## Motor Power Calculation

## Motor Power Calculation

(OP)

Hi All,

I want to calculate the Power supplied by the motor to the pump shaft and using voltage and current I want to calculate the flow delivered by the pump. I have a situation here where the we are not able to measure the flow delivered by the pump accurately. We have pressure gauges on the pump suction and discharge using which I can calculate the hydraulic or brake horse power assuming certain flow rate and pump efficiency at this flow rate from the pump curve. We also the amperes supplied to the pump, voltage, pump efficiency and power factor. From this can we calculate the power consumed the power consumed by the motor and the fraction of it that is supplied to the pump?. If I can equate the power calculated in these ways and determine the flow rate delivered by the pump.

Thanks and Regards,

Pavan Kumar

I want to calculate the Power supplied by the motor to the pump shaft and using voltage and current I want to calculate the flow delivered by the pump. I have a situation here where the we are not able to measure the flow delivered by the pump accurately. We have pressure gauges on the pump suction and discharge using which I can calculate the hydraulic or brake horse power assuming certain flow rate and pump efficiency at this flow rate from the pump curve. We also the amperes supplied to the pump, voltage, pump efficiency and power factor. From this can we calculate the power consumed the power consumed by the motor and the fraction of it that is supplied to the pump?. If I can equate the power calculated in these ways and determine the flow rate delivered by the pump.

Thanks and Regards,

Pavan Kumar

## RE: Motor Power Calculation

Study this material and you will get your answer.

You should be a fresh graduate , check with your electrical engineer , he should have a power meter to clamp directly , this will give your the power consumption .

This should be part of your toolbox .

Good luck

Pierre

## RE: Motor Power Calculation

Thanks for sending the paper to support my calculations. I actually did the calculations before you sent the paper, which I did not post earlier. I knew of the electrical side of the power calculation but was not sure. I verified the formula that I used for the power delivered by the motor to the shaft with the one in your paper and it was correct. I calculated the flow delivered by equating the shaft power ( using pump differential head and efficiency) and the power supplied to the shaft by the motor. The calculations are shown in the attached spreadsheet. I calculated a flow of 2161.5 US gpm while the pump curve gave me 2450 US gpm. There is a difference of 288.5 US gpm which we are asking the pump vendor to clarify.

Thanks and Regards,

Pavan Kumar

## RE: Motor Power Calculation

## RE: Motor Power Calculation

Also is the curve from a performance test or generated by the supplier from his standard curves? Accuracy of standard curves is 3-5% in many cases.

I assume you meant goal to zero by adjusting cell E11?( flowrate?)

Also they are pretty low pressures. How are they being measured? Where are the gauges or transmitters? Have you adjusted the input to align with the centre line of the pump inlet nozzle?

Do you have the motor efficiency and power factor vs load? These are not flat figures - what is the rated power / FLA of the motor?

Remember - More details = better answers

Also: If you get a response it's polite to respond to it.

## RE: Motor Power Calculation

Yes the pump curve is based on the standard pump curve given by the pump manufacturer and not the performance test curve. I shall request the vendor to send the performance test curve. The pump curve is attached. The impeller size is 10.3".

Yes I calculated the flow delivered by the pump by goal seeking cell E11 ( sorry there was typo which E13.)

The pump suction and discharge pressures were measured using pressure gauges which are brand new. These gauges are immediately at the pump suction and discharge and match with the center line of the pump.

I took the motor efficiency(93%) and Power Factor(0.83) from the motor nameplate. I don't have the PF vs. Load curve. I will ask these from the pump manufacturer today. The motor is rated for 30HP. The FLA of the motor is 28.6 amperes. We are measuring 28 amperes at the flow we are measuring. The motor is rated to work at 575 volts. Initially we doubted if the correct size impeller was installed. To rule this out I measured the pump shut-off head by closing the discharge valve on the running pump for a short duration( less than a minute). The shut-off matched with 10.3" impeller the one installed by the vendor during our purchase. We measured the flow using a portable Ultrasonic flow meter and measured a flow of 1630 US gpm at 37.29 feet differential head. The flow rate as per the pump curve for 37.29 ft differential head is 2450 US gpm. Since the flow meter can be in error, I used the power calculation methodology to calculate the flow rate. I calculated the flow rate as 2161.5 US gpm, still short of 2450 US gpm by 288.5 US gpm. We need to know the reason for this difference. The pump vendor who inspected the pump said that smaller pump suction line could be the problem. The pump suction line is 10" We know it is supposed to be 14", which we will replace during a one day shutdown. The pump is not cavitating with NPSHA is 50 ft when the NPSHR is 11 ft. I seriously doubt if the 10" line size ( which I admit is not correct size as it was installed before my time) is the real problem. I will do pressure drop calculation on the pump suction side today and check how much we are losing. The higher the loss higher the differential head and lower the flow delivered by the pump. Let me know your thoughts. The reason we are so worried about this pump is that it is feeding the shell side of a product condenser after being cooled through a Cooler which is underperforming. We think we can enhance the Cooler's performance by increasing the flow rate of this pump which is on the shell side of the cooler as the shell side is controlling.

Thanks and Regards,

Pavan Kumar

## RE: Motor Power Calculation

a)You need to get the performances test report of your pump .

b)New gauges do not mean they are calibrated , similar comment for flow meter

c)Use a power meter as already explained in my 1rst reply to avoid duplication of errors .

Note : Curves are missing , Power vs Flowrate , quid Efficiency , What about system curve ?

Good luck

Pierre

## RE: Motor Power Calculation

If you didn't ask for a performance test I doubt there is a specific one for that pump.

However some comments on it

1) It doesn't show efficiency or Power curve which is usual - search the documents you've got for your size impellor to see if its there.

The motor looks right on the limit and with a FLA of 28.6 you are at 28 so not much room for more power there...

2) There is what looks like a system curve? Thin black line which seems different to reality? That seems to be how someone got the duty point of 3000 GPM

Your motor is only just about sized for max flow (98%?). Someone has really cut this pump and motor to the bone.

Efficiency

The single data point provided is at 3000 gpm and it's 79.15% It actually say BEP is 2876 so that could be nearly 80%

At your flow rate of 2450, I would estimate therefore that efficiency at 2450 to be at least 76%, not the 70% you used in your spread sheet unless you've got more data than you've let on.

Using 0.76 as efficiency I get 2350 GPM so only 100 GPM adrift.

That's well within the 3-5% accuracy of standard pump curves.

SO I don't think you have an issue with the pump

I also don't think you've got much spare in the pump before you trip the motor.

But yes - your 10" suction line is too small. At least 12" and pref 16" might give you a bit more flow without overloading the motor.

So as usual in these instances, there is nothing wrong with the pump, but you have some other losses which were not allowed for when choosing the duty point of the motor. Find those flow restrictions and you will get more flow. but watch those AMPS!

Remember - More details = better answers

Also: If you get a response it's polite to respond to it.

## RE: Motor Power Calculation

Thank you very much for your replies. They are very helpful to me. Please see my response below in blue font.

Thanks and Regards,

Pavan Kumar

## RE: Motor Power Calculation

Shows at 2500 GPM an efficiency of about 77%. So even closer to your required flow.

Remember - More details = better answers

Also: If you get a response it's polite to respond to it.

## RE: Motor Power Calculation

I think you did the calculation the wrong way around.

Look at the head difference. That gives you the flow the pump is supposed to be doing. That gives you the efficiency at that flow (2450). That matches the power going in.

So there is nothing wrong with the pump.

If there is fouling on the HX or extra pressure drop in the inlet line then the flow will drop as the differential head rises.

Remember - More details = better answers

Also: If you get a response it's polite to respond to it.

## RE: Motor Power Calculation

The Head-Efficiency- Flow curve I posted for this pump is little different from the one you posted above. Our current Diff head is 37.29 ft based on suction and discharge pressure gauge readings. From the pump curve the flow should be around 2450 US gpm and efficiency at 76% and power around 28 HP. The shaft power calculated with the current load is 28.82 hP. If I put these numbers then the flow calculated by matching shaft power with electrical power supplied by motor is close to 2450 US gpm. I manually changed the pump efficiency as I iterated the pump flow rate. It looks like I did not enter the pump efficiency correctly and that is what lead to the difference in the flow. Is there a way I can get Efficiency vs. flow in tabular format? Can the pump vendor supply to me? I always wanted this.

I can use it to iterate the flow correctly next time.

From this exercise I see the pump is flowing 2450 US gpm. When we measured the flow the flow meter measured 1630 US gpm which looks like it was way off. Since the power consumed is matching with the flow rate of 2450 US gpm, I will consider the flow delivered by the pump us 2450 US gpm. I now tend to believe that there is nothing wrong with pump. I will check if I can put a bigger motor with higher RPM ( of 1800 RPM ) against the current 1200 RPM to get more flow. We will also install a VFD to control the flow as per our requirements. I first got check with pump vendor if the pump can support 1800 RPM motor.

Thanks and Regards,

Pavan Kumar

## RE: Motor Power Calculation

Before you go round making changes first work out what flow you want at what differential head. Then get the pump to match, not the other way around.

I think you're probably better off with a larger impellor and 40 or 50 hp motor in the same range at the same speed. And a bigger inlet pipe.

Let us know how it goes, even in 6 months time.

Remember - More details = better answers

Also: If you get a response it's polite to respond to it.

## RE: Motor Power Calculation

It's always a good habit to select a pump with the max impeller size thus you have the ability to machine the impeller if needed .

Note : You still have the possibility to change for a bigger one.

Good luck

Pierre

## RE: Motor Power Calculation

More HP equals more current.

Then you find your cable isn't big enough

Then the MCC starter isn't big enough

Then there isn't room for a bigger one in the switchboard.

So a "simple" change of motor costs 10 times what you think it will.

So find out the max power motor your existing electrical system will cope with and then work out what that means.

Remember - More details = better answers

Also: If you get a response it's polite to respond to it.

## RE: Motor Power Calculation

Then you find out the MCC is at full load anyways, requiring an MCC upgrade.

Then you’ll find the transformer is undersized for the new MCC, necessitating a transformer upgrade from the power company, requiring all new power supply cables.

Congratulations! Your $2,000, 4-week upgrade project just turned into a $100,000 4-month upgrade project.

That seems to be how it goes for me, anyways…

## RE: Motor Power Calculation

Remember - More details = better answers

Also: If you get a response it's polite to respond to it.

## RE: Motor Power Calculation

Thank you all for your invaluable inputs. It really helped me. I will do my homework and work through this.

My supervisor is proposing another solution which is running two pumps parallelly to get more flow rate (the current one and the standby one.) The standby pump is the smaller one. I will plot the system curve and determine the operating point and make sure that the operating point is not at the shut-off head of the smaller pump. This is the second option we are working on currently. If the motor sizes and system curve support this option we will go this route after understanding all the risks. I will come back with some questions on parallel pump operation, if needed.

Thanks and Regards,

Pavan Kumar