Figure 1 Applying the MSS Theory (illustrated here) means constantly driving superheat to the lowest point where stability is reached, thereby maximizing efficiency by maximizing utility of the evaporator.

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Stepper MotorPulse Width Modulation

Read about the pros

and cons between

two primary EEV

technologies to

decide which valve

technology to use.

For the last century, thermostatic expansion valves have been used to control refrigerant flow to the evaporator to match cooling capacity to the load. In the 1970s, electronic expansion valves were introduced,

boasting higher efficiency and better control, albeit at a higher cost. Over the years, electronic valves have been further developed and production scale economies have increased, improving performance and driving down costs. Today, it is common to see EEVs in many applications. This article explores the two primary EEV technologies available, stepper motor and pulse width modulation, and discusses when they could be the right option for an appli-cation, and when a mechanical valve could be the best choice.

To begin, why choose electronic over mechanical valves? The primary rea-sons are to improve efficiency by minimizing superheat, to better control hu-midity by dynamically managing superheat, and to respond more quickly to changing capacity requirements. Some manufacturers also see electronic valves as a means to differentiate, both by offering system benefits and by the per-ceived value of applying the latest technology. One additional reason for us-ing electronic valves is to control systems using CO2 as a refrigerant, for which there are currently no mechanical expansion valves on the market.

General benefits and challengesTo understand how EEVs can minimize superheat, it is important to understand a principle called the Minimum Stable Superheat Theory developed in the 1980s, and since applied in electronic-controller algorithms to drive optimal

All images courtesy of Danfoss Inc.

By JonAThAn hollowAy

Figure 2 A detailed cutout of a stepper motor valve.

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Both stepper motor and pulse width modulation valves also

have had successful operating histories for decades, so

any questions about system life have been resolved.

“performance from electronic expansion valves. The normal purpose of the evaporator is to absorb heat, changing the refrigerant phase from liquid to vapor, but the process in-volves moving through a range of instability with drops of liquid turbulently whirled through the vapor, eventually leav-ing only pure superheated vapor at the end of the evapora-tor (see Figure 1). If a temperature sensor is placed on the areas of pure vapor or liquid, it will have a smooth signal, but if there is a mixture, the signal will jump sporadically due to the volatile change of refrigerant state. Applying the MSS Theory means constantly driving superheat to the lowest point where stability is reached, thereby maximizing efficien-cy by maximizing utility of the evaporator.

In addition to improved efficiency, EEVs can also control dehumidification by actively managing the effective surface area of the evaporator and the cycle time when the coil tem-perature is below the dew point. This is important in comfort cooling, and crucial in some refrigeration applications where humidity levels can impact food safety and quality.

EEVs offer a wide control range, and in contrast to me-chanical valves they require no superheat to open. Both step-per motor and pulse width modulation valves also have had successful operating histories for decades, so any questions about system life have been resolved.

To achieve all these benefits, there are some added system costs. Normally, one pressure and one temperature sensor are required to ensure proper calculation of superheat. It is pos-sible in some applications to use two temperature sensors, but what is gained in cost savings is offset by lower control accu-racy. The sensors feed data to a controller, which calculates any needed adjustments and sends commands in real time to the valve.

Stepper motor technologyAs opposed to conventional motors, which rotate as long as power is supplied, stepper motors operate by rotating a set distance and stopping, based on electrical pulses from a con-troller (see Figure 2). As the motor rotates, a screw is turned, driving an orifice up or down, open or closed; the resulting action is similar to a mechanical expansion valve. Stepper motor technology is commonly applied in a variety of appli-cations, and generally handles single-circuit evaporators more smoothly than pulse width modulation valves.

Stepper motor valves can have hundreds or thousands of steps, enabling very precise control of refrigerant flow and smooth adaptation. Many stepper motor valves ensure sole-noid tightness upon closing, eliminating the need for an ad-ditional solenoid valve, assuming there is backup power to drive the valve closed in event of power failure. These valves are comparatively more effective than pulse width modula-tion valves at very low-load conditions by holding steady at just a few percent of capacity.

Though the valves are designed to make precise step changes to adapt to system needs, at times the controller’s commands are not received and executed by the valve, result-ing in a missed step, which over time can aggregate to a sig-nificant gap between where the controller “thinks” the orifice

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is and where it actually is. However, overdriving the valve closed during an off cycle realigns controller setting and ori-fice position. Another challenge for stepper motor valves that have thousands of steps is that it can take tens of seconds to go from fully open to fully closed.

Pulse width modulation technologyPulse width modulation technology is based on a principle of fully opening and closing a solenoid valve over a set period of time. As an example, to achieve 50% flow with a period of six seconds, the valve will open once, remain open for three seconds, close for three seconds, then continually adapt based on refrigerant-flow needs. The design of these valves offers some unique benefits.

First, the turbulence created by opening and closing im-proves evaporator-circuit distribution, thereby increasing the usable heat-transfer surface area of the coil. The flow charac-teristics also improve oil return, which can be a particular challenge for stepper motor valves in low-load conditions. Electrical connections are simpler, with two wires for most pulse width modulation valves vs. four or five wires for most stepper motor valves. Since pulse width modulation valves are normally closed, they eliminate the need for an addi-tional solenoid valve. Lastly, these valves can adapt faster to dramatic capacity changes, moving from fully closed to fully open and vice versa in milliseconds.

Pulse width modulation valves are often applied in refrig-

eration applications, particularly with multi-circuit evapora-tors. One challenge for these valves that preclude their use in some applications is the pulsing created in the system, which is why they are not seen in residential A/C units for example. Another comparative drawback of pulse width modulation valves is power consumption. Where stepper motor valves use pulses only when changes are required, pulse width modula-tion valves use solenoid holding power for the open portion of the cycle.

ConclusionsOverall, pulse width modulation valves have greater value in multiple-circuit units—such as display cases and cold rooms—where more turbulence improves refrigerant distri-bution. On the other hand, in systems with one circuit, in-cluding most A/C applications or for injection control for cascade systems, stepper motor valves are the best choice to avoid excessive pulsation. Pulse width modulation valves can improve oil return and can adapt more quickly to fast changes in capacity needs, but stepper motor valves can be more effective in low-load conditions and consume less en-ergy in operation. Lab tests show that both types control superheat similarly, so the other factors mentioned should guide selection.*

Clearly, there are places for both types of valve, but equally important are the controllers that drive them. Fig-ure 4 illustrates the impact of applying the MSS Theory to various valves. Regardless of the choice of valve, apply-

Figure 3 A detailed cutout of a pulse width modula-tion valve.

Figure 4 This figures illustrates the results of the MSS Theory being applied to various valves.

Clearly, there are places for both types of valve, but equally important are the controllers that drive them. “

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Circle Reader Service No. 48

ing the MSS Theory can make systems operate with 2°–4° less superheat in most applications (in some cases much more) than those with a fixed-superheat setting, which do not achieve optimal efficiency, and can even result in lower effi-ciency than a TXV. As such, once a pulse width modulation or stepper motor valve is chosen based on the application, it is imperative to understand the control algorithms to ensure the desired results are achieved.

*Lab tests performed at Danfoss Inc.

Jonathan Holloway is the Marketing Manager for North Amer-ica at Danfoss Automatic Controls. For more information, visit www.danfoss.com.