Squirrel Cage Efficiency When Coupled with Pump

The pump and motor operating points are defined by the operating curves (or their intersection point).

On this point you will get the power drawn, speed and the corresponding efficiency of the motor. How are you sure that 500HP motor will be enough when you were earlier with 700HP?

The power required by any fluid mover is equal to the rise in pressure multiplied by the volumetric flowrate, all expressed in compatible units, divided by the efficiency. If the answer exceeds, oh, say 450hp, then "don't do it, Ethel" is applicable.

The motor and pump efficiencies are 2 different things.The pump has to be selected with max efficiency for the given head and flow.The motor efficiency at 100,75 and 50 % will be known from catalogs.Near 100% load (Practically85 and above) the efficiency will be close to full load efficeicy.for the motor rating mentioned you should choose one with atleast 95% efficiency.Also motors are availble with effieciency remaining constant (almost) from 60-100%) So any minor variation in load does not make the motor operate at lower eff.points.Now the efficiency of the total system that is that of motor and pump together will be product of the eff.of motor and pump.If pump eff. let's say is 80% and motor is 95% system eff of the two will beonly 0.76 or 76%.Now have you studied the existing pump operating points and analysed the need for change.lot more has to be done on these things.

Dear Sir

Well answered and thank you.We have design calculation for 500 HP after converting Exisiting 700 HP yet to be modified with different stator winding.The efficiency calculation are 125 %---94.6, 100 %--- 94.5 , 75 %---94.2.We are interested to know, as informed by you, knowing pump efficiency by actual measurement and design efficiency of motor will ensure the motor is operated at full efficiency.How we can check by physical measurement of voltage, current etc that motor efficiency can be worked out to prove that the modified design 500 HP Motor will run at maximum efficiency

Regards

Sadasivam

The design calculation is one thing and actual will be slightly different.The correct procedure will be to test the motor and determine the eff. by type test.But for such large motor and that too vertical it may be difficult to locate a place where u can type test.Further in a motor the out put is mechanical so eff. determination becomes difficult in such cases where it is rewound and original eff. is not known.One way is to determine combined eff by measuring the pump out put Measure motor input power .If u have determined pump eff then u can easily determine motor efficiency.it is an indirect method.

Dear Sir

Thank you for your prompt reply.We do understand design efficiency and actual efficiency will not be one and the same,As indicated we have to measure input power and pump output , compare with earlier design 700 HP motor drive pump practically on load at site

Regards

Sadasivam

also compare with 500Hp design Eff and see the variation.Any way if u had measured the energy consumption u can measure now and compare.if the no of units are lower than earlier then it is an improvement.best wishes

After carrying out the actual testing on the pump and recording the power at different loads (including the worst load scenario) - may be the overload test, decide on the motor.

If you can accommodate withing 500HP, you will not only save in motor cost, but also in the efficiency as well as power factor. A motor loaded at 70% load is not going to be good on either aspect.

Yeah, who cares about the efficiency if you don't have the power to produce the flow/pressure you require.

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You need to provide much more system information for any one to advise you here. Keep in mind that motors draw the energy they need to do the work load applied. You may use more energy with the smaller motor due to overload conditions of the 500 hp motor. Please inform us info on the motor and its controller and info on the volume and pressure that you are working with so we can better advise you.