In many automation systems, solenoid valves are not frequently cycled. Instead, they remain energized for long periods to maintain flow, pressure, or system status. In such applications, coil temperature rise is often overlooked, yet it is one of the leading causes of premature solenoid valve failure.

Why Continuous Energization Causes Coil Heating

A solenoid coil converts electrical energy into magnetic force, but part of that energy inevitably becomes heat. In intermittent operation, this heat dissipates during off cycles. However, in continuous duty applications, heat accumulates over time.

Several factors intensify this effect:

◆ Excess supply voltage above the rated level

◆ Low coil resistance leading to higher current draw

◆ Poor heat dissipation in enclosed panels or near hot media

Under these conditions, coil temperature can rise steadily, even when the valve appears to function normally.

How High Temperature Affects Coil Insulation

Solenoid coil failure rarely occurs instantly. More often, insulation degradation develops gradually. Elevated temperatures accelerate aging of the enamel insulation on the winding wire, reducing insulation resistance.

As insulation weakens, the coil becomes prone to:

◆ Partial short circuits

◆ Increased leakage current

◆ Unstable electromagnetic force

These issues reduce reliability long before complete burnout occurs.

Long-Term Impact on Solenoid Coil Lifespan

Most solenoid coils are rated under insulation classes such as Class B, F, or H, with maximum allowable temperatures typically ranging from 130°C to 180°C. These values represent absolute limits, not ideal operating conditions.

When a coil operates near its thermal limit for extended periods:

◆ Service life decreases exponentially

◆ Failure timing becomes unpredictable

◆ Unplanned downtime risk increases

For industrial automation, this uncertainty poses a significant operational concern.

Common Misunderstandings in Continuous Duty Applications

Many selection decisions focus on valve body material or port size, while coil duty cycle is overlooked. Typical mistakes include:

◆ Using intermittent-duty coils for continuous energization

◆ Ignoring the thermal differences between AC solenoid coils and DC solenoid coils

◆ Underestimating ambient temperature effects

As a result, valves may operate reliably at first but suffer recurring coil failures months later.

Engineering Approaches to Reduce Overheating Risk

Managing coil temperature does not require complex solutions. Effective practices include:

◆ Selecting low power DC solenoid coils for continuous duty

◆ Ensuring stable and correct supply voltage

◆ Using thermal isolation or extended stems near high-temperature media

◆ Allowing sufficient airflow around the coil

These measures significantly improve long-term solenoid valve reliability with minimal cost impact.

Maintenance Indicators of Coil Overheating

Coil overheating usually provides early warning signs. During routine inspections, engineers should watch for:

◆ Coil surface temperatures noticeably higher than similar valves

◆ Slower response or incomplete actuation

◆ Localized heat buildup inside control cabinets

Recognizing these signals early allows corrective action before failure occurs.

(FK9025)