When specifying a solenoid valve there are several important characteristics of the assembly the machine designer must take into account.
All coils will come with a rating that indicates the voltageß and the power consumption in Watts. Establishing the requirement for a machine seems straightforward but there are pitfalls that must be avoided.
Most coils are advertised as continuously rated provided they are working within set parameters. When a solenoid is energized it will generate heat, the amount of heat experienced by the coil will depend on the power being applied, the design of the coil and any cooling effects. The temperature of the valve is important because it affects the performance of the encapsulating material. The materials used will have a class given to them.
For example Class 'H' guarantees the material up to 180º C while class 'F' up to 155º C. If the ambient temperature plus this increase in coil temperature rises above the rating of the insulation and encapsulation material then failure will occur. Most catalogs will indicate an ambient temperature of between -20º C and +40º C.
Change in temperature also affects the resistance of the coil and hence the force it will exert, but more of that later.
The second most important consideration is the application and the exposure of the coil to the elements. Coils and tube assemblies are designed to withstand different levels of water ingress. This is called the 'IP' rating. The connectors used will carry their own IP rating such as DIN 43650 at IP65, most Deutsch connectors are IP67. The level of water ingress protection increases with the higher number. Details can be found in BS EN 60529. Some manufacturers also seal the coil/tube joint with an "O" ring to prevent corrosion and possible weakening of the tube.
On machinery where the valve is mounted a long way from the power source voltage loss along the wires has to be accounted for. The coil is rated to give a performance to a valve specified with a tolerance on the voltage. Typically +/- 10% of the nominal voltage. If the voltage at the coil drops then full performance from the valve will not be achieved. Proportional valves in particular require the availability of a constant current to operate effectively and predictably. And it should be remembered that as the coil warms up the resistance will change affecting the voltage and so the reaction of the valve to change in input signal. Utilizing Pulse Width Modulation (PWM) will minimize these adverse effects. Hysterisis within the valve can also be reduced by applying PWM to the signal. Feedback loops within the electronic control can also make the valve more accurate.
Most hydraulic companies will specify a performance figure for their valves that is available at a percentage of the maximum power usage. This is to allow good operation when the coil heats up and there is a resultant loss of magnetic force, therefore when testing a machine it is important to verify the valve function at stabilized temperature/most extreme service conditions.
A coil is an inductor – it stores energy and resists change, so when switched off it will generate a brief high voltage signal which potentially can damage other electronics devices on the vehicle or system.
To protect against these, coils or connectors can be fitted with diodes to permit the energy to dissipate safely.
As yet there are not generally recognized standards as to how response times should be measured, some manufacturers will quote figures from the time when the power is switched to the point where the armature reaches the end of its stroke, others from the time the power is switched to the point where the hydraulic fluid reacts. When looking at different manufacturer's products it is important to compare like for like.
Note: Response times are usually different for switching the power on or off.
With mechanical valves the performance is often based on the pressure drop across the valve at a set flow. With electrically operated valves it is more likely that the balance between the solenoid force, the internal spring force and the flow forces will dictate the working envelope of the valve.
Force on valve components due to pressure is usually straightforward but forces are also generated by the fluid passing over the surface of the valves components, these flow forces can either act with the coil force to help the valve to stay in its operated condition or act against the coil force. There have been occasions where they have caused a valve to switch back to its original position as soon as the flow force overcomes the magnetic force causing an actuator to change direction in the middle of its stroke. Performance data often indicates that a valve will allow more flow in one direction than the other. This can be directly attributed to the flow forces within the valve.
In the case of most solenoid valves the pressure limitation will be determined by the tube design and the factor of safety employed by the manufacturer. But in some cases over pressurization of the valve may cause the valve to open, as the force created overcomes the magnetic force exerted by the coil. A typical example of good hydraulic design using electrical operation is shown in Figure 7.