Centrifugal pump & motor issue...

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TRW0930

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Situation:

50 HP motor, 45 FLA, 3550 RPM, resistance of windings .33 - .35 ohms.

This motor is drawing 58-60 amps when running a chilled water pump, needless to say it will trip the OLs. Removed the coupling from the motor and pump, amps drop to 10-11 amps. Pump appears to turn freely by hand. Discharge of the pump when running is 80-85 psi. Motor checks are all fine...installed a new motor of the same size...still having the same issues. One would think it would be the pump but with it moving so freely...I'm kinda confused...any help??? Thanks for your ideas.
 
mcclary's electrical

mcclary's electrical

Senior Member
Location
VA
vein pumps will not develope much resistance until it gets up to full speed. This is true on even some geared pumps. so freeturning by hand is normal,,,however,,, I still suspect a problem with the pump or discharge piping.
 
eric9822

eric9822

Senior Member
Location
Camarillo, CA
Occupation
Electrical and Instrumentation Tech
Try choking down on a valve on the discharge side of the pump. Less flow = less work = less current. Pump may be ok, you may just be outside the pump curve for the available horsepower.
 
C

Cow

Senior Member
Location
Eastern Oregon
Occupation
Electrician
mcclary's electrical said:
I still suspect a problem with the pump or discharge piping.
Click to expand...

You need to verify the motor is in fact sized to the pump and sized to the discharge piping, and all specs are correct. You don't want to come into this thinking it's been that way for 10 years or whatever and that it should be fine. Verify everything, don't assume.

If that means getting a pump tech out there, then that's what it takes.

One other thing, have you verified voltage at the motor with the pump running?
 
T

TRW0930

Member
I didn't check the voltage at the motor directly...checked it at the starter though...This is great people...keep the ideas comming...Thanks
 
Jraef

Jraef

Moderator, OTD
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eric9822 said:
Try choking down on a valve on the discharge side of the pump. Less flow = less work = less current. Pump may be ok, you may just be outside the pump curve for the available horsepower.
Click to expand...
Give him a cookie...

Most centrifugal pumps will require more HP as the flow through the pump increases. The power varies by the cube of the flow (look up "affinity law").

So there are most likely 2 scenarios:
1) The pump was designed and sized to be used with a specific pipe or some other means of flow restriction (i.e. a valve), then someone thought they could get more flow by removing or changing it. They did, without realizing that it also means exponentially more current.

2) The pump and piping was designed for a 50Hz application somewhere else in the world, then it came to North America (you must be in Canada) and is connected at 60Hz. They dealt with the voltage issue, but failed to recognize that at 60Hz the motor is going to spin 20% faster, and when it does, the flow increases 20% and the power the pump pulls increases by 1.2 cubed, or 173% of normal.
 
Besoeker

Besoeker

Senior Member
Location
UK
TRW0930 said:
Situation:

50 HP motor, 45 FLA, 3550 RPM, resistance of windings .33 - .35 ohms.

This motor is drawing 58-60 amps when running a chilled water pump......
Click to expand...
Hmm...
Assuming it's a 460V motor with a power factor of 0.85 and an efficiency of 95%, 58A would be about right for a 50 hp motor.
Unless it's an odd voltage motor, the 45A FLC doesn't seem right.
 
T

TRW0930

Member
Update....

Ya, I'm from Canada and the voltage is 600...
Anyways, during my swing shift going from days to nights, as I exhausted the motor theroy of being faulty...all basic tests of the electrical
components...starter...heaters...breaker...etc passed, the mechanics decided to check the pump...turns out they replaced a couple of bearings and the amperage dropped to 42 amps....

Again thanks for the input....
 
eric9822

eric9822

Senior Member
Location
Camarillo, CA
Occupation
Electrical and Instrumentation Tech
TRW0930 said:
Update....

Ya, I'm from Canada and the voltage is 600...
Anyways, during my swing shift going from days to nights, as I exhausted the motor theroy of being faulty...all basic tests of the electrical
components...starter...heaters...breaker...etc passed, the mechanics decided to check the pump...turns out they replaced a couple of bearings and the amperage dropped to 42 amps....

Again thanks for the input....
Click to expand...

As usual in an industrial setting, All problems are electrical until proven otherwise. Glad it worked out,
 
Besoeker

Besoeker

Senior Member
Location
UK
TRW0930 said:
Update....

Ya, I'm from Canada and the voltage is 600...
Anyways, during my swing shift going from days to nights, as I exhausted the motor theroy of being faulty...all basic tests of the electrical
components...starter...heaters...breaker...etc passed, the mechanics decided to check the pump...turns out they replaced a couple of bearings and the amperage dropped to 42 amps....

Again thanks for the input....
Click to expand...
My apologies - I should have considered the possibility that it was a Canadian installation.
The only things we have done there were a few drives in a paper mill in Thunder Bay and some for cranes Canadian Coast Guard ice breakers. Interesting project that was - but off topic.

If bad bearings caused the current by such a margin they must have been getting almightily hot. Anyway, I too, am glad that the problem has been resolved.
 
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