Understanding Pump Curves
Overloading and Non-Overloading
The two curves below illustrate the fact that at a fixed speed, the impeller diameter determines the load placed on the motor at any given flow rate, and thus how much power the motor will draw from the service at that flow rate.
Impeller diameter can be reduced or "trimmed", such that the motor cannot be overloaded nor loaded into the motor service factor at all points on the pump curve. Irrigation Craft trims all pumps over 5 horsepower to a non-overloading diameter, to provide longer motor life and to allow for field problems such as voltage unbalance and low voltage.
Below are two pump performance graphs for the same pump driven by a 30 horsepower motor. The only difference between the two graphs is the impeller trim.
The first graph shows the pump with an OVERLOADING impeller trim.
The second graph shows the same pump with a NON-OVERLOADING impeller trim.
The hp curve shows the power required to run the pump. The horsepower curve is the bottom green curve line corresponding to the bhp scale just to the right of the green curve line. "bhp" stands for brake horsepower.
Graph A - Overloading 30 hp Pump Curve
Above is a 30 horsepower pump curve. Follow the green horsepower curve from left to right and notice how the horsepower required to operate the pump goes above the 30 horsepower scale line at 270 gpm. This means that flow rates greater than 270 gpm cause the motor to operate at greater than 30 horsepower (into the service factor or beyond). Worse yet, when the pump is operating at BEP (Best Efficiency Point), the motor is operating in the service factor. That means if you operate the motor at the flow rate where the pump is most efficient, you are operating the motor in the Service Factor.
Many pump manufacturers provide their standard pumps with impellers that use the service factor of the motor. If you wish to have the impeller trimmed to a non-overloading diameter, you must specifically state that desire and you may pay extra for the custom trim.
Irrigation Craft does not normally allow this on our pumps. See the pump graph below to see the same pump as trimmed to Irrigation Craft specifications.
Graph B - Non-Overloading 30 hp Pump Curve
Above is a 30 horsepower pump curve showing the performance of an impeller trimmed to Irrigation Craft specifications. Follow the green horsepower curve from left to right and notice how the horsepower curve goes exactly to the 30 horsepower scale line but does not exceed 30 hp at any point on the graph. This means that the motor cannot be loaded beyond 30 hp at ANY flow rate.
The advantages of Impeller Trimmed to Non-Overloading Diameter
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High Flow Rates do not cause the motor to overload. |
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Electrical problems (i.e. low voltage, bad connection, unbalanced voltage, etc.) may cause this non-overloading pump motor to operate in the service factor, which is available because the Service Factor was not used. |
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When problems occur on a system with another manufacturer's overloading pumps that were normally operating in the service factor, the problem now causes the pump motor to draw power beyond the service factor where damage can occur quickly. |
DID YOU KNOW?
If you install the same 20 horsepower pump wet end on a 20 horsepower motor and also on a 5 horsepower motor, the pumps will perform similarly? If the 5 horsepower motor could get spinning, the big difference would be that the 5 horsepower motor would quickly burn out because it would draw the amount of current required to handle the load, 20 horsepower's of electricity flowing through a 5 horsepower motor.
NEC® allows a 30 horsepower motor with a 1.15 service factor to run out to 34.5 horsepower. Because NEC® allows it, many manufacturers use the service factor in their products. According to NEMA® MG1 Section 9.15.1 motors operating continuously in the service factor may have a shorter life expectancy, depending on ambient temperatures and on duration in the Service Factor. A motor operating in a hot environment is particularly sensitive to operation in the service factor.
Learn more about this subject by following the links below.
Related Subjects on this Website:
Glossary - Full Load Amps (FLA)
Glossary - Service Factor (SF)
Follow the links below to pages focusing on other types of information provided in the Pump Characteristic and more, including graphs and explanations.
