--- title: "Classes of duty by load variation, with examples" subject: "Utilization of Electrical Energy" chapter: 2 key: "semester_7/utilization_of_electrical_energy:motor-duty-classes-load-variation-examples" source: https://www.bctengineer.com/q/semester_7/utilization_of_electrical_energy/motor-duty-classes-load-variation-examples site: BCT Engineering --- # Classes of duty by load variation, with examples Utilization of Electrical Energy · Chapter 2: Electric Drive System - Asked in: 2080 Baisakh, 8 marks; 2073 Shrawan; 2071 Shrawan (asked 3 times) - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/utilization_of_electrical_energy/motor-duty-classes-load-variation-examples - Whole chapter: https://www.bctengineer.com/md/semester_7/utilization_of_electrical_energy/chapter-2 ## Question For the selection of various types of motor, what are the classes duties to be performed by the motor on the basis of load variations? List out some examples of the driver/machine applicable to various classes of duties. ## Short answer Motor duty classes S1 to S8 define load patterns: S1 continuous (pumps, fans); S2 short-time (valve actuators, mixers); S3 intermittent periodic (cranes, presses); S4 with starting (lifts); S5 with starting and braking (mill auxiliaries); S6 continuous intermittent (pressing machines); S7 continuous with starting and braking (reversing mills); S8 with periodic speed changes (multi-speed drives). ## Answer The **duty** of a motor is the pattern of load, rest, starting and braking it experiences with time. Since motor heating depends on this pattern, the rating must be chosen for the duty class; a motor sized for continuous duty would be oversized for short-time duty, and a short-time-rated motor would overheat on continuous duty. IEC 60034-1 defines the standard duty types; the eight classic duty classes used for motor selection are S1 to S8 (newer editions also add S9 and S10 for non-periodic and discrete-constant loads). #### Classes of duty | Class | Duty (IEC/IS) | Load pattern | Examples | |---|---|---|---| | S1 | Continuous duty | Constant load long enough to reach thermal equilibrium | Pumps, fans, compressors, conveyors, paper mill drives | | S2 | Short-time duty | Constant load for a short time, then rest long enough to cool to ambient | Lock gates, bridge-opening motors, valve actuators, battery-charging motors, household mixers | | S3 | Intermittent periodic duty | Identical cycles of load and rest; no time to reach steady temperature in either; starting heat negligible | Cranes, hoists (some), pressing and drilling machines | | S4 | Intermittent periodic duty with starting | As S3 but starting period significant | Metal-cutting machines, lifts, hoists, cranes | | S5 | Intermittent periodic duty with starting and electric braking | Start, run, electric brake, rest | Billet mill, rolling mill auxiliaries, centrifuges, lifts | | S6 | Continuous duty with intermittent periodic loading | Load and no-load periods, no rest | Pressing, cutting, drilling machines, conveyor with intermittent feed | | S7 | Continuous duty with starting and braking | Start, load, electric brake, no rest | Blooming mill, reversing rolling mill main drive | | S8 | Continuous duty with periodic speed changes | Load at one speed, then another speed, no rest | Multi-speed drives, machine tools with gear/pole changing | #### Load-time sketches ```text S1 continuous S2 short-time P |______________ P |____ | | |__________ rest +-------------> t +-------------> t S3 intermittent S6 continuous, intermittent load P |__ __ __ P |__ __ __ | |_| |_| |_ rest | |_| |_| |_ no-load +-------------> t +-------------> t ``` #### Notes on each class - **Continuous duty (S1)**: the motor reaches its final steady temperature; rating = continuous rating (CMR). Load may be constant (fan) or variable continuous; for variable load, the **equivalent (rms) current/torque/power** method is used to size the motor. - **Short-time duty (S2)**: the motor can deliver more than its continuous rating because it never reaches final temperature; standard periods 10, 30, 60, 90 min. - **Intermittent periodic duties (S3, S4, S5)**: specified by **cyclic duration factor** (CDF) = (load time)/(cycle time), e.g. 15, 25, 40, 60 %; cycle time usually 10 min. Starting (S4) and braking (S5) losses add heating and must be included. - **Continuous with intermittent loading (S6)**: motor never stops; no-load periods give partial cooling. - **S7 and S8**: continuous running with frequent electric braking/reversal or speed changes; heavy rotor heating, so inertia and number of operations per hour matter. #### Applying to motor selection 1. Identify the load cycle of the driven machine. 2. Select the matching duty class. 3. Find the equivalent power (rms) over the cycle and check maximum torque against motor pull-out torque. 4. Choose a standard rating with that duty class marked on the nameplate (e.g. "S3 40 %"). Correct duty matching avoids both overheating (insulation damage) and over-sizing (poor efficiency and power factor). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why must the motor rating be chosen for the duty class? A. Because heating depends on the load pattern with time B. Because colour depends on duty C. Because supply frequency changes with duty D. Because duty decides the number of poles only Answer: A. Motor heating depends on the pattern of load, rest, starting and braking, so rating must match the duty. (Key point: Meaning of duty) 2. What distinguishes S2 short-time duty from S1 continuous duty? A. S2 carries load until thermal equilibrium B. S2 has constant load for a short time, then rest to cool to ambient C. S2 has no rest and continuous starting D. S2 has periodic speed changes Answer: B. In S2 the motor never reaches final temperature and then rests until cool, so it can give more than its continuous rating. (Key point: S1 vs S2) 3. Which statement about S3, S4 and S5 is correct? A. S3 includes significant starting; S4 does not B. S4 adds significant starting heat and S5 adds starting plus electric braking C. S5 has no rest period D. S3 includes electric braking Answer: B. S3 is intermittent with negligible starting heat; S4 adds significant starting; S5 adds starting and electric braking. (Key point: S3 vs S4 vs S5) 4. Which example is correctly matched to its duty class? A. Fans and pumps - S2 B. Lock gate motors - S1 C. Reversing rolling mill main drive - S7 D. Household mixer - S8 Answer: C. Fans/pumps are S1, lock gates and mixers are S2, and blooming/reversing mills are S7. (Key point: Examples of duty classes) ## Related in Utilization of Electrical Energy - [Classes of motor duty](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Aclasses-of-motor-duty) — Utilization of Electrical Energy, chapter 2, asked 2× - [Motor selection characteristics; rheostatic and regenerative braking](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Amotor-selection-characteristics-braking) — Utilization of Electrical Energy, chapter 2 - [Induction motor with square-law load: voltage at 1300 rpm](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Ainduction-motor-fan-load-1300rpm-voltage) — Utilization of Electrical Energy, chapter 3 ## Related in other subjects - [Basis and advantages of decoupled load flow](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Adecoupled-basis-advantages) — Power System Analysis II, chapter 2 - [Electric drive and motor selection factors](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aelectric-drive-motor-selection) — Industrial Power Distribution and Illumination, chapter 5 - [Series motor load start; shunt motor speed control](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adc-series-load-shunt-speed-control) — Electrical Machines, chapter 4 - [Efficiency variation with load and maximum efficiency condition](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aefficiency-variation-max-condition) — Electrical Machines I, chapter 2