The Lowdown On Electric Valves
Since electric valves are assigned their function in the system by the software in their controller, an electric valve can be used anywhere in the system — as an expansion valve, discharge gas bypass valve, evaporator control valve, heat reclaim valve, head pressure control valve, or crankcase pressure control valve. Several designs have evolved to meet different applications.
TYPES OF ELECTRONICALLY CONTROLLED VALVESFour basic types of electric valves have been offered to the marketplace: solenoid (or pulse), analog, heat motor, and step motor valves.
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 must be considered.
Analog valves are almost infinitely variable in their modulation, but the programming needed to accurately position the plunger is very complex. The valves are subject to high hysteresis, internal friction, residual magnetism, and poor repeatability. Properly designed analog valves have been very successful in transport refrigeration but are uncommon in other applications.
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 extremely fine control can be obtained.
The digital circuitry used by step motor controllers can respond quickly and accurately. Step motors often run at a rate of 200 steps per second and can be made to return to an exact position.
THE NEED FOR STEP MOTORSStep motors have existed for many years but were limited to very specialized (and usually expensive) applications. When the personal computer industry expanded, the need for small, reliable, and inexpensive step motors increased dramatically.
Step motors permitted the repeatable precision movement needed for high-speed printers and computerized engine management in automobiles. In the 1980s, Sporlan experimented with step motor technology, and production step motor valves were offered in the early 1990s. Initial designs incorporated unipolar motors, but later designs employed the more efficient bipolar style.
STEP MOTOR THEORYUnlike traditional motors, which will 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 principle that opposite poles attract and like poles repel. These poles are called North (N) and South (S).
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.
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.
There are two general types of step motors: unipolar and bipolar. In a unipolar style (such as the Sporlan SEO valve), the black lead is always at ground and each of the other three colors is in turn connected to a positive voltage supply. The 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. Bipolar is the style of choice for all modern step motor valves.
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, or DLA. DLAs are used to convert rotation on a push/pull, 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, then 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 6 degrees, the resolution is ±3 degrees. Analog electric valves or TEVs have better resolution because they open and close 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, limiting 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 resolution of an EEV is governed by the stroke and number of steps in that stroke. Such valves are available in a variety of sizes for a multitude of applications.
Dolin is product manager, Mechatronic Products. For more information, contact Sporlan Valve Co. at 636-239-1111; www.sporlan.com (website).
Publication date: 11/04/2002