AC1, AC2, AC3, and AC4 duties are standardized classifications used to define the appropriate application and performance requirements for contactors and relays when switching alternating current (AC) loads. These categories ensure that the selected electrical switching device can safely and reliably handle the specific electrical characteristics of the connected load, protecting both the equipment and the system.
Understanding AC Duty Categories for Contactors
Contactor duty categories, primarily defined by international standards such as IEC 60947-4-1, specify the type of load a contactor is designed to switch. This classification is crucial because different types of electrical loads present unique challenges to contactors, particularly concerning inrush current, breaking current, and frequency of operation. Choosing the correct duty category ensures longevity and safe operation of the control gear.
AC1 Duty: For Resistive and Non-Inductive Loads
AC1 for resistive loads signifies the lightest duty for a contactor. This category is designated for loads that are primarily resistive, meaning they draw a current that is relatively stable and does not have significant inrush current during switching.
- Load Type: Primarily resistive or slightly inductive loads.
- Switching Conditions: The make and break current are approximately equal to the rated operational current. Arcing during switching is minimal.
- Typical Applications:
- Heaters (e.g., resistance ovens, radiant heaters)
- Incandescent lighting
- Distribution circuits for non-motor loads
- Resistive furnaces
- Isolation switches for general purpose loads
AC2 Duty: For Slip-Ring Motors
AC2 for slip-ring motors is a category designed for the starting and occasional stopping of slip-ring motors. These motors have a more complex starting characteristic compared to resistive loads.
- Load Type: Slip-ring motors, also known as wound-rotor motors.
- Switching Conditions: Contactors in AC2 duty must be capable of making current up to 2.5 times the rated operational current and breaking the current at voltages that can be higher than the supply voltage due to rotor circuit resistance.
- Typical Applications:
- Cranes and hoists
- Pumps with high inertia loads
- Fans with large flywheels
- Applications requiring smooth acceleration and speed control via rotor resistance.
AC3 Duty: For Squirrel-Cage Motors (Starting and Running)
AC3 for starting and running squirrel-cage motors is one of the most common duty categories. It addresses the high inrush currents associated with starting standard squirrel-cage induction motors and then breaking the current once the motor is running at full speed.
- Load Type: Squirrel-cage induction motors.
- Switching Conditions:
- Making Current: When starting, squirrel-cage motors can draw an inrush current typically 5 to 7 times their rated operational current. The contactor must safely close on this high current.
- Breaking Current: Once the motor reaches full speed, the contactor breaks only the motor's full-load running current, which is significantly lower than the starting current.
- Typical Applications:
- Most industrial motor applications (e.g., pumps, fans, conveyors, compressors)
- General machinery where motors start and run continuously without frequent stops or specialized braking.
AC4 Duty: For Severe Squirrel-Cage Motor Operations
AC4 duty is the most demanding category for squirrel-cage motors and is used in applications involving frequent starting and stopping, inching, plugging, or reverse current braking. These operations impose significant thermal and mechanical stress on the contactor.
- Load Type: Squirrel-cage induction motors requiring frequent or severe switching operations.
- Switching Conditions:
- Making Current: Similar to AC3, making current can be 5 to 7 times the rated operational current.
- Breaking Current: Unlike AC3, the contactor might be required to break a high current (multiple times the rated current) while the motor is still decelerating or even reversing. This generates severe arcing conditions and significant contact wear.
- Typical Applications:
- Inching/Jogging: Repeated short-duration starts to move a motor slightly.
- Plugging: Reversing the motor direction to quickly stop it.
- Counter-current braking: Similar to plugging, using reverse current to brake.
- Machine tools, packaging machines, hoists with dynamic braking.
Summary Table of AC Duty Categories
To provide a clear overview, the table below summarizes the key characteristics of each AC duty category:
| Duty Category | Load Type | Description of Switching Conditions | Typical Applications |
|---|---|---|---|
| AC1 | Resistive or slightly inductive loads | Makes and breaks current approximately equal to the rated operational current. Low arcing. | Heaters (resistance ovens, radiant heaters), Incandescent lighting, Distribution circuits for non-motor loads, Isolation switches, Capacitive loads (requires specific considerations for inrush current, often classified separately or handled with specialized contactors). |
| AC2 | Slip-ring (wound-rotor) motors | Makes current up to 2.5 times rated current. Breaks current at high voltage (due to rotor resistance) but typically at or below rated current. | Cranes, Hoists, Centrifugal machines, Pumps and fans with high inertia, Applications requiring smooth acceleration control (e.g., with starter resistors). |
| AC3 | Squirrel-cage motors (starting & running) | Makes high inrush current (5-7 times rated current) during motor start. Breaks only the full-load running current once the motor is up to speed. | Most general industrial applications: Conveyors, Pumps, Fans, Compressors, Elevators, Air conditioning units, Machine tools where motors start and run without frequent reversing or jogging. |
| AC4 | Squirrel-cage motors (severe operations) | Makes high inrush current (5-7 times rated current). Breaks high current (multiple times rated current) while the motor is decelerating or reversing (e.g., during inching, plugging, or reverse current braking), leading to severe arcing and contact wear. | Machine tools with frequent reversing, Inching or jogging applications, Packaging machinery, Textile machinery, Hoists with dynamic braking, Any application requiring rapid and frequent start/stop or directional changes where the motor is still at high speed when the current is broken. This duty is highly demanding and often requires oversized contactors or specific contactor designs. |
Why Duty Categories Matter: Practical Insights
Selecting the correct contactor for a specific AC duty is paramount for several reasons:
- Equipment Longevity: Using a contactor rated for a lower duty (e.g., AC1) in a more demanding application (e.g., AC3 or AC4) will lead to premature contact wear, overheating, and eventual failure of the contactor.
- Reliability: An appropriately rated contactor will operate reliably over its intended lifespan, minimizing downtime and maintenance costs.
- Safety: An undersized or incorrectly selected contactor can fail catastrophically, leading to arcs, short circuits, or even fires, posing significant safety hazards to personnel and property.
- System Performance: The right contactor ensures that the load starts and stops smoothly and efficiently, contributing to the overall performance of the electrical system.
When selecting a contactor, always consider the motor type, the starting and braking methods, the frequency of operation (how many operations per hour), and any specific environmental conditions. Consulting the manufacturer's data sheets and adhering to relevant electrical standards (like IEC 60947-4-1) is crucial.
Related Standards and Resources
For detailed technical specifications and deeper understanding of contactor duty categories, refer to the following standards:
- IEC 60947-4-1: Low-voltage switchgear and controlgear - Part 4-1: Contactors and motor-starters - Electromechanical contactors and motor-starters. This standard defines the construction, performance, and testing requirements for contactors.
- (Note: This standard is available for purchase from the International Electrotechnical Commission webstore and is often referenced in manufacturer technical documentation.)
- Many reputable manufacturers (e.g., Siemens, Schneider Electric, Eaton, ABB) provide extensive technical guides and catalogs that explain these duty classifications in detail, often with practical examples and selection tables.
