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).


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). Courtesy: Grundfos Pumps Corp.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.

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