Two contactors can have identical power contacts and still not be interchangeable, because their coils are different. The coil voltage, the type of current and the frequency must all match the control circuit, and every manufacturer encodes them in its own way in the part number. This article explains what to check so the replacement closes reliably and does not burn out. For the basics of how a contactor works, see Contactor - application.
AC coils: inrush and holding
When an AC coil is energized, the magnetic circuit is open: there is a large air gap between the fixed and moving parts. The coil's inductance is low, so it draws a high inrush current. Once the armature closes, the gap disappears, the inductance rises and the current falls to a much lower sealed (holding) current. Catalogs give both values, usually in VA.
Two practical consequences:
- Control transformers and power supplies must be sized for the inrush of all the coils that can close at the same time, not just for the holding power.
- If the armature cannot close completely (dirt, a mechanical obstruction, voltage too low), the coil keeps drawing inrush current and overheats quickly.
AC magnets also use a shading ring on the pole face to keep the armature from vibrating each time the current passes through zero. A broken shading ring or worn pole faces cause a loud hum or chatter.
AC coil inrush and holding current, schematic.
DC coils
With direct current, the steady current is limited only by the coil's resistance, so there is no inrush caused by the air gap in the same way. To get enough pull-in force without overheating while held closed, DC contactors often use a larger magnet, a dual winding (pick-up and hold) with an economizer contact or circuit, or an electronic coil control. A DC coil and an AC coil of the same nominal voltage are not interchangeable.
50 Hz, 60 Hz and dual-rated coils
An AC coil's current depends on frequency, because its impedance does. As a general rule:
- A coil designed for 50 Hz used on 60 Hz at the same voltage has a higher impedance, draws less current and produces less force; it may close weakly or chatter at low voltage.
- A coil designed for 60 Hz used on 50 Hz draws more current and runs hotter, which shortens its life.
Many coils are therefore dual-rated, with a marking such as "110 V 50 Hz / 120 V 60 Hz" or "220-230 V 50 Hz / 240 V 60 Hz". The voltage is slightly higher at 60 Hz to compensate for the higher impedance. If your plant is on 60 Hz and the coil shows only a 50 Hz rating, check the manufacturer's data before using it.
The product standard for contactors, IEC 60947-4-1, also defines the range of control voltage over which a contactor must close and the range in which it must drop out. The range commonly quoted for closing is 85% to 110% of the rated coil voltage; check the current edition of the standard and the data sheet for exact limits.
Why coil codes differ by manufacturer
There is no universal coil code. Each manufacturer adds a group of characters to the contactor reference to identify the coil, and the same letters mean different voltages from one brand to another. For example, Schneider Electric TeSys D contactors use a letter code at the end of the reference to indicate the coil; other manufacturers place the coil code in the middle of the catalog number or use a separate coil part number. Always decode the coil from the manufacturer's own catalog for that exact product range. Never assume that a letter means the same thing on two brands, or even on two product families of the same brand.
When you send a request for a contactor, give the complete reference including the coil code, plus the actual control voltage and frequency of your panel. See how to read part numbers and nameplates for more tips.
How to read the coil marking
On most contactors the coil carries its own marking, often visible on the front or on the coil body once the cover is removed. Look for:
- The rated voltage or voltage range (for example 24 V, 120 V, 100-250 V).
- AC, DC or AC/DC, often shown with the symbols ~ and =, and the frequency (50 Hz, 60 Hz or 50/60 Hz).
- A coil part number, when the coil is a replaceable spare part.
- Sometimes the inrush and sealed power in VA or W.
- The coil terminals, usually marked A1 and A2.
If the contactor's coil is replaceable, a spare coil can be a quicker fix than a complete contactor, provided it is the exact coil for that frame size.
Reading a coil marking (fictitious values).
What happens with the wrong coil
| Mistake | Typical result |
|---|---|
| Coil voltage lower than the supply | Excessive current, overheating, coil burns out quickly |
| Coil voltage higher than the supply | Weak pull-in, chatter, or no closing; an AC coil that does not seal keeps drawing inrush and overheats |
| AC coil on DC | Current limited only by resistance, far above rating; coil burns out |
| DC coil on AC | Reduced current and force, hum or chatter, unreliable operation |
| 50 Hz-only coil on 60 Hz, or the reverse | Weak closing (50 Hz coil on 60 Hz) or overheating (60 Hz coil on 50 Hz) |
For guidance only; the manufacturer's data defines the allowed range.
Electronic coils
Many recent contactor ranges use an electronic coil: a small circuit that rectifies the supply and controls the magnet. Benefits often include a wide voltage range (one coil for AC and DC over a range), lower holding power and less sensitivity to voltage dips. Some versions accept a low-power signal straight from a PLC output. Points to check when replacing a conventional coil with an electronic one, or the reverse:
- The voltage range and whether it covers your control voltage.
- Off-state leakage from triac outputs, solid-state relays or long control cables, which can keep a low-consumption coil from dropping out.
- Whether a coil surge suppressor is needed or already built in.
Coil surge suppressors
When a coil is switched off, its stored energy creates a voltage spike that can disturb electronics or wear PLC output contacts. RC modules, varistors and diodes (for DC) limit it. A plain diode on a DC coil noticeably increases the drop-out time, which can matter in fast sequences; check the manufacturer's recommendation for your application.
Work on electrical equipment must be done by qualified personnel following the applicable electrical code and the manufacturer's instructions.
Key takeaways
- Match coil voltage, AC or DC, and frequency to the control circuit, not just the contactor size.
- AC coils draw high inrush on closing; size control transformers for it.
- Coil codes are manufacturer-specific: decode them from the right catalog.
- A wrong coil either burns out or fails to close properly.
- Electronic coils offer wide ranges but can be sensitive to leakage current.
Related parts at 3JIndustry
Find used contactors and spare coils in our Contactors collection; also read IEC utilization categories to choose the right rating. All parts are used, with a 30-day warranty.
Looking for a discontinued part? Send the full part number to sales@3jindustry.com or use our contact page.
