Elements of Electric Valves for Refrigerant Control
First of Two PartsFour basic types of electric valves have been offered to the marketplace: solenoid or pulse, analog, heat motor, and step motor.
Solenoid valves are commonly used as shutoff valves and are simple in design. When a coil surrounding a plunger is energized, the magnetic field lifts the plunger (Figure 1). The valves are designed to have the plunger. This type of valve is either open or closed and has no ability to modulate flow.
To be successful as a modulation valve, a solenoid must be opened and shut rapidly in response to a signal generated by a controller. The term pulse width modulation (PWM), or simply pulse, is used for this design. Mechanical limits of the design confine the load following ability, or resolution, to a very narrow range.
Because of the sudden starts and stops of the refrigerant flow created by this type of valve, “water hammer” or vibration may occur and cause valve or system damage. Applications of pulse valves have been moderately successful; however, long-range effects on valves and systems have not been determined.
Analog valves (Figure 2) are related to pulse-style valves in that they basically have a solenoid design. The main difference is that the coil and plunger are specifically designed to create a variable magnetic field. A stronger field will open the valve more; a weaker field will allow the valve to close.
Analog valves are almost infinitely variable in their modulation, but the programming needed to accurately position the plunger is relatively complex. The valves can be subject to high hysteresis, internal friction, residual magnetism, and poor repeatability. Some analog valves have been successful in transport refrigeration, but are uncommon in other applications.
Heat motor valves (Figure 3) are similar in construction to thermostatic expansion valves (TEVs). TEVs have a bulb filled with a substance, usually a mixture of refrigerants or other fluids that expand when heated. The heat causing this expansion, and subsequent opening of the valve, is transferred from suction piping to the bulb.
In a heat motor valve, a small, electric heater immersed in the bulb fluid is energized by variable amounts. The heat generated expands the bulb fluid to modulate the valve.
Step motor valves (Figure 4) are the most sophisticated design. In this type of valve, a small motor is used to open or close the valve port. The motor that is used does not rotate continuously, but instead rotates a fraction of a revolution for each signal sent by the controller.
These discrete “steps” give the motor its name. The number of step signals sent by the controller is “remembered” by the controller, and the controller can return the valve to any previous position at any time. This repeatability is almost absolute, and fine control can be obtained. In addition, the digital circuitry used by step motor controllers can respond quickly and accurately.
How Step Motors OperateUnlike traditional motors, which rotate as long as the proper power is supplied, step motors rotate a known amount of arc and then stop. When power is removed and then reapplied, the step motor will rotate another fixed amount, or step, and again stop.
This cycle may be repeated infinitely, within mechanical limits, in either direction.
While seemingly complex, this start/stop motion is mechanically simpler than induction or commutated motors. Step motors, like almost all motors, are based on the magnetic principal that opposite poles attract and like poles repel. These poles are called North (N) and South (S). (See Figure 5.)
If the center magnets above and below are free to rotate, then the previous orientation will always occur. If electromagnets are used, then a pivoted magnet or rotor can be made to align with the magnetic fields created when the electromagnets are energized. If power is left on, the magnetic poles will align and no further motion will take place.
If multiple groups of electromagnets are placed around a freely rotating permanent magnet rotor and each is energized in series, then the rotor will step to each alignment position and a step motor is created.
In reality, step motors may have 24 to 100 virtual electromagnets arranged around the rotor. Simple arithmetic shows these motors to have 15- to 3.6-degree step angles, or increments of rotation.
Step Motor TypesThere are two general types of step motors: unipolar and bipolar.
In the unipolar style, current flows in only one direction. Drive circuitry is simpler, but torque and efficiency are lower than bipolar designs.
A bipolar motor is powered by signals that change polarity. For the first step, the black lead may be negative while the white is positive; for the second step, the black becomes positive while the white becomes negative. This push/pull increases torque and efficiency for motor size and power input.
Digital Linear ActuatorsSmall increments of rotation may be useful in print head drives or for signaling purposes, but often a linear movement is more desirable.
In the case of electric refrigerant control valves, not only is linear motion needed, but significant linear force is also needed to close a port against high pressure. The solution to both these needs is a digital linear actuator (DLA). DLAs are used to convert rotation to a push/pull action, often with a large increase in output force.
The force increase is derived from a simple gear train and may account for a fivefold increase in mechanical advantage. This torque increase is used to turn a drive screw or threaded shaft. A drive nut, or coupling, is threaded onto the shaft but prevented from turning by keyways, or specially shaped guides. Since the drive nut cannot turn, it must move forward or backward, depending on the rotation of the threaded shaft.
ResolutionResolution is defined as the ability of the valve to meet flow requirements accurately.
In a pulse-type valve, only two stages of resolution are possible: fully open or fully shut. Theoretically, if a valve needs to meet a 50% load, it may remain shut for half the time and be fully open for half the time. The control of temperature and superheat will be “jumpy” as the valve alternately floods and starves the evaporator. If the swings are 6Â°F, we say the resolution is ±3Â°.
An analog electric valve or TEV has better resolution because it opens and closes smoothly. In both valves, however, there is hysteresis.
Hysteresis is the internal friction of any system. In a TEV, it takes more force or pressure to deform the diaphragm in the opening direction than in the closing direction. This hysteresis has an effect on the resolution of the TEV, and limits its ability to precisely meter refrigerant over widely changing head pressure and evaporator load conditions.
Balanced-ported TEVs have a much greater ability to follow load than conventional TEVs, but still not to the extent that electric expansion valves (EEVs) can. The stroke and number of steps in that stroke govern the resolution of an EEV. The piston or pin of a particular EEV moves the same linear distance for each step, 0.0000783 in. This small change in the distance the pin moves away from the seat is reflected in a minute amount of refrigerant flow increase or decrease. Other step motor-operated valves with fewer steps for the same stroke, or pulse-type valves with open-and-shut capabilities, will have inferior resolution.
A simple analogy is comparing an on/off light switch, which has two steps of resolution, and a dimmer switch, which may have thousands.
Actual control hardware for the valves may take a variety of forms. The most complex and expensive utilizes discrete or individual transistors for each switching function. This design requires the use of eight transistors, labeled Q1 through Q8, connected as shown in Figure 6.
Transistors are solid-state switches. “Solid state” means they are fabricated from a solid chip of silicon and have no moving parts. They act as switches or relays by using a small electrical signal to turn a large signal off and on.
The microprocessor, or small computer, used in the controller has the ability to sequence signals to the “base” of each transistor. As shown in Table 1, this sequence of signals turn the transistors on and off in pairs to step the valve open or shut.
Bipolar Drive SequenceTransistors are available as bipolar (not to be confused with motors of the same name), which control current, and MOSFET (Metal Oxide Semiconductor Field Effect Transistor), which control voltage.
In each type, transistors also are used to turn off the supply voltage or the ground. Full exploration of these differences is beyond the scope of this article, but drive circuitry using each of these types has been used successfully.
Permanent-magnet step motors will maintain position when power is removed. This “brake” effect allows controllers to be simpler and use less energy. (I suggest that all voltage be removed from the motor when not actively stepping to minimize heat and power consumption.)
More than 130 lb of force is needed to cause the motor to turn when not powered. This is not possible in any proper application of the valve.
NEXT WEEK: EEVs and superheat settings.
Dolan is with Sporlan Valve Co., 206 Lange Dr., Washington, MO 63090; 636-239-1111.
Publication date: 09/11/2000