Machine Design

Power management boosts solenoid-valve performance

Next-generation solenoids draw less power and produce more force.

Solenoids have long been used to control valves and fluid flow. A typical solenoid converts an input voltage to magnetic force that attracts a moveable core which, in turn, opens or closes a valve. Different size solenoids provide specific flow and pressure ratings, and are often customized to meet customer requirements.

RedHat Next Generation solenoid valves have built-in power-management capabilities. The two, three, and four-way valves are designed for demanding fluid-control applications with air, water, light oil, and inert gas at temperatures from -40 to 200°C. The valves are available in brass, stainless steel, plastic, and aluminum with diaphragms and seats in a wide range of resilient materials. Sizes range from 1/8 to 3 in.
Despite their proven use in fluid-control applications, the downside is that most solenoids do not have power-management capabilities -- and this wastes energy. That's because solenoids require a lot of power to open a valve but substantially less to then hold the valve open. But today's solenoids are designed to continuously draw maximum power, and most of the time the excess power converts to heat.

Normally a solenoid valve opens in a fraction of a second and then is left on for several seconds, minutes, hours, or even days. A typical application involves a flow-control valve where a solenoid is energized for 10 min while a tank fills. The solenoid requires 10 W for the first 60 msec and then only 2 W to hold the valve open. But because it draws 10 W continuously, the solenoid wastes 8 W of power for virtually the entire operating cycle.

Next-generation solenoids with electronic power-management capabilities eliminate this drawback. An ASIC (Application Specific Integrated Circuit) built into the solenoid provides a simple timing function. It permits high in-rush current for the 60 msec needed to shift the valve, and then throttles back power to the level required to hold the valve in place.

An obvious benefit is that this can substantially reduce power consumption and cut operating costs. Equipment manufacturers can also downsize power supplies and switching devices to save money without sacrificing safety or performance.

Power management offers several other advantages. One is that it substantially increases dc pressure ratings. Manufacturers often prefer 24 Vdc because it is safer and wiring simpler than 120 Vac. But in the past users often had no choice but to use ac solenoids that produce higher forces and can operate higher-pressure valves. Force output is directly related to temperature rise in the coil, and dc ratings are generally one-fourth to one-third that of ac ratings. For example, an ac solenoid valve might be rated for 150 psi and 140*F maximum ambient temperature, while a comparable dc valve would be rated for only 40 psi and 104*F.

Because a power-management circuit minimizes temperature rise in the coil, efficiency improves dramatically. The result is dc pressure ratings that now meet or exceed ac levels.

This particularly benefits users of fieldbus-controlled devices. Ac produces interference on dc communication lines, so as users migrate to fieldbus controls, ac signals must be carried on separate lines or shielded from the communication lines. A dc system eliminates this headache.

Another benefit involves inductive loads. De-energizing a solenoid causes the magnetic field to collapse, which in turn causes an induced voltage spike to oppose the change in current. This inductive voltage spike travels through the power line and can damage solid-state or mechanical switches. Users often add an RC "snubber" circuit to protect the switch.

Asco has found that power-management circuits attenuate the inductive spike before it leaves the coil. Switching devices do not see an inductive load, extending life and reliability and eliminating the need for snubber circuits. For example, the company's Next Generation solenoid valves effectively increase switch life by up to four times.

Housing the power-management circuit within the solenoid protects the electronics from the environment -- whether harsh chemicals, washdowns, or rain and sleet. And it makes the transition to next-generation solenoids transparent to the user. The solenoid platform also offers other electronic benefits, such as bus communications and the flexibility to add control, safety, and diagnostic capabilities in the future.

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