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Discussion Starter · #1 ·
Fellas, give a little back ground info on what I have. I am putting in a slurry pump attached to a 600 gallon or so tank (catches water from our wash plant). The slurry pump will be attached to a VFD. I will try to get the flow rate correct with the VFD, but at times when aggregate flow changes, so will the rate at which water fills the tank. I need to have an auto off at low water level, and an auto on at high level water in the tank (to the VFD). I would have no problem doing this with out the VFD, but I am a bit confused with the VFD. Trying to figure out the best way to do this. Any info helps greatly! Thanks, A.J.
 

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To maintain the tank level between two levels would only require the drive to be wired in "3 wire" control mode- the same as if it were a starter, with with a greater than low level closing the "stop" input and a high level switch closing the "run/start" command momentarily. The speed reference, in this scenario, would be a drive preset, based on trial and error, to determine the pump speed that would work for the application.

If you wanted to maintain a certain level, you would need some sort of analog transmitter wired to the drive to vary the motor/pump speed based on level or weight, for the drive speed reference.
 

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I'd probably go with a level transmitter, rather than a float switch. The level transmitter, in a tank application, would typically be a pressure transducer installed low in the tank. Analog input from the level transmitter to the VFD.
 
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To maintain the tank level between two levels would only require the drive to be wired in "3 wire" control mode- the same as if it were a starter, with with a greater than low level closing the "stop" input and a high level switch closing the "run/start" command momentarily. The speed reference, in this scenario, would be a drive preset, based on trial and error, to determine the pump speed that would work for the application.

If you wanted to maintain a certain level, you would need some sort of analog transmitter wired to the drive to vary the motor/pump speed based on level or weight, for the drive speed reference.
I've done similar but used the floats to fire ice cube relays with a separate control power source. Then it's just a two wire input for the drive. I almost wired them the way you mentioned but I was afraid if I used the drives 24v directly, one day the floats may get damaged and I didn't want to take a chance it would short the drives 24v control power.
 

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If all you have is two float switches, you are wasting your money on a VFD. Just run it across the line.

If you want a variable resonse because of the variable flow coming off the wash plant, do like Varmint said. Use a level transmitter as an analog signal to a PID loop in the VFD and set up a Set Point of the level to maintain. I've done this dozens of times on wash plant recovery tanks, it works great. Program your set point to be at the minimum level at which your pump does NOT suck air with a vortex. Then you might have to tweak the gain on the PID loop to make sure it keeps up with the maximum rate at which the walk plant flow can increase, which generally is not that fast. You will not likely need the D function in the PID loop, so don't worry about it.

A couple of words of advice.

If you are not very familiar with using PID loops for control, get a drive that has very simple to use setup procedures and a good text display. I recommend an AB PF7x family or an ABB ACS 550, both make that really easy to implement. The cheaper the drive, the harder it will be. And stay AWAY from Schneider and Siemens drives for this, they are the WORST for setting up PID loops in my experience, even for seasoned vets.

Use an ultrasonic level transmitter, it's the most reliable for this. Make a still well out of a piece of 4" drain pipe and aim the transmitter into it to prevent having the turbulence screw you up. Good programmers can do it without the still well, but it's too much of a PITA if you ask me just to avoid a $10 piece of pipe and a couple of brackets.

Lastly, make sure your pump is capable of pumping the maximum volume at the necessary head to keep up with the maximum flow of the wash plant. Trust me, the pump guys screw this up a LOT and will ask you to "tweak the VFD output to make it pump more", which if anyone should know will not work, it should be them. But they often get excited about looking like they will save money for their user by selling them a smaller pump. The worst one I encountered was when the PID loop was pegged at maximum drive speed, the tank was still gushing over and washing away the base. I was blamed for the VFD programming, yet the motor was at full speed! Turned out they had yanked out a 400HP pump and replaced it with a 100Hp pump per the pump suppliers recommendation, then I just got the contract to supply and install the VFD. I pointed out the folly of thinking a 100HP pump could keep up, the pump guy insisted that I over speed his pump, that would take care of it. If you don't know, centrifugal pumps use power at the CUBE of the speed change, whereas the motor HP only goes up by the same ratio as the speed change. So if you run a pump at just 20% over speed, the pump power required goes up to 172% of the original motor rating, but yor motor is only developing 120% power, so you end up overloading it by 52%.... So to do the work of a 400HP pump, I would have had to run that small pump at something like 120Hz, which would have needed a 400HP motor. I had them put the old pump and motor back in and used a 400HP VFD, which only used the HP it needed at low flow but had the maximum capacity if they needed it. They sued the pump supplier over the additional costs, who settled out of court...
 

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Discussion Starter · #6 ·
Wow thanks for all the help fella's. Every one of the responses helped. I don't need a constant level on the tank, just an emergency shut off/pump start. I should be able to tweak the VFD frequency to get it close enough where the pump won't turn off often at all.
So an easy way/cheap way to do it would be use the 24 volt out of the VFD through a side mount float http://www.gemssensors.com/Products...eered-Plastic/LS-7-Series-Type-5-Level-Switch on the top of the tank to the start side of the VFD. And then the similar float at the bottom to the stop side?
I have one more question. I will be using a very small dosing pump that will administer flocculant into the slurry pump outflow. If I would like this unit to shut off when the other VFD does, could I just Jump a line from the Start of one VFD to the start of the Other and same with the Stop? Thanks again for all the help. -AJ
 

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I have one more question. I will be using a very small dosing pump that will administer flocculant into the slurry pump outflow. If I would like this unit to shut off when the other VFD does, could I just Jump a line from the Start of one VFD to the start of the Other and same with the Stop? Thanks again for all the help. -AJ
Use the programmable relays in the drive and drive a separate relay to stop/start other pumps as needed...Mixing controls between 2 or more drives without relays won't work...
 

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Why not use two floats and have the top float run at a higher speed? Maybe I'm not understanding the application.
 

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Discussion Starter · #10 ·
Use the programmable relays in the drive and drive a separate relay to stop/start other pumps as needed...Mixing controls between 2 or more drives without relays won't work...
Also what I meant was if another VFD receives a 24 volt monetary current, won't it start too? Meaning you have a 24 volt current going out to the float switch. When the switch closes, it sends the 24 volts back. If that line were spliced and sent one 24 volt to one VFD and one to another, it wouldn't start both?
 

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Discussion Starter · #12 ·
http://www.tesensors.com/us/products/hyde-park-sc-sm-vc-vm-xx
these are great, super easy to program both levels, should be perfect for your application.
Any suggestions as to which model I would need? We are dealing with about a 4 foot high to low level. I'm new to this kind of stuff so my apologies for the ignorance. I am guessing these units have a 24 volt power supply then when the water is in the high level, it closes the switch and sends 24 volts to one (Start side of VFD) and when it hits low level it closes the switch and sends 24 volts to another line (Stop of VFD). Thanks again A.J.
 

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You'll have to do some research and find the right sensor for your application, you can do that best since its your project, just make sure it can handle the environment its in. Also, you can run multiple pumps of the same drive, just make sure the drive HP is a little more than all the pumps combined. Signal to drive enable and signal to start, should be home free. Keep us updated.
 

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I've done similar but used the floats to fire ice cube relays with a separate control power source. Then it's just a two wire input for the drive. I almost wired them the way you mentioned but I was afraid if I used the drives 24v directly, one day the floats may get damaged and I didn't want to take a chance it would short the drives 24v control power.
While using the drive's built-in control power would work, I never do it that way. I always use the field device to switch a relay, and run the drive control through that. It adds one more point of failure, but it also isolates the drive I/O in case something down the line spazzes out. If a little ice cube relay blows out it's quick and easy to replace.
 

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Also what I meant was if another VFD receives a 24 volt monetary current, won't it start too? Meaning you have a 24 volt current going out to the float switch. When the switch closes, it sends the 24 volts back. If that line were spliced and sent one 24 volt to one VFD and one to another, it wouldn't start both?
I would have a separate power supply and run that through the float switch contact. That would operate a 2 or 3-pole relay or whatever, then you could route all of your drive start/stop inputs through the relay.

Your drive should also have relay output options that you can configure to actuate open or closed whenever your drive starts or stops and so forth. Use that to drive a relay, run a control circuit for the dosing pump through that.

You can wire a lot of that stuff directly to the drive, and it will probably work, I just prefer to keep the drive wiring close.
 

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Discussion Starter · #16 ·
While using the drive's built-in control power would work, I never do it that way. I always use the field device to switch a relay, and run the drive control through that. It adds one more point of failure, but it also isolates the drive I/O in case something down the line spazzes out. If a little ice cube relay blows out it's quick and easy to replace.
Thanks again guys. I picked up a ultrasonic pump out sensor. Basically when the water level is past the Near point, it closes a switch to one line. When the water level is pumped out and drops past the far point, it opens another line. This setup would effectively run my VFD as is.

After hearing multiple suggestions of ice cube relays, I think I will go this route. Again, a new territory for me and I have never dealt with the actual correct wiring of an Ice cube relay. Could you possibly give me a little wiring advice on this. Sorry for my ignorance and this is greatly appreciated (I have a different profession title each day, but it allows me to always know how to fix things when they go wrong. From our concrete plant to trucks to aggregate plant, I have managed to save a lot of money and grow great knowledge doing it this way). Thanks again! A.J.
 

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After hearing multiple suggestions of ice cube relays, I think I will go this route. Again, a new territory for me and I have never dealt with the actual correct wiring of an Ice cube relay. Could you possibly give me a little wiring advice on this. Sorry for my ignorance and this is greatly appreciated (I have a different profession title each day, but it allows me to always know how to fix things when they go wrong. From our concrete plant to trucks to aggregate plant, I have managed to save a lot of money and grow great knowledge doing it this way). Thanks again! A.J.
Each relay will come with a wiring diagram on the side of it.. 2 words of advice.. Don't go with the cheapest one on the market - you get what you pay for... Also don't find a "great deal" on one and end up with the only one in the county.. Find something that is common in your plant/supply houses...
 

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If you really want to make this a fun learning project, build a panel, add a Micrologix1000 plc, if you have RSlogix software, else i think AB makes a low end plc that comes with free software. Add a couple of motor starters in your panel and wire the coils to your plc outputs, your sensor signals to your inputs. This way you can add or modify in the future, no need to use any 60's relay technology. Maybe JRaef will get you an employee discount if you ask him nicely.
 

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Thanks again guys. I picked up a ultrasonic pump out sensor. Basically when the water level is past the Near point, it closes a switch to one line. When the water level is pumped out and drops past the far point, it opens another line. This setup would effectively run my VFD as is.
...
I still don't get the need for a VFD here.
 
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If you really want to make this a fun learning project, build a panel, add a Micrologix1000 plc, if you have RSlogix software, else i think AB makes a low end plc that comes with free software. Add a couple of motor starters in your panel and wire the coils to your plc outputs, your sensor signals to your inputs. This way you can add or modify in the future, no need to use any 60's relay technology. Maybe JRaef will get you an employee discount if you ask him nicely.
ab plc are the most expensive plc, just get a click plc or teco smart relay they are some models under 100$ all with free software
 
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