What Is a Contactor?
A contactor is an electromagnetic switching device used to open and close power circuits remotely. It is common in motor, heater, lighting and capacitor-bank circuits.
How Does a Contactor Work?
When the coil is energised, a magnetic field pulls the armature and closes the main contacts. When coil power is removed, springs return the contacts to their normal state.
Key Selection Criteria
Correct sizing checks load current, utilisation category, motor power, operating voltage, coil voltage, electrical/mechanical life and panel temperature.
For motors, the contactor should be coordinated with an overload relay or motor protection breaker. The contactor switches; the protection device handles overload and fault risks.
AC-1, AC-2, AC-3 and AC-4 Utilisation Categories
AC-1
Resistive or slightly inductive loads such as heaters.
AC-2
Starting and stopping slip-ring induction motors.
AC-3
Standard starting and stopping of squirrel-cage motors; the most common motor duty.
AC-4
Inching, plugging, reversing and frequent start-stop duty; much harsher for the contactor.
Contactor Selection by Motor Power
A contactor should not be selected by current alone. Load type, utilisation category, coil voltage, auxiliary contacts and switching frequency must be considered together.
| Motor power | Approx. current | Example contactor |
|---|---|---|
| 0.75 kW | 2 A | 9 A class contactor |
| 1.5 kW | 4 A | 9 - 12 A class |
| 3 kW | 7 A | 12 - 18 A class |
| 5.5 kW | 11 A | 18 - 25 A class |
| 7.5 kW | 15 A | 25 - 32 A class |
| 11 kW | 22 A | 32 - 40 A class |
| 15 kW | 30 A | 40 - 50 A class |
Voltage and Coil Selection
Correct sizing checks load current, utilisation category, motor power, operating voltage, coil voltage, electrical/mechanical life and panel temperature.
| Criterion | Contactor |
|---|---|
| Line voltage | Voltage and current carried by the main contacts. |
| Coil voltage | Selected from the control circuit: 24 VDC, 24 VAC, 110 VAC, 230 VAC or 400 VAC. |
| Frequency | Check 50/60 Hz compatibility for AC coils. |
Auxiliary Contact Selection
1NO
Used for run indication to PLCs or signal lamps.
1NC
Used in interlocking, safety and feedback circuits.
Mechanical interlock
Prevents two reversing contactors from closing together.
Contactor and Relay Differences
A contactor is an electromagnetic switching device used to open and close power circuits remotely. It is common in motor, heater, lighting and capacitor-bank circuits.
| Criterion | Contactor | Relay |
|---|---|---|
| Function | Switching power circuits | Switching signal or control circuits |
| Current level | Higher load currents | Lower control currents |
| Contact structure | Main power contacts plus auxiliaries | Mostly auxiliary contacts |
Common Mistakes
Using AC-1 rating for a motor
The contactor may be undersized for AC-3 duty.
Wrong coil voltage
The contactor may fail to pull in or the coil may burn.
Ignoring panel temperature
Service life and capacity can decrease.
Missing auxiliary contacts
Extra modules may be needed during commissioning.
Conclusion
A contactor should not be selected by current alone. Load type, utilisation category, coil voltage, auxiliary contacts and switching frequency must be considered together.
FAQ
What does a contactor do?
It switches a load circuit remotely and safely.
Which category is used for motors?
AC-3 is commonly used for standard squirrel-cage motors.
Why is coil voltage important?
It must match the control circuit for reliable operation.
Does a contactor provide protection?
No. It switches; overload and short-circuit protection require separate devices.