Pump power reading problem or something else?

[mjpetrag]

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

 

[ione]

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-? 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-? conversion fuses rated 160 amps are needed.

 

[mjpetrag]

It's 480 V 3 ph.

-Mike

 

[waross]

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

 

[tr1ntx]

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.

 

[mjpetrag]

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

 

[Artisi]

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.)

 

[maytag]

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

 

[mjpetrag]

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