460 VOLT MOTOR PROBLEM

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bure961

Senior Member
Location
Farmingham, MA
I been having trouble with this motor 460v 3phase and I megged it and got 1.8 G on each of the 3 leads. It starts up and runs for ten minutes stops by water pressure switch. When it trys to restart the starter pulled in but it seemed as if it did not get the full 460 to the motor. The motor turned very very slowly and tripped out at that time. I took a volt reading at both sides of starter and it appears to read 430v only at time of slow running . When it is running the amp reading is below full load and appears it is fine, but won't start back up until 10-15 minutes later. No control power or switch problem that I can see. Megged motor 3x. 20hp. When running fine 460 at starter.

Checked all connections in motor and starter and disconnect. Any ideas ????
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Is it there is pressure on the discharge side of the pump when tries to start?

Class 10 or 20 overload?

I they even make a class 30 overload.

I would also check to make sure the overload is set correctly.
 

kwired

Electron manager
Location
NE Nebraska
Pressuer at start was my first thought as well, but if is a centrifugal pump I don't believe that matters. They will just spin. Positive displacement pump would change that. Check bearings.

I agree, but also find it somewhat hard to believe this motor couldn't accelerate the load, but if voltage is dropping to 430 - maybe it won't, got to be a long or undersized or both conductor, or even a source that is not stout enough to deliver this starting load without significant voltage drop.
 

Smart $

Esteemed Member
Location
Ohio
I agree. It is about 6.5% - assuming that it was actually 460V in the first place.
Should be close to 480V when not running. That's the commonplace nominal voltage over here. NEMA motors are rated at 460V to allow for supply line voltage drop. Of course there are tolerances associated with those numbers... so it appears 430V is the compounded lower tolerance for the motor of discussion.
 

kwired

Electron manager
Location
NE Nebraska
Wouldn't that depend on what other loads are on the supply?

I would still expect that to run up a 460V motor on a centrifugal pump assuming that's what the load is.

I think many of us would expect it to be able to start if it wer a centrifugal pump, we are assuming it must be some kind of positive displacement pump. And yes even then it should probably be able to get it going with only 430 volts applied, unless the characteristics of this pump/media being pumped are such that it will need more torque to accelerate the load then the chosen motor can deliver, or motor is borderline undersized to begin with, then voltage drop will only make matters worse.

The low voltage is in many ways no different then starting this motor with a soft starter, but a soft starter will allow output voltage level to rise over a pre-determined time, where this motor will only see a voltage rise as it accelerates and current goes down as a result.
 

Smart $

Esteemed Member
Location
Ohio
Wouldn't that depend on what other loads are on the supply?

I would still expect that to run up a 460V motor on a centrifugal pump assuming that's what the load is.
It will... and also the "stiffness" of the supply.

And you are not the only one. But the OP's situation is empirical, not speculation. There are quite a few variables which factor into what is plainly evident. We do not know them all. Just the fact the motor started and completed one cycle without failure is evidence the motor worked when parameters were favorable. The fact it has diminished restart says the parameters changed to unfavorable. With what design information that has been imparted to us, we can only speculate on what changes in parameters are the cause.
 

bure961

Senior Member
Location
Farmingham, MA
I would give you more information on this problem I have but I will not be back to work until Monday evening. I can not think of any other issues to post until i'm back to work after going over this again and use some of your ideas.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
I would give you more information on this problem I have but I will not be back to work until Monday evening. I can not think of any other issues to post until i'm back to work after going over this again and use some of your ideas.

So will you be able to tell the difference between a centrifugal pump and a positive displacement pump? Let us know if you're not sure, we'll give you some tips on what to look for.
 

Sahib

Senior Member
Location
India
''Stopped by water pressure switch''-those words from the OP suggest it is positive displacement pump; closed valves/clogged pipe lines might have operated the pressure switch;low voltage might be due to high current drawn by motor attempting to start again.
 

kwired

Electron manager
Location
NE Nebraska
''Stopped by water pressure switch''-those words from the OP suggest it is positive displacement pump; closed valves/clogged pipe lines might have operated the pressure switch;low voltage might be due to high current drawn by motor attempting to start again.
Doesn't have to be be positive pump. What he described could easily be your typical domestic water well, which is usually a multistage centrifugal pump, and is started/stoppedbased on state of a pressure switch.

But such pumps will have little problem starting against pressure because they are not a positive displacement pump and there is no pressure differential across the pump inlet and outlet while stopped with such a pump design.

A common positive displacement type of pump that could easily start against pressure is an air compressor, but a good design in that kind of application allows for pressure to bleed off at the pump somehow when it is stopped so that when it starts the next time it doesn't have to overcome pressure when starting.
 

meternerd

Senior Member
Location
Athol, ID
Occupation
retired water & electric utility electrician, meter/relay tech
Maybe I missed it, but did you read the current? If starting current is 5 to 7 times running just during a normal start, sounds OK. If it stays high when running slow, you may have a winding problem or a connection problem. Did you take off the "pecker head" cover and check all of the connections? Check current on all 3 phases. They should be almost equal if voltages are equal. Megger only tests for grounds. Over current is what trips breakers. If all else fails, put on a recording ammeter to see what's happening during starting on each phase. Sounds like single phasing to me.
 
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