What Is the Purpose of a Solenoid? Actuator and Relay Selection Guide

What Is the Purpose of a Solenoid? (The Core Physics)

The direct answer: a solenoid is an electromechanical transducer designed to convert electrical energy into precise, linear mechanical force. When current flows through the tightly wound copper coil, it generates a concentrated magnetic field that pulls a ferromagnetic plunger toward the center of the coil.

In practical applications, this linear motion serves two distinct purposes. First, as a raw linear actuator, the plunger directly manipulates a physical mechanism—think of the solenoid valve controlling water flow in an irrigation system, or the locking pin in a commercial door strike. Second, and more relevant to electrical panels, the solenoid acts as the prime mover for relays and contactors. In this configuration, the magnetic pull closes or opens heavy-duty electrical contacts, allowing a low-power control circuit to safely switch a high-power load. Understanding which role your solenoid plays dictates how you wire, rate, and test it.

Coil vs. Contact Side: Wiring the Two Halves

The most critical concept when wiring a solenoid-actuated relay or contactor is recognizing the galvanic isolation between the control side (the coil) and the load side (the contacts). They are mechanically linked but electrically separate.

• The Coil Side (Control): Typically labeled A1 and A2. This circuit only needs to supply enough current to energize the electromagnet—usually between 10mA and 100mA, depending on the physical size of the contactor. You can wire this to a PLC output, a toggle switch, or a microcontroller relay shield.
• The Contact Side (Load): Typically labeled L1/T1, L2/T2, etc., for main power, and NO/NC (Normally Open/Normally Closed) for auxiliary control contacts. This side carries the full amperage of your load, requiring appropriately sized AWG wire and torque-specific terminal tightening.

Rating Table: Which Column Governs Your Load?

A common mistake on the bench is looking only at the maximum amperage printed on the contactor's faceplate and assuming it applies to all loads. It does not. The governing rating column depends entirely on the IEC 60947 utilization categories. Inductive and motor loads generate massive inrush currents and inductive kickback that destroy contacts rated only for resistive heating.

Which column governs? If your load has a motor or a transformer, the AC-3 (Motor) or AC-15 (Control) column governs. Never size a contactor for a motor using the AC-1 resistive column; the inrush current will weld the contacts closed on the first startup.

Load Selection Decision Path

Use this decision tree to select the correct solenoid-actuated switching device for your specific application. Follow the logic down to your concrete part selection.

How to Test a Solenoid: Dead and Live Diagnostics

When a solenoid actuator or contactor fails to pull in, you need to isolate whether the failure is electrical (coil) or mechanical (plunger jam). Always follow lockout/tagout procedures before performing dead tests.

1. Dead Testing (De-energized)

• Coil Continuity: Set your multimeter to Ohms (Ω). Place probes across A1 and A2. A healthy AC control coil typically reads between 10Ω and 100Ω. A DC coil might read higher (100Ω to 500Ω). If you read OL (Open Line), the internal copper winding is burned out. If you read 0.0Ω, the coil is internally shorted.
• Contact Isolation: With the coil de-energized, measure across L1 and T1. You must read OL. If you read near 0Ω, the contacts are welded shut from a previous overload event—a critical safety hazard.
• Mechanical Bind: Use a non-conductive probe to manually press the solenoid plunger or contactor crossbar. It should move smoothly and snap back crisply under spring pressure. Gritty movement indicates dust ingress or a melted armature.

2. Live Testing (Energized)

Safety Note: Only perform live testing if you are qualified to work on energized panels and are wearing appropriate PPE.

• Coil Voltage: Set your meter to AC or DC Volts. Measure directly across A1 and A2 while the circuit is commanded 'ON'. The voltage must be within ±10% of the coil's nominal rating. A 24VDC coil will chatter or fail to pull in if the voltage drops below 19V due to undersized control wiring.
• Contact Voltage Drop: With the solenoid energized and the load running, measure the voltage across the closed contacts (L1 to T1). A healthy contact pair will drop less than 50mV. If you read several volts across a closed contact, the silver-alloy surface is heavily pitted or carbon-fouled, generating excessive heat.

Repair vs. Replace: The 100A Threshold

Solenoid-actuated devices are subjected to intense mechanical shock and electrical arcing. Knowing when to rebuild versus when to throw the unit in the scrap bin saves time and prevents catastrophic panel fires.

The Hard Rule: For any solenoid relay or contactor rated under 100A, always replace the entire unit. The cost of labor to disassemble, clean, and re-tension the contacts far exceeds the $20 to $80 replacement cost of a modern sealed unit. Furthermore, sub-100A contactors are not designed with user-serviceable contact tips; attempting to file down pitted contacts with sandpaper removes the silver cadmium oxide plating, guaranteeing the contacts will weld shut on the next motor start.

For heavy industrial contactors over 100A (e.g., NEMA Size 3 and larger, or IEC equivalents handling 50HP+ motors), rebuilding is standard practice. These units feature bolt-on, replaceable contact tips and accessible solenoid coils. In this tier, you replace the contact tips when the wear indicator groove is no longer visible, and you replace the solenoid coil if it shows thermal discoloration or reads out of the manufacturer's specified resistance tolerance (usually ±10%).

When in doubt, swap it out. A $40 contactor is cheap insurance against a $4,000 motor replacement caused by a single welded contact failing to disconnect power during a fault.