What is a contactor?
A contactor is an electromagnetically operated switching device that uses a small control signal to switch a power circuit for motors, heating or lighting on and off remotely, and that is built for high currents and millions of operations. A coil of, for example, 24 V DC pulls in the main contacts, so a PLC or push button can start a motor of tens of kilowatts. Contactors are the workhorses of every control cabinet and every MCC, and are covered by the product standard IEC 60947-4-1.
A contactor switches, but it does not protect. Only in combination with thermal overload protection and a circuit breaker or fuses does it become a complete motor starter.
🧠 How does a contactor work?
A contactor consists of an electromagnet, a moving armature carrying contact bridges, fixed main contacts and an arc chamber. It works in four steps:
- Energising the coil - the control voltage (for example 24 V DC or 230 V AC) on terminals A1/A2 creates a magnetic field. Under IEC 60947-4-1 a contactor must pick up reliably between 85 and 110 percent of its rated control supply voltage.
- Closing the main contacts - the armature pulls the contact bridges against the fixed contacts. The three main poles (L1-T1, L2-T2, L3-T3) connect the load to the supply.
- De-energising the coil - a spring pushes the armature back and the contacts open. The arc that forms on breaking is cooled and extinguished in the arc chamber.
- Reporting status - auxiliary contacts (normally open NO or normally closed NC) switch along and report the position to the PLC, or form an interlock with a second contactor.
An AC coil has a shading ring in the magnet core that prevents humming and chattering when the current passes through zero. Modern contactors often have an electronically controlled coil with a wide voltage range and low holding power. Switching off a coil produces a voltage spike, so an RC network, varistor or diode across the coil protects the PLC’s transistor output.
🔄 What is the difference between a contactor and a relay?
Both switch by means of a coil, but they are designed for different jobs. A common rule of thumb is that a relay switches control currents up to about 10 A and a contactor switches power currents above that. The essential difference lies in construction and standard.
| Feature | Contactor | Relay |
|---|---|---|
| Product standard | IEC 60947-4-1 | IEC 61810 (control relays: IEC 60947-5-1) |
| Typical current | Approx. 6 A to well over 1000 A | Milliamps to approx. 10 A |
| Poles | Usually 3 main poles plus auxiliary contacts | 1 to 4 changeover contacts |
| Arc quenching | Arc chamber, double break per pole | Limited |
| Load | Motors, heating, capacitor banks | Signals, coils, small loads |
| Rating | Current per utilisation category (AC-1, AC-3) | Contact rating in A at a given voltage |
A time-delay relay or safety relay often drives the coil of a contactor: the relay does the thinking, the contactor switches the power.
🔧 Which ratings determine the selection?
The most common mistake is to look at the highest current in the catalogue. A contactor does not have one fixed current rating but a current per utilisation category:
| Utilisation category | Load | Effect on selection |
|---|---|---|
| AC-1 | Resistive loads such as resistance heating | Highest permissible current |
| AC-3 | Squirrel-cage motors: starting and switching off while running | Standard value for motors |
| AC-3e | High-efficiency motors with higher inrush current | In the standard since 2018 |
| AC-4 | Inching and plugging (reverse-current braking) | Much lower current or service life |
The same contactor may switch 40 A at AC-1 but only 25 A at AC-3, because the starting current of an induction motor is six to eight times its rated current. As an illustration: in the smallest frame size of a common product range, a 9 A contactor switches a 4 kW motor at 400 V, and a 16 A type a 7.5 kW motor.
Other important ratings:
- Mechanical endurance - the number of operations without load; for small contactors typically 10 to 30 million.
- Electrical endurance - the number of operations under load, a fraction of the mechanical endurance and strongly dependent on current and category. Manufacturers publish curves for this.
- Control voltage and coil type - AC or DC, and whether the PLC output can handle the pick-up power.
- Coordination type - with type 2 under IEC 60947-4-1, the starter must remain suitable for further use after a short circuit, with only lightly welded contacts that are easily separated permitted; with type 1 it may be damaged and may need repair or replacement.
Worn or burnt contacts cause voltage drop, arcing and heating, and a pole that no longer conducts leaves a motor running on two phases, which leads to motor damage. Periodic inspection should therefore cover contact wear, coil operation and the tightness of the terminals.
🏭 Where are contactors used in industry?
- Motor control - as a direct-on-line starter (DOL), in a star-delta starter with three contactors, or as a reversing starter with two contactors that are electrically and mechanically interlocked.
- Bypass and mains isolation - as a bypass for a soft starter after run-up, or as a line contactor upstream of a variable frequency drive.
- Heating and lighting - heating elements, ovens and lighting groups, usually at AC-1.
- Pumps, fans and conveyors - in industrial automation, building automation and water management.
- Medium voltage - vacuum contactors for large motors and capacitor banks, under IEC 62271-106.
For loads that switch very frequently, such as heating elements with pulse control, there are semiconductor contactors without moving parts (IEC 60947-4-3).
🔐 What role does a contactor play in machine safety?
In many machines the contactor is the final element of a safety function: an emergency stop or door switch de-energises the coil via a safety relay or safety PLC, after which the motor loses power. This is stop category 0 in IEC 60204-1. Because contacts can weld, extra requirements apply:
- Redundancy - for a high Performance Level under ISO 13849-1 or a SIL under IEC 62061, two contactors are placed in series.
- Mirror contacts - an NC auxiliary contact to IEC 60947-4-1 Annex F cannot be closed while any main pole is closed. The safety relay reads this contact back and refuses a restart if a contactor has welded.
- Mechanically linked contacts - under IEC 60947-5-1 Annex L, NO and NC auxiliary contacts can never be closed at the same time.
- B10D value - manufacturers publish the number of operations until 10 percent fail dangerously, for example about 1.37 million at nominal load. You use it to calculate the failure probability of the functional safety function.
A contactor is not an isolator. During maintenance under Lock-out Tag-out and NEN 3140, you lock the main switch or local isolator, never only the contactor, because a PLC fault or welded contacts can bring the voltage back.
📡 How does the contactor relate to IT/OT convergence?
Traditionally a contactor is a dumb component: coil on, contacts closed. Modern ranges such as Siemens SIRIUS and Schneider Electric TeSys offer variants with IO-Link or a fieldbus module. These report at least the switching state to the PLC; add-on current monitoring relays and intelligent motor starters add current, operating hours or an operations counter. This enables condition monitoring and predictive maintenance: you replace the contactor based on actual wear rather than after a failure.
This also makes the motor feeder part of the OT network. An attacker who gains write access to the PLC or the fieldbus network can switch contactors and so start or stop pumps or conveyors. Segmentation and access control under IEC 62443 therefore apply down to the lowest layer, while hardwired safety functions remain independent of the network.
❓ Frequently asked questions
What does AC-3 mean on a contactor?
AC-3 is the utilisation category in IEC 60947-4-1 for starting and switching off squirrel-cage motors while running. The AC-3 current is the value you use to size a contactor for a motor. For high-efficiency motors with a higher starting current, AC-3e applies.
Why do some contactors hum?
A contactor with an AC coil can hum when the shading ring in the core is damaged, the pole faces are dirty or the control voltage is too low. The contactor then does not pull in fully and the contacts may start to chatter. Check the control voltage and replace the contactor if the humming persists.
Can I drive a contactor directly from a PLC?
A small contactor with a 24 V DC coil can often be connected directly to a PLC transistor output, provided the coil power is within the output current and coil suppression is fitted. Larger contactors or AC coils are driven through an interposing relay. Always check the datasheets of the contactor and the PLC output module.
What is the difference between a contactor and a motor starter?
A contactor only switches the current to the motor on and off. A motor starter combines a contactor with overload protection and usually short-circuit protection, so the motor is also protected. The contactor is therefore one component of the motor starter.
How long does a contactor last?
The service life of a contactor depends on the number of operations under load, not on time. Small contactors reach 10 to 30 million operations mechanically, a fraction of that electrically at AC-3, and far fewer when inching (AC-4). With an operations counter via IO-Link or the PLC, you can plan replacement.
Can I use a contactor to isolate an installation?
No, a contactor is not an isolator and must not be used to make an installation safely dead for work. Use the main switch or a local isolator that can be locked in the off position instead. This follows from IEC 60204-1 and, in the Netherlands, NEN 3140.
📌 In summary
A contactor is an electromagnetically operated switching device that uses a small control signal to switch motors, heating and other power loads, and is selected under IEC 60947-4-1 by its current per utilisation category. Size it at AC-3 or AC-3e for motors, combine it with overload and short-circuit protection to form a motor starter, use mirror contacts in safety functions, and treat contactors with IO-Link as part of the OT network.