Problem with several Pleuger Submersable Motors

[sparkview]

Hi all,

We currently are conducting a Root Cause Analysis on a Pleuger Submersible motor witch soul function is to pump well water (salt water) to a header where other similar Submersible motor are connected. Here is the story:
On August 9th, 2015 at 11:39 PM the panel operator noticed that Beach Well pump stopped for no apparent reason. It was noticed that a trip code indicates Fault code 17 (input phase loss). Prior the fault the Beach well pump was running at a normal rate with no process change. Trend graphs indicate that the Amperage was running at a continues rate between 98-102A and then suddenly went to 110A at the time incident. Trend graphs also indicate that the well-water level and flow were at an acceptable rate. Initial motor measurement indicates a shorted lead to ground which triggered a complete disassembly.
On disassembly of the motor it was noted that the rotor contained excessive damage and "flash"(see attached picture). As described below the motor runs in potable water which was tested during disassembly. Lab results indicate Salt water intrusion most probably from the well-water. Additionally, high turbidity and high iron content is also noted.

Your opinion and or observation of what could of caused such rotor damage will be highly appreciated. Many thanks for your support.

Submersible motor info.
The so-called “wet” electric motor is a water-filled three-phase AC squirrel-cage motor with a watertight winding, which is operating in water and is designed especially for direct drive of submersible pumps. The motor filling-water cools the winding and bearings as well as lubricating the thrust and radial bearings.

Stator winding
The stator winding consists of winding wire provided with special insulation. The power cables are connected to the winding by a special watertight splice and leave the motor through stuffing boxes.

Rotor
The rotor winding consists of induction welded copper bars with short-circuit rings. The rotors are dynamically balanced and are protected against corrosion by a protective varnish. The drive shaft end is made of stainless steel.

 

[jraef]

With nothing else to go by other than the pictures, it looks like a voltage stress event, as if the stator winding insulation failed and arced into the rotor, burning through its insulation. Hard to tell though.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[sparkview]

Thanks for the reply jraef. I have accumulated additional information. Motor is connected to a VFD an uses and output reactor. I have also attached the trend graph indicating Amps, Flow and Level. (Pump no. 120)

Graph can be viewed larger in the attachment.
1. Initial motor measurement indicates a shorted lead to ground.
2. Trend graphs indicate that the Amperage was running at continues rate between 98‐102A and then suddenly
went to 110A at time of incident.
3. Trend graphs also indicates that the well‐water level and flow where at an acceptable rate.
4. Voltage and frequency data are not recorded.
5. Visual inspection indicate no power cable damage.
6. Visual inspection indicate Pump/ motor shaft moves freely and does not show blockage.
7. Cable‐ties have been noted trapped between rotating and stationary parts (refer to the attachment, page 2).
These cable‐ties does not seem to cause a major restriction on the rotation of the pump/ motor shaft. Cable ties
are used to attach the power cables to the pump and piping, however they seem to break loose.
8. A lab analysis has been performed on the motor filling‐water composition (see attachment for lab results
page 1). The results indicate the following:
a. Salt water intrusion is noted most probably from the well‐water.
b. High turbidity and high iron content is also noted.
9. Motor filling‐water level has been measured at about 4.7 gallons (according to the Operation & Maintenance
document, this is sufficient).
After pump/ motor dis-assembly
10. Mechanical seal seems to be intact with marginal wear (refer to attachment, page 3).
11. Outer physical appearance of the motor indicates a “flash” pressure has been released true the vent valve
and at the bottom section (refer to the attachment, page 4).
12. Rotor indicates excessive damage with concentration on the outer sections (refer to the attachment, page 5
and 6).
13. Bearing clearance has been measured, however adequate clearance specification where unable to be
acquired in the “Operation and Maintenance documents”.

 

[zlatkodo]

At first glance, the cause could be a bearing.
Can you attach some photos showing the stator demage?

Winding Design and Repair

 

[sparkview]

Thanks for the reply zlatkodo. The funny part is that the stator does not appear to have sustained extensive damage compared to the rotor. See pictures of the stator below (note that winding is dirty due to the arc flash of the rotor and well water entering the motor). Furthermore, recent findings indicate that at the winding connection leads the "rubber tape" appears to be pressed against the motor housing. Additionally, where leads have been taped it appears to stick together, most likely due to heat. See pictures below.

 

[sparkview]

Any idea somebody?

 

[waross]

Three suggestions:
1: A voltage event as jraef suggests. Check the reactors for shorting. A shorted reactor May allow high frequency, high voltage harmonics to reach the motor.
2: Insulation damage by salt water incursion.
3: Rubbing damage from wearing bearings.
This leads to several questions.
1> Are the reactors in good condition? Is there any indication of high frequency high voltages on the motor leads?
2> How did the salt water get in the motor? Over a long period of time? Over a short period of time? As a result of the arcing damage?
(Subsequent to the expulsion due to the fault, there may be a fairly high vacuum developed in the housing as the vapors condensed. This may have overcome the seals and allowed the incursion of the salt water.)
3> Bearing wear. Are the shiny spots on the rotor due to rotor displacement due to worn bearings? In the absence of specs, you may want to slide the rotor into the stator and measure the gap between them. Bearing play should be considerable less than half of this value.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[X49]

I have come across submersible motors that were required to have dv/dt or even sine-wave filters installed when operated with a VFD, according to the manufacturer. Load reactors were apparently not sufficient. These motors had an insulation rating of only 850V for a 575V (rms) application.

That being said, I can't say whether or not that damage was caused by voltage transients.

 

[sparkview]

Hi all,

As a follow-up on the issue, me and my colleague decided to do a complete autopsy of the motor and determine where exactly and how severe was the shorted lead. After completely disassembling the motor and completely removing the stator winding, we found
one winding wire completely open and exposed copper true the isolation (see pictures below). A second defective winding wire were found showing signs of burnt isolation but no exposed
copper (refer to pictures below). I am not sure what could of caused such damage but do suspect sea water (incl. solid particles) entered the motor through the open mechanical seal. These particles got between the winding wires and created a hot spot which caused the burned isolation at two points (and caused the ground to earth fault). If any of fellow formers have additional suggestions or opinions please feel free to advise. Many thanks for your support.

Jairo

 

[waross]

I saw similar damage to an underground conductor in a wet location. The cause may have been a pinhole in the insulation.
I suspect the insulation may have been punctured by a voltage transient.
The intrusion of sea water would hasten the failure.

Bill
--------------------
"Why not the best?"
Jimmy Carter