Changing Variables

CONSULTING-SPECIFYING ENGINEER: Pumps, drives and motors are a good example of integrated engineering in action. What are some of the best applications for such combinations? Where are they not appropriate?

RISHEL: The best applications for variable-speed pumps are systems with very broad load swings such as from 500 to 10,000 gallons per minute. The poorest applications are for load where the flow in the system seldom varies, such for computer centers.

PEREGOY: Any pump application that either benefits from reduced flow or pressure, or requires varying flow or pressure to regulate the process is a good candidate for an adjustable-speed drive (ASD). Assuming head pressure is constant, pumps produce flow rates that correlate linearly to the motor speed that drives them, so it is very simple to design the system to control the flow rate. Centrifugal pumps offer the added advantage of significant energy savings when operating from ASDs at less than full speed. The energy saved when operating a centrifugal pump at half speed is greater than 80% of the energy used operating at full speed and load.

ZAK: Any hydronic system with fluctuating loads in the range of 30% to 100% is the best candidate for variable-flow applications. It is important for the designer to consider the overall efficiency of the pump and motor assembly. They should not be evaluated as single components.

Another consideration is the part-load efficiencies of the primary equipment, such as the chillers and boilers. Matching system variable flow with the equipment’s part-load efficiencies ensures that the system will always operate at its optimal level as the loads fluctuate. A misapplication occasionally occurs when variable-speed pumps are used without consideration for the overall system operation.

CSE: On the subject of ASDs/variable-frequency drives, how necessary are they in relation to improvements in HVAC technology that now allow some systems to better handle varying flow rates and pressures?

RISHEL: Obviously, constant-flow systems do not need variable-speed pumps. But I don’t think the proliferation of variable-speed devices has created any systems that no longer need variable speed.

PEREGOY: In the applications where the speed required is less than 100% but doesn’t change, it is better to size fixed-speed components to operate at the desired speed than to apply a drive and never change the speed. In addition to the added cost of a piece of equipment that might not be necessary, certain problems become more likely to manifest themselves when the speed is not changed.

ZAK: Variable-speed pumping systems are not the answer to rescuing system flow problems, nor are they the ultimate answer to efficient pumping. For example, a system with 50-hp pumping requirements and multiple air-handling units with chillers would certainly benefit from a variable-speed application. But, a system with two small boilers and a 1-hp pumping requirement would not be cost effective with respect to variable-speed pumping. That’s not to say that the smaller system would not benefit from variable flow. However, the equipment costs would result in a higher payback period.

CSE: For variable-speed systems, is it absolutely necessary to utilize motors specifically designed for use with VSDs and ASDs?

YATES: Any three-phase induction motor will operate on an ASD. The question is for how long. If one is unfamiliar with all the nuances of a particular application, buying a motor with built-in insurance is a good idea. Of course, having flood insurance on a mountain bluff would be pretty ridiculous, but someone will sell it to you.

ZAK: It is important for the motors to be capable of maintaining maximum efficiency throughout the design operating range.

PEREGOY: Another important factor to consider is stress. Pulse-width modulated drives induce stress on motors that is not present when operating from 60-cycle sine wave power. There are two main reasons: reduced cooling airflow over the motor due to the lower operating speed and the synthetic output sine wave the drive produces. Of the two, stress caused by the synthetic sine wave is the most difficult to handle. Most motor manufacturers have done empirical testing to determine acceptable minimum speed ranges and resulting temperatures at which motors may operate. What are much more difficult to guard against are the voltage spikes—referred to as dv/dt in the industry—associated with IGBT output power devices, used in most present-day drive designs.

CSE: What should an engineer look for to avoid some of these stress problems?

PEREGOY: Some drives produce output waveforms that are more stressful than others due to motor insulation systems. Manufacturers had to develop enhanced insulation to guard against voltage spikes, so the length of the leads from the drive to the motor has a significant impact on the electrical stress created in the motor. Drive manufacturers publish their maximum recommended lead length for their respective products and ratings, and engineers should verify that their designs fall within the manufacturer’s guidelines.

CSE: One of the challenges in bringing variable speed into the pump design equation is that it is difficult to predict performance. Part of the problem seems to be that there isn’t a simple way to access and calculate data that would help a specifier determine true motor-to-drive efficiency at partial load and speed. What, if anything, can be done to facilitate this task?

YATES: Typical application data is available, but in most cases, specific pump/motor/drive data is what is of interest. Unfortunately, not too many companies manufacture all three pieces, so matched testing is usually not available. A pump curve can be very complicated when all the system variables are accounted for, then multiplied by the variations created by the motors and drives. It is possible to create, but difficult to present in an easy-to-read format for a reasonable cost.

ZAK: Several factors are necessary to predict performance and make a selection. First, pump head capacity curves at variable speed must be analyzed. Typically, most HVAC pump manufacturers will certify pump performance to around 40% of full speed. Beyond that point, the pump performance becomes more of a variable and is difficult to predict.

The good news is that most pump manufacturers have software for analyzing performance and selection. When analyzing the pump curves, the designer should pay particular attention to the parabolic path of the best efficiency point curve. This will help the designer understand the variation of pump efficiency with speed changes. For larger applications using parallel pumping, it is advantageous to have the pump operation and loading characteristics factory-tested and certified prior to installation. This will provide the engineer with a composite system curve, which will aid in optimizing the system’s operational control points. For example, it may be more efficient to operate two pumps at 20% capacity each, than one pump at 40% capacity.

PEREGOY: As Mr. Zak notes, most ASD manufacturers have software tools that calculate energy savings based against other methods—either restricting the input or the output for controlling flow rate. The programs are based on a number of input parameters such as duty cycle—how long the unit runs at different speeds, the efficiency of the motor and the cost of energy. Some programs even address financial requirements like internal rate of return.

There are also some third-party software packages available. For example, the Electrical Power Research Institute has produced software for evaluating applications. In addition, pump manufacturers publish software that helps calculate efficiency and performance under various operating conditions.

I have even heard rumors that some pump companies are revising their application software tools to incorporate a more intuitive and user-friendly way to determine performance when operating at varying speeds.

CSE: Without turning to software, what other options do designers have for delivering the greatest level of efficiency?

PEREGOY: Engineers are expected to be experts in a multitude of different products, but in reality, it is very nearly impossible—especially in the ever-changing power electronics world. Keeping up with drive technology, alone, is a full-time job. The other option is to have a reliable source to help. Drive/motor and pump manufacturers have application expertise for their respective products, but neither is an expert at the other’s product.

YATES: Sticking to the most efficient motors and drives will generally give the best results in overall operating energy efficiency. But quantifying exact levels requires additional testing with the specific components of pumps, motors and drives.

ZAK: Also keep in mind that pumps, drives and motors, although three distinct devices, are in reality, only a single component in establishing system efficiency. It is possible that the pumps and motors are the most efficient devices in an HVAC system. But this efficiency will rapidly diminish as a result of poor flow control, substandard piping design and failure to properly match the variable flow with the part-load efficiencies of the primary equipment.

CSE: To that notion, what kinds of new controls procedures are out there that might be able to enhance system efficiency?

PEREGOY: A supervisory pump controller, serially connected to the drive unit, has been developed that regulates motor torque and subsequently, pressure in the pumping system. It is a much more accurate way of controlling system efficiency, and we are seeing better overall system efficiency with this configuration than that achieved by controlling speed alone.

RISHEL: In the HVAC field, the advent of adaptive control for programming pumps ensures that the most efficient number of pumps are operating at any load on the system. Factory set points no longer have to be relied upon.

ZAK: Once again, there is software that will calculate the theoretical pumping power requirements and compare this criteria to the actual measured power. The program then determines the optimal efficiency point with single or multiple pumps operating in a parallel configuration. The software will then evaluate and select the most efficient operating point using composite system curves, which were developed during the design and testing of the pumping system prior to the installation. Finally, the program selects single or multiple pumps to meet the demands of the HVAC system.

Participants

Pete Zak, P.E. , Group Leader, Graef, Anhalt, Schloemr & Assocs. Milwaukee

Burt Rishel, P.E. , Owner, Pumping Solutions, LLC Cincinnati

Scott Peregoy, ASD , Product Manager, Toshiba International Houston

Roddy Yates , Manager of Power Electronics and Controls, Baldor Electric Co. Fort Smith, Ark.

A Case for Variable Frequency Drives

Motor systems consume 63% of all electricity in the U.S. industrial sector, and manufacturers logically start their savings search here. To determine savings potential, vendors need to consider how efficiently energy is delivered to the pump motor during startup, and how efficiently the pump motor operates. Implementing AC drives, improved efficiency motors and soft starters reduce equipment wear and lead to significant energy savings.

For example, in the city of Sandusky, Ohio, the water department’s lines were frequently breaking, causing recurrent disruptions in water service, road closures and an increased risk of system contamination. At the root of the problem was a costly, inefficient and antiquated pump process. This labor-intensive process required pump changing throughout the day to meet fluctuating water district needs. High-service pump startups used excessive electricity, and pump changes created water hammer—a phenomenon where sudden pressure fluctuations can cause pipes to shake and possibly break.

The answer was to install AC drives to control pump motors and provide precise water pressure control, leading to significant energy savings. The AC drives decreased the need for time-consuming pump changes, reduced electrical costs by more than $31,000 over a 12-month period and lowered water main breaks by 76%. This resulted in more reliable water service to all end users. Reduced pump startups also translate to more constant water flow to satisfy demand, resulting in better water treatment, chemical dispersion and longer pump life. The AC-drive-based system increased Sandusky’s return on assets and lowered its total cost of ownership.

Targeting variable costs head-on by using AC drives demonstrates sound energy conservation and business acumen. Proactive motor system management will help the best-performing facilities minimize necessary energy investments in a world where steadily increasing costs are a certainty.

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