VFD installers: Mix & match brands? Use 1-phase motor? Test well pumping level?

[Pawpaw]

Hello again,

Here are some questions for the VFD installers out there:
1) Are you hesitant to mix & match the brands of a motor & VFD?
2) Are you hesitant to install a VFD designed to control a single-phase, 230V, 3-wire motor?
3) Do you test the well's pumping level in order to set the VFD's minimum frequency?

Details:
I have a pump short-cycling issue that's discussed in another thread (link). I am (slowly) assessing some of my options, including installing a variable frequency drive (VFD) or changing the pump. The purpose of this thread is not to debate those choices (I hope that happens on my earlier thread, once I update it with a better assessment of my present choices).

I let someone else request quotes for installing a VFD on a single-phase pump, and I'm getting the impression that the well folks prefer to a) match VFD & motor brand, and b) install 3-phase motors with VFDs. At this moment, I anticipate that brand matching may be important for warranty compliance, and/or pre-programmed avoidance of resonant frequencies. Are those realistic concerns, are there other concerns, or is the brand matching mostly a vendor lock-in technique? Admittedly, I have no experience with VFDs, but am I wrong to believe that brand-matching is not practiced with the industrial VFDs in widespread use?

In the realm of submersible well pumps, is there any advantage to a system using a three-phase motor and appropriate VFD, compared to a system using a single-phase motor and appropriate VFD? Have VFDs for single-phase motors developed a bad reputation?

Lastly, is it best to know the well’s pumping level so that the VFD’s minimum speed can be set to avoid zero-flow spin? Is it common just to sacrifice some low end frequency and program for static lift from the depth of the pump? Without a flow meter, is there another way the VFD would avoid zero-flow spin?

Thanks in advance.

 

[Valveman]

I don't sell Cycle Stop Valves because I don't know anything about VFD's. Just the opposite, I studied Electrical Engineering and was building and programming VFD's in the late 80's and early 90's. The CSV came about as a dependable and inexpensive way to get constant pressure without the all the problems that go with a VFD.

The whole purpose of the new single phase VFD's is so you don't have to pull the pump and replace the single phase motor with a three phase motor. If you have to pull the pump, simply replace it with a regular single phase pump sized to not build too much back pressure, then the CSV would be a better choice than a VFD.

From the age of your pump, even though it is a Goulds, it probably has a Franklin motor. Using one of the Franklin single phase VFD's would be the easy thing to do. None of the single phase VFD's have been around long enough to have a track record, but they are not likely to last or work any better than the three phase VFD's.

Again, you are way ahead of most installers by even asking about about the water level and talking about spinning at zero flow. But you are exactly right! The water level and pressure required should be tested to know the minimum hertz that will still pump water, and the VFD controls need to be set accordingly. However, most installers don't have a clue what this means or how to figure it. So, most of the domestic size VFD's are set up to work without proper setting.

For instance the Pentek PID is very popular. However, it doesn't even work like a VFD, more like a pressure switch with a soft starter. To keep from having to test and set the minimum speed for each application, they simply turn off the pump after it has run for 1 minute at any given speed. The system is then being fed by the pressure tank until the pressure drops and ramps the pump up again. So, the pump runs for a minute and shuts off, then restarts again, over and over. The only difference in this and a regular pressure switch system is the VFD gives a slow ramp up or a soft start, which the company claims will let the pump survive many more starts than usual. However, this is not necessarily true. Soft start of a submersible motor is not good, as the thrust bearing has no lubrication until the pump is up to at least 50% speed. The longer the motor takes to get to 50% speed, the more wear on the thrust bearing.

I believe most of the newer single phase VFD's do something similar. Which is to just shut the pump down every once in a while, see if the pressure falls, then it knows you are still using water and ramps up the motor again. Otherwise yes, you need to test for the minimum speed that will still produce water and set the VFD to shut down at that minimum frequency after a short time. Boyce and VA may know how to do that, but the vast majority of pump installers have no clue about any of that.

I believe the Pentek Intellidrive and the Yaskawa VFD are adjustable in that way. But they are expensive and need to be set precisely to work with each system differently.

I would suggest any of the single phase drives so you don't have to change to a three phase motor. If the VFD hastens the demise of your existing pump, you can then switch to a pump that builds less back pressure and go back to using a CSV, which as you said worked well until it failed from the extra high pressure pump.

 

[Sponsor]

 

[Valveman]

Reading back I see your pump is only about 3 years old. That would mean it has a Goulds motor on it not a Franklin. But it is three wire so I don't know that any particular brand of VFD would be required.

 

[Pawpaw]

Here are some questions for the VFD installers out there:

I don't sell Cycle Stop Valves because I don't know anything about VFD's.

Sorry Valveman, it was not my intention to exclude you or others. Thanks for the response.

The whole purpose of the new single phase VFD's is so you don't have to pull the pump and replace the single phase motor with a three phase motor.

I agree, that is why I'm surprised to be getting suggestions for a three-phase system when requesting quotes for single-phase stuff. I'm trying to figure out if there are good reasons for that from a consumers perspective.

I believe most of the newer single phase VFD's do something similar. Which is to just shut the pump down every once in a while, see if the pressure falls, then it knows you are still using water and ramps up the motor again.

Everyone at this residence is well acquainted with city water pressure regulated internally. Nonetheless, the present 50/70 PSI control has not bothered anyone. I see no reason to pay dearly for a soft-starting "constant-pressure" control system that has it's own short-cycling issues. I will absolutely be on the lookout for that.

Did I understand you correctly that while some VFDs stop the motor to check for flow, other VFDs intentionally slow down into no-flow conditions, and then promptly spin back up if flow is determined? Perhaps the latter is not too bad for the motor & pump if done quickly? Then again, the hunting behavior does feel like a hack, and it would have to be done rather frequently to avoid a long period of unintentional operation at no-flow.

Reading back I see your pump is only about 3 years old. That would mean it has a Goulds motor on it not a Franklin. But it is three wire so I don't know that any particular brand of VFD would be required.

Yes, I expect it is a CentriPro motor. Are you implying that brand matching may be needed when running a 2-wire motor, but not for 3-wire (single-phase) motors?

 

[Valveman]

Yes I think brand matching is important on 2 wire motors as Franklin uses a biac switch where Pentair and Goulds use a spilt capacitor type motor. But I don't think three wire would be important to match the brand except that the manufacturer would like to sell both.

To set up a VFD properly the math or testing would need to be done to figure the minimum speed the particular pump could spin from the specific depth to water and deliver 50 PSI as needed. Then you set this minimum frequency for the correct minimum speed and the VFD goes to sleep as they say. Then "sleep mode" will let you set the restart for 10 PSI or so lower than 50 PSI. However, every pump is different, every well has a different water level, and some houses want more or less than 50 PSI. This means every VFD needs to be set according to the specifics of each individual pump system. I can probably count on one hand the number of pump installers who would even know how to do that.

Setting the minimum speed too high would cause the pump to cycle on and off at low flow. Set the minimum speed too low and the pump would never shut off until it melted down. So, most of the domestic VFD's don't even work like a VFD. They just turn the pump on until it hits a high pressure setting or stays at any given speed for 60 seconds, and then shuts the pump off. Then because water is still being used and they have a very small pressure tank, the pump comes back on in just a few seconds. Some like the Franklin Subdrive turn the pump on and off 45 times a minute to see if you are still using water. This kind of pressure hunting is a "hack" for not having to know how a VFD works. If they could only sell VFD's to the people who know how they work, I could count all their customers on one hand. Lol!

Everyone wants you to switch to a three phase motor as they do work much better with a VFD, and the single phase are very new and still unknown. I believe the single phase VFD's are just another "hack" to keep you from having to pull the pump and replace the single phase motor with a three phase one.

Again, if you have to pull the pump just put one back that doesn't have more back pressure than a CSV can handle, and you can save yourself a lot of trouble not having to use a VFD at all.

 

[Pawpaw]

Everyone wants you to switch to a three phase motor as they do work much better with a VFD

Would you please expound on that?

 

[Valveman]

Well other than the obvious which is varying the voltage and frequency will have a negative impact on the torque available from the single phase motor, there are other concerns. Usually high amperage at full flow is the biggest problem. I also see all these VFD's use the "bump and test" method to see if you are still using water. They will just regularly bump the pressure higher and turn off the pump. If the pressure drops they know you are still using water and start the pump again. If the pressure stays up they go to sleep or turn off the pump. Like I said this is really a hack to keep from having to figure the minimum speed for each pump system. Anyway you look at it these VFD's are still cycling the pump on and off over and over. Here is an article I found interesting as there is not much available yet, which should also tell you something.

https://www.wolfautomation.com/blog/vfds-for-single-phase-motors/

 

[Pawpaw]

Sorry for the long silence

I've gotten more details about the quotes. Only one service tech thought that brand matching of motor & VFD is needed in my situation, others just left open the possibility that other situations might require it. That sounds good for my situation, but still, if resonance is a practical concern in this situation, it's not clear how a VFD would avoid problematic speeds without advance knowledge.

I had seen that Wolf Automation page, and it sounded like the single-phase VFD market was pretty sparse in 2016. This time through, I noticed that it seems to consider "capacitor start" & "any motor with separate starting winding" as incompatible with a single phase VFD. Thus my single-phase, 3-wire CentriPro M10412 attached to a Franklin 2801084915 (capacitor start, induction run (CSIR)) box would seem to be excluded. Is the Wolf Automation info limited in scope or inaccurate?

varying the voltage and frequency will have a negative impact on the torque available from the single phase motor

Doesn't the torque required by a centrifugal pump drop quite rapidly as the speed decreases? At very low speeds, I can imagine there are some practical limits to just how soft a soft-start is. But are you saying that adding a VFD to a single-phase motor intended to run a given pump may lead to insufficient torque conditions above, say, half speed?

I was perhaps a bit too rough calling the hunting behavior a hack, though I still don't consider it a satisfying control solution. Insofar as time is money, making a VFD install easier & faster is likely to reduce upfront cost to the consumer, but it'd certainly be a trade-off if the VFD causes excessive pump cycling. A number of well techs thought I'd be unhappy trying to use a VFD to reduce pump cycling; they seemed to consider "constant-pressure" as the primary goal of a VFD. When I inquired about configuring a VFD to be a solution to pump short-cycling, there seemed to be less confidence and experience (if any) doing so.

I think I'd want a VFD setup to use the same 20 PSI differential (50-70 PSI) that I'm using now, so that the existing pressure tank can help minimize pump cycles. That alone seemed like it could be a hurdle for two reasons. First, VFDs seemingly get installed with tiny pressure tanks. If the pressure is buffered by a non-tiny pressure tank, then any VFD that detects flow by measuring pressure decline will be more apt to erroneously decide that there is no flow and turn off. Secondly, some of these VFDs come with pressure "sensors" which, if I'm not mistaken, are merely switches with a "constant pressure" differential. To operate at a 20 PSI differential, one might need to buy the "transducer" upgrade. A transducer might allow the VFD to have smoother control behavior, but without a flow sensor or properly set minimum speed, I'd think the no flow detection challenges would remain.

 

[Valveman]

The Monodrive will work with 3 wire single phase motors. Yo can also use a regular 40/60 pressure switch with the Monodrive instead of the 2 PSI differential switch. The Monodrive is basically just a soft starter, as normally just bounces the pump on and off with only 2 PSI differential. Using a regular 40/60 switch will greatly reduce the cycles. But it will still cycle more than 100 times per day even with a huge tank the same way you described the regular 40/60 pressure switch systems. All you are getting with the Monodrive is a soft start, which can be done by just sizing the wire to use the longest length of the smallest wire possible to get a reduced voltage soft start with a standard pump and controller.