Pump Motor

What u mean to say is that the present driven equipment is with a driver having speed of 1450rpm. Now u propose to replace the driver with a motor having a syn speed of 3000 rpm. Please buy a suitable gear mechanism to couple the driven and driver equipment, Else God save the driven machine.

Hello ahmed952

As Current Maarey says above, the machine will be driven at twice normal speed, if you use a motor which rotates twice as fast as the original.

Most applications do require a motor rated at a certain RPM - in your case 1450 RPM.

You have not advised what type of machine your motor is driving.

Many machines may fly to pieces, or cause injury/death if driven at twice the intended speed.

It would be best to use a reduction gearbox, V-belt pulley or similar to halve the speed of your replacement motor.

If you advise what type of machinery your motor is to drive, it may be easier to advise you.

Reply here, with

Kind Regards....

The motor of 3000 rpm sync is not suitable .Pump will run at much higher speed and will not meet performance desired.More detailed analysis will have to be done to know effects with pump curves.You have to use a motor with sync rpm of 1500 which may run at 1480 or near about depending upon the duty point.Using a gear box or pulleys is the most inefficient method.Basically 3000 rpm is not the right choice.By the way how this has happened?

The question is different.You are replacing the pump set ie both pump and motor of 1450rpm with a set of 2900 rpm.This is ok as long pump has similar characteristics.It is in conceivable that pump set of 1450 rpm is not available.Check with other manufacturers .(By the way it appears u are dealing with a distributor or agent.)Compare the pump dataatleast head, Flow which should be on the name plate.The only disadvantage of a 2900 rpm pump is more wear and tear compared to a 1450 rpm pump.But as long as u are replacing the set it is in order.The question gave us the impression that you are only replacing the motor retaining the original pump which is inadvisable.By the way plenty of pumps in 1500 rpm are available.

Thanks for the response, the existing pump has 1450 rpm motor but this is not available in the market so, the manufacturer is recommending for another pump set with motor running at 2900 rpm and the flow rate is the same. Motor rating is as follows.

Power output: 3.1 kw.

voltage : 380v

Speed : 2900 rpm

phases: 3

frequency: 50 hz

Its a submersible pump with flow 10 L/s\

Head: 1.5 bar

So, kindly can anyone let me know is it advisable to change the pump from 1450 rpm to 2900 rpm with the specification above. Thanks.

I've been watching this thread to see if enough information might surface to answer your question. I'm not sure it has yet, but I believe there may be enough to give you a few answers, one of which may be yours.

It now looks like you have a 1450 RPM motor on a submersible pump that delivers (for my use and ease of figuring, I've converted everything to US measure) 156 GPM @ 50' TDH. You want to replace that pump with one that has the same characteristics, 156 GPM @ 50' TDH, but operates at 2900 RPM. You wonder if that is acceptable. The answer to that question is "Yes". The speed of the pump makes no difference to you or your electrical requirements as long as the pump has an impeller that is smaller than that of the slower motor and is sized for that application as shown on the manufacturer's pump curve.

Now, the other two scenarios that may be interpreted from your given information is that you have a pump that is now operating at 1450 RPM and delivering 156 GPM @ 50' TDH and using a 4.2 HP motor. You want to know if you can run THAT pump with a 2900 RPM motor. The answer to that is a resounding "NO". Operating that pump at 2900 RPM would deliver 312 GPM at a TDH of 200 feet and require 33.3 HP. That is not possible with the motor you describe.

The last scenario is the oposite of the one above, having a 2900 RPM motor on a pump delivering 156 GPM @ 50 TDH and using 4.2 HP. The answer as to whether you can run THAT pump with a 1450 RPM motor is "Yes, you can." The result would be that the pump operating at 1450 RPM wold deliver 78 GPM at a TDH of 12.5 feet and it would do that using only 0.5 HP.

As you can see, the flow as well as the head are diminished considerably by cutting the operating speed in half, but they are increased exponentially by operating the pump at twice the design speed.

I hope that one of these answers suits your question. If not, get back and we will start over.

It seems that the whole submersible pump unit (set) will be replaced.

In that case the new unit would have been selected for the specific duty and it should function ok.

The difference would be in the pump design. the impellers may be smaller or thinner with different vanes and a different number of stages.

Ask the supplier for the two pump curves and verify the duty points for yourselves.

Thanks, Hendrik for your response your assumption is correct the whole submersible pump set will be replaced.

Being a submersible pump motor combination it would be easier for you to replace the whole pump motor assembly togther. You can replace the existing 1450rpm system with a 2900 rpm system. Just bear in mind that you will only be able to match the pump characteristics at one operating point to meet your deisred output of head (in metres) and capacity (in cubic metres per hour), but you will not be able to match the operating characteristics across the entire operating range (Remember that the Head-Capacity curves of pumps vary from manufacturer to manufacturer as well as from model to model)

Hi ahmed952

Poster above is correct. the set should be suitable across the full range of use.

You should also check on the efficiency (power use) of the pump at the duty point(s).

If it draws more power than the previous pump you should complain because it will cost more to operate.

Don't do it, Ethel. Replace the motor with a similarly-rated one that runs at the same speed.

As you are replacing the "unit" and not just the motor, the conciderations would be as follows;

Pump curve should be verified against your existing unit.

Motor power consumption and sf to be at or bellow your existing unit.

I assume your old unit is out dated and needs to be converted.

There are motors available that can be coupled with a VFD (Variable Frequency Drive) to allow for chaning rpms with changing system conditions to maintain operation at your BEP (Best Efficiency Point).

What is driving your desire to change?

Interesting responses so far, but no-one has mentioned a VFD. The fact that a 2900 rpm motor is being considered probably means that motor is already purchased, or is the easiest to purchase in the location of ahmed952. Use the 2900 rpm motor with a properly sized VFD programmed to limit max frequency to 25 Hz. Usually a very simple task. Then you have a 2900 rpm motor that will run at 1450 rpm maximum. The 50% de-rating of speed will reduce torque a bit, but we often use pumps run at 10% of max frequency in pharmaceutical applications with no serious problems. Modern VFD's have torque compensation built in to allow running at slower speeds with minimal loss of power.

VFD can be considered if he retains the pump of 1450 rpm and decides to replace the motor alone with a 2900 rpm instead of 1450.He has decided to replace the pump set (reasons Unknown)and hence no need to consider VFD.In pump applications or fan VFD is useful only if there is a variation in duty .Otherwise it is better to use the correct rpm motor.Apart from the cost for fixed duty using VFD to run the motor at half the sped is very inefficient.

If you read my original post, I assumed he:

1. wanted to keep the original pump, as it still functioned

2. has the 2900 rpm motor already, or is a readily available motor in his location

I suggested the VFD to fit above assumptions so he did not have to scrap a second motor, or buy a new pump, not as the "perfect" solution. Obviously getting a whole new pump set is better, but not if you've already got a 2900 rpm motor setting there with a perfectly good 1450 rpm pump. The original post by ahmed952 left that impression with me. Further re-definitions have changed that.

Yes I agree with you.That was my understanding too and I raised it in my first posting and reply was post 5.I still feel cost new motor may be cheaper or at best equal to the VFD and so it is better to go in for a new motor.

Best regards

i don't think you have to worry about spec the pump with 2900 rpm instead of 1450 rpm. the pump should work if it is designed to run at 2900 rpm with a 10 l/s capacity at 1.5 bar and driven by a 3.1 kw motor.

be sure to check how many stages the new pump will have. since your head requirement is very low, 1.5 bar, i suspected that your new pump will not have that many stages. meaning the heat reject is not great and efficiency may not be so high. according to my calculations, a 3.1kw motor for 10 l/s at 1.5 bar gauge, is very tight for the design capacity.

what are you planning to buy? grunfos or gould?

You have already received detail explaination to your query. I think, VFD is a costly solution due to high initial cost. Better to replace pump unit with new one if operating point with better efficienty achieved by new pump unit.

AMEN, Make it happen and move on to bigger and better things. As improvements are made changes are sometimes necessary.

Go ahead and use the pump with 2900 rpm. Only confirm that the pump characteristic matches.

Thanks for the responses.

We have asked the manufacturer to provide us with the same RPM motor .If he still fail to provide the same configuration then we may ask him to go for an alternate manufacturer but with the same RPM. I will try to update you all. Thanks again for your contribution.

Regards

Hi ahmed 952,

By increasing the speed of the motor to 3000 rpm, this would increase the output flow of the pump, thus shifting the operating point of the pump towards the right of the pump curve. This will move the operating point of the pump from Best efficiency zone to a zone towards the right of the curve(pump curve). This will result in the pump/ motor having the following effects namely: cavitation, shaft deflection (both motor and pump shafts). consult the pump curves of the pump you wish to install, this will have all the curves for the flow/head, kw values for chosen head/flow. According to the affinity laws, Power ( brake horse power of kw) changes directly proportional with the cube of the change in velocity. Also the flow (Q) changes directly proportional to the change in velocity(N or rpm ). Please select the motor/pump that best suits your system curve using the appropriate pump curve. Hope this helps.

If you check I believe the thought was to change the unit, not just the motor.

If the supplier can provide the same pumping and load values as the OLD UNIT. Why not do this and get rid of the old stuff?

This could have been done already and the problem behind the man having an updated piece of machinery.

Dear Mr. ahmad952,

You should not use 2900 RPM motor, in a pump to be driven by 1440 RPM, because the speed will be twice to that of the original speed.

If speed is doubled for a pump (or for a fan) The law of affinity comes into picture, which states

1. Capacity is proportional to speed, C α N

2. Head is proportional to the square of the speed, H α N²

3. Power is proportional to cube of the speed, P α N³

You have not indicated, the actual capacity, head of the pump, H.P. of the motor, and the equipment in the 1^st instance and you mentioned subsequently.

If you use the 2900 RPM motor, new capacity will be 2900/1440 = 2.01 times more, the Head will be 4.055 times more, and the power demand will be 8.167 times more equal to 40.83 HP.

Had it been any other equipment, you can use with suitable gear box, or belt pulley drive,.

Since it is a submersible pump – do not not use 2900 rpm motor.

DHAYANANDHAN.S