have been used for more than 100 years in many diverse flow control
applications across many industries. But their underlying technology
has not changed a great deal until recently. Even so, the physics of
their operation is not always well understood, leading to
misapplications or suboptimal performance. For example, not all users
realize that simple solenoid coil excitation with AC rather than DC
power usually allows for higher forces, watt-for-watt, in opening a
poppet valve. So AC may be helpful against higher differential
pressures of the fluid, for instance. However, using DC power often
provides other advantages, such as ease of wiring.
|Figure 1. Simplified poppet-valve cross-section.|
First, let’s examine the operation of a simplified normally
closed poppet valve (Figure 1). When the coil is not energized, no
electromagnetic forces are generated; the conical spring (f) presses
the disk (a) with its attached core (c) against the orifice (b),
stopping flow. The disk is typically made from an elastomeric material
to ensure a tight closure. When electrical current is passed through
the coil (d), a magnetic field (h) is induced in the magnetically
permeable circuit, comprised of the stationary plugnut (e), movable
core (c), and yoke (j). The resulting electromagnetic force attempts to
close the air gap (g) by moving the core towards the plugnut. When the
core lifts, compressing the spring, the disk moves away from the
orifice and flow commences. A larger orifice selection allows a greater
flow coefficient (or Cv) — the amount of flow for a given pressure drop.
The simplified explanation above works equally well for AC or DC
excitation (ignoring a few subtleties of internal construction). But
let’s take a closer look at what happens when the valve has just
started to open. Force from the spring helps keep the valve closed, but
so does hydraulic force caused by the pressure drop from the inlet to
the outlet of the valve. This hydraulic force is proportional to the
square of orifice size, since
the area of pressure imbalance is equal to pi times the orifice radius
squared (remember that class in geometry?). This is why such
direct-acting valves with greater flow often take a disproportionately
larger coil and lifting force to open.
|Figure 2. AC current (magnitude vs. time)|
However, once the valve is open and flow commences, this
hydraulic force decreases dramatically. So an ideal solenoid valve
should have just enough initial magnetic lifting force (from electrical
power to the coil) to overcome both spring and hydraulic forces. But
once flow starts, the valve should decrease its magnetic
lifting force (and
lower its electrical power consumption), only compressing the spring.
Asking anything more of the coil simply creates waste heat.
The AC Advantage
Here’s where AC excitation is usually more efficient for solenoid valves, from a power consumption standpoint. Typically, an end-user will apply
a voltage to the coil, for example, via a mechanical or solid-state
switch. But it’s the current passing through the coil (multiplied by
the number of coil turns) that actually creates the magnetic field.
This current is equal to the voltage divided by the coil impedance. For
an AC sine wave (remember Circuits 101?), this impedance is calculated
as R + j*2*pi*f*L, where R is the coil resistance, L is its inductance,
f is the AC frequency, and j is a mathematical operator that results in
a 90-degree phase shift. As the solenoid valve opens, the air gap
quickly narrows (the core accelerates as the magnetic circuit becomes
more efficient). That makes the coil’s inductance, L, (and thus the
impedance) increase dramatically. As a result, the current decreases
after an initial inrush or spike. So using AC excitation achieves the
desired result — to open the valve against pressure, then cut down on
|DC current (magnitude vs. time)|
With DC excitation, the case is reversed. When a DC voltage is
applied to a solenoid coil, the current will increase to eventually
(asymptotically) equal that voltage divided by the coil resistance. The
time it takes to get to that steady-state level is determined by the
time constant of L divided by R (coil inductance divided by
resistance). Thus DC excitation, with its relatively slow buildup of
current, creates exactly the opposite of what is wanted in a solenoid
Considering the Tradeoffs
excitation is typically more efficient, it usually comes in the form of
high voltages (120/240 V AC) and can bring accompanying user issues,
such as wire segregation, shock hazards, infringement of electrical
safety codes, and so on. (Low-voltage AC is of course available, but
this necessitates a transformer with its own power/heat issues.) Nor is
AC excitation without intrinsic losses. Since the voltage/current is
cycling in an AC solenoid, substantial “iron losses” (hysteresis and
eddy currents) can account for half of its power dissipation.
Also, modern industrial applications using PLCs, DCSs, and so on
to automate fluid control systems often provide easier connectivity to
DC loads. They may offer more outputs per plug-in module, or the user
may want to share one output module between different load types. And
DC power busses are often more readily available (e.g., 24 V DC with
auctioneered backups in many process plants).
|Figure 4. Simple DC spike-and-hold circuit.|
Ideally we want the current waveform “shape” of AC solenoid
excitation, without all its problems. Of course, using additional
electronic components, we can change the shape of the DC coil current
waveform to be closer to the mark. These additional circuits —
sometimes called spike and hold — are included within some
solenoid valve packages. The simplest example is shown in Figure 4.
When the coil is first energized, current will spike through the
capacitor C until that becomes charged, after which resistor R limits
current flow through the coil. Problem: With current
flowing through an additional resistor or similar heat-
generating component, these approaches are typically confined to smaller-sized
solenoid valves. Yet larger solenoids are where we need power conservation most. We want a means of generating a high-current pulse from
a DC voltage rail, then reducing that current to conserve power, while
maintaining the solenoid valve open — without dropping the steady-state
current through a resistor (or linear-mode transistor, etc.) and consuming more power.
New technology currently on the market uses an
integral switching system to accept a wide range of input voltages
(either AC or DC). The system is programmed to supply increased current
to the coil during initial excitation and then drop back to maintenance
current. Such valves can deliver much-improved fluid performance, with
higher flowrates and pressures, at a fraction of the steady-state
electrical power consumption of conventional designs.
Can’t Take the Heat
|Figure 5. New switching circuit design.|
solenoid valve solutions can generate considerable energy costs and
require large, expensive, separate power supplies. But power delivered
to the solenoid is not the only application consideration. That
power of course turns into substantial, and troublesome, heat. For
example, a bank of 10 conventional 11-watt solenoids in a sealed cabinet can quickly
raise internal temperatures beyond the specifications of other
electronics in the cabinet. Removing heat from instrumentation systems
can require fans, air-conditioning, and so on.
Solenoid coil heat also reduces a valve’s life expectancy. Differing grades of
magnet-wire/insulation allow for larger or smaller tolerances for
ambient system temperatures, as well as for the increased temperature
caused by a valve’s own heat of operation. But the 10 C rule of thumb
(based on Arrhenius’s theory and a typical activation energy) suggests
that a coil’s thermal life approximately doubles for every 10 C
reduction in operating temperature, all
other things being equal. For example, if a conventional 11-watt
solenoid coil can be reduced to two watts by the current switching
system mentioned above, the coil temperature will typically
decrease by 40 C, with a lifetime increase of 2^(40/10) = 16.
Assuming such results, it is possible to extend the service life
of a solenoid coil from five years to 80 years.
Users should take care to optimize performance of a simple
solenoid valve to the fluid flowrates and pressures being controlled.
Advanced designs now available can significantly reduce power
consumption via switched current management, allowing either DC or AC
applications to run even more efficiently than previous AC-only coils.
These new solenoid valve designs also allow for more widely ranging
voltage supplies. In addition, they significantly reduce heat
generation and consequent failure rates.
Stephen Glaudel is
VP of engineering for Emerson Electric Corp.’s ASCO Valve business unit
in the Americas. Previously, he held the same position at Brooks
Instrument, also part of Emerson. Prior to joining Emerson, Mr. Glaudel
held positions at Leeds & Northrup and Westinghouse. He earned a
bachelor’s degree in Electrical Engineering & Biosystems from
Syracuse University and an MBA from St. Joseph’s University. Mr.
Glaudel can be reached at firstname.lastname@example.org
or 973 966-2543.
For More Information: www.ascovalve.com