Motor Current Discrepancy

Are the pumps piped in parallel?

Still awaiting an answer!

There are 2 sets of parallel. The process and mechanical have concluded the issue is electrical, and want us to replace the motor, I checked all electrical aspects, correct motor connection, no loose connections, all phases currents are balanced. I even checked no load current which was 25A per phase.

I asked the question in case I missed something.

Too many cooks.

IMHO, if your steady state case temps are similar, you don't have a problem.

But a flow check, switching each motor on and off, might give you the answer.

As usual, Electrical has to prove there is a mechanical or process component.

New motor, new pump(?) I will bet the new motor does exactly the same thing - try swapping motors between two pumps and see what happens - oh, but that would be a mechanical issue and shot down before you begin.

Like it!!

Me too...

http://insights.globalspec.com/article/2560/pumping-in-parallel-is-not-always-simple

Excellent link....

It seems that the new motor is providing just a little less than its rated mechanical power. The current ratio between old and new motors is close to SQR(3) so I wonder if the AC power to these motors is wired identically.

Sounds like you have a 50hz motor running on 60hz....or 3 60hz motors running on 50hz...

Maybe there has been impeller erosion in the older pumps, so they pump less flow and thus less load.

I agree with your answer. Congratulations.

Adding to the theme here, the current drawn by a motor is the current required by the load, plus any conversion losses. It's highly unlikely that the new motor is that much less efficient than the 30 year old motors, in general the trend has been going the other way; motors have been getting more efficient. So what's left is that the load is somehow different. So the older pumps having worn impellers is very plausible. Some other change / difference in flow would result in a difference in load, because in a centrifugal pump, load = flow.

Check the steady state (running) case temperature - your figures seem to indicate new motor is less efficient, if so it will show up as heat - also 'exhaust' heat.

Can you turn the motors off independently, while measuring the flow? Output should be proportional to the current draw -

Many good ideas here already.

Can you show us how the fluids are connected to all motors?

Is it possible to somehow monitor the fluid quantity pumped with each motor?

Is it possible that a valve is not fully open, probably on the pressure side of the new motor?

Andy, we had a parallel chiller system with this problem. After some expensive time we found somebody put a bottle of soda to cool it and it worked iets way into the supply line to the pump and the moment the pump start suction it drew the bottle into a socket and sealed the supply 1/2 way.

War stories!

I was commissioning the electrics and software for a fancy hydraulic system and someone didn't remove the temporary plastic pipe plugs.

1500 PSI pushed the plastic plug along until it hit the most expensive PQ valve.

I was giving the valve the 4-20ma signal, confirmed the electrics, and it still took about 24 hours before we could get the mechanical people to take it apart - they insisted it couldn't possibly be their problem - they always do 100% perfect work.

"I was giving the valve the 4-20ma signal, confirmed the electrics, and it still took about 24 hours before we could get the mechanical people to take it apart - they insisted it couldn't possibly be their problem - they always do 100% perfect work."

Preaching to the choir here. It's like having an automated inspection reject 'too many parts' when the inspection is set up right, but the PRESS is making a bunch of bad parts. Instead of fixing the problem, they blame the equipment that 'tattled' on them.

WOW!!

compare the discharges among old and new pumps.

check the terminal links inside the motor terminal box for old and new (Star or Delta)

what is the existing utility system voltage in your country?

Was the new motor a replacement for an "old motor" and so pump and associated pipework and valves are same as the "original" three?

Is the new motor part of a complete new installation of pump and associated pipework?

Thank You

I have noticed newer motors have less slip than older motors. This would possibly cause the new motor to run faster and hence draw more load (and provide proportionally more flow).

We can only assume it has been connected correctly and the frequency ratings of all the motors etc are the same.

20 year old motors are not "old" in the scheme of things. - Many motors in the paper industry are pushing 50 to 100 years old. Usually with bearing changes every few years.

Probably a combination of all the points mentioned here.

Maybe some minor changes in impeller & casing design from 20 years ago, along with a high efficiency (less slip) motor, then possiblly some impeller wear on the old pumpsets, all meaning that the new motor is taking more than it's share of mechanical pumping load.

Have you seen a drop to 70 amps with the old motors? Or was that the reading before?

If you are in a position to swap motors it might give a clue.

In my opinion the new motor is loaded close to the rated [55 kw].The old only 43 kW.

It could be the old motors are only 45 kW rated?.

http://www.tpomag.com/editorial/2016/03/save_money_with_integrated_pump_monitoring

A good link for the OP I feel.....