Why does higher voltage results in higher current drawn?

[Crowbar]

In the factory test, the 460V, 60Hz, 3ph, FLA of 100A pump gives 300 litre/min. The current measured was 100A. When installed on site with a 480V, 60Hz, 3ph power supply, the pump now gives 330 litres/min and the current measured 110A.
Gievn some voltage drop across the cables which I am sure it is within 3%, shouldn't a slightly higher voltage results in a lower current drawn? Why do I measure a higher current instead?
Please let me know if you need more information. Thank you.

 

[kwired]

In the factory test, the 460V, 60Hz, 3ph, FLA of 100A pump gives 300 litre/min. The current measured was 100A. When installed on site with a 480V, 60Hz, 3ph power supply, the pump now gives 330 litres/min and the current measured 110A.
Gievn some voltage drop across the cables which I am sure it is within 3%, shouldn't a slightly higher voltage results in a lower current drawn? Why do I measure a higher current instead?
Please let me know if you need more information. Thank you.

If after the voltage increase the output remained at 300 litre/min and efficiency and power factor also remained the same then current should drop.

But you said output went up to 330 litres/min, that is an increase in load of 110%, and increase in voltage of about 104%. You need to look into differences in power factor and efficiency at both load levels to see the rest of the puzzle.

 

[Fulthrotl]

In the factory test, the 460V, 60Hz, 3ph, FLA of 100A pump gives 300 litre/min. The current measured was 100A. When installed on site with a 480V, 60Hz, 3ph power supply, the pump now gives 330 litres/min and the current measured 110A.
Gievn some voltage drop across the cables which I am sure it is within 3%, shouldn't a slightly higher voltage results in a lower current drawn? Why do I measure a higher current instead?
Please let me know if you need more information. Thank you.

instead of an inductive load of a motor, think of a resistive load.
if you increase the voltage, the current will also increase.

in your case, voltage, current, and work done all increase with the voltage increase.

 

[Jraef]

10% is a lot to expect from a simple voltage difference. 1-2% maybe, 3% possibly, 10% unlikely.

It's far far more likely that a slight difference in static head, slightly different pipe size, fewer turns and bends in the piping providing less fluid friction or some other aspect of the piping resulted in the increased flow, which then in a centrifugal pump results in an increase in current.

 

[GoldDigger]

The key factor is that when, for whatever reason, the flow rate of a centrifugal pump increases the power delivered by the motor increases and therefore the current drawn by the motor (for a fixed voltage) has to increase too.
Since the current is above the nominal FLA of the motor, it is a very strong indication that the motor is mismatched to the combination of pump and load and is being used at higher than its nominal maximum mechanical power output. Unless there is a service factor on the motor, that is at least a technical abuse of the motor.

 

[Ingenieur]

Post the pump curve
Jraef imo hit the nail on the head
the system curve is different, a bit less head than the test point

throttle/close the discharge valve down a bit while measuring i and Q

 

[kwired]

10% is a lot to expect from a simple voltage difference. 1-2% maybe, 3% possibly, 10% unlikely.

It's far far more likely that a slight difference in static head, slightly different pipe size, fewer turns and bends in the piping providing less fluid friction or some other aspect of the piping resulted in the increased flow, which then in a centrifugal pump results in an increase in current.

Didn't really think too much about that before, but yes. As long as frequency remained same in both situations, pump speed and would remain the same, and pump load would remain same if piping, head, etc remained the same, so very good chance the media being pumped has different characteristics in some way at the site application then it had in the factory test.

 

[junkhound]

pump speed and would remain the same, and pump load would remain same if piping, head, etc remained the same, .

Not quite due to slip ? Am assuming an induction motor?
As Ign.. said, need the pump curve. Motor curve would be good also. Or at least type of motor, pump speed would stay the same if sycn motor, but then would not expect power draw to increase.

Flow went up, assuming piping stayed the same, that means head increased slightly also.
Power proportional to CUBE of pump speed.
From pump curve, would be able to see how much slip of motor (hence slightly increased speed) decreased.
OP said kVA went up 14%, from which one could crank numbers and infer that the increased flow velocity in the piping to increase head, etc.....
Cube root of 14% is 4.4% increase in speed (approximately), say 3400 to 3550 RPM, which seems possible unless synchronous motor.

 

[kwired]

Not quite due to slip ? Am assuming an induction motor?
As Ign.. said, need the pump curve. Motor curve would be good also. Or at least type of motor, pump speed would stay the same if sycn motor, but then would not expect power draw to increase.

Flow went up, assuming piping stayed the same, that means head increased slightly also.
Power proportional to CUBE of pump speed.
From pump curve, would be able to see how much slip of motor (hence slightly increased speed) decreased.
OP said kVA went up 14%, from which one could crank numbers and infer that the increased flow velocity in the piping to increase head, etc.....
Cube root of 14% is 4.4% increase in speed (approximately), say 3400 to 3550 RPM, which seems possible unless synchronous motor.

I guess what I meant was synchronous speed would be the same if frequency was the same, which means motor will be aiming for same speed, it is voltage and load differences that will cause a change in slip. If pump is turning at the same speed in both locations and the pump, piping, and media characteristics are identical, then the driven load will be the same in both locations - then an increase in voltage should see a decrease in current if the same work is being done, since OP saw an increase in current, as well as an increase in flow rate, and presumably same reference frequency which should give same base motor speed, we do not have the same load conditions in both places.

 

[Jraef]

Exactly. At 100A that 100HP motor was not fully loaded, even at 110A I doubt that it is. That to me removes the likelihood of this being related to a difference in slip speed. Yes, at a slightly higher voltage the torque will be slightly higher, but the motor is not very likely to be more than 3-5% slip under normal operating conditions, so going from 5% slip to 4% slip will not make that much difference, unless the motor was in high slip the fist time because of being overloaded, and the evidence at hand doesn't show that to be the case.

This is my favorite graphic for cases like this, showing the effects of voltage variation on an AC induction motor. Notice that the curve labeled as speed doesn't vary enough to warrant this being relevant here.


I believe it is load related and the increase in current is the result of an increase in flow, but the increase in flow is not the result in that slight voltage variation. Remember, a motor labeled as 460V is DESIGNED to operate at 480V; 460V +-10%

 

[Crowbar]

Thank you everyone for your views. I was hoping to get a curve before I post again but looks like I will not have it so soon. I had also checked the medium used in factory and on site, I was told they are generally the same.

More information - This is a positive displacement pump and i learnt that the same flow meter was used (despite some months later) for both factory test and on site. Even the manner of connecting the flow meter was the same, they removed the connecting hard pipes and attached the flow meter with a 1 meter hose that goes from supply to return outlet directly. The pressure was kept about the same (~1.5% different from factory) and I supposed the head remains the same? Are there any other mechanical parameters that I have missed out or I should measure?

Both frequency are the same ~ 60Hz and they measured almost the same rpm. The power factor for the power supply are almost the same too (0.8 - 0.85)

I think the only main difference is the power being supplied. In factory, they claim they use 460V while on site, the power supply is about 480V before 1-2% voltage drop. Hence, I was puzzled why a slightly higher voltage led to higher current measured.

It is currently working in the service factor range and we obviously hope it will be within the full load current.

 

[Ingenieur]

If the flow meter is accurate the only thing that can account for an increase in power is an increase in Q
and for a given pump curve that means lower head
this imho is not an elec issue but mech/hydraulic

 

[GeorgeB]

This is a positive displacement pump ...

With similar fluids and pressures, flow rate is directly proportional to speed. That the current only went up the same 10% says the pressure is "the same". Remember that pressure times flow times a system of units constant is power ...

Both frequency are the same ~ 60Hz and they measured almost the same rpm.

10% higher flow rate says the speed ___OF THE PUMP___ is 10% higher.

Can you give us a pump model?

 

[GeorgeB]

If the flow meter is accurate the only thing that can account for an increase in power is an increase in Q
and for a given pump curve that means lower head
this imho is not an elec issue but mech/hydraulic

Positive displacement pump ... no typical pump curves, Q=k*(rpm)

 

[Jraef]

Yes, it being a PD pump makes this more complicated, assuming the given information is accurate. It's true, the only way the current can be that much higher is if the flow rate is that much higher, which in a PD pump must be related to the speed. +5% (or even less) voltage at the motor terminals is basically insignificant.

I'm now thinking more along the lines of measurement error. Values of current and/or voltage and or frequency in one of the tests was assumed, not measured, or measured incorrectly. How / where are the measurements being taken?

 

[Ingenieur]

We know this
v ~ the same
speed constant
i + 10%
Q + 10%

although the pump 'curve' is linear with a nearly vertical slope the operating point varies based on the system curve
sith all the various measurements imosafe tosay the only thing that changed is the system curve

 

[retirede]

Based on the last from OP, the test voltage of 460 is assumed, not measured.
I'm also going with some kind of measurement error.

 

[junkhound]

PD pump throws out most of previous discussions prior to that significant bit of information.


Unless same electrical meters used at both locations and piping nearly the same, agree that measurement error most likely culprit - unless some valve position was different in the 2 tests, eh?

Hmm - direct drive or belt driven? Anyone replace an old worn slipping belt, etc.. etc...

 

[kwired]

I would think a lot details need to be nearly identical at both locations for a PD pump to have the same load on it at both locations then what has to be identiacl for centrifugal pumps to have same load at both locations.

Centrifugal pumps are primarily loaded by flow, PD pump loading is effected more by changes in pressure then a centrifugal pump, so if a line is different size, length etc. it can have a bigger impact on loading of a PD then a centrifugal pump.

 

[retirede]

I would think a lot details need to be nearly identical at both locations for a PD pump to have the same load on it at both locations then what has to be identiacl for centrifugal pumps to have same load at both locations.

Centrifugal pumps are primarily loaded by flow, PD pump loading is effected more by changes in pressure then a centrifugal pump, so if a line is different size, length etc. it can have a bigger impact on loading of a PD then a centrifugal pump.

Correct about the effects of pressure on loading. But he also reported that the flow increased by 10% post installation. The only thing that can cause a PD pump to do that is to proportionately increase the speed.

I think we are presuming a standard AC induction motor and constant speed transmission (direct coupled, belted, etc.).
I still think something is wrong with the data collection.