At Holland we size pumps for sanitary applications every day. After selecting the pump type and model, determining the size of the motor required to drive the pump is the other half of the problem and the part that people tend to struggle the most with. This can be especially challenging with centrifugal pumps. Sanitary centrifugal pumps follow the affinity laws, which we covered in a previous post. The Affinity Laws tell us that as discharge head decreases, flow increases, and so does power draw. So selecting a pump and motor combination that will function under all system conditions is essential. This post will take a look at what is non-overloading motor power and why it is critical in sizing a centrifugal pump.
Non-overloading power is not just another number on your pump flow curve. A motor is overloaded when the required horsepower exceeds the motor capacity, causing the motor to trip its overload. A larger motor which can sustain itself at these conditions is considered non-overloading. So the non-overloading power of a centrifugal pump motor is a motor of sufficient size to be able to sustain itself at any point along the pump’s curve.
To understand non-overloading power when viewing a pump curve, first find the curve for your impeller size. Next, locate the dashed horsepower lines. If the point on your pump performance curve (a specific flow and pressure) is to the left of the dashed line, the motor size of the dashed line is not going to be overloaded. If your point is to the right of the dashed line, the motor will be overloaded.
In This Application, there is a 7 1/2 Horsepower Requirement at the Duty Point, but the Non-Overloading Horsepower is 15
Now, follow your pump curve all the way out to its maximum flow. The last motor size which every point along the pump curve falls to the left of the dashed horse power is the non-overloading motor size. This also corresponds to the motor size a program like Pumpflo will specify- because as you can imagine, overloading a motor is not good.
Selecting a non-overloading motor usually means selecting a motor 20-30% larger than the power required at the design point. This means that we will pay a slight efficiency penalty for selecting a non-overloading motor and also a higher upfront cost. The operational problems and costs we avoid, however, far offset these relatively minor upfront costs.
So, do we think you should always use a non-overloading motor when sizing your sanitary pump assembly? It depends. If you really know your application and are sure that the pump will always run at what you have chosen as your duty point, using a non-overloading motor may be overkill and not necessary. A good example of this would be a pump that has a flow orifice downstream of it that will always restrict the flow rate. But if you think there may be times when the flow rate of the pump will run out the curve for short period (think starting up a pump when the lines are empty), choosing a non-overloading motor can be cheap insurance.
Another potential advantage of selecting a non-overloading motor is flexibility. At some point, even after your best efforts of designing a system, you flow requirements may change. If your flow requirements increase, meeting the new duty point can be as simple as putting a larger impeller on the pump…..if you have a large enough motor. If you size your motor for your exact duty point, it may not be large enough to handle the new duty, resulting in having to buy a complete new pump assembly.
So for your next centrifugal application, take into account the range of system conditions and how this will affect the pump. Be mindful of the affinity laws and use them to understand how your pump is going to perform. As a general rule of thumb, select a motor size which is to the right of every point along your pump curve. Even if this is 2 or 3 sizes larger than your duty point, the operational cost savings may far offset the higher initial cost and efficiency. If you have any questions about selecting a motor for your sanitary pump application or need help getting started, contact a Holland Sales Engineer today.