Finding the Cause of Solenoid Coil Burn-Out
FLUID POWER - Design Data Sheet 30
Perhaps occasionally a coil on a solenoid valve may burn out
because of a defect in its manufacture. But usually the cause can
be traced to some abnormal condition either in operating conditions
of the machine on which the valve is installed, or to unusual
environmental conditions. This becomes evident if the burn-out
should occur more than once at the same coil location.
Checklist for AC Solenoid Valves
Burn-out is more common on valves with AC coils than on those with
DC coils because of the high inrush current. Until the armature on
the solenoid can pull in and close the air gap in the magnetic
loop, the current is often 5 times as high as the steady state, or
holding, current after the armature is seated. Inrush is
approximately the same as holding current on a DC solenoid
1. Coil Does Not Match Operating Voltage
Improper match between the electrical source and the coil rating
is sometimes a cause for coil burn-out:
- Voltage Too High. The operating voltage should
not be more than 10% higher than the coil voltage rating. Excessive
voltage causes excessive coil current which overheats the
- Voltage Too Low.
Operating voltage should not be more than 10% below coil rating.
Low voltage reduces the mechanical force of the solenoid. It may
continue to draw inrush current without being able to pull in.
The low voltage test should be made by measuring the voltage
directly on the coil wires while the solenoid is energized and with
its armature blocked open so it is drawing inrush current. Energize
the solenoid just long enough to take a voltage reading. Also take
a no-load reading with the solenoid disconnected from the feed
wires. A difference of more than 5% between these two readings
indicates excessive resistance in the wiring circuit, or
insufficient volt-ampere capacity in the control transformer, if
one is used.
- Frequency. Operation of
a 60 Hz coil on 50 Hz causes the coil to draw above normal current.
Operation of a 50 Hz coil on 60 Hz causes the coil to draw less
than rated current and it may burn out from inability to pull
2. Overlap in Energization
On some double solenoid valves, if both solenoids are energized at
the same time and held in this state for a short time, the last
coil to be energized will burn out from the excessive inrush
current, which is about 5 times holding current.
A solenoid can burn out if both
solenoids are energized
at the same time on a double solenoid valve
like this one in which solenoids are mechanically
coupled to opposite ends of a common spool.
The burn-out condition described above will occur only on double
solenoid valves where the two solenoids are yoked to opposite ends
of a common spool as shown in the figure. If each solenoid can
immediately close its magnetic gap, neither will burn out if
Careful attention should be given to electrical circuit design
to make certain that the operator, through accident, cannot
energize both solenoids simultaneously.
Even with correct circuit design and interlock circuits a relay
with sticking contacts or slow release could be responsible for a
momentary overlap of energization on each cycle and eventual coil
burn-out. A simple device for de-tecting this condition is
described in Design
Data Sheet 18.
3. Too Rapid Cycling
Since the inrush current may be up to 5 times the holding current,
a standard AC coil on an air-gap solenoid may overheat and burn out
if required to cycle too frequently. The extra heat generated
during inrush periods cannot escape fast enough. The gradual
build-up of heat inside the coil winding may, in time, damage the
High cycling applications can be roughly defined as those
requiring the solenoid to be energized more than 5 to 10 times per
minute. On those applications, oil immersed solenoid structures
should be used. Conduction of heat through the oil surrounding the
winding allows the coil to operate at a lower temperature.
In plants where instrumentation is available, a thermocouple can
be placed on the surface of the winding in two identical valves of
the type to be used. One valve can be continuously cycled for
several hours at the proposed cycle rate while the other is
continuously energized. A difference of more than a few degrees in
surface temperature of the two coils indicates a need for oil
4. High Electrical Transients
If the current for the solenoid valves is taken directly from a
power line which is supplying large inductive devices such as
electric motors, the switching of these motors may cause high
voltage transients which may break down the insulation of solenoid
valve coils. A 'thyrector" should be installed across each coil to
"short circuit" these transients. Thyrectors are available at
industrial electrical supply houses.
5. Dirt in Oil or in Atmosphere
A small solid particle lodging under the solenoid armature may
keep it from fully seating against the core, causing coil current
to remain higher than normal during the holding period. Be sure
that solenoid dust covers remain tightly in place to protect
against dust deposited from the air.
Small dirt particles in the oil may lodge on the surface of the
spool, glued there by "varnish" circulating in the oil, or the
varnish itself may cause excessive spool drag and excessive coil
current. "Varnish" forms in systems where the oil is allowed to run
too hot. Heat accelerates unwanted chemical reactions. Reduce oil
temperature with a heat exchanger.
6. Environmental Conditions
Abnormally high or abnormally low ambient temperatures to which a
solenoid is exposed for an extended time may cause a solenoid to
- High Temperature. Coil insulation may be
damaged and one layer of wire may short to the next layer. A heat
shield or baffle will give some protection against radiated heat.
High temperature or oil immersed solenoids are the best protection
against heat conducted either through metal surfaces or from
surrounding high temperature air.
- Low Temperature. Cold ambient temperatures
cause oil to become more viscous, possibly overloading solenoid
valve capacity (See Item 9). Mechanical parts of the valve or
solenoid structure may distort, causing the valve spool to stick
and burn out the solenoid coil. Use an oil more suitable for the
low temperature, or use an oil immersed or high temperature coil to
handle the greater load imposed by the abnormally low ambient
7. Dead End Service
Fluid circulating through a solenoid valve helps to carry away
electrical heat. Some valves depend on fluid flow to keep excessive
heat from accumulating, and if used on dead end service, where the
solenoid remains energized for long periods without fluid flow, the
coil may burn out from this effect, possibly in combination with
8. Atmospheric Moisture
High humidity, coupled with frequently changing ambient
temperature, may form corrosion on metal parts of the solenoid
structure, causing the armature to drag or the spool to stick.
Humidity also tends to deteriorate standard solenoid coils, causing
shorts in the winding.
Change to molded coils or oil immersed solenoids. Keep solenoid
protective covers tightly in place, and perhaps seal the electrical
conduit openings after the wiring is installed.
9. Excessive Flow Through Valve
Pressure drop through the spool of a direct-acting solenoid valve
caused by the flow of fluid, creates a force unbalance tending to
cause the spool to move in an axial direction. This phenomenon is
described in Design
Data Sheet 18.
In circuit design, be very careful not to overload such a valve
above manufacturers flow rating. It should be de-rated when used
with fluids of high viscosity, or fluids having a high specific
gravity (fire resistant fluids, etc).
Checklist for DC Solenoid Valves
AC solenoid valves are far more common for in-plant industrial
applications, but DC solenoids may in some cases offer a specific
advantage. AC current may be run through a full-wave rectifier to
obtain a DC supply. A filter capacitor may have to be added to
eliminate chatter or hum.
- Inrush Current. On DC solenoids, inrush
current is the same value as holding current. Therefore, some of
the burn-out conditions previously described may not apply.
- Fast Cycling. Because of the low inrush
current, DC solenoid valves can usually be cycled at higher rates
than AC solenoid valves without overheating and coil burn-out.
- Repeatability. Shifting time on a valve with
DC solenoid repeats accurately from cycle to cycle. On AC valves,
shifting time may vary each cycle according to the state of the
line current at the moment the valve is energized - whether at
maximum, minimum, or in between.
- Limit Switch Contacts. DC solenoids usually
burn up switch contacts faster than do AC solenoids. Energy stored
in the coil inductance must dissipate when the coil is
disconnected, causing an arc across switch contacts on their break.
Much of this energy can be safely dissipated by wiring a diode
across the coil, with + on diode connected to + side of coil
voltage. The diode should be rated for at least 2 to 3 times the DC
A capacitor wired across the switch contacts on AC or DC
solenoids will help absorb the released energy. The best value of
capacitance can be determined by trial, either by observing the
intensity of the arc, or by measuring the voltage spike with an
oscilloscope as various capacitors are tried.
To reduce switch arcing, a
capacitor may be wired across
the contacts, or a diode may be wired across the
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Design Data Sheet 30 - Finding the Cause of Solenoid Coil
© 1990 by Womack Machine Supply Co. This
company assumes no liability for errors in data nor in safe and/or
satisfactory operation of equipment designed from this