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Savings with Variable Speed Control Russ McIntosh, PE, CEM, LEED AP®

ABSTRACT Variable speed control is a common method of achieving energy savings in buildings, usually by controlling pumps and fans that are part of the heating, ventilation and air conditioning (HVAC) system. Electronic variable speed drives (VSDs) that can be retrofitted to a common alternating current (AC) motor have been available for decades, and have improved in reliability and performance over the years. This article discusses many, but not all, energy-saving strategies using variable speed control of motors in HVAC systems and related topics, such as energy-saving calculations and details of variable speed drives (VSD) application. Energy-saving strategies include the elimination of balancing valves that limit flow and replacement of less efficient air volume controls, such as inlet guide vanes. Applications also include controlling pump and fan speeds (along with the necessary system conversions) to reduce speed to meet partial loads. These include converting a hydronic system with three-way valves to one with two-way valves, and converting a constant-volume air system to variable air volume (VAV). Accurate calculation of projected energy savings requires understanding certain key details of the systems involved. Some common assumptions can lead to unrealistically high savings projections. Some precautions and extra steps may be necessary to ensure a reliable and trouble-free installation and compliance with fire safety codes and other regulations. BASIC ENERGY SAVING STRATEGIES Pulse Width Modulated Control Variable speed control can be used to achieve energy savings in a variety of ways. A variable-speed drive (also called a variable-frequency drive or an adjustable speed drive) can control the speed of a common

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AC squirrel-cage induction motor as part of a retrofit or an energy efficient design. The most common of these is the pulse-width modulated (PWM) system that uses transistors to simulate the sine wave voltage pattern of a standard AC current. Coupling Drives Mechanical and hydraulic speed control devices are available in a variety of forms, but most are not commonly specified as an energy-saving retrofit. However, some coupling-type drives, which use magnetic induction to transfer torque from one shaft to another, are available, and are installed between the motor and the output shaft. Similar devices are used as soft-starters, reducing wear on motors and equipment by avoiding the stress of sudden starting. Some of these control speed by changing the area of proximity of the torque transfer areas, relying on a mechanism to move one part of the drive over the other. “Eddy current drives” can vary speed by modulating the current used to induce the magnetic field that provides traction between the two couplings. ECM Motors and Older DC Motors Before modern VSDs were available, it was more common to see a brushed direct current (DC) motor used for variable speed control, which was achieved by varying DC voltage. DC motors require commutators, brushes that transfer electric current to the motor rotor. A brushed DC motor typically requires frequent maintenance of its commutator and was used for elevator applications where variable speed control, operation at low speeds, and high starting torque were all advantageous. New brushless DC motors or electronically commutated (EC) motors are now common, but they are used far more often for small motor applications in new equipment, and not as commonly for an energysaving retrofit to an existing building’s HVAC system. Elevators Some old and new variable speed technology is used in elevators, including old DC motor-generator sets, brushed DC motors and newer EC motors and PWM-type VSDs with class D squirrel-cage AC induction motors. However, due to low-run-hours, elevators alone are not usually the source of an energy-saving retrofit. Elevator motors, drives and controllers are more often replaced for reliability and maintenance reasons, while energy savings may be an ancillary benefit.

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Benefits of VSDs Additional advantages beyond energy savings include: • Lower stress on mechanical systems due to lower speeds • More quiet operation • Improved power factor • Reduced stress on motors and mechanical systems by providing a soft start Typical Applications Common energy-saving retrofits to an HVAC system include modulating fan or pump speed to meet load. This typically involves replacing other existing systems that reduce fan or pump output to meet load, such as: • •

Fan inlet guide vanes or (more rarely) an outlet damper system at a fan A pump throttling valve, e.g. a triple-duty balancing valve In some cases, variable speed control may be part of a larger retrofit to the existing water or air system, such as: 1. Replacement of three-way valves with two-way valves and associated modifications 2. Conversion of a constant-volume reheat or dual-duct system to a variable air volume system

Energy engineers tend to be familiar with these savings strategies. This article will focus on specific technical points related to these practices, both related to how savings are calculated and the installation and application of variable speed drives. ENERGY SAVINGS PRINCIPLES The principle behind energy savings with VSDs is to slow a system to match the load. For a retrofit application, energy savings are a function of both the system that was in place before the VSD was applied and the original design intent. The energy used by a system with a VSD depends on how that system responds to a change in speed, and the associated changes in

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flow rates and other system parameters. Well-publicized fan and pump affinity laws typically show power use proportional to the speed cubed: This is based on

Power ~ speed3 Power ~ pressure * speed

And the assumption that pressure is proportional to the speed squared: Pressure ~ speed2 In general, the following two conditions need to be met for pressure to be proportional to the square of the speed of the fan or pump, or the square of the flow rate of the fluid being moved: 1)

Inertial effects must dominate the load. This is generally true for ordinary HVAC equipment, such as centrifugal pumps or a propeller or squirrel-cage fans. This is not necessarily the case for positive displacement pumps or situations where viscous effects and/ or static head dominate the load, but most of the typical applications of VFDs in HVAC systems do not experience these effects. One exception is cooling tower pumps, where static head can be more significant.

2)

The system curve needs to remain constant. This is not the case for many typical HVAC systems, such as VAV air systems or piping to chilled water coils. For most HVAC applications, fan and pumps speeds are regulated to meet a constant static or differential pressure set point, and the assumption that fan or pump power consumption decreases with the cube of the speed can significantly exaggerate energy savings.

This is well-documented in many technical publications, including ITT Corporation Bulletin No THE-685 (2). As explained by Tillack and Rishel (1), when a pump or fan is controlled to meet a pressure set point, the pressure decrease that a fan or a pump experiences as a result of a reduction in speed depends on where the pressure transducer is located within the system. If the static or differential pressure sensor is at, or close to, the

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pump or fan, that pump or fan will experience little or no reduction in pressure as speed decreases. In this case, power will be proportional to speed, not speed cubed. Locating a pressure transducer away from the pump or fan that it controls farther down stream in the system, may allow for a greater decrease in pressure with speed, but how this is done depends on system configuration and other considerations. If a VAV system is controlled with a static pressure reset, then the static pressure will drop as the load drops, and fan power is closer to a function of the speed cubed. This explains why static pressure reset provides greater savings than operation at a constant pressure set point. Fixed, Static and Dynamic Pressure As explained by the sheet metal and air-conditioning contractors’ national association (SMACNA) (5), fan or pump power is a function of the differential total pressure at the inlet and outlet of the fan or pump: Fan total pressure = Ptotal discharge – Ptotal supply In addition: Ptotal = Pstatic + Pdynamic The power that a fan or pump consumes is a function of the total pressure delivered to the ductwork or piping that it serves: Ptotal = Pfixed + Pbefore + Pafter + Pdynamic Where: • • • •

Pdynamic = velocity * density/2g, also known as velocity pressure Pfixed = fixed (constant) head of system, sometimes referred to as static head, but not to be confused with static pressure set point, which is pressure not including dynamic pressure Pbefore = pressure drop of the system attributable to ducts or pipes before the transducer (between fan or pump and transducer) Pafter = pressure drop of the system, attributable to ducts or pipes after the transducer (the pressure reading delivered to the controls system, and the pressure for which pump or fan speed is typically controlled)

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It is important to define both Pbefore and Pafter as pressure attributable to flow, resulting from friction, and therefore equal to zero when there is no flow. The pressure that is measured by a transducer in a controls system for the purposes of fan speed control is often called static pressure (Pstatic), as named in the previous equation. This means that the air pressure probe is designed to pick up pressure that does not include dynamic pressure. In this context, static pressure refers to all pressures listed above, except dynamic pressure: Pstatic = Pfixed + Pbefore + Pafter However, the static head in a pumping system is actually just Pfixed in the equations above. This could represent the lift required for a cooling tower pump to bring water to the top of the tower, approximately equal to the elevation difference between the nozzles and the sump. This is not the same as static pressure sensed by a transducer in an air system.

Figure 1: Fixed Or “Static” Head In Cooling Tower Pumping System

Dynamic pressure will also decrease in proportion to the speed squared. However, for most HVAC applications this dynamic pressure is usually not a large part of the total pressure that a pump or fan must deliver. For example, airflow of 2400 fpm comprises a velocity pressure (dynamic pressure) of only 0.36 inches of water. Much higher velocity is different; airflow of 7200 fpm comprises a velocity pressure of 3.24

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inches of water, but this would also mean a friction of 2.25 inches of water per 100 feet of duct (through a 20- by 30-inch duct), and therefore may be less common. However, it may still be worth getting an idea of the velocity where the pressure transducer is, since: 1)

Fan energy use is a function of total pressure (Ptotal)

2)

Fan speed control is normally a function of static pressure (Pstatic)

In an ordinary piping system, dynamic pressure is not read by the controlling differential pressure transducer, since it will be the same on both supply and return lines. Because water (or other fluids being pumped) is relatively incompressible, dynamic energy will be the same in the return as the supply. When operated on a VSD, this total pressure (Ptotal) can be calculated as a function of speed: Ptotal = P1fixed + P1after + ((P1before + P1dynamic)*%flow2) Or to account for some viscous effects and other friction that is not a function of speed squared: Ptotal = P1fixed + P1after + ((P1before + P1dynamic)*%flow(sf)) Where: P1 = pressure under full-load conditions (when fan or pump speed is at its maximum) sf = speed pressure relationship factor that is typically 1.8 to 2.0 to account for viscous effects and any friction that is not a function of the flow rate squared Figures 2 and 3 show a simple illustration of the effect of the percentage of fixed pressure set point for a pump system, where

% Constant Head = P1fixed + P1after

Figures 2 and 3 show idealized curves, assuming head that is not constant varies in proportion to the percent flow squared (SF = 2).

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Figure 2: Percent Head By Percent Constant Head

Figure 3: Percent Power By Percent Constant Head

The Effects of Pump, Motor and Drive Efficiency As described by Bernier and Bourret (4), the efficiency of pumps, motors and drive (the VFD itself) all tend to vary with speed. The impact of these efficiencies, which typically decreases with load, should be included in the calculations of energy use. The theoretical power can be calculated for air and water systems under ordinary conditions using the following formulae: Theoretical power for pumps: 𝐹𝑙𝑜𝑤 𝐺𝑃𝑀∗𝐻𝑒𝑎𝑑 𝐹𝑡∗𝑆𝐺 𝑃𝑜𝑤𝑒𝑟 [𝐻𝑃] = ———————————— 3960

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Where: SG = the specific gravity of the fluid being pumped Theoretical power for fans: 𝐹𝑙𝑜𝑤 𝐶𝐹𝑀∗𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒[𝐻2𝑂] 𝑃𝑜𝑤𝑒𝑟 [𝐻𝑃] = ———————————— 6340

In either case, actual energy use depends on input power: 𝑇ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝑃𝑜𝑤𝑒𝑟[𝐻𝑃] 𝑃𝑢𝑚𝑝 𝐼𝑛𝑝𝑢𝑡 𝑃𝑜𝑤𝑒𝑟[𝐻𝑃]= ————————————— µ 𝑝𝑢𝑚𝑝 ∗ µ 𝑚𝑜𝑡𝑜𝑟 ∗ µ 𝑑𝑟𝑖𝑣𝑒 𝑇ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝑃𝑜𝑤𝑒𝑟[𝐻𝑃] 𝐹𝑎𝑛 𝐼𝑛𝑝𝑢𝑡 𝑃𝑜𝑤𝑒𝑟[𝐻𝑃]= ———————————— µ 𝑓𝑎𝑛 ∗ µ 𝑚𝑜𝑡𝑜𝑟 ∗ µ 𝑑𝑟𝑖𝑣𝑒 Where: µ fan = fan efficiency µ pump = pump efficiency µ motor = motor efficiency µ drive = drive efficiency

Effect of Controls on Energy Efficiency An example set of calculations was run to illustrate the effect of reduction in fan speed with a VFD at different percentages of total pressure drop before the transducer. This set of calculations was run for a VAV system using: • • • •

Chicago weather data A balance-point temperature of 50°F A fan speed-pressure power relationship (sf) of 1.9 Four inches static pressure from fan at full-load conditions

The calculations used a load profile with fan speed decreasing from 96% to 61% with cooling load, and then to 55% in the coldest outdoor temperatures, as shown in Figure 4. Figure 5 shows lower energy use when the pressure transducer is closer to the end devices that are served, instead of near the fan. It is common for a design intent document to call for a pressure transducer

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to be “two-thirds down the ductwork.” Although a design will often not specify exactly where that is, the actual location of the transducer sometimes winds up being closer to the fan. For the previous example, Figure 6 shows how the fan curve moves as VAV boxes close in response to a decrease in load. This illustrates that in systems without a static pressure reset, a lower fan speed does not necessarily result in a lower pressure. The Effects of Static Pressure Reset Control If static pressure reset control is used, however, the pressure of the air delivered to the system tends to drop off with load, while VAV boxes tend to stay more open than a conventional (constant pressure set point) VAV system. As a result, this will reduce the shift in the system curve illustrated by Figure 6, and increase fan energy savings.

Figure 4: Fan Speed Profile

Figure 5: Annual Fan Energy Per 1000 CFM of Capacity (kWh/Yr)

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Figure 6: Fan Curve Moves as VAV Boxes Close

Pumping Systems With pumping systems, the closing of two-way valves serving end devices on a pumping loop will shift the system curve inward as load decreases. As in a VAV system, the location of the differential pressure transducer farther down into the system may: 1) 2)

Help control and meet the differential pressure needed by the end devices Save energy by making a larger portion of the pump load variable with the flow, instead of constant

As noted by Tillack and Rishel (1), more than one differential pressure sensor might be recommended for a reverse-return system, where no one coil has the longest run of pipe from the pump. As the load varies: • If the pressure transducer is close to the fan or pump, distant enddevices will see some pressure fluctuation • If the pressure transducer is far from the fan or pump, end-devices near to the fan or pump will see some pressure fluctuation Cautions include: 1) Placing the pressure transducer at the end of a long branch in a system with long runs to areas with different loads and schedules could result in inconsistent supply pressure at end devices in other areas. This is one reason that the location of the pressure transducer

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is typically recommended to be two-thirds down the ductwork, instead of all the way down the ductwork. As a retrofit, simply moving the pressure transducer farther away from the fan or pump will increase energy use because the fan or pump will have to work harder to meet the same pressure set point. An appropriate reduction in pressure set point is needed to represent the pressure drop of the additional distance down the ductwork or piping.

APPLICATIONS FOR VSDS

Common applications for energy savings using VSDs include:

•

Replacement of three-way valves with two-way valves within pumping systems

•

• • • •

Retrofit of a VFD for pumps on balancing valves

Conversion of air systems from constant volume to variable volume Cooling tower fans

Replacement of inlet guide vanes

Kitchen exhaust hood and makeup air fan speed control

This is not meant to be an exhaustive list. Other opportunities may be found, depending on the facility and the systems involved. Retrofit of a VFD for Pumps on Balancing Valves Pumps are often set up with a balancing valve throttling the flow of water delivered. The retrofit of a VSD can save energy by allowing the operation of the pump at partial speed, with balancing valve opened up. Figure 7 shows a pump curve with pump operation at three points: • • •

Point 1: unrestricted full-speed flow, showing the pump delivering more flow than desired at full speed with balancing 100% open Point 2: flow restricted by balancing valve, where the added head imposed by the balancing valve rides the pump curve up to a higher head and lower flow rate Point 3: VFD operation, where the pump is slowed to meet the required head and flow, with balancing valve opened

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Figure 7: Flow From a Pump, Unrestricted, Restricted and at Partial Speed (Curve reprinted with permission from Xylem, Inc.)

The pump curve shown in Figure 7 is published by the manufacturer in the document Bell & Gossett Curve Booklet B-260G. Page two of the same booklet shows pump speed having essentially the same effect as pump impeller diameter. The sheet metal and air conditioning contractors’ national association (SMACNA) (5), and other technical resources, explains the affinity law that shows the pump speed and its relationship to impeller diameter: 𝐵𝐻𝑃1 𝐷2 ——— = —— 𝐵𝐻𝑃2 𝐷1 Where: BHP = pump brake horsepower D = diameter of the pump impeller This shows that reducing the size of the pump impeller could produce similar savings to speed reduction, and, in some cases, is selected as an alternative to a VSD for this particular situation. Shaving the pump impeller instead of installing a VSD may also have a lower capital cost, avoid drive energy losses, and some of the risks of using a VSD. The risks of using a VSD are addressed later in this article. However, shaving the pump impeller also permanently removes the extra capacity of the pump, while a VSD could be enhanced if more pumping head is needed in the future due to expansion or other system changes.

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Determining the size of the impeller, which can reveal the pump curve that should be used, and the position on the curve at which the system is operating, can be a challenge to the energy engineer looking to specify this retrofit. Pressure gauges, provided that they are accurate, may help display the pressure differential delivered by the pump, and possibly to the system if one exists downstream of the balancing valve. The pump’s model number can show what set of curves should be used, and can sometimes also indicate the impeller size. The deadhead pressure can also reveal a pump’s impeller size, as shown by Figure 8.

Figure 8: Dead-Head Pressure Revealing the Impeller Diameter (Curve reprinted with permission from Xylem, Inc.)

This test involves shutting the isolation valve downstream of the pump and reading the deadhead pressure. A number of precautions should be taken when performing this test: 1. 2. 3. 4. 5.

Make sure that the piping valves, fittings, and all components, including the isolation valve and pump are in good condition to withstand the higher deadhead pressure Verify that the valve can be operated without any problems Verify that the water flow from the pump can be disrupted without causing any problems to the system served Dead-head the pump for a short time, only long enough to get a pressure reading. Leaving the pump dead-headed is likely to cause water within the pump to boil, resulting in pump failure Although deadhead pressure is often not that much more than the

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6.

operating pressure (as is the case with the illustration provided in Figure 8), a careful look at the pump curve to verify this is warranted. This type of test is not for positive displacement pumps, or any type of pump that responds with very high pressure, or where trouble may result from deadheading. The viability of this test depends on the accuracy of the pressure gauges.

Descriptions of this retrofit are part of retro-commissioning training provided by the Portland energy conservation institute (PECI) and may also be included in training delivered by Bell & Gossett through its little red schoolhouse program, among other resources available. Replacement of Three-Way Valves with Two-Way Valves Within Pumping Systems Three-way valves within piping systems ordinarily require constant flow. They function only to re-direct flow from a piping loop through an end device such as a cooling or heating coil, as shown in Figure 9.

Figure 9: Simple Schematic of Pumping with Three-way Valves

This naturally means that full flow is required, and that the replacement of the three-way valves with two-way valves is needed to gain the energy savings associated with variable speed control of the pumping. A number of practical considerations should be made when evaluating the replacement of three-way valves with two-way valves. As shown in Figure 10, this conversion will require the addition of a pressure transducer, which is used to modulate pump speed to meet a differential set point. This retrofit also typically needs a differential bypass valve, usually set at a pressure slightly higher than the differential pressure set point. The differential bypass is meant to control for a pressure close to set point if the pump reaches minimum speed and the differ-

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Figure 10: Simple Schematic of Pumping with Two-way Valves

Figure 11: Piping with Three-Way Valves Converted to Two-way Valves—Not Recommended

ential pressure still exceeds set point, or if the VSD is disabled and the pump operates at full speed. The conversion of three-way valves to two-way valves by simply capping or closing off one of the ports or legs of the three-way valve, as illustrated by Figure 11, is not recommended. Three-way valves have a different construction from two-way valves, and they are designed to re-direct the flow of water rather than to throttle or shut off that flow. Three-way valves converted to operate as two-way valves will often leak, potentially wasting energy, reducing control, and causing other issues. Of course it is important to make sure that flow rates stay within acceptable limits of any chiller or other piece of equipment served by the piping loop. Newer chillers can handle more variation in flow, and new designs of variable primary flow chilled-water systems can save on both energy and first costs. It may be possible to save more with judicious application of dedicated pumps to serve selected air handler or other systems, if there can be a commensurate reduction in energy at the main loop pumps. Taylor

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(3) goes into the details of this and other useful information regarding chilled-water piping and plant design. Conversion of Air Systems from Constant Volume to Variable Volume VSDs are sometimes included in the conversion of constant-volume air systems to VAV systems. A VSD may still be used to control for static pressure in a constant volume system, but this may be considered more essential for VAV. A VAV system may be more efficient when using a static pressure reset, which can allow the supply fan to decrease in speed and deliver a lower pressure to the supply ductwork until one VAV box is all the way open. This requires a control system that has real-time knowledge of the position of each VAV box, however. Variable Speed Control of Cooling Tower Fans VSDs can be used to control cooling tower fans, potentially varying speed to meet a condenser water temperature set point. However, because most chillers will operate more efficiently with colder condenser water, it is often worth the extra energy associated with running the tower fan at full speed. As a result, it is common for a VSD retrofit of a cooling tower fan to drop out of an energy project. Replacement of Inlet Guide Vanes VSDs are often retrofitted to fans that use inlet guide vanes to control for airflow. Inlet guide vanes spin the air in the direction of the fan rotation, reducing fan output in the process. Inlet guide vanes tend to be less efficient at controlling fan output than a VSD, and more efficient than the much less commonly used simple outlet damper. The efficiency of inlet guide vanes depends in part on the fan used. Energy savings are best calculated using performance curves from the manufacturer, if available. Generally, inlet guide vanes compare favorably with VSDs for high outputs, when little flow reduction is required, while VSDs are more efficient when larger flow reductions are needed. Variable blade pitch control is used to control airflow from some vane-axial and propeller-type fans. Because that is a relatively efficient way to control flow, a VSD is often not a cost-effective retrofit for fans with variable blade pitch control. Minimum speed requirements for VSDs can place a low limit on airflow from fans retrofitted with a VSD. Leaving the inlet guide vanes

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in place can allow control to lower flow rates, once the VSD has reached minimum, if a wide range of airflows is needed. It should be noted that rebates available from state government and local utilities might require the removal of the inlet guide vanes. Some air handler fans, such as those that serve laboratories, may be over-sized. This may be done to overcome the resistance of dirty filters or another obstruction or problem to ensure adequate airflow to areas where ventilation airflow is critical. Replacement of inlet guide vanes with VSDs may produce substantial savings in this situation. Kitchen Exhaust Hood and Makeup Air Fan Speed Control Specialized systems are available to provide real-time control of the speed of commercial kitchen exhaust fans and make-up air units, based on the temperature and exhaust air needs. Savings for reduced heating and cooling requirements can be far greater than fan energy savings, depending on the conditioning of the make-up air. Special controls may be needed when make-up air is delivered from a space adjoining the kitchen, and/or from an air handler that must serve additional loads besides the kitchen exhaust. Makeup air units may have minimum air flow requirements. APPLICATION SPECIFICS PWM-type VSDs have been used for many years, but potential problems can arise as a result of installation issues. Codes and other methods to avoid overheating, electrical problems, and motor and equipment failure are worth considering when installing VSDs. Fire Protection Codes National Fire Protection Association (NFPA) codes may impose some additional requirements on the installation of VSDs. For example, a fire alarm system might have addressable communication with system input devices, such as smoke detectors, and system output devices, such as relays, in series, controlling the permissive operation of fans that have internet protocol (IP) addresses and communicate over an ethernet loop with the main fire alarm panel. In this case, the loop is supervised, meaning the fire alarm panel continuously monitors and detects a component or loop wiring failure (open loop). In the case of an open loop, the com-

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munication continues from either connected end of the loop back toward the point where the circuit became open. Wiring that is not supervised by the alarm panel is limited to one meter in length to minimize the possibility of breaking the circuit. The installation of a VSD may mean that wires that connect the fire alarm relay contacts to the safety circuit terminals of the VSD control circuit fall into this category as unsupervised wiring. A scenario like this might have existing magnetic starters located in the motor control centers and fire alarm relays that stop the fans in the event of a fire located in a panel mounted on the motor control center. In this situation, the installation of VSDs may require moving the fire alarm relays to within one meter of the safety contacts of the VSD, because the VSD became the new motor controller. Compliance may be most easily accomplished by rerouting the fire alarm system supervised loop wiring and conduit to pass through every VSD. Electrical Issues and Motor Reliability Over time, the quality of VSDs has improved, and many complications and problems associated with their use have been solved. The PWM type of VSD is used for the vast majority of VSD applications to control a standard squirrel-cage AC induction motor, and these are commonly applied without a problem. However, there are risks. Reflective wave harmonics can produce problems for motors, cabling and connections on VSDs. In recent years, manufacturers have made VSDs with faster-switching transistors, improving drive performance and efficiency. However, the fast voltage “rise time” of these faster-switching transistors can cause a “standing wave” voltage spike that develops between a VSD and motors it serves, caused by a pulse from the VSD adding to a pulse reflected back from the motor. This voltage spike can be several times the line voltage, and can cause damage through corona discharge within the motor, and other systems. The magnitude of the problem can be a function of the voltage rise time and voltage spike magnitude. A number of factors can influence reflective wave harmonics, including cabling impedance (length and type), and other factors. Solutions to this include: •

Use of cabling with high impulse voltage breakdown levels, and thermoset insulation material

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•

Use of an inverter-duty motor, or verifying that the motor to be run on the VSD meets a certain minimum insulation class (class F or H, depending on the situation) Keeping cabling distance between motor and VSD low (some recommend no greater than 15 feet) Line reactors, filters and electronics that can remove or abate voltage spikes Controls programmed to prevent operation at excessively low speeds

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Bearing Currents Voltage transients from the operation on a VSD can cause electric current through the motor shaft to ground. These electric currents may cause bearing damage and motor failure. Some guidelines specify grounding rings, and grounding of other parts of the motor and associated systems to avoid bearing damage resulting from voltage transients. Cabling and connections can also have an effect on bearing currents. General Motor Stress The operation of a VSD can be a little tougher on a motor in some other respects. Motors typically run hotter when connected to a VSD. It may be wise to make a note of any potential sources of trouble, such as motors that are already stressed, over-current, running hot or loud, or are not really the right type for the job, for example, an open drip proof where there should be a totally enclosed fan-cooled motor. Harmonics and Nuisance Tripping Harmonic distortion produced by VSDs can potentially cause trouble with the electrical system that serves them, and sometimes with other equipment on the same system. In some cases this can result in issues with electronics, and nuisance tripping of circuit breakers. An analysis of harmonics and vulnerability to problems like this may be warranted. Manufacturers of VSDs have some reliable technical information on these topics. In general, referring to the manufacturer’s guidelines for installation is recommended. Technical References for Electrical Topics Useful technical material is available from the Institute of Electrical and Electronics Engineers (IEEE), and some major manufacturers.

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Publications from IEEE include: • “Installation Considerations for IGBT AC Drives” (by G. Skibinski of Rockwell Automation) •

“A Practical Guide to Understanding Bearing Damage Related to PWM Drives” (by Don McDonald and Will Gray, Toshiba International)

U.S. Department of Energy (DOE), Energy Efficiency and Renewable Energy (EERE) Information Center—Industrial Technologies Program • Motor Tip Sheet #14—When Should Inverter-Duty Motors Be Specified? American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (ASHRAE) • “Shaft Grounding—A Solution to Motor Bearing Currents,” by H William Oh and Adam Willwerth, ASHRAE Transactions Vo. 114, Part 2 Publications from Allen Bradley include: • Wiring and Grounding Guidelines for Pulse Width Modulated (PWM) AC Drives •

Power Quality Manual

Publications from ABB include: • Technical Guide No. 5—Bearing currents in modern AC drive systems •

“Bearing currents in AC drive systems” TDM001 EN Rev B 2007

A helpful technical guide is available from AutomationDirect at: http://motors.automationdirect.com/Information/compare.html SUMMARY AND CONCLUSIONS A number of common applications for controlling AC induction motors with VSDs provide reliable energy savings. However, the savings might not match the fan or pump “affinity laws,” which state that power is a function of speed cubed. It is more realistic to determine how

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pressure will vary, and then calculate fan or pump power based on pressure times flow rate. Some situations require retrofits to an HVAC system in order to take advantage of the savings that VSDs provide. Considerations for these include: • •

• •

Locating the pressure transducer away from the fan or pump that it controls and selecting a good pressure setting can both optimize energy savings. Knowing where the existing system is on the pump curve for an accurate estimate of savings from replacing a throttling valve with a VSD. A deadhead test can help to indicate this, if the size of the pump impeller is not known, and not implicit from the pump model number or other data. Installing new two-way valves instead of trying to convert the three-way valves, to avoid trouble with valves leaking when converting the piping system from three-way valves to two-way. Using a static pressure reset for additional fan savings if converting a constant volume system to VAV. This requires a control system to be able to know the position of each VAV box.

Important considerations for a reliable and trouble-free installation include: • A VSD can make it more difficult for a motor to operate. An existing motor that is in rough shape, undersized or misapplied may warrant replacement. • An inverter-duty motor, filters and/or specific cabling may be warranted to avoid trouble with voltage spikes caused by reflective wave harmonics, particularly at cabling lengths of more than 15 feet. • Grounding rings on the shafts of motors served may be used to avoid trouble with bearing damage due to eddy currents. • Set up controls to avoid operation below recommended speeds. • Follow the manufacturer’s recommendations for installation and operation. The author is available to discuss this and other technical energy topics and can provide additional information and technical backup for statements made in this article. Contact information is provided below (page 31).

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References 1. 2. 3. 4. 5.

Larry Tillack and James B. Rishel, P.E., “Proper DP Control of Pumps” ASHRAE Journal, November 1998 ITT Fluid Handling and Training Department, “Variable Speed/ Variable Volume Pumping Fundamentals”, Bulletin No. THE-685, ITT Corporation, 1985. Taylor, Steven F, PE, Fellow ASHRAE. “Optimizing Design and Control of Chilled Water Plants.” ASHRAE Journal, July 2011. Michel Bernier, Ph.D., and Bernard Bourret, Ph.D., “Pumping Energy And Variable Frequency Drives”, ASHRAE Journal, December 1999 SMACNA., HVAC Systems, Testing and Adjusting Manual, 2002

———————————————————————————————— ABOUT THE AUTHOR Russ McIntosh, PE, CEM, LEED AP®, has more than 20 years of experience in building energy efficiency. Since receiving a bachelor’s degree in Mechanical Engineering from the University of Massachusetts at Amherst in 1987, Mr. McIntosh has performed energy engineering and project development work for commercial and industrial customers for NORESCO as well as numerous other energy efficiency organizations. He worked as a senior energy engineer performing energy and water savings analysis during his career and spent several years as a senior software engineer, developing web applications for energy and water savings. Contact information: [email protected], P 508.614.1080, M 508.614.5370, F 508.836.9988, www.noresco.com, One Research Drive, Suite 400C, Westborough, MA 01581.