Your questions answered: Designing for Minimum Flow With VFD Operation

Jim Swetye, senior technical trainer at Grundfos Pumps Corp. in Ohio tackled unanswered questions from the June 14, 2016, webcast on minimum flow considerations when using variable frequency drives (VFDs).

By CFE Media June 21, 2016

Jim Swetye, senior technical trainer at Grundfos Pumps Corp. in Ohio tackled unanswered questions from the June 14, 2016, webcast on minimum flow considerations when using variable frequency drives (VFDs).

Q: From the affinity laws, what is the equation for calculating the required frequency if a lower pump speed is known?

Jim Swetye: The affinity laws are not needed to make this calculation. Instead, consider a 60 Hz motor designed for a nominal 3,600 rpm. Note that dividing 3,600 rpm by 60 Hz equals 60. Slowing the motor to 50 Hz results in a nominal speed of 3,000 rpm. Note that dividing 3,000 rpm by 50 Hz also equals 60. This value of 60 is the key to the answer to your question. If you know that "the required frequency" at a "lower required pump speed" is 2,760 rpm on the 60 Hz nominal 3,600 rpm motor, then 2,760 rpm divided by 60 equals 46 Hz.

Q: Are the control curves you showed programmed at the control system or preprogrammed into the VFD?

Swetye: This varies by manufacturer. Some are able to preset the control modes, while other systems require field setting. Go to Grundfos Technical Institute/Training Catalog/Recorded Webinars: "Variable Speed Pumping" to learn more about variable control options.

Q: What is the problem with matching a VFD with a non-inverter duty rated motor?

Swetye: That question has a complicated answer. A simplification is that for centrifugal pumps, an inverter duty motor is not required as long as you use a premium efficiency motor. Any other type of motor has the potential to be damaged by temperature rise. Go to Grundfos Technical Institute/Training Catalog/Recorded Webinars: "What is Inverter Duty Anyways?" to learn more about these options.

Q: What are your recommendations for using "sensorless" pumps with VFDs for a closed system?

Swetye: This is a good alternative to positioning a sensor at the farthest emitter from the pump. The most accurate scenario would be to have a sensor prior to the pump, and a second sensor at the far end of the critical circuit. The second best scenario would be to have either a differential pressure sensor on the pump, or sensors before and after the pump. A sensorless system will monitor differential pressure, temperature, and other parameters across the pump, and will adjust pump speed and flow output to attempt to match the system head curve. The sensor approach does this directly, while the sensorless approach does it with calculation. The sensorless approach has simplified installation, lower cost, and less potential for maintenance issues, but does not have the exacting degree of ideal performance as does the sensor type system.

Q: Are there better VFDs that work more efficiently with pump motors? in other words, are there better matched VFDs and motors?

Swetye: The state-of-the-art concerning VFDs has improved dramatically in the last 20 years, so a wide variety of VFD manufacturers have created products that work well with a wide variety of motors. A recent trend has been to integrate the VFD directly onto the motor. This probably has the best potential for optimizing the interaction of the drive/motor combination.

Q: How good are VFDs at providing a soft start for a pump to avoid pressure surges?

Swetye: This is an area in which the drive can provide great benefits. In effect, the VFD is also a soft-start device. With many VFDs, you can alter the ramp-up time to full operating speed. However, there are limitations.

Q: Will a motor fail prematurely if operated below 20% of its rated speed?

Swetye: With general-purpose motors, this should not be a problem because of low torque loads when using a centrifugal pump.

Q: How do you determine minimum flow?

Swetye: This varies widely by pump model. I have seen it range from perhaps 30% of best efficiency point (BEP) flow all the way down to 10%, or even less. But because this is a critical topic and pump damage can occur as a result, you should always consult with the pump manufacturer before proceeding.

Q: What are the VFD’s effect on pump efficiency?

Swetye: This is another complex question. If you are operating in a system with no static head, such as a closed-loop hydronic system, the system curve will begin a 0 gpm flow and 0 gpm head. In this case, the pump’s constant efficiency curve will virtually match the system curve. Therefore, the pump efficiency will almost be constant from maximum flow to minimum flow. On the other hand, in an open system that has some degree of static head, the efficiency will drop, moving to the left of the BEP on the curve. There is some good training out there on this topic. One training resource is

Q: How can a VFD be programmed to operate close to the BEP of the pump?

Swetye: This is a great question and there are many ways to do it, depending on the application and many system specifics. There is at least a dozen or so control methods that provide various ways of optimizing efficiency. Go to Grundfos Technical Institute/Training Catalog/Recorded Webinars: "Efficient Pump Selection and Control" to learn more about these options.

Q: What is relation between pump selection, flow control valves, and controlling pump speed? What are the best ways to deal with flow that is not needed at pump minimum flow?

Swetye: This is another complex question, and the answer may vary somewhat depending on the application. I’ll use boiler feed as an example. As steam demand and water level in the boiler vary, a traditional system might use a fixed-speed pump with a feed valve between the pump and the boiler, and a bypass line back to the pump’s suction source, which is usually a de-aerator tank. When the water level in the boiler starts to rise, the feed valve will begin to modulate closed. This causes the pump head to rise, which causes a reduction in flow. However, there is a minimum flow required by the pump. So a bypass line is installed between the pump and the feed valve. This sends a regulated amount of minimum flow back to the de-aerator so the flow to the pump does not drop too low. However, this system wastes head across the valve, which wastes kilowatts. It also wastes the bypass flow back to the tank, which also wastes kilowatts. A better choice is to use a variable speed pump that receives its modulation signal directly from the boiler level sensor. The pump speeds up and slows down to meet demand. There is no feed valve to waste head, and there is no bypass piping to waste flow. All of those kilowatts are saved. If the demand for flow gets too low—even for the variable speed pump—a minimum speed can be programmed into the drive so that the unit shuts down during periods of very low demand.

Q: What are the criteria for stepping down the number of operating pumps—flow (gpm) or head (feet)?

Swetye: There are many different options for sequencing in or out of pumps in a multiple-pump parallel system. These include sequencing based on flow, current, speed, and efficiency. Sequencing based on efficiency has the potential to provide the lowest overall operating costs while avoiding cavitation and other issues.

Q: Please discuss sensorless VFD control for pumps.

Swetye: There are many applications in which sensorless control can be used to good advantage. One example is in a secondary piping system for heating or cooling in which there are multiple fan coils. As valves for each coil modulate open and closed, the friction head in the system changes, which causes an associated flow change out of the pump. In a traditional system, a sensor might be located at the last coil, and the pump speed, flow, and head would be controlled accordingly. But friction loss head values are pegged directly to flow rates, and these changing friction head values can be calculated and anticipated. Consequently, it is not necessary to use a sensor to detect changes. Instead, programming in the drive can anticipate the estimated reductions in friction head and modulate the pump speed to achieve the desired flow without the use of a sensor.

Q: What is the future of variable flow pumping systems?

Swetye: The future looks bright because of the vast potential for energy savings and system optimization.

Q: What will be the minimum required torque?

Swetye: Because centrifugal pumps are variable torque devices, the lower the speed, the lower the torque; and the higher the speed, the higher torque. There is not necessarily a minimum torque that is required, but there are certain minimum speed considerations, which are covered in this webcast.

Q: What is the potential energy savings when using multiple pumps with VFDs?

Swetye: Many studies by private industry and government departments have indicated that energy savings of 20% to 50% or more are possible.

Q: What are the recommendations regarding trimming impellers on VFD applications?

Swetye: It is almost always not recommended to cut the impeller diameter when using variable speed. Pump speed can be regulated to achieve almost any flow and head, so why waste the machining time and expense? There are exceptions, but they are somewhat rare. Go to the free Grundfos Technical Institute/Training Catalog/Recorded Webinars: "To Trim or Not to Trim? That is the Question" to learn more about these options.

Q: Hydraulic Institute recommends selecting pumps in the preferred operating region (POR). Does variable speed operation impact POR at low flow operation?

Swetye: In systems with no static head, such as closed loops for hydronic heating, the POR remains pretty constant from maximum speed to minimum allowable speed. However, in systems with static head-and the more static, the greater the variation—the pump can easily move out of the POR when operating to the left of the BEP. Go to Grundfos Technical Institute/Training Catalog/Recorded Webinars: "Efficient Pump Selection and Control" to learn more about these options.

Q: What are the design guidelines for when VFDs should be specified and when they can be omitted?

Swetye: If neither the flow nor the head varies, a VFD probably will provide no energy-saving benefits. However, many or even most applications in this world DO have either varying flow or head. In these scenarios, a VFD should be considered because there are many benefits—including reduction in energy use.

Q: When should you use VFDs? What pump modifications are required when using a VFD? Also, discuss how to properly size a pump when VFDs are used.

Swetye: A VFD can be used with benefit in almost any application with varying flow or head. As a rule, no modifications are needed for a bare centrifugal pump when operated in variable speed mode. Because there are so many different applications and other variables, it is impossible to answer in this short space. We suggest you seek additional training on this topic. One training resource is

Q: Please discuss how to read and interpret a pump curve.

Swetye: There are around 19 key factors on pump performance curves that must be thoroughly understood to ensure the proper application of VFDs to centrifugal pumps. Go to Grundfos Technical Institute/Training Catalog/Recorded Webinars: "How to Read a Pump Curve" to learn more about these options.

Q: I was taught that brake horsepower is the available horsepower available before system losses. Am I missing something?

Swetye: I would define it this way: Brake horsepower is the load the pump is putting onto the motor. And then, because the motor is less than 100% efficient, it is putting a somewhat higher load on the drive if one is used. Because the drive is not 100% efficient, it is putting a slightly higher load on the incoming power system.

Q: Please comment on centrifugal pumps operating in parallel. Does this really add to flow?

Swetye: It does add to flow, but to what degree can vary widely. If you have a "flat" system curve, there can be a significant increase in flow that can approach twice as much gpm for systems with two identical pumps. However, a very "steep" system curve might result in very little increase in flow through the system.

Q: I have two questions:

(1) In selecting a pump, what is the acceptable range for BEP?

(2) What is are advantages and disadvantages of lower speed (1,450 rpm) versus higher speed pumps?

Swetye: (1) See the Hydraulic Institute standard on the allowable operating region for a comprehensive look at this topic. In general, the allowable operating region is defined by the manufacturer, and is found near the extreme left and right sides of the curves, but the exact locations can vary widely. The POR varies, and you must consult the standard for particulars. It can range from 70% of the BEP flow to 120% of BEP flow, but it also can have other values, depending on pump design specifics.

(2) There are many advantages to both higher and lower speed pumps depending on the application. Higher speed pumps sometimes can reduce the initial cost. Some will argue that a higher speed pump will wear out faster, but others will argue the opposite. You must conduct what is known as a "lifecycle cost analysis" to enable you to make a sound decision.

Q: Is energy saved by operating two "headered" pumps in parallel?

Swetye: This arrangement offers the potential for energy savings, but it will not do so in all circumstances. There are many factors involved in making the determination. Among other things, you must consider the system curve, sequencing method, and control mode.

Q: During design of a system with a VFD, can you predict the harmonic frequencies?

Swetye: The value for the harmonic frequencies will be the same whether you are using a fixed speed pump or a VFD because they depend on the physical structure of the system, and not the speed of the pump. These can be calculated by a properly trained engineer.

Q: You mentioned system natural frequencies, something I have never heard of before. How does one go about determining the natural frequency (or frequencies) of a particular system?

Swetye: The value for the harmonic frequencies are based on the physical structure of the system, and can be calculated by a properly trained engineer. These can be very involved calculations.

Q: For a typical end suction HVAC pump, is there a rule-of-thumb for minimum speed to avoid pump seal damage—or at least an order of magnitude—is it 50% or 10% speed and seals will be OK?

Swetye: I have seen some manufacturers of small end suction pumps allow minimum speeds of 50% of full speed, while others allow 30% or less. As mentioned, it is best to check with the manufacturer.

Q: Can you elaborate on control curves? On the constant pressure curve, how did you get from the straight control curve to a minimum acceptable speed?

Swetye: On the constant pressure curve question, find a multispeed curve that shows a variety of speeds from maximum to the minimum allowed. This curve also must identify the minimum allowed operating line. Find your specified duty point (usually on the maximum speed curve) and draw your horizontal constant pressure line from the duty point back to the point where it intersects with the minimum allowable operating line. At this point, you should be able to interpolate between the curves at various speeds to determine the minimum acceptable speed.

Q: What is the energy cost for using a VFD—at any speed?

Swetye: For most, but not all, applications, energy can be saved by using variable speed pumping. If the flow and head never change, a VFD is not an energy saving solution. But if there is variability in flow and/or head, there is potential for energy savings with a VFD. If the range of change is small, the potential for energy savings is less. The greater the variability, the greater the potential for savings. In many applications, energy use can be reduced by 40%, 50%, or even 60%, and a payback of under a couple of years can often be experienced.

Q: What are some more common issues for running at 50% below nominal rpm?

Swetye: The following list includes some of the potential issues:

  • Operating outside of the allowable operating region
  • Poor pump efficiency
  • Lubrication problems
  • Solids handling issues
  • Self-priming pumps fail to prime
  • Motor bearing loads and lubrication
  • Motor cooling on submersibles
  • Motor thermal protection issues.

Q: How does one obtain and construct a pump control curve on pump chart?

Swetye: Most modern pump selection software programs enable you to automatically add a system curve to the performance curve. However, you must input the correct data concerning your systems static and dynamic head. Most pump manufacturers are happy to provide training that demonstrates how to use this software function. However, before you use the software, it is highly recommended that you learn how to do the calculations and manual plotting. Again, most pump manufacturers provide training on this.

Q: Can VFDs be used to compensate for overdesigned pumps or should the offending pump be replaced?

Swetye: It depends on how severely oversized the pump is. This requires some careful cost analysis. In many cases, this is a good option, but there are many factors involved in making the decision.