In industrial automation and electrical engineering, three-phase induction motors serve as the driving core, with their performance and reliability directly impacting the stable operation of entire systems. However, motors don't operate at constant power in all applications but rather exhibit various duty cycle patterns according to operational requirements.

The "duty type" of a motor refers to the load cycle pattern it experiences during operation, including stages such as starting, electrical braking, no-load operation, and power-off rest periods, along with their duration and sequence. Duty type serves as a crucial basis for motor selection and application, directly affecting the motor's temperature rise, lifespan, and reliability.

According to IEC 60034-1 standards, motor duty types can be classified into continuous duty, short-time duty, and periodic duty, among others. The cyclic duration factor represents the percentage of loading time relative to the total cycle time. The selection of duty type is the responsibility of the purchaser. If unspecified, manufacturers should default to S1 (continuous running duty).

A motor's rating refers to the performance parameters declared by the manufacturer under specific operating conditions, such as power, voltage, current, and speed. When determining ratings, manufacturers must select an appropriate rating class, defaulting to continuous running duty if unspecified.

The IEC 60034-1 standard provides detailed classification and definitions of motor duty types, offering essential references for motor selection and application. Below are the 10 major duty types defined by this standard:

• Definition: Operation at constant load sufficient to reach thermal equilibrium.
• Characteristics: Constant load, unlimited operation time.
• Applications: Scenarios requiring continuous operation, such as fans, pumps, and conveyors. Represents about 90% of motor applications.
• Selection Criteria: Choose appropriate power based on continuous load requirements to prevent overheating.

• Definition: Operation at constant load for a limited time insufficient to reach thermal equilibrium, followed by sufficient rest to cool to ambient temperature.
• Characteristics: Constant load, limited operation time, adequate cooling periods.
• Applications: Equipment requiring short operation periods followed by long rests, such as cranes and welding machines.
• Selection Criteria: Specify operation duration (e.g., S2 40 minutes). Consider short-time overload capacity and cooling capability.

• Definition: Operation in identical cycles, each comprising constant load operation and power-off rest periods, with negligible starting effects on temperature rise.
• Characteristics: Periodic operation, constant load, rest periods, negligible starting impact.
• Applications: Equipment requiring periodic starts and stops, such as machine tools and punch presses.
• Selection Criteria: Specify cyclic duration factor (e.g., S3 30%).

• Definition: Operation in identical cycles, each comprising significant starting time, constant load operation, and rest periods.
• Characteristics: Periodic operation, constant load, rest periods, significant starting time.
• Applications: Equipment requiring frequent starts/stops with long starting times, such as conveyors and mixers.
• Selection Criteria: Specify cyclic duration factor, motor inertia (JM), and load inertia (JL).

• Definition: Operation in identical cycles, each comprising starting time, constant load operation, electric braking, and rest periods.
• Characteristics: Periodic operation, constant load, rest periods, electric braking.
• Applications: Equipment requiring frequent starts, stops, and braking, such as cranes and elevators.
• Selection Criteria: Specify cyclic duration factor, motor inertia (JM), and load inertia (JL).

• Definition: Operation in identical cycles, each comprising constant load operation and no-load operation, without rest periods.
• Characteristics: Periodic operation, constant load, no-load periods, no rests.
• Applications: Continuous operation with periodic load variations, such as ventilators and compressors.
• Selection Criteria: Specify cyclic duration factor (e.g., S6 30%).

• Definition: Operation in identical cycles, each comprising starting time, constant load operation, and electric braking, without rest periods.
• Characteristics: Periodic operation, constant load, electric braking, no rests.
• Applications: Continuous operation with periodic load variations and frequent braking, such as rolling mills and winches.
• Selection Criteria: Specify motor inertia (JM) and load inertia (JL).

• Definition: Operation in identical cycles, each comprising operation at predetermined speed/load combinations, without rest periods.
• Characteristics: Periodic operation, varying load and speed, no rests.
• Applications: Continuous operation with periodic load and speed variations, such as machine tools and fans.
• Selection Criteria: Specify motor inertia (JM), load inertia (JL), and parameters for each speed/load combination.

• Definition: Operation with non-periodic variations in load and speed within permitted ranges, including frequent overloads exceeding reference load.
• Characteristics: Non-periodic load/speed variations, permitted frequent overloads.
• Applications: Equipment with irregular load/speed variations and potential overloads, such as excavators and crushers.
• Selection Criteria: Select motors with sufficient overload capacity based on actual conditions.

• Definition: Operation at a specified number of discrete loads, each maintained long enough to reach thermal equilibrium.
• Characteristics: Multiple discrete loads, sufficient operation time at each to reach equilibrium.
• Applications: Equipment with multiple stable operating states, such as multi-speed fans and multi-stage compressors.
• Selection Criteria: Specify relative load values (p/Δt), insulation system thermal life expectancy (TL), and rest period load (r).

Beyond duty types, these factors are crucial in motor selection:

• Power: Slightly exceed load requirements to ensure normal operation with safety margin.
• Speed: Match load requirements; use gearboxes if necessary.
• Voltage and Frequency: Match power supply specifications.
• Protection Class: Match environmental conditions.
• Insulation Class: Match operating temperatures.
• Starting Method: Choose based on grid capacity and load characteristics.
• Efficiency: Higher efficiency reduces operating costs.
• Manufacturer: Reputable brands ensure quality and service.

Proper understanding and selection of motor duty types are essential for reliable operation and extended service life. This article has detailed the 10 major duty types defined by IEC 60034-1 standards.

As industrial automation and smart manufacturing advance, motor applications will become more diverse, demanding higher performance and reliability. Future motor design will emphasize energy efficiency, intelligence, and customization, while new materials and technologies will further enhance motor capabilities.