An expansion valve is a flow-restricting device present in a refrigeration system that causes a pressure drop of the working fluid. The valve needle remains open during steady-state of operation. The size of the opening or the position of the needle is related to pressure and temperature of the evaporator. When set and controlled properly, an expansion valve will keep the evaporator active throughout its operation. There are two types of electric expansion valve technologies popular in the market place - pulse width modulated valves, and stepper motor driven valves.
Pulse width modulated expansion valves (PWM)
Pulse width modulated expansion valves use simple solenoid valve circuit to control the refrigerant flow. These valves can open or close completely for a set period of time, on receiving a signal from the controller. As an example, a PWM expansion valve may remain open for first five seconds and shuts down for next five seconds to achieve 50% flow within ten seconds.
Such valves can adapt to changing loads, moving from a fully closed to a fully open position and vice versa, in a span of a few milliseconds. A major drawback of such valves is excessive power consumption. Though such a valve pulses only when changes are required, it uses up solenoid holding power for the entire open portion of the cycle. They may also create excessive pulsation during start-up if used in single circuit low-tonnage systems.
Stepper motor driven expansion valves
As opposed to a permanent magnet DC motor that rotates as long as power is supplied, a stepper motor moves in discrete steps, using magnetic fields to move in fixed increments. Depending on the step size of the miniature motors and step pattern of the controller, stepper motors can achieve extremely accurate positioning.
A conventional stepper motor provides rotational movement in small steps, which can be used in several industrial and medical applications. However in case of motors for valve actuation, not only is linear motion needed, but also a significant linear force is required to close the valve port against high system pressure.
STEPPER MOTOR BASED EEVs
Selecting the right stepper motor
Several factors affect the choice of a stepper motor for valve actuation applications, such as output torque or force, speed, step resolution, and drive system. The output torque or force from a stepper motor is a function of motor size, duty cycle, motor winding, and the type of driver used. In a manufacturer’s data sheet, the pull-in torque curve shows the maximum load torque that a motor can start with, at different stepping rates without losing any step. The pull-out curve shows the total available torque when a motor runs at constant speed at a given stepping rate.
Understanding the exact force requirement can be a challenge, as valves operate against significant back-pressure and under varying load conditions. Many designers prefer to keep at least a 50% safety factor over application torque at any given speed. Any rotary stepper motor datasheet has pull-out and pull-in torque vs speed curves. Depending on the loading pattern in the application, we need to choose either of the torques for calculation. If the motor has to accelerate and is loaded from start, the pull-in torque has to be considered, which is typically the case for expansion valves.
The following example can be considered to select a motor for a given valve of force and speed.
- Minimum Force to be achieved by Valve: 150 N
- Actuation Linear Speed required: 0.4 mm/sec
- Linear Resolution of the Valve: 0.002 mm/step
- Voltage Rating: 12 VDC
Since the valve needs a very fine linear resolution, a motor with large no. of steps per revolution is an ideal selection. Portescap offers 42mm motors with 100 steps/revolution which is suitable for most of the expansion valve applications.
Assume a gearing system with reduction ratio of 12.25: 1 is used with the motor to improve the available torque for linear actuation. A suitable leadscrew is selected for linear actuation which can safely operate at higher force and provide better transmission efficiency and desired linear resolution. The lead of the screw in this case will be 2.45mm.
The datasheet provides the torque vs speed curves. To determine the operating point of the motor, we need to calculate the PPS (Input frequency) needed to achieve the desired linear speed.
Number of steps per revolution of Motor = 100
Gear Reduction ratio = 12.25
Linear Speed of valve (mm/s) = 0.4
Lead of Screw (mm) = 2.45
Then, PPS (full steps/sec) of motor = [100* 12.25*0.4]/ 2.25 = 200
From the datasheet of Portescap 42M100D2B motor, the available pull-in torque is 0.024 Nm. Assume the transmission efficiency of gears is 90% and that of lead screw is 50%.
Torque available at Lead screw = 0.024*12.25*0.9 = 0.265 Nm
Output force of the Valve (N) = [0.265*2*3.14*0.5*1000]/[2.45] = 340
From the above example, we can see that the motor has a minimum safety factor of 3, to achieve the desired force at the desired speed.
To precisely control the refrigerant flow during the dynamic load conditions, stepper motor in the valve is designed to provide linear motion to the valve needle in smaller steps. The resolution of a stepper motor system is affected by several factors—
- The stepper motor full-step length i.e. step angle of the motor
- The selected driver mode (full-stepping, half-stepping, or micro stepping)
- The Gear reduction ratio
- Lead screw pitch
Thus, there are several different combinations which can be used to get the desired resolution. Stepper motor valves can have hundreds of steps, thus enabling an extremely precise control of refrigerant flow and smooth adaptation. These valves are comparatively more effective than pulse width modulation valves due to their ability to respond accurately to changing load conditions.
There are three commonly used excitation modes for step motors; full stepping, half stepping and micro-stepping.
In a full step operation, the motor moves through its basic step angle, for example a stepper motor having 7.5 degree step angle would take 48 steps per motor revolution. Half step excitation results in steps that are half the basic step angle. Due to the smaller step movement, this mode provides twice the resolution and smoother operation. Half stepping produces roughly 15%-20% less torque than dual phase full stepping. Modified half stepping eliminates this torque decrease by increasing the current applied to the motor.
The micro-stepping controller is used for applications that require smoother resolution at lower speeds. It is actually a driver that sends pulses to the motor in an ideal waveform for smooth rotation. The idea is for the driver to send current in the form of sinewaves. Two sinewaves that are 90 degrees out of phase is the perfect driver for a smooth motor. If two step coils can be made to follow these sinewaves, it results in a perfectly quiet, smooth motor with no detectable “stepping”.
Most of the todays EEV’s use extremely fine micro stepping to solve noise and resonance problems, and to increase step accuracy and resolution. Looking at criticality of the system, the drivers are integrated into a sophisticated controller which accepts inputs from temperature and pressure sensors located at the upstream of the evaporator to maintain optimum system balance.
The primary reasons for using an electronic expansion valve is to improve system efficiency by minimizing superheat (SH), and to respond more quickly to changing capacity requirements. Though both valve technologies are capable of meeting these requirements, stepper motor valves if designed correctly can be more effective at very low load conditions, by holding steady at just a few percent of capacity. Stepper motor design provides some unique advantages such as lower running cost, simplified design and ability to respond quickly to changing load conditions, thus making them the first choice for system designers. Portescap provides geared can stack and direct-drive linear actuator solutions with a custom sub-assembly capability that allows for streamlined integration into the valve body, yielding precision flow control of refrigerants.
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