when the water is high.

I am talking about 2 switches N/O make on water rise.

you have a relay with a second N/O contact (might be a side switch on a big contactor)
Wire the upper water switch to the coil
Also wire the lower water switch to the coil but in series with the N/O side switch.
Water rises, bottom switch makes, nothing happens yet because contactor N/O is still open. Then upper switch makes, that makes the contactor. N/O switch is made. Water starts to fall. Top switch opens but the contactor is still held up by lower switch through the N/O. When lower switch opens the contactor drops out.

Greg Fretwell

#181117 - 09/23/08 08:33 AM Re: Help with irrigation control circuit design [Re: gfretwell]  
KJay  Offline
Joined: Nov 2007
Posts: 763
What about maybe adapting a 3-electrode type mV control that is often used for cooling tower make up water?
They normally have one common electrode, one to open the fill valve and one to close it off after the level is reached. You would need a good ground for this system to work and would also have to either cut off the electrodes at the height you needed or could try using the existing ones.
A contactor to control the pump motor would be needed since these aren’t really designed to handle motor loads. Baltimore Aircoil or Warwick Controls should still have these available.

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