How to Convert a Motor's rpm to a Pump's GPM

Reliable operation of an irrigation system is essential for good crop yields.

Efficient, reliable operation of a farmer’s irrigation system is vital if he is to nurture his crops through critical growth stages. However, most agricultural pumping plants achieve only 50 percent efficiency, according to a 2010 field study by Colorado State University, and is often poorly matched to the pumping load. A pump’s capacity to deliver water stems from the maximum head, or pressure, that it can generate, which in turn results from the speed of the motor that drives the pump. Pump flow rate in gallons per minute, or GPM, is directly proportional to motor speed in revolutions per minute, or rpm.

1. Check the manufacturer and model number of your pump. Identify the corresponding pump data sheet.

2. Read the nominal pump motor speed in rpm and nominal flow rate in GPM from the data sheet. Make a note of these values.

3. Note the motor speed in rpm that you wish to convert to GPM. Divide this value by the nominal pump motor speed and record your answer. For example, if the required motor speed is 900 rpm and the nominal speed is 1,000 rpm, the answer is 0.9.

4. Multiply your answer by the nominal flow rate for the pump to give the converted flow rate. For example, if your answer is 0.9 and the nominal flow rate is 4,200 GPM, the converted flow rate is 3,780 GPM.

5. Check your math. Record the result, which is the desired motor speed in rpm converted to pump flow rate in GPM.

Tip

You can generally find data sheets online if you don’t have the correct one for your pump.

Warning

If you intend to increase pump speed to get a higher flow rate, check that the motor can provide enough power to generate the necessary head. Power is proportional to the cube of the motor speed and thus increases more rapidly than flow rate. Running a pump outside its optimum working range will reduce its service life.