High amperage readings

Keith_8

We replaced (2) 1 horsepower 3 ph pump motors this fall that had been burn't out. We assumed that the motors failed due to losing 1 leg of the 3 phase power. The existing heaters in the starters were way oversized and offered no protection.

After we changed out the motors we installed the correct heaters but found that the motors are drawing in excess of the 4 amps the nameplate indicates.

With the exception of the heaters tripping the pumps perform OK.

What would cause these motors to over draw in amperage?

Thanks for your input,

Keith

Ken_8

Could be anyone of these things

Either the pump should have had the impellor cut to match the actual load; the piping is too large allowing huge GPM rates that exceed the assumed system resistence; you mis-read the amp load rating from 230 1-phase assuming it was actually the 208 3-phase (or whatever voltage actually exists); perhaps the voltage is much less than assumed (check the voltage of each phase to ground); or the pump is being run without any head restraint.

If the latter, simply apply restriction to the discharge side of the pump (presumably there is an isolation valve on the outlet?) until the amp draw falls in line.

Believe it or not, the amp draw is REDUCED by restricting the flow. The reason being, the less water the impellor can pass - the less work involved!

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will smith_2

How's the bearing assembly? Could it be causing the motor to drag?

jerry scharf_2

confused

Ken,

I'm confused. I always thought that a three phase moter ran a lowest power at full speed and no load. Why would having too little resistence cause the the motor to draw too much? As I see it, adding resistance can only cause the impeller to spin slower, which means more load.

I understand trimming the impeller would reduce the load.

jerry

jim lockard

Phase monitors

this has nothing to do with Amp draw but have you installed a phase monitor on this motor.
On this pump is every thing inline? how do you know check with dial indcator, laser? Are the bearings overheating?
Where are you testing at the motor or at the breaker? could you have a loose connection? Faulty breaker? What is the voltage phase to ground for each leg with the motor off ? with the motor running? after startup? after 1 hour of run time? Same question with the amp draw is the amp draw the same after cold start as it is after running for 1 hour? Best Wishes J.Lockard

zeke

Use a current probe to get the current in each of the three phases without load ( ie decouple the pump from the motor). This noload current should be quite small. Also the three currents should be within 10% of each other. Repeat under load conditions and also get your phase to phase voltages(should be within 10% of 220V) to make sure you have a balanced electrical system and they meet the motor requirements. It looks to me,from my tables, that your motor is rated for 220V 3 phase. Finally, if you determine that the phases are proper then I would look for a mechanical overload situation like bad bearings in the pump or too much pumping power for the system and, if the latter and is required, go to a larger motor.

Blackoakbob

Check the amps...

as you are closing the outlet valve and set it at the full load amps and then decide if this flow will be adequate for the system needs. If these motors are being fed by a electrical panel that has alot of inductive load i.e. other motors, fluorescent lighting; as in many large buildings you may need to look into a power factor problem on the incoming line this can cause excessive heating of the motor windings and an increase in operating costs.
Best Regards,

Ken_8

You're half right...

The term load refers to the amount of water being moved.

If you restrict the output, the load will be reduced. The motor will "slip" and not be allowed to "bite" as much water since the capacicty of the pump will be reduced.

Remember, this is NOT a positive displacement pump. If you dead head the pump, it will pump nothing and merely "rotate with no "work" taking place!

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Keith_8

overamping

Thanks for the feed back guys,

We have choked down the triple duty valves about 75% and it does reduce amperage draw almost to within the nameplate data.

I'm reluctant to leave it this way for the winter without going through the building and checking gpm flow at the end devises.

Electrical connections were reviewed and all look solid.

We will follow up with the other suggestions and report back.

Thanks,

Keith

jerry scharf_2

The post was about the electrical load

I agree with you completely regading the hydraulic work output of a pump being a function of flow and load.

The question is does the motor slow down more when deadheaded or when flowing through light resistance. I've not studied this, but on quick observation the electrical demand from the motor should be highest at deadheading. This is based on my minimal feel for the volute/impeller flow characteristics and the general dynamics of motors.

It would be much better to measure this on a 3 phase motor, since PSC motors are inefficient and the effect might be masked. Then again, it it's masked the person asking wouldn't care ...

jerry

Mark Eatherton1

Don't follow your gut on this one Jerry...

Ken put it quite sucinctly. Dead headed pumps build a lot of pressure, but are not really exerting themselves. The RPMs should be slighlty lower depending upon the horsepower of the motor, during ful flow conditions.

People get REALLY upset if they see you deadheading a pump. Truth be known, it's OK for about a minute depending upon the design, size etc. I know my partner freaks out, even with the little wet rotor circulators we deal with. I calmy explain to him that I've been doing it for most of my hydronic life, and that I've seen some that were deadheaded for 3 winters. He still get nervous when I do it.

Now, if its a BIG pump, the amount of time is CONSIDERABLY less. You're dumping a LOT of mechanical energy into water thats not going anywhere. The water temperature has a tendency to rise pretty quickly, gets super heated, and then when it sees a drop in pressure, becomes steam and tries to occupy 1700 times as much space as it used to. This generally results in loud noises, and some parts moving at terminal velocities.

Don't feel bad about your gut feeling though. Other than the PONPC, this is probably the second hardest concept for technicians to understand. They equate pressure with work, and such is not the case with CIRCULATORS. Now, if its a pressure PUMP, thats a beast of a different color...

ME

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Ken_8

Remember,

the electrical load and hydraulic load are lock-stepped one to the other!

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Mark Eatherton1

\"With the exception of the heaters tripping the pumps perform OK

Thats a pretty subjective comment...

Pump performance, in my experience is generally major overkill. Oversized pumps can lead to the problems you're dealing with. Over loaded motors, motors need frequent replacement, small bore pipes springing leaks etc.

Throw some gauges at it and convert the pressure differential into feet of head, then using the manufacturers perfromance curve, see where on their curve the pump is running.

What exactly is this pump doing?

Got a schematic?

We'll get you fixed, one way or 'tuther.

ME

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Weezbo

You smart *~/:)

there are quite a few variables with Three Phase motors ...and there are some special variables involved in various processes...capacitor. . banks .. voltage balancing loads,wired for the correct roatation how simple it all is when you know immediately what the problem is... volt ohm clamp on am meters all very useful tools as well.some machines need heavier Heat to start and others require brakes:) in this application circuit setters are handy devises.to be able to choose the right thing to do first isnt as intuitive as someone might think. The fact that you jumped up and dialed in the powerfactor cool. it Could Be not all neuterals are created equal ....sometimes on larger buildings a maintinace guy might come along and dial a few valves when a guys not looking and flow aint gonna happen no matter What size motor and pump and impeller or vane is installed...the valves might not even be in that room! sometimes a guys gottabe justa bout Clairvoyant ) or if you do have flow it may be around and around in a fourty foot section of pipe ) really though the lighting thing is a very good point.

Dale

A few more thoughts

First to make your life more interesting when investigating breaker trips, especially in rehab buildings you need a true RMS voltmeter and clip on ammeter. Non linear loads like PC
's can reall throw the numbers off. And, besides the non linear problem make sure you measure voltage with normal office loads on. As the other post said unbalance lighting loads can get you. I agree with the other posts especially about restricting output, your problem IS probably too big a pump for the head actually in place. This also works with any centrif pump including your basic warm air furnace squirrel cage, choke off the inlet or outlet, like a dirty filter or A coil and the amps go Down. Propeller fans like on a condensing unit are opposite, choking off the outlet and amps go up. With 3 phase be VERY careful about inlet voltage, just a few volts difference can overheat a motor severly, in the phase with the highest current the percent increase in temp is about twice the SQUARE of the percent of voltage unbalance. For instance a 3 1/2 % unbalance increases temperature about 25%. % volt unbalance = 100 x volt difference from average volts divided by average volts. Get a Copeland book from your wholesaler to see a good example of the math involved.