Who has the authority? Pump or Motor 8

[CrazyHorse81]

Ok. Lift station grinder pump. So does the pump tell to motor how much to work or does the motor say "I'm only going to give you what I'm made for!"

I think that the pump dictates the motor how hard to work, but don't exactly why.

Q: What would cause the pump to pump more flow at a lower head? A: A lower TDH. So at some point in time, the lower TDH relates to no discharge at the manhole. Then what happens? More cycles?

Please enlighten me!

 

[zdas04]

I think the answer is "yes". A pump guy will say that for a given volume and a given head, the shaft input has to be "x". A motor guy will tell you for a given rpm and a given amp draw the shaft output has to be "y". If the motor rpm changes, then the pump volume and/or head must change. If a pump volume or head change, the motor must either change speed or draw different current load.

You can't have chickens without eggs, and you can't have eggs without chickens. I think your the answer to your question is approximately that indeterminate.

David

 

[Artisi]

The power required by the pump for any given condition dictates the demand made on the motor.

Power = flow x head x pump efficiency

In metric terms
Q (l/s) x H (metres) x Spec. Gravity / 102 (constant to round up all the variable terms etc) x pump efficiency (as a decimal at the pump duty point) = Kw

 

[zdas04]

See, now we need to hear from a motor guy.

 

[Artisi]

For more clarification on your: Q. What would cause the pump to pump more flow at a lower head? A: A lower TDH. So at some point in time, the lower TDH relates to no discharge at the manhole. Then what happens? More cycles?

To the first part the answer is yes, lower head equals a higher flow rate until NPSHr exceeds NPSHa or the drive motor overloads and trips the overload breaker or destroys itself (burns-out).

For the second part of the question, if you pump the "hole / sump" down at a rate higher than the inflow then the pump will run dry until such times as the inflow is sufficient to restart pumping, not usually recommended as there will be insufficient motor cooling - this could result in motor burn-out. If this is a possible senario, fit a level switch to control the sump level.

 

[electricpete]

The power required by the pump for any given condition dictates the demand made on the motor.

That sums it up IMO.

As a very good first approximation, the motor is a constant speed device (slip is a very small fraction of full speed). It will increase it's output power (within limits below the trip setpoints) to whatever level is required to keep the pump running at that speed, which is determined by the pump curve and operating conditions.

A very small secondary effect is the small amount of motor slip. As we increase load on the motor, it's speed droops very slightly and this of course shifts the pump curve slightly so that flow and dp and fluid power will decrease slightly for a fixed flow resistance.

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[electricpete]

By the way I assume there is no vfd in the picture.

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[CrazyHorse81]

electricpete: you assume correctly.

The reason i ask this stems from a lift station evaluation project. The design report and the first evaluation report (we are doing a 2nd eval) list a pump model number that is said to be 3hp. Even the model number supplied by the electric maintenance company was reporting what the design report did.

Well the mfg says told me that this model number is 5hp min and they don't come in a 3hp. And upon further investigation of the first evaluation report, they say three but back it up with a 5 hp pump curve in the appendix. Strange I thought.

So then when we did our site investigation of the lift station, the amp reading on the lead pump was 25 amps, and the second was 13 amp. And from what I understand that these amps relate to a 5hp and a 3hp pump.

Well could it be that there are one of each (one 5hp and one 3hp)? Two 5 hp? Two 3 hp? I think could all be possibilities.

 

[BobM3]

You made two measurements on the same pump/motor and came up with the two different amp readings? Were they done years apart or minutes apart? Or are these two different motors on the same pump?

 

[CrazyHorse81]

1st reading on pump 1 25 amps

1st reading on pump 2 13 amps

then the pumps alternate lead/lag after one cycle

2nd reading on pump 2 13 amps

2nd reading on pump 1 25 amps

 

[electricpete]

I assume you are saying you have reasonable confidence that the two pumps are operating under similar fluid conditions. (deserves more discussion but others are more qualified to answer than me...I'm a motor guy).

At no-load / low load, there can be substantial difference in current draw depending on the motor size. Specifically a higher horsepower has higher no-load magnetizing current). As we load up the motor (let's say above half load), the no-load magnetizing current becomes small in comparison to the load component of the current which depends directly on the shaft power.

I was trying to figure out what fraction load you are on these motors from the Amps. At my plant a 3 or 5 hp motor would be 460vac, but the full load currents would be much lower. What voltage/type are these motors... 220vac single phase? Maybe they have different voltage/configuration, which would also explain the current difference. Alternatively, what are the full load amps on the nameplates?

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[electricpete]

Also even if nameplate voltages are the same, if for some reason one of the motors is for some reason fed by a lower actual voltage, then it will draw more current to reach the same output power.

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[BobM3]

Are the motors turning at about the same speed?

 

[djack77494]

My opinion-You turn on the motor and the shaft spins up to speed. Fluid starts passing through the pump and things move pretty quickly to equilibrium. At this point, you can refer to your pump curve to figure out the power being consumed by the pump. The motor should have been purchased as being fully capable of meeting the pump's power requirements. Most typically, the spec will call for the motor being capable over the entire pump curve. So in answer to the OP, the pump tells the motor how much power to apply (if properly designed).
Opinion of a process engineer who is probably more "pump-oriented" than "motor oriented".

 

[ScottyUK]

Interesting question. There are two related answers to the question:

The electrical supply determines how fast the motor, and implicitly the pump, will try to rotate. In the 50Hz world the common speeds are nominally 3000 rpm, 1500 rpm, 1000 rpm, 750rpm, 600rpm. In the 60Hz world add 20% to those values. The actual motor speeds are slightly lower than the nominal speeds because an induction motor has to run slightly slwoer than sync speed to develop any power. The exact speed varies a little from no load to full load conditions, but essentially:

1) The motor determines the speed the pump rotates at.


The pump moves a certain amount of liquid and discharges at a certain pressure when it operates at the speed determined by the motor. The load presented to the motor therefore depends on the power input required by the pump to meet the flow and pressure demand, so the pump determines the motor power requirement.

2) The pump determines the motor power rating.




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If we learn from our mistakes I'm getting a great education!

 

[Artisi]

When the pumps alternate the lead,is the flowrate the same for pump No 1 as when pump No 2 is the lead unit?

 

[ccfowler]

CrazyHorse81,

Assuming that "Pump 1 & Motor 1" is functionally identical to "Pump 2 & Motor 2," I would look carefully at the suction and discharge piping arrangements. The most likely problem is something effectively throttling the flow of Pump 2.

If Pump 2 is not making nasty cavitation noises, the problem is likely to be in the discharge piping. I'm thinking mainly in terms of some item of construction debris being in a position in the piping to restrict the flow from Pump 2. It could be anything: a wrench, wrench socket, nut, smaller pipe coupling, or who-knows-what. Don't overlook the possibility of a check valve that may not be opening fully.

You may be able to find likely locations for a blockage by using a mechanic's stethoscope to listen for excessive flow noise caused by the high local velocity at the flow restriction. If you want to spend much more money on the test equipment, I'm sure that you can find fancy electronic noise sensors or ultrasound sensors, but I freely admmit to being very old-fashioned and a bit cheap.

If the configuration of the discharge piping for Pump 2 is very different from that of Pump 1, that could explain the problem.

Usually, or at least very commonly, pump problems arise from piping or control valve problems rather than from the pump or motor themselves.

As is usually the case, Artisi, electricpete, and the other discussion participants have all provided excellent advice. My experience indicates that an induction motor always do all that it can to put out whatever magnitude of torque that may be demanded by the pump without regard to its own health and well-being without protective devices to protect it from itself. If you actually have a 3 hp motor installed with 5 hp protection, it will thoroughly cook itself trying to put out much more than 3 hp. Since you have not mentioned smelling thoroughtly cooked insulation or other nasty issues, I doubt that either motor is being seriously overloaded.

 

[waross]

One pump may have an impeller sized for a 3 HP motor even though a 5 HP motor is supplied.

Other than that, look for an obstruction or piping issue.
Hint. An obstruction will (counter intuitively) be in the pump system with the LOWEST current.

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

 

[CrazyHorse81]

electricpete: i don't have the nameplate information, i will dig into that

BobM3: i don't have any reason to assume that they were turning at different speeds, same pump, constant speed, same age...

Artisi: the drawdown in the wet well took about the same time when each pump was lead, so i would say that the flow rates were similar for each pump...

ccfowler: there were no odd, loud, suspicious noises or odors like burning. the lift station from our observations was in good operating condition.

waross: i agree. an obstruction means limited flow which throws the operating point to the left on the curve where less power is consumed.

Thanks all for the fun and good input!

 

[Artisi]

" I agree. an obstruction means limited flow which throws the operating point to the left on the curve where less power is consumed." If this was the case, then how is drawdown the same which ever pump is lead?

Seems there is something else being overlooked, is it possible that the 25 amp pump is drawing more current due to a mechanical loss, ie, impeler rubbing on the case, something jammed in front / behind the impeller?

If flow and head are the same which ever pump is lead, power will have to be similar irrespective of speed or motor size, as stated earlier - power = flow x head.