There’s a bank of pneumatic solenoid valves on a filling machine that kept burning coils. Same valve, same position, three coils in five months. The coil was a 24V DC, 4.8W. The voltage at the terminal was 24.3V. The power supply was fine. The customer replaced the coil each time and it failed again. They assumed the valve manufacturer was shipping bad coils. The coils weren’t bad. The valve body was.
The coil that fails with correct voltage
A solenoid coil fails from heat, and heat comes from more than applied voltage. The coil’s rated temperature rise assumes the valve body conducts heat away. If the valve body can’t dissipate heat, the coil cooks itself. On the filling machine, the valve manifold sat directly above a heat-seal station. The ambient temperature around the manifold was 55°C. The coil’s rated ambient was 40°C. The coil was running 15°C above its design point. The insulation on the magnet wire degrades. It shorts. The coil burns out.
The voltage reading was fine. Voltage doesn’t tell you the coil temperature. A coil can burn out at exactly its rated voltage if the ambient is too hot, the duty cycle is too high, or the valve spool is sticking and the coil is holding harder than designed.
What the replacement history showed
All three failed coils were on the same valve position — the one that fired every cycle, 40 times a minute, continuously. The other eight valves on the manifold ran half as often and never failed. That’s a duty-cycle problem, not a quality problem. The coil was rated for 100% ED (continuous duty) but the heat from continuous cycling at 55°C ambient stacked up. Coil heat builds faster than it can dissipate in a hot environment. The 100% duty rating assumes 40°C ambient, still air, and a valve body at ambient temperature. Real conditions rarely match.
The fixes that worked
1. The manifold was moved away from the heat-seal station. The new location had 35°C ambient. The coil’s temperature rise dropped. The next coil lasted 14 months.
2. The duty cycle was reduced. The valve was firing 40 times a minute because the PLC was doing two pulses per cycle — an unnecessary double-fire added during a previous commissioning. Removing the second pulse cut the cycling in half. Fewer cycles, less heat, longer life.
3. The coil voltage was re-checked under load. The 24.3V reading was at idle. Under load, the voltage at the coil dropped to 22.8V — still within tolerance, but the holding force at 22.8V is lower. On a slightly sticky spool, the coil draws inrush current longer. The solution here wasn’t bigger wire. It was a clean, dry air supply so the spool never got sticky in the first place.
The coil temperature rule
For every 10°C above the rated ambient, the coil life halves. That’s the rule that governs solenoid life. If your coils are failing faster than the MTBF sheet says, measure the actual ambient temperature at the coil, not the room temperature. A $10 thermocouple on the coil surface tells you more than a $300 multimeter. If the coil surface runs above 80°C, something is wrong. Fix the heat, the duty cycle, or the spool stickiness — the coil is just the messenger.
Coils burn out from heat, and heat comes from ambient, duty cycle, and spool condition — not just voltage. The three failed coils sat over a heat-seal station, fired twice per cycle, and never got a chance to cool. Move the manifold, cut the double-fire, measure the coil surface temperature. The replacement coil isn’t the fix. The environment is.
