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Thread: Burnt Pump Motor

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    DIY Member traveller's Avatar
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    Default Burnt Pump Motor

    I think I spoke a little bit about this submersible pump in another thread but, since uncovering more info, I have some new questions.

    Here are the basics. The well is a 55' drilled well in sand and gravel with a 6" casing. The water table is 17' below ground level, and only changes a few inches between summer and winter. Although no drawdown test was ever done on this well, other drilled wells close by, in the same gravel aquifer, have almost instant recharge and, therefore, little drawdown. A screen was installed at the bottom of this well. Also installed at the bottom of the well was a 3 phase 3 hp Pentek pump and motor, rated at 50 gpm. The intake of the pump is 51'4" below ground level.

    The pumphouse is 8' from the well and the water is delivered via a 2" black "Polytube". Inside the pumphouse is a Pentek Intelli-Drive connected via a pressure sensor. A 14 US gallon bladder tank is installed after the pressure sensor.

    The well was drilled just over a year ago and put into operation shortly afterwards. Luckily, there is a shallow well and jet pump in the pumphouse, as well, as the owner has had to rely on it most of the time. The well and large pump were intended to supply a large irrigation system and the owner's house. I spoke with the designer of the irrigation system and he says it is a "trickle" system with a few hundred 1/4" nozzles supplying water to many acres of saplings. He also says the system requires 35 gpm when fully open and, because there are so many nozzles, provides no backpressure to the pump at all. As the whole property is on level ground, I calculate that the pump, with the irrigation system wide open, is pumping against a 17' or 7 psi head. Is this correct? Or, with the intake at 51', is the pump working against a 51' head?

    Anyways, the pump has been shutting down on a steady basis since it was installed. The last time around, it showed "Ground Fault" and would not start at all.

    I pulled the pump from the well and was a little startled at what I saw. I was even more startled today when we cut the pump loose from the Polytube, and we were able to examine it more closely. On one side of the motor, on the upper half of the motor, the stainless steel casing is blue, as if someone took an oxy-acetylene torch to it. Our water contains a large amount of calcium, and the entire length of the pump on the same side as the blue metal has a 1/8" thick layer of precipitated calcium scale on it. I couldn't understand why no water came out of the Polytube, when I sawed it off the top of the pump. I soon found out why. The built in check valve at the top of the Pentek pump had gotten so hot, it had melted the plastic (rubber?) seat in the check valve. I was able to pour a gallon of water in through the check valve and watch it pour out the intake of the pump. And lastly, the hose clamps were melted into the Poly tube for about a 1" circumference of the Polytube.

    Well, needless to say, something got very hot here. The first thing I thought was the pump, with no backpressure, had run away and pumped many times its rated flow of 50 gpm and managed to pump the well dry. But, if this were true, why did only one side of the pump and motor overheat? A technician at Pentair thought it possible the pump was not hanging straight in the well casing, due to the natural curl of the Polytube, and one side of the motor was hard against the casing, preventing the flow of cooling water. After I finished reading his e-mail, I thought about the pump sitting at the bottom of the well, and wondered if the pump motor was not only up against the side of the casing, but sitting on the top of the screen rim, as well; further blocking cooling water to that side of the motor. The tech from Pentair felt the "Ground Fault" indication on the Intelli-Drive could be the windings having gotten so hot, they are shorting out to the motor casing.

    Anyways, the only solution I can see is to order a new pump and motor and to raise it up 10-15 feet from its present position, testing for drawdown after installation. I also thought of getting rid of the 2" Polytube downpipe and replacing it with 2" Schedule 80 PVC pipe to make the pump hang straight. Also, I have a 5" flow inducing shroud (sleeve in the USA?) I was thinking of installing over the motor, with centralizing pins to keep the motor in the centre of the shroud opening.

    The real question I have, and the one thing the tech from Pentair did not touch on, is this: With no backpressure from the irrigation system and the pump only working against a 17' head, should I install a 50 gpm (or smaller?) flow restrictor somewhere on the line to increase the head?
    Last edited by traveller; 09-11-2013 at 09:40 PM.

  2. #2
    Jack of all trades DonL's Avatar
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    I would verify that your 3 phases are properly phased.

    If not then excessive heat will occur.

    But if that was the case then 50 GPM would never be achieved, I would not think.


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  3. #3
    DIYer, not in the trades LLigetfa's Avatar
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    Default

    Quote Originally Posted by traveller View Post
    As the whole property is on level ground, I calculate that the pump, with the irrigation system wide open, is pumping against a 17' or 7 psi head. Is this correct?
    ...
    Also, I have a 5" flow inducing shroud (sleeve in the USA?) I was thinking of installing over the motor, with centralizing pins to keep the motor in the centre of the shroud opening.

    The real question I have, and the one thing the tech from Pentair did not touch on, is this: With no backpressure from the irrigation system and the pump only working against a 17' head, should I install a 50 gpm (or smaller?) flow restrictor somewhere on the line to increase the head?
    Yes, it has only a 17' head. The flow inducer sleeve will help cool the motor.

    I don't know VFD pumps and whether you can induce artificial head. Maybe valveman will weigh in on that.

  4. #4
    Moderator valveman's Avatar
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    There are so many possible problems I don’t know where to begin. Blue color on the side of the motor is heat from a lack of flow. It is also shorted to ground now, which is the reason it shows a ground fault code. It never should have been installed below the top of the screen without a shroud or flow inducer sleeve. But that is most likely not the cause of failure.

    One of the biggest problems with variable speed drives or VFD’s is that nobody knows how to figure the minimum possible speed. A 4” motor in 6” casing controlled by a VFD needs a minimum flow of 13 GPM to keep from overheating. With a 17’ water level and guessing at a pressure setting of 50 PSI, the minimum speed for that pump to produce 13 GPM is 77% of 3450 RPM, or 2657 RPM, which is a minimum of 46 Hz. If the minimum speed setting was anything less than 46 Hz, then that pump can’t produce the 13 GPM it needs to stay cool. My guess is someone just set the minimum speed to 40 or even 30 Hz, and the pump was running so slow it couldn’t pump any water and it melted down fairly quickly.

    Also the pulsing DC voltage from a VFD and the reflective wave from the length of wire in the well, causes the 230 volt motor to see spikes that can exceed 1000 volts. This can also cause heat in the motor.

    Then the harmonic content of the power created by a VFD can also add additional heat to the motor windings.

    Next, if the VFD is set to soft start the motor too slowly, less than 50% speed in 1 second, the thrust bearing in the motor is running dry and will drop the rotor and short out the motor.

    Lastly the system could have been set up with a very small pressure tank, and any small leaks would cause the pump to come on often enough to produce that kind of destructive heat.

    A 3HP, 50 GPM pump can produce 35 GPM at 80 PSI from 17’. So the pump should have been slowed down to produce 35 GPM at 50 PSI. But the 35 GPM nozzles are the restriction for a 50 GPM pump, and the pump should be running on at least 50 PSI. If the pump was unable to maintain 50 PSI while running the 35 GPM nozzles, then it is also possible that you could have been pumping the well dry, which would also overheat the motor. However, if it were set properly, the VFD should know the amps are too low and shut the pump off showing a Dry well condition fault or “low amps”.

    The main problem is that VFD’s are designed to make money for the manufacturers, which is the opposite of being a good thing for the end user. They are hard on the motor, expensive, short lived, not repairable, and since it supplies desperately needed water, must be replaced quickly after a failure. VFD’s are so complicated that nobody knows how to set them properly, which further shortens the life of the pump and system, and helps even more of your money to flow easily to the manufacturer as intended.

    The definition of insanity is doing the same thing over and over and expecting different results. So if you just replace that 3 phase motor, be expecting the same results. I hope you can get some warranty on that. But the warranty on most VFD’s has gone from 5 years, to 3 years, and now I hear it is only 1 year. I wonder why?

    You would be much better off with a standard single phase motor and control box. Then you could control it with a Cycle Stop Valve and a 44 gallon size tank and have a dependable, long lasting system. Without the CSV I would use about 3 large tanks (80 gallon size). Even though three tanks would still cause the pump to cycle, it would still be preferable to a VFD.

  5. #5
    Sprinkler Guy Wet_Boots's Avatar
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    To be honest, it sounds like the entire irrigation system needs to be trashed. Actual drip irrigation does give a pump something to push against, as there is usually at least 30 psi in the drip tubing.

    Anything that makes the existing system work will certainly be off the beaten track. I believe there are 'specialty valves' that can variably restrict the flow, and always give the pump something to push against. Figure a thousand bucks or so for whatever Cla-Val has to deal with the problem.

  6. #6
    DIY Senior Member VAWellDriller's Avatar
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    I agree with Valveman...the discoloration is caused from overheating, which is for sure due to being set and the bottom of the well. You should also do a drawdown test on the well so you know for sure what you have and where your pump really needs to be. Right now, you have a 4" pump, so you should install a 4" sleeve on it, not a 5" sleeve, if you are determined to set the pump in the screen. Also, if you're replacing everything as you should, and the flow and pressures required are as you described, a 2 HP can easily do this job, either Goulds 40GS20 or 55GS20 would be enough.....Also, drop the poly and plastic; for 50 gpm, hang it on 2 sticks of 2" steel pipe!

  7. #7
    DIY Senior Member craigpump's Avatar
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    Although I have installed a few VFD's, I don't anymore. Too much aggravation, the boxes are too expensive and try getting a box on Christmas or New Years Eve......

    Last I knew, Franklin Sub Drive boxes had a 3 yr warranty while the motors had a 5 yr warranty.

    Imagine telling Joe homeowner that his Sub Drive box is blown and it's a $1000.00+ to replace it?

  8. #8
    Sprinkler Guy Wet_Boots's Avatar
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    The description of the system makes it sound like a disaster in the making. Now, if it were 1/4-inch OD on tubes feeding plants, with emitters at the ends, that could make sense. Actual 1/4-inch nozzles in the hundreds would flow so freely that there would be no load whatsoever for a pump to push against beyond the 2-inch pipe itself. You could water it by gravity draining a tall water tank.

  9. #9
    DIY Member traveller's Avatar
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    Quote Originally Posted by Wet_Boots View Post
    The description of the system makes it sound like a disaster in the making. Now, if it were 1/4-inch OD on tubes feeding plants, with emitters at the ends, that could make sense. Actual 1/4-inch nozzles in the hundreds would flow so freely that there would be no load whatsoever for a pump to push against beyond the 2-inch pipe itself. You could water it by gravity draining a tall water tank.
    I was a little shocked, too. I spoke to the fellow that designed and installed this computerized irrigation system, and he assured me that not only did it require 35 gpm, it also provided no back pressure whatsoever to the pump. This meant that the 17 foot head was only supplying 7 psi back pressure to the pump.

    I was ordering a new pump and motor this afternoon and the tech there recommended installing a gate valve in the pump house with a pressure gauge upstream of it. He then said to open up the irrigation system and every tap in the two houses connected to this well and, when the pressure drops to a seriously low level, begin closing the gate valve until 30 psi is registering on the gauge. He then recommended removing the handle from the gate valve to keep anyone from trying to adjust it.

    He said the gate valve would likely wear in a few years but was, after all, cheap to replace or rebuild.

  10. #10
    Moderator valveman's Avatar
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    It is not possible for that pump to be at 0 PSI when putting out only 35 GPM. With only 17’ of head on the pump, it would be producing about 75 GPM. Either your drip system is putting out 75 GPM, the pump isn’t as large as you think, or there is more pressure on the pump than you think.

    Adding a gate valve or a Cla-Val type pressure sustain valve to hold back some pressure on the pump is going to starve the irrigation system. If the irrigation system is letting out 75 GPM or even 50 GPM, and you further restrict the flow with a valve, now only part of the irrigation system is getting water. Water will take the path of least resistance, so the lower parts of the irrigation line will get water and any lines slightly higher will not be getting water.

    The drip system needs to be nozzled to let out only 35 GPM, or at the most 50 GPM. And when nozzled to produce 50 GPM, that pump is still going to produce 60 PSI.

  11. #11
    Jack of all trades DonL's Avatar
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    How can a pump moving that much water Over heat and burn up ?

    It sounds to me that it may have took a lightning strike. Over Pumped or power phase is wrong.

    I would just install a 2 HP pump and a CSV, for that size tank, that would make sense.

    And be a lot cheaper in the long run.


    I am no expert, I just want to be a plumbers helper helper.


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    Last edited by DonL; 09-13-2013 at 07:57 AM. Reason: Added Helper that needs help
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    DIY Member traveller's Avatar
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    Quote Originally Posted by valveman View Post
    It is not possible for that pump to be at 0 PSI when putting out only 35 GPM. With only 17’ of head on the pump, it would be producing about 75 GPM. Either your drip system is putting out 75 GPM, the pump isn’t as large as you think, or there is more pressure on the pump than you think.

    Adding a gate valve or a Cla-Val type pressure sustain valve to hold back some pressure on the pump is going to starve the irrigation system. If the irrigation system is letting out 75 GPM or even 50 GPM, and you further restrict the flow with a valve, now only part of the irrigation system is getting water. Water will take the path of least resistance, so the lower parts of the irrigation line will get water and any lines slightly higher will not be getting water.

    The drip system needs to be nozzled to let out only 35 GPM, or at the most 50 GPM. And when nozzled to produce 50 GPM, that pump is still going to produce 60 PSI.
    Hello Valveman

    Thank you for the reply. Unfortunately, I do not think the customer will change any part of the irrigation system to provide a source of back pressure. I believe he paid quite a bit of money for it and is very impressed with the fellow who designed and installed it. And I do not hold the designer at fault, either. He lives in a large urban area where his systems are typically fed by municipal water and there is no reason to consider the problems we are facing.

    That being said, what do you think of a 2" Dole Flow Valve (Restrictor?) set at 50 gpm (the rated flow of the pump)? Would this not have the same effect as nozzling the drip irrigation system, and still let the system have the maximum flow of 50 gpm?
    Last edited by traveller; 09-13-2013 at 10:01 AM.

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    DIY Senior Member BobL43's Avatar
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    Quote Originally Posted by DonL View Post
    How can a pump moving that much water Over heat and burn up ?

    It sounds to me that it may have took a lightning strike. Over Pumped or power phase is wrong.

    I would just install a 2 HP pump and a CSV, for that size tank, that would make sense.

    And be a lot cheaper in the long run.


    I am no expert, I just want to be a plumbers helper helper.


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    Have a Great Friday the 13th.
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    Jack of all trades DonL's Avatar
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    Quote Originally Posted by traveller View Post
    Hello Valveman

    Thank you for the reply. Unfortunately, I do not think the customer will change any part of the irrigation system to provide a source of back pressure. I believe he paid quite a bit of money for it and is very impressed with the fellow who designed and installed it. And I do not hold the designer at fault, either. He lives in a large urban area where his systems are typically fed by municipal water and there is no reason to consider the problems we are facing.

    That being said, what do you think of a 2" Dole Flow Valve set at 50 gpm (the rated flow of the pump)? Would this not have the same effect as nuzzling the drip irrigation system, and still let the system have the maximum flow of 50 gpm?

    Sorry for clutter in your thread.

    But the customer was happy with something costing that much and it only lasted a year ?

    I think I would be changing the design, because that track record sucks.


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    Moderator valveman's Avatar
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    Maybe it is pumping 75 GPM and pumping the well dry. If the well only makes 35 or 50 GPM, and you are pumping 75 GPM, it will start sucking air, lose prime, and usually melt the plastic impellers in the pump and/or burn up the motor.

    This is more in line with the helper I see.


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