here's the story, the plumbers in the building maint. department say there is a clog in the 8inch pipe down the line from the discharge.well when the pump runs,after about 30mins, it heats up and the amp reading are 6.4amps on each phase. the nameplate on the motor is 9.5amps on a 480volt connection.
question, if there was a clog on the pipe wouldn't the amps be higher than 9.5amp and than trip the overloads?
The fall off in amps indicate that the pump is not working, and that something is blocking it. When a pump works, the motor draws amps to do the work. If you know the power factor, you can calculate the work the pump is doing, and compare that with the published pump curve to make an evaluation. Your data tells me this pump is way back up (to the left of) its curve.
The heat also indicates that the flow is stopped, and the electrical energy required to turn the pump is heating up the fluid, possibly flashing it. The amps verify this.
It is when the pump is actually flowing a lot of fluid that the amps go up, and you worry about tripping the overloads.
Your conditions, lacking other information, say blockage. Buy the plumbers lunch.
Is the pump actually delivering any water at all - can you check this point, if it is only delivering partial flow then you can probably assume that there is a blockage either in the pipework or in the impeller itself or the impeller is badly worn, BUT partial flow will not usually result in the pump "heating up".
If no flow, you can assume there is a blockage in the pipework or impeller vanes or the pump is running in wrong direction, also check that the pump is not airlocked, being airlocked can cause the fluid to increase in temp., and as the pump is doing work on whatever water is in the pump casing this will show on your amp meter as a pump load.
Before doing anything, check that the pump is running in the right direction, it wouldn't be the first time that the sparkies have done some work on power supply / cabling or the control box and phases have been switched.
International College
Naresuan University
Phitsanulok
Thailand
twins,
As Artisi and rmw have suggested, the low amps and high temperatures indicate lack of flow through the pump.
If you have a control or isolation valve in the discharge line, then close that with the pump still running and see what happens to the motor amps. If readings are much the same with the valve closed then you have confirmed that you have a blockage somewhere (if there is flow then you will likely hear this in the valve as it is being closed, the amps should drop off further, and you will still need to find the partial blockage).
A second pump running in parallel can "dead head" its mate, but there is nothing in your post to suggest that is the problem here.
I agree with all of the above comments by Artisi, rmw and JohnGP.
I think that the easiest way for you to check the pump performance is to obtain a copy of the manufacturers Head-Capacity-Efficiency-BHP curve. This will confirm for you whether the BHP in fact does drop as the flow rate is reduced resulting in zero or low flow.
The shape of the BHP curve varies significantly depending on the specific speed of the pump. At low specific speeds (up to about 1500 to 2000) the BHP is low at low flow rates (high differential head), and is highest near the best efficiency point of the pump, and then drops off again at pump runout condition. This is characteristic of radial flow impellers. Mixed flow impellers have higher BHP power requirements at shut off head. With Axial flow impellers (specific speed > 5,000) maximum BHP occurs at shut-off head.
The comments by Artisi, rmw and JohnGP all refer to low specific speed pumps and based on your comments that the pump is in a building application, I would expect that they are right in making this assumption. If this is a waste water pump, you can be 99% sure that it will be a low specific speed pump.
However, the easiest way to check the pump is with the knowledge that you have compared to the manufacturers performance curves.
I presume that the motor is a 3-phase induction motor with a squirrel cage rotor as 98% of the motor drives are of that kind as you are also aware.
Since it is induction type, the amount of loading (ammeter reading) depends on the amount of “SLIP” (relative speed with respect to the synchronous speed) that the rotor is subjected to. That means,
“Stator loading (Ammeter reading) is Directly proportional to the amount of SLIP or the speed that the rotor is subjected to “
Therefore when the motor is running at no-load, as an example with the pump uncoupled, we can see only a small loading in the ammeter just because that the rotor is running ONLY against the bearing friction and fan windage which is very very low.
But if somebody tries to stop the rotor by holding it forcibly using a piece of wood or some other arrangement then the rotor is allowed to turn slowly with the full voltage applied to the stator. That means the so called “SLIP” or the relative speed to the syn. speed is increased. As a result the stator takes more “Amps”. If we lock the rotor at this moment it takes about 6 x full load name plate current till the protection operates.
Now your motor is not subjected to either no-load or locked rotor condition as explained earlier. But it takes about 67% ( 6.4 Amps) of the name plate current of 9.5 Amps and also balanced in all three phases. Where does that energy go? It is used to push the flow not to the original amount but to an amount proportional to 64% motor loading. But there may be a small clog or block down the discharge line and NOT at the impeller, which has reduced the nominal flow by 37%.
So make arrangements to remove the clog or the block down the line and then the motor will give you the original flow that means the original ampere meter reading.
Therefore the ammeter reading shows you whether the motor is running or not.
My conclusions about the blockage were based on the heat up over time. I took from the post that the fluid being pumped was not normally hot, or it would not have taken 30 minutes to heat up, rather it would have been hot immediately.
Having said that, I have to point out that most pumps do not work at max amps, so the drop of in amps was not as much as an issue to me, just another clue. A pump doing just what it is supposed to might draw well below rated amps. A pump which would work at max amps would indicate a pump that was pumping about all it could pump, if the motor was chosen correctly.
As a general statement, motors are selected to provide enough HP to keep the motor from overloading if the flow runs out on the curve to the maximum amount. Therefore, a pump operating at BEP would be drawing less than rated amps.
Question then becomes, what was the HP of the pump design operating point, and are the amps given by twins normal or abnormal. He has to tell us what HP the pump would draw if it was operating normally, and then Kiribanda can do the "slip" evaluation.
I would need to know the pump to make specific statements, meaning I would need the pump curve showing head, flow, and power plotted up together. That tells the story.
Without that specific information, then generally the following applies:
1
If the pump is low Specific Speed (say around 3000 or less American System Ns), then amperage increases as flow increases.
2
If the pump is high Specific Speed (over 5000 American System or Ns), then power decreases as flow increases.
Many pumps have a flat power curve where amperage remains constant over the entire flow range.
3
On most centrifugal pumps, any and all intake problems always result in lower amperage draw. Matters not if the intake is clogged, air leaking in, whatever, power draw always drops.
4
I almost hate to say this because it can be misleading in the hands of a careless person, but here goes. Most common pumps with low horsepower (less than 10 horsepower), draw more current with increased flow. But again, there are exceptions and principally this relationship between flow and current is based on the pump Ns value being low (less than 3000).
5
Ns describes the shape of the impeller. Impellers with values below 3000 Ns look like discs and are often called pancake type impellers. Values above 8000 or so start to look more like propellers except they may have shrouds enclosing the vanes.
Need more information just ask us. Some smart guys on this thread.
Sorry about repeating what you had said, I am lazy at times and do not carefully read the posts. Actually I have not seen you on the forum for awhile, and few others reference Specific Speed, so I just assumed. But repeating is good too, makes the readers take what was said more seriously.
Business is great this year. We have a lot of very good customers that are fun to work with, chased off the ugly ones.
from the plumber's point of view... if they say there is a clog in the pipe... ask them if by chance they could have forgotten a rag or something in the piping and now they are afraid to come fwd with a "mea culpa"...
the last one who touched any system is the responsible party...
if the pumps were running fine before and are playing games now... something changed... find out what was the last thing they did in the system and start there your investigation...
HTH
It would also be worthwhile to check that the inlet side of the pump is not blocked / restricted. If it was possible that sufficient solids / foreign material could enter the pump system to block an 8 inch discharge line, it would more than likely block the inlet side / impeller first, assuming it is not a solids handling pump unit.
From my experience it is always better to look for the most simple cause initially and then work towards the most complex.
International College
Naresuan University
Phitsanulok
Thailand
