Control logic of high pressure water pumping system

[SillyBoy]

Dear Team,

I need help to develop a control logic for high pressure water pump cascading system, this system will be used in hotel water supply system.

There are total 3 Nos of Pumps and Panel consist 1 VFD.

 

All pumps are connected to two parallel power contactors. One contactor gets direct power and one contactor got VFD output power.

All pumps output is connected to common supply pipe which consist 1 Pressure transmitter.

 

VFD-------Contactor K1--------PUMP1

MCB------Contactor K2--------PUMP1

VFD-------Contactor K3--------PUMP2

MCB------Contactor K4--------PUMP2

VFD-------Contactor K5--------PUMP3

 

 

Now come to logic part.

- When the auto cycle starts, Contactor K1 and VFD are turned on. So pump 1 runs on vfd power and try to maintain the set pressure using PID.

-If the pressure does not reach the set value in desired time, the contactor K1 will off, then contactor K2 will be turned on instantly. So the pump1 will be now work on full speed mode.

-Now contactor K3 will be turned on, and pump 2 starts on VFD to maintain the set pressure using PID.

--If the pressure does not reach the set value in desired time, the contactor K3 will off, then contactor K4 will be turned on instantly. So the pump1 and pump2 will be now work on full speed mode.

-Now contactor K5 will be turned on, and pump 3 starts on VFD to maintain the set pressure using PID.

 

If the pressure increases then the set point and VFD runs on 0 speed for set time, the reverse sequence will work.

- If pressure remain high for some time and VFD at 0 speed then, contactor K5 and K4 will be off and contactor K3 will on. So pump 3 will stop and pump 2 will run on VFD to maintain pressure.

 

This is basically a cascading system. Which should work on both direction.

 

I am pretty clear about how the logic will be done, but I am not sure about how to manage the acceleration and de-acceleration time during switchover.

If I am starting a new pump, VFD should start from 0Hz and if I am stopping the running pump and switchover to last pump then VFD should be moved with 50 Frequency.

 

Please help if anyone have experience of such system. 

[SillyBoy]

@panic mode

@Michael Walsh

@BobLfoot

Dear Admin,

Can you please give any suggestions from your experience ?

[BE]

I will preface/caveat my comments with a couple of points:

• What you are trying to do is use Bypass Contactors on VSD driven motors. I have never seen this done, although a Google Search indicates that it is used in some applications.
• Consult with the VSD manufacturer before implementing anything, to ensure that your arrangement will not cause any issues with the VSD
• I have not consulted with my VSD reps to see if this is possible, and if it was something I wanted to implement, I would be consulting with them BEFORE implementing or designing anything.
• I personally can't see any reason to run something this way, unless there are concerns around the heat dissipation from the VSD's. It would be simpler to leave the pumps running at 50hz from the VSD.

With the VSD's I typically use, they will 'ramp up' (or ramp down) even if there is no load connected. So the physical motor isolator could be off in the field, but the VSD will still "Ramp Up" to the setpoint. When the Isolator is switched on, the motor starts at that setpoint. This isn't something I typically do, but it is occasionally a necessity during commissioning and maintenance operations. Based on that, if your motor was running on the mains contactor (ie. Direct to mains power), I would think a switchover would simply require you to ramp up the corresponding VSD to 50hz, and then switch the mains contactor off, and switch the VSD contactor on. At this point, you can ramp the motor down to the speed required.

Again, and I can't stress this enough, consult with the VSD manufacturer. Motor control is what they do for a living, and they are going to be able to give you the specific guidance and requirements for their VSD's that are required to implement what you want to do.

[Joe E.]

In my experience, most VFDs don't like having 480V applied to their output terminals. Like, "let the smoke out" don't like. I've seen soft starters with bypass contactors, either internal or external depending on size, but not VFDs. Keep that in mind while you're designing your logical and physical interlocks.

What's the reasoning behind switching from VFD to across-the-line? Is it an efficiency thing where you want the VFD's losses to be eliminated while running at line speed?

Reading your sketch, each pump motor has 2 contactors feeding it. One contactor is supplied with line power, the other with the output of a VFD. In that case, taking pump 1 as an example, with contactors K1 and K2, I would use a mechanically interlocked contactor, which is basically 2 contactors side-by-side with a mechanical latch making sure only one can be closed at a time. They're generally sold as reversing contactors, but you obviously wouldn't use it to reverse the pump; it would instead select between the 2 sources. Note: You would NOT want to do this while the VFD was supplying power! VFDs generally don't like having load side disconnects or contactors opened while energized. To do a "live" switch, you would need to configure the drive for coast-to-stop, then stop the drive, de-energize K1, then energize K2. To transition back from K2 to K1 could be even trickier. When I used a PowerFlex 525 to power a flywheel press, I had to enable "flying restart", which imposes a delay of a few seconds before the drive starts supplying power to the motor. You would have to turn off K2, turn on K1, then enable the drive and let it match itself to the motor's speed.

Frankly, I don't think that's all worth the effort and risk. Just let the drive run at full speed. Definitely take @BE's advice to contact the drive's manufacturer for advice. This isn't really a typical use that I've seen in person or in any manuals.

[Prochot]

I have done several parallel pumping systems for pressure maintenance and agree with the other posters about not using bypass contactors. If you want to lock pumps at 100%, it is better to use the equivalent of software manual or an output hold in the PID. I have designed a couple systems that work the way you describe in that when the running drives are maxed, they lock at 100% speed and a new pump is started to modulate and maintain pressure, but I have found that it is more efficient, and safer, to allow all pumps to modulate with a PID at all times. This helps to prevent over-pressurizing the system if demand drops suddenly. Also note that depending on the system pressure, your pumps potentially won't actually start moving liquid until they are at 50% - 70% speed, so look at your pump curves and do some testing to set the minimum speed (or feed forward) in the PID just under where your pumps actually come up to pressure.

[BobLfoot]

My Suggestion for revision:

When the auto cycle starts, Pump 1 runs on vfd power and try to maintain the set pressure using PID.

-If the pressure does not reach the set value in desired time, then contactor K3 will be turned on instantly. So the pump 2 will be now work on full speed mode.

-Pump 1 still on VFD will try to maintain the set pressure using PID.

--If the pressure does not reach the set value in desired time, then contactor K5 will be turned on instantly. So the pump2 and pump3 will be now work on full speed mode.

-Now Pump 1 still on VFD will try to maintain the set pressure using PID.

 

If the pressure increases then the set point and VFD runs on Low (20hz)  speed for set time, the reverse sequence will work.

- If pressure remain high for some time and VFD at Low (20hz) speed then, contactor K5 will be off. So pump 3 will stop and pump 1 will run on VFD with Pump 2 to maintain pressure.

- If pressure remain high for some time and VFD at Low (20hz) speed then, contactor K3 will be off. So pump 3 and pump 2 will stop and pump 1 will run on VFD to maintain pressure.

 

This avoids the dangers of bypass contactors and the fact that most VFD do not like their output interrupted.

[BE]

5 hours ago, BobLfoot said:

My Suggestion for revision:

Providing they can get away with the DOL starting of the pumps (or maybe use soft starters), this is a good idea. Only downside would be if the VSD ever failed, there would be no redundancy with the other 2 pumps to regulate pressure (ie. they are either 0% or 100%). Putting 2 pumps on VSD's would alleviate that, but if that level of redundancy was required it would probably just be easier to run all 3 on VSD's 🙂.

[BobLfoot]

On 4/27/2025 at 6:09 PM, BobLfoot said:

My Suggestion for revision:

When the auto cycle starts, Pump 1 runs on vfd power and try to maintain the set pressure using PID.

-If the pressure does not reach the set value in desired time, then contactor K3 will be turned on instantly. So the pump 2 will be now work on full speed mode.

-Pump 1 still on VFD will try to maintain the set pressure using PID.

--If the pressure does not reach the set value in desired time, then contactor K5 will be turned on instantly. So the pump2 and pump3 will be now work on full speed mode.

-Now Pump 1 still on VFD will try to maintain the set pressure using PID.

 

If the pressure increases then the set point and VFD runs on Low (20hz)  speed for set time, the reverse sequence will work.

- If pressure remain high for some time and VFD at Low (20hz) speed then, contactor K5 will be off. So pump 3 will stop and pump 1 will run on VFD with Pump 2 to maintain pressure.

- If pressure remain high for some time and VFD at Low (20hz) speed then, contactor K3 will be off. So pump 3 and pump 2 will stop and pump 1 will run on VFD to maintain pressure.

 

This avoids the dangers of bypass contactors and the fact that most VFD do not like their output interrupted.

Couple of comments as I reflected on this.  

1) Each time a full speed pump kicks in you will want to return pump 1 to minimum speed with feed forward and fairly quickly to avoid a major over pressure event.

2) The converse will be true as well pump 1 will want to "jump" from min to max each time a full speed pump is turned off.