# motor formula

Discussion in 'Homework Help' started by spark8217, May 7, 2012.

1. ### spark8217 Thread Starter Member

Aug 29, 2011
64
2
How to calculate the size of a motor to pump continuosly @ 500rpm?

delivers flow of 0.05 m3/s
pressure difference of 350 kpa
overall efficiency of 70 %
efficiency of reducer is 95 %
assume the centrifugal pump has a approx quadratic torque - speed characteristic:
would appreciate a helping hand for a formula to work with ? i have tried to research the question for a solution or a formula to work with but to no avail and also - what is meant by quadratic/ torque speed ?

thanks

2. ### spark8217 Thread Starter Member

Aug 29, 2011
64
2
is this correct ?
Water power is a simple way to equate the pump to mechanical power based on mass, distance lifted against gravity and time.

Power_W = (Mass_kg x Gravity_m/s/s x Height_m) / time_s

Where:
Gravity is 9.81m/s/s
Mass is 0.05 * 1000kg for one cubic meter of water
Height (head) is 35.7m for 350kPa.
Time is 1s, for flow per second.

Now increase the power for the losses = Power_W / (0.7 * 0.95)

This is the mechanical power from the motor.
You have stated the speed is constant @ 500 rpm, so torque - speed is not needed. The torque from the motor (used by the pump) can be calculated as:
torque_N.m = (60000 * Power_KW) / (2pi * RPM)
cheers !!!

3. ### mlog Member

Feb 11, 2012
276
36
Power is equal to volumetric flow times pressure. All you need is to account for losses and to get your units in order.

spark8217 likes this.

Aug 29, 2011
64
2