×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Pump power reading problem or something else?

Pump power reading problem or something else?

Pump power reading problem or something else?

(OP)
I have a 175 hp pump motor that is drawing around 210-220 amps during operation.  Fuses are rated for 190 amps +/- 10%.  The 210-220 amp reading was taken with a clamp on ammeter and confirmed with another clamp on ammeter.  We decided to put a power monitor on the MCC panel for this motor (split core current transformer off one phase from the power panel) and it reads around 170 amps during operation.  Now the 170 amp reading would match up with the pump power curve, but I'm wondering if anyone has had similar experiences with clamp on ammeters reading higher than expected.  Maybe the 210-220 amp draw is correct and the current transformer needs to be recalibrated?  I realize it's past the amp draw to trip the overloads but maybe something else is going on.
 

-Mike

RE: Pump power reading problem or something else?

For a 175 hp 3ph – 500V – 60 Hz motor the nominal current drawn should be 184 amps. For direct start fuses rated 250 amps and for Y-Δ conversion fuses rated 200 amps.

For a 175 hp 3ph – 690V – 60 Hz motor the nominal current drawn should be 134 amps. For direct start, fuses rated 250 amps are needed and for Y-Δ conversion fuses rated 160 amps are needed.
 

RE: Pump power reading problem or something else?

(OP)
It's 480 V 3 ph.

-Mike

RE: Pump power reading problem or something else?

Canadian Electrical Code guidelines for a 175 HP motor extrapolate to 210 Amps.
Is the power monitor on the same phase that the clamp-on meter readings were taken from? A voltage unbalance can easily result in a large current unbalance.

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

RE: Pump power reading problem or something else?

I agree with waross, it's probably large current imbalances between the 3 legs.  Did you get the 210-220A off just one leg?  And then perhaps put the power monitor on a different leg?  There are power monitors that connect to all 3 and thus give "true power" accounting for the phase shift.  I can't remember the brand name.

RE: Pump power reading problem or something else?

(OP)
All three legs read between 210 and 220 with the clamp on.  


Just for clarification, I'll describe the set up.  We have a transfer pump connected to a tank that varies between 25'-4' depending on how long the pump is running.  When the tank is full, the pump draws about 220-225 amps, sometimes even higher.  When it is nearing 4', it draws about 200-210 amps.  

Matching up the pump curve with operation points, the power draw when the tank is full should be 150 hp, and 140 hp when the tank is almost empty. Converting with a power factor of .95 and motor efficiency of .9, the draws should be 157 Amps and 147 amps, respectively.

Pumping bypasses were checked and everything was closed.  If the draw was 220-225 amps, we would be pumping an extra 1000 GPM somewhere (operating point is around 2100 GPM)

-Mike

RE: Pump power reading problem or something else?

Not sufficient information to make any qualified comments.

You need to supply inlet and outlet conditions, pipe work configuration, relative levels of source and discharge point, pump curve etc flow rate, pressures on both sides of the pump etc etc - from here we might be able to make some sense of it.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)  

RE: Pump power reading problem or something else?

I would lean toward the clamp-on ampmeter readings being correct especially when two different ones were used.  The motors of this size that we use have a FLA of 222 which is within the range you have stated.  Some things that work on paper just don't work out that way in the real world applications.

Tom

RE: Pump power reading problem or something else?

(OP)
I think I found the problem.  The power factor/efficiency at the operating point is .85 and .92, respectively, which I didn't correct for on the pump power curve.  210-220 amps looks to be correct for the clamp on.  The power monitor will have to be recalibrated.

-Mike

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources