Expansion valves are flow-restricting devices present in any refrigeration system. The valve needle remains open during steady state of operation. The size of the opening, or position of the needle, is related to the pressure and temperature of the evaporator. When set and controlled properly, an expansion valve will keep the evaporator active throughout its operation.
There are different types of expansion valve technologies popular in the marketplace such as:
- Non-motorized solution using temperature- sensing bulb
- Motorized solution using: Simple solenoid driven in Pulse Width Modulation (PWM) Motor and lead screw combination, with motor being a stepper in most cases
PWM Expansion Valves
Pulse width modulated expansion valves use a simple solenoid valve circuit to control the refrigerant flow. These valves can open or close completely for a set period of time upon receiving a signal from the controller. As an example, a PWM expansion valve remains open for the first five seconds, and then shuts down for the next five seconds to achieve 50% flow within ten seconds.
Such valves are used in multi-circuit evaporators due to their ability to 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 PWM valves is 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.
As opposed to PMDC motors that rotate as long as power is supplied, a stepper motor rotates in discrete steps using magnetic fields to move in fixed increments. Depending on the step size of the motor and the step pattern of the controller, stepper motors can achieve extremely accurate positioning.
Selecting the Right Stepper Motor
A conventional stepper motor provides rotational movement in small steps, and can be used in several industrial and medical applications. However, in the case of electronic expansion valves (EEVs), not only is linear motion needed, but a significant linear force is required to close the valve port against high system pressure.
Several factors affect the choice of a stepper motor for valve applications, such as:
- Output torque/force
- Step resolution
- 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 and pull-out torque are specified as functions of the stepping rate for various types of motor and driver combinations.
The pull-in torque curve shows the maximum load torque that a motor can start with, at different stepping rates, without losing any steps. 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 the valve operates against significant back pressure and under varying load conditions. Many designers prefer to keep at least a 50% safety factor above application torque at any given speed.
Motor Selection Example
Any rotary stepper motor datasheet has pull-out and pull-in torque versus speed curves. Depending on the loading pattern in the application, either of the torques must be chosen 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 force and speed:
- Minimum force to be achieved by the valve: 150 N
- Actuation linear speed required: 0.4 mm/sec
- Linear resolution of the valve: 0.002 mm/step
- Voltage: 12 VDC
Since the valve needs a fine linear resolution, a motor with a large number of steps per revolution is an ideal selection. Assume that 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 lead screw is selected for linear actuation that 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 2.45 mm.