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KarlJ
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| Posted: 07 February 2010 at 8:54pm | IP Logged
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15HP 3 phase above ground bore pump motor
here is the bill from the local electrical boys.
replace overload relay
replace contactor
unit tripped on first attempt to start but ran OK on all other attempts
Fault has to be with motor
suspect broken rotor ring.
Parts $791 labout $468 certificate of electrical safety $25
Total $1285 and not fixed.
what is the rotor ring?
is it repairable?
Edited by KarlJ on 07 February 2010 at 8:54pm
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Robb
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| Posted: 08 February 2010 at 2:28pm | IP Logged
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Is the pump one of these. Mono for example. They were common 20+ years ago with above ground electric motors or diesels. These days everyone seems to use submersibles down the bottom of the hole. Some even powered by diesel gen sets.
Sounds like the electrical guys are saying the load (pump or shaft assembly) may have a fault therefore stalling the motor.
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Downwind
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| Posted: 08 February 2010 at 5:14pm | IP Logged
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It had me stuffed what a rotor ring was so i had a search and found very little.
See if this helps.
* Squirrel-cage rotor
The most common rotor is a squirrel-cage rotor. It is made up of bars of either solid copper (most common) or aluminum that span the length of the rotor, and those solid copper or aluminium strips can be shorted or connected by a ring or some times not, i.e the rotor can be closed or semiclosed type. The rotor bars in squirrel-cage induction motors are not straight, but have some skew to reduce noise and harmonics.
................
The current practice for manufacturing copper rotor cages for large AC traction motors uses brazing for assembling the longitudinal copper rotor bars to the end rings. The end rings and copper bars which position the interior laminated steel core are assembled and held in a fixture for the joining operation. The difficulty in welding heavy section copper components is due to the high thermal conductivity of copper which conducts the heat away from the surface to be joined. First, the entire assembly and fixtures are placed into an oven and preheated to 650° F. After soaking to a uniform
temperature, the assembly is removed from the oven and the hand brazing operation is begun. After flux has been applied, each copper bar is brazed individually to the lower end ring. When all the bars have been brazed to one end ring, the assembly is inverted and the other ends of the bars are brazed to the other end ring using a hand torch. This brazing operation is costly and time consuming. In addition, the process is susceptible to producing defects and weak brazed joints since brazing the bars singularly produces uneven heating that generates thermal stresses.
I would think the island dweller might know more on this.
One question that comes to mind is what are the motor bearings like???
I have seen bad bearings cause starting problems.
Pete.
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oztules
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| Posted: 08 February 2010 at 8:14pm | IP Logged
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Glad I fix my own motors.
Induction motors. ... The stator acts like a primary in a transformer. It effectively induces an EMF in the secondaries. These secondaries are in fact the rotor bars. They are explained as in the above from Pete..
Now once we have induced this EMF in the secondary (bars of copper in the rotor), we short it out. This means all the EMF is lost in this piece of "wire" shorted at both ends by the ring that joins them up. Because we have shorted this EMF, we get a current flowing proportional to the EMF and resistance in the wire/bar. (I=E^2/R) This flowing current does what all currents do, and makes a magnetic field around itself... which will be in opposition to the inducing field.
So the resistance of the bars will have an effect on the stator current (not the induced EMF), and the slip and the start torque. A compressor duty motor will have very low R, so start currents are huge, but torque is large, and slip is smaller. If we use resistive bars, we can lower the start currents and torque, but the slip will be larger... centrifugal water pump duty... low torque start.
Now if we have one or more of the bars break it's connection with the shorting ring, then we lose that magnet circuit, and less current should flow in the stator from the remaining bars to reflect this "loss of short circuit), and the slip will increase....and the current may rise again to try and keep up speed but if load is greater than the motor torque can supply, we stop and huge current can then flow in the remaining bars,
Or if the start torque is not enough to get the rotor bars out of the way fast enough (stationary) then the current may trip the contactors instead from more current in the remaining bars.
If the start torque is small, losing a bar or two should not stop the operation, as it will start on the others. There are lots of them in there. you can see the "lines" for each spiral down the rotor.
If there has been significant movement for reasons I cant begin to guess, you may have broken a few or a lot of bars..... I have never seen this actually happen... so my predictions may be less then accurate, but I can't see why they would be.
As I recall it...
From start.....Once the rotor starts to move, the angle of the primary flux working on the secondary wires changes, and the induced EMF starts to drop as there is less magnetic lines cutting the rotor bar at the right time/angle... current drops to something sensible (E^2/R again)and we are running
At synchronous speed, EMF would be nil.... so that won't happen, and so the slip angle will be a reflection of the resistance in the bars, and the angle required to cut them and develop a back MMF that keeps the rotor running at this equilibrium.
At least thats how I remember it anyway.
Edit:
It seems a highly unlikely scenario. I have never seen evidence of or heard of an instance of rotor bars leaving the rings... although I recall in an old motor book there was a procedure for finding out using a growler I think.
It smells.....
...........oztules
Edited by oztules on 09 February 2010 at 4:26am
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GWatPE
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| Posted: 08 February 2010 at 9:24pm | IP Logged
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I doubt that an electrician would be able to determine if the squirrel cage rotor was damaged, broken bars in the squirrel cage, without dynamometer type testing, or other efficiency testing, or inspection and visible signs of patchy rotor overheating.
If the pump, was a "mono" type, then these have much higher motor start current requirements, to a multi stage centfifugal type. Especially if the pump has not been used for a while. I think the "mono" ones I was involved with "submersible bore hole type", back in the 1980's, required 3-5x the start current of a centrifugal. Once the pump had been run for a while, subsequent starts are a litte easier on the current required. I had a successful experience, designing an inverter that allowed, starting and running 1.8kW 415V 3phase mono type pumps, from Solar panels.
I don't recall any terminolgy of motor rings in any normal 3phase induction motors either.
15kW, is a decent size. I guess for irrigation. What was the SWL, or head required to be pumped? How many L/s?
Gordon.
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KarlJ
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| Posted: 08 February 2010 at 10:44pm | IP Logged
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mmmm
The sparkie is now somewhat annoyed with me for telling him that it was unacceptable to replace the overload relay with one that did not fit the original housing.
(the reset button now cannot be pressed without removing the cover)
I've told dad to stick up for himself and the sparkie to grow up and get over it, fix it properly, then we can get on and fix the pump!
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KarlJ
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| Posted: 09 February 2010 at 6:51pm | IP Logged
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BTW, the pump is at the bottom of the hole , connected by a shaft that runs in bearings down the 5"casing.
Used for dual purpose, both irrigation, runs a dozen or so sprays on the flat (that go pretty hard) or pump to the tank ontop of the hill which is 70M head + the distance down the hole which is at least 20m.
Pressure gauge reads 140psi when pumping to the top of the hill and about 30-40psi when irrigating.
Pump runs every night in summer, with a few extra hours on the off peak over the weekends to "catch up" with pumping tot he tank on the hill or irrigate the "green band" which is a dozen sprays in front of the house used as a fire break.
I've never seen 2" poly pipe like it (going up the hill) must be at least 8mm wall thickness, there was a time when the troughs on the flat were bursting pipes all the time as they did not realise everything at the bottom of the hill needed to deal with 140PSI. (which incidentally rips apart blueline pipe like a bannana!)
To make matters worse I priced up an 11KW contactor from my local ideal electrical (not by any means the cheapest out there) and the contactor -retail for me off the street was $320....
a fair difference to $577 these rogues charged us.
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KarlJ
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| Posted: 12 February 2010 at 1:06am | IP Logged
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WIN WIN WIN WIN
Sparkie returned my call today and apoligised for the bill
the excuse was that his new 18 year old girl doing the invoices was told the prices fr the parts and automatically added their "normal" margin.
Unfortunately the prices quoted to her already included the margin.
So he should be knocking off a few hundred bucks off that bill!
Pump however is still causing grief so i'll get it apart next week for a look see with some luck.
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isaiah
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| Posted: 17 February 2010 at 9:31am | IP Logged
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KarlJ
' '
Im having some trouble here Why do you need a bore motor at all?
You guys say you live downunder if so you should be able to just drill a hole up in the ground and the water should just run out. Like drilling a hole in the bottom of a water barrel. If you lived up here then you have to drill down and pump it up.''
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neil0mac
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| Posted: 17 February 2010 at 4:30pm | IP Logged
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Riiiight, Isaiah.
You'd get some belly laughs down 'ere if it wasn't for the gravity of the situation.
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Downwind
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| Posted: 17 February 2010 at 5:59pm | IP Logged
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isaiah
We use the pumps to keep the water in the ground not to get it out. 
If we didnt you side of the world would drain dry.
Pete.
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isaiah
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| Posted: 18 February 2010 at 1:13am | IP Logged
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Hey is that where the term sh*t UP comes from.
Do you guys use water or compressed air to flush your stools down there?
Sounds like it would be a real pain keeping that stuff in the ground .
Somebody said that you have to use a special stool also?
Thats probably why we get all that stuff in the air and ground when it rains up here.
Edited by isaiah on 18 February 2010 at 1:16am
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