motor size calculation 3

[gilles56]

Hi,
knowing the Total head for a pump, how do you find out the coorect motor.
I detremined the power consumed by the pump but i am not too sure about the efficiencies to work it out with the efficiencies (pump and motor)

 

[electricpete]

From a sizing standpoint, you don't need to take the motor efficiency into account. The rated horsepower of a motor is expressed in terms of motor output power delivered to the shaft.

If motor will operate continuously at full load, I believe it is common practice to select motor approx 133% larger than required. This allows some margin for errors in the calculation or unexpected factors which may increase motor load. (better to get one a little to big than a little too small and have to run it overloaded). Also this means that if calculated load is correct, motor will be operating approx 75% - which is often the most efficient point (from motor standpoint) within the motor range.

 

[jbartos]

Suggestion: If the motor - pump are married tight, the motor power factor and efficiency will be higher and also the sensitive protection of the motor will protect the pump from hidden mechanical malfunctions or signal mechanical deteriorations. Another aspect is a motor standard size. A custom motor size is not recommended.

 

[Matador]

Here is what I do.

Using the Cameron Hydraulics handbook calculate the brake horsepower. I assume you know the flow in GPM so use the formula bhp=(gpm*H (ft)*sp gr)/(3960*eff) The efficiency number is the pump efficiency and can be found on the curve.

I use two checks.
(1)- I use the 90% rule which means under operating conditions the Hp should not exceed 90% of the nameplate Hp. Talk to your process people about the highest possible flow demand which could involve a larger size impeller in the future. Take this into account when choosing a motor.
(2) - Where small motors are involved I select a non overloading motor. Use the pump curve for the chosen impeller diameter and determine the Hp for the maximum possible flow. Then select the next larger size motor.

Hope this helps

 

[jbartos]

Suggestion: It appears that the 90% rule applies to standard motors, SF=1.0. If a higher SF motor is used, one could possibly get the tighter match for the power factor and efficiency.