Solenoid coil burnout occurs when abnormal current, poor heat dissipation, or mechanical sticking causes the temperature of the internal conductor to exceed the limit of its insulation class (Class F or Class H, for example), degrading or melting the insulation, causing turn-to-turn shorts, and ultimately leaving the solenoid unable to generate enough magnetic force to function. As coil temperature rises, the enamel insulation on the magnet wire can age, carbonize, crack or melt — leading to turn-to-turn shorts, an open circuit or an abnormal resistance reading. The coil can then no longer produce the force needed to shift the spool, and the valve fails.
In a hydraulic system, energizing the coil creates a magnetic field that pulls in the armature and shifts the spool to redirect flow. Under normal conditions the heat the coil generates dissipates continuously through its housing, keeping temperature within the design range.
In automated equipment, CNC machine tools, and hydraulic systems, coil burnout is one of the most common failures encountered in the field. Most plants have experienced a machine stopping mid-cycle, only to find a burnt-out solenoid coil. In reality, the burnt coil is rarely the root cause — it is the consequence of something else going wrong in the hydraulic system, the power supply or the way the equipment is used. Replace the coil without eliminating the real source, and the same failure is likely to recur.
This article covers the five most common causes seen in industrial settings and how to prevent each, to help plant and maintenance staff find the root cause quickly and reduce downtime.
Why do solenoid coils burn out? The causes behind the overheating
Contaminated hydraulic oil causing the spool to stick
After extended operation, hydraulic oil accumulates metal wear particles, seal debris, sludge and oxidation products. These contaminants circulate with the oil into the valve and settle in the precision clearance between spool and body.
Once contamination reaches a certain level, the spool can begin to stick, move sluggishly or fail to shift fully. At that point, even with the coil correctly energised, the magnetic force cannot pull the armature fully home.
On an AC solenoid valve, an armature that does not seat fully leaves an air gap in the magnetic circuit. Coil inductance falls and operating current rises. Held in that high-current, high-temperature state for any length of time, the insulation ages rapidly, and the coil eventually overheats, shorts or burns out. Many failures that look like coil defects therefore trace back to oil contamination and a sticking spool.
How to prevent it.
- Manage the hydraulic oil properly: schedule regular cleanliness testing and change oil and filter elements according to the equipment specification.
- If a valve starts behaving abnormally — for example, with hesitant shifting or unusual noise — check the spool and the circuit immediately before the problem escalates.
- A sound oil management regime reduces failure rates far more effectively than repeatedly replacing coils.
High hydraulic oil and ambient temperatures
Many hydraulic power units are installed in poorly ventilated spaces, or run at high load for long periods, driving oil temperature steadily upward. Sustained oil temperatures above 60°C — particularly above 70°C or 80°C — accelerate oil degradation and raise the valve's operating temperature.
Because the coil is mounted directly on the valve, heat from hot oil passing through the body conducts to the coil through the armature tube. When that external heat source is added to the coil’s own temperature rise from being energised, internal temperature can keep accumulating, accelerating insulation ageing. Operating in high ambient temperature for extended periods increases the risk of shorting, failure and burnout.
Insulation class matters when specifying a coil.Tai-Huei coils are wound throughout with high-grade PEWH polyesterimide enamelled copper wire, providing additional thermal margin even under continuous duty or high oil temperature.
The table below compares the common insulation classes used for industrial solenoid valves with the temperature ratings of the materials used by Tai-Huei:
Insulation Class | Maximum Allowable Operating Temperature (°C) | Recommended Industrial Applications |
|---|---|---|
Class B | 130°C | Suitable for equipment operating under relatively low loads, intermittent duty cycles, and well-ventilated ambient conditions. |
Class F | 155°C | A commonly used industrial insulation class, suitable for most automated production lines and general-purpose CNC machine tools. |
Class H | 180°C | Suitable for heavy-duty hydraulic systems operating with high oil temperatures, continuous energization, or in relatively enclosed installation spaces. |
Additional Note: According to the international standard IEC 60085, the maximum allowable temperature for Class H insulation is 180°C. Based on the Class H insulation rating, Tai-Huei uses PEWH polyesterimide enamelled copper wire rated for temperatures up to 200°C to manufacture its solenoid coils. This provides approximately 20°C of additional thermal margin beyond the standard Class H rating, giving the coil greater thermal buffering capacity and enhanced operating reliability under demanding conditions such as high hydraulic oil temperatures.
How to prevent it.
- Monitor oil and reservoir temperature regularly, and make sure the power unit has adequate ventilation and heat dissipation.
- Check that coolers, heat exchangers and related equipment are working properly, so that reduced cooling efficiency does not drive system temperature up.
- Where operating conditions allow, avoid running equipment above capacity for extended periods, and specify valves and coils rated for the actual thermal environment.
Abnormal supply voltage
Operating a solenoid valve with a supply voltage that is too high, too low, or unstable can affect coil performance and increase the risk of overheating and failure.
Overvoltage: When the supply voltage remains above the coil's rated voltage, coil current and heat generation increase, causing the internal temperature to rise. Prolonged overvoltage accelerates insulation ageing, leading to shorting, failure or burnout.
Undervoltage: when supply voltage falls significantly below the rated value, the coil may not generate enough force, leaving the armature unable to seat fully or the valve shifting erratically. On an AC solenoid, an unseated armature keeps the coil in a high-current state, generating still more heat. Left unresolved, this accelerates insulation ageing and raises the risk of burnout. On a DC solenoid, an unseated armature does not usually cause a marked current rise, but the coil can still overheat and fail through prolonged energisation, poor heat dissipation or overvoltage.
How to prevent it.
- Measure supply voltage regularly to confirm it matches the equipment specification, and check that power supplies, transformers and wiring are functioning correctly.
- Where the line includes large motors, variable-frequency drives or other high-power equipment, avoid sharing an unstable supply with solenoid valves.
- If a valve runs abnormally hot, shifts erratically or fails repeatedly, check supply quality alongside the valve itself.
High switching frequency — inrush current and accumulated heat
On automated presses, injection molding machines, punching equipment and robotic handling, solenoid valves often cycle at high frequencies. When switching frequency is too high, the heat the coil generates cannot dissipate quickly enough, internal temperature keeps rising, and the coil can overheat or burn out.
On AC solenoids, every energisation produces a brief inrush current well above normal operating current. If actual duty exceeds the coil’s recommended duty cycle (ED%), heat accumulates faster than it dissipates and the failure risk rises.
How to prevent it.
- For high-cycle equipment, assess the coil’s duty cycle (ED%), energized time and cooling conditions in addition to confirming the valve specification, and avoid running beyond design load for long periods.
- Where equipment requires sustained high-frequency operation, specify valves rated for continuous energisation or high cycle counts, and pair them with good installation and cooling conditions to extend coil life and improve system stability.
Oil leakage and moisture ingress leading to short circuits
Prolonged exposure to high pressure and vibration can age valve body seals such as O-rings, allowing hydraulic oil to leak. Humidity, water vapor, or coolant can also enter through gaps in the coil housing or terminal box. Once liquid or moisture reaches the coil interior, insulation absorbs moisture and degrades, reducing the dielectric strength of the magnet wire and its insulation.
Failures of this kind are usually progressive. Early symptoms may be no more than erratic operation or intermittent faults, developing later into a short circuit or full burnout.
How to prevent it.
- Inspect seals and valve body sealing regularly so that ageing or wear does not admit oil or moisture.
- Where the working environment allows, avoid leaving equipment in high humidity, oil mist or coolant spray for extended periods.
- In harsh environments, specify connectors rated IP65 or better.
- Clean oil and coolant residue from valve surfaces regularly to limit corrosion and ingress paths.
- If a coil shows signs of moisture, terminal corrosion or abnormal heating, stop the machine and investigate immediately before the fault worsens.
Further reading: Hydraulic Solenoid Valve Connectors: Comparing 5 Wiring Types, IP Ratings and Selection Criteria
How to reduce the risk of coil burnout
Once the common causes are understood, the risk can be reduced in three key areas: selection, oil management, and electrical protection.
1.Specify the coil and ingress protection for the actual duty
Select the coil specification during design or maintenance based on the actual operating conditions. For continuous duty or high oil temperature, choose a coil designed for 100% ED (continuous energisation), preferably with Class F insulation or better. In wet, oil-mist or dusty environments, specify IP65 or better ingress protection to reduce the risk of moisture and oil reaching the coil.
2.Maintain oil quality and control operating temperature
Oil quality and temperature are among the biggest influences on valve life. Fit appropriate filtration, change filter elements on schedule, and keep the oil clean so that fewer contaminants reach the valve. Where oil temperature runs high, control it with an oil cooler or by improving heat dissipation, and keep oil temperature within the range specified for the equipment to limit heat conducted into the coil.
3.Improve electrical protection
Adding appropriate protection to the control system can significantly reduce the risk of coil damage caused by abnormal electrical conditions. Protective devices in the control circuit provide overload protection when abnormal current occurs. For DC solenoids, fit a flyback diode or snubber to suppress the inductive voltage spike generated at de-energisation and protect the coil insulation.
Further reading: How to Select Hydraulic Solenoid Valve Size: Flow Rating Chart and ISO 4401 Mounting Interfaces
A four-step diagnostic procedure
When a valve fails to operate, operates erratically or the coil runs abnormally hot, work through these four steps to locate the source quickly.
Step 1 — Measure coil resistance.
Start by checking the coil itself. Measure the resistance across the coil terminals with a multimeter to check for coil burnout, a short circuit, or an open circuit. An abnormally low reading suggests an internal short; an infinite reading suggests an open circuit. Confirm the power is isolated before measuring, to protect both equipment and personnel.
Step 2 — Confirm supply voltage.
If the coil resistance is normal, check the control signal and supply voltage. Measure the actual voltage at the valve connector and confirm it matches the coil rating (AC 110V, AC 220V, DC 12V, DC 24V, etc.). Insufficient voltage leaves the coil unable to develop enough force; excessive voltage can drive the coil to overheat and burn out.
Step 3 — Check spool movement manually.
With the electrical side confirmed, move on to the mechanical side. Isolate power and system pressure, then use the manual override to check that the spool moves freely. If the spool is sticking, common causes include oil contamination, foreign matter, internal wear, or seizure after a long shutdown.
Step 4 — Check hydraulic oil cleanliness.
If both coil and electrical control are sound but the valve still operates abnormally, examine oil quality and system cleanliness. Metal particles, contaminants or sludge in the oil can block the spool clearance, causing sticking, internal leakage or erratic operation. Maintaining the filtration system properly helps reduce valve failure rates and extend service life.
Following the sequence of electrical check → signal check → mechanical check → oil analysis helps avoid unnecessary valve replacement, identify the root cause quickly, and reduce both downtime and repair costs.
Case study: what was really behind the repeated coil failures
At an injection molding plant in northern Taiwan, the same solenoid coil on an auxiliary hydraulic power unit burned out four times in six months. The initial response was straightforward: replace the failed coil, restart the machine, and resume operation. But each replacement lasted only three to five weeks before failing again. The problem kept recurring and was never resolved.
After the fourth failure, the site supervisor began to suspect the coil was not the problem and asked the Tai-Huei engineering team to assess the machine on site. Our engineer first measured coil resistance and confirmed it was within the normal range, ruling out a manufacturing defect. Disassembling the valve revealed heavy dark-brown sludge on the mating surfaces between spool and body, with the spool clearly binding as it moved. Further inspection of the power unit reservoir uncovered the root cause: oil quality had gone unmanaged for a long time. The hydraulic oil had not been changed in over two years, the reservoir had accumulated substantial sludge and metal wear debris, and the filter element was blocked and no longer filtering effectively. These contaminants were carried into the valve with the oil, obstructing spool movement and holding the coil in an abnormal operating state until temperature rose far enough to destroy it.The recommended remedy was a complete oil change, new filter elements, and a proper oil management regime — replacing the previous replace-on-failure approach with scheduled maintenance and regular oil cleanliness testing. Nearly six months of follow-up showed no further coil failures on that valve.
The case illustrates the point again: a burned-out coil is usually the result of a system fault, not simply a component quality issue. Replacing the coil without eliminating oil contamination or spool sticking leaves the failure free to recur.