Pump Torque vs. Speed Calculation and Motor Selection. 1

[Pavan Kumar]

Hi All,

I have sized a Cooling water pump and determined a 75 HP. Our plant inventory has a 75 HP motors rated for 1780 RPM. I have the motor's Torque vs. speed curve, as attached. I want to calculate the pump's required Torque vs. speed curve and overlap it with the motor's Torque vs. speed as suggested in the link below in order to make a correct motor selection. The motor will be operated with a VFD and I would need to a speed of 641 RPM to get my duty point.

https://www.pumpworks.com/centrifugal-pump-motor-torque-speed-curves/#

1. I want to know how I calculate the plot the pump torque vs. speed curve. Please suggest me reference material that I can read and work out of.
2. I want to know how the VFD will affect the motor selection and as it would affect the Torque generated by the motor?.
3. I want to know if the motor torque as suggested in the link above is above the pump torque curve until the full load speed then is it ok to say the selected motor is good enough?.

Thanks and Regards,
Pavan Kumar

 

[1503-44]

Eso es.... almost.
Power reduces, but torque remains constant. That is because speed also falls exactly proportional to the drop in Hp. Thus the torque remains constant until reaching full load.

Torque = Hp * 5252/ Speed

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Pavan Kumar]

Hi 1503-44,

Eso es.... almost.
Power reduces, but torque remains constant, since speed is also proportionally reduced.

Yes that is correct. The vendor said the same thing. He said that motor torque remains constant at 437.5 ft-lbf between 5 - 60 HZ. I need 523.7 ft-lbf which the motor will never generate. The only option seems to be to use a belt drive if I have to use a 75HP, else I need to go to 100 HP motor(which I don't want to).

 

[1503-44]

Exactly Correct.

A belt or gear box with a 2.5:1 ratio will do the trick.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Pavan Kumar]

Hi 1503-44,

I understood the point after so much discussion. As mentioned earlier we already have 75HP , 1780 RPM motor in our warehouse. I can use that with 2.5:1 sheave diameter ratio.

N2/N1 = (D1/D2)

700/1780 = 1/ 2.5 = D1 / D2.

We can put a VFD on to this to fine tune the RPM lower than 700 as we need.

Thanks and Regards,
Pavan Kumar

 

[1503-44]

Yes that's right.
Glad we got there.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Pavan Kumar]

Hi All,

I want thank every body in this thread especially 15034-44 and Littleinch to help me understand this concept. For this project I am going to buy a 75 HP, 1200 RPM motor and belt drive and VFD t get the required Torque.

Thanks all again.

Thanks and Regards,
Pavan Kumar