--- title: "Classes of motor duty" subject: "Utilization of Electrical Energy" chapter: 2 key: "semester_7/utilization_of_electrical_energy:classes-of-motor-duty" source: https://www.bctengineer.com/q/semester_7/utilization_of_electrical_energy/classes-of-motor-duty site: BCT Engineering --- # Classes of motor duty Utilization of Electrical Energy · Chapter 2: Electric Drive System - Asked in: 2082 Baisakh, 6 marks; 2079 Baisakh, 8 marks (asked 2 times) - Video: [WHAT IS MOTOR DUTY | WHAT IS S1, S2, S3, S4, | USE OF S1,S2,S3,S4 MOTOR AND WORKING PRINCIPLE |](https://www.youtube.com/watch?v=nDH0wFuNOjE) — Electro - 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/classes-of-motor-duty - Whole chapter: https://www.bctengineer.com/md/semester_7/utilization_of_electrical_energy/chapter-2 ## Question Explain different classes of motor duty with waveform and area of applications. ## Short answer Motor duty classes, defined by IEC, range from S1 continuous duty (fans, pumps) to S8 continuous duty with periodic speed changes (multi-speed drives). Rest periods and starting/braking losses determine temperature rise and required rating. Examples: S2 for vales, S3 for cranes, S4 for lifts, S5 for rolling mill auxiliaries, S6 for presses, S7 for reversing mills. ## Answer The **duty class** of a motor describes how its load varies with time: continuous running, short running, periodic on/off, starting, braking and speed changes. Because temperature rise depends on the duty, the motor rating is specified for a duty class (IEC 60034-1; the eight classic classes are S1 to S8, and newer editions add S9 and S10). #### Classes with waveforms In the sketches, P = load, $P_v$ = losses, $\theta$ = temperature. **S1 Continuous duty**: operation at constant load long enough to reach thermal equilibrium. ```text P |_________________ th | ___----------- (reaches steady temp) +-----------------> t ``` Applications: pumps, fans, compressors, conveyors. **S2 Short-time duty**: constant load for a short time $t_L$ (10, 30, 60, 90 min), then rest until the motor cools to ambient. ```text P |_____ | |__________________ rest th | /\ | / \______ (never reaches final temp) +-----------------> t ``` Applications: sluice gates, swing bridges, valve actuators, kitchen mixers. **S3 Intermittent periodic duty**: identical cycles of constant load and rest; starting losses negligible; temperature fluctuates around an average. ```text P |__ __ __ | |__| |__| |__ th | /\/\/\/\/\/\ (saw-tooth) +-----------------> t on off ``` Specified by cyclic duration factor $\text{CDF} = \dfrac{t_{on}}{t_{on}+t_{off}} \times 100\,\%$ (15, 25, 40, 60 %). Applications: cranes, hoists, presses. **S4 Intermittent periodic with starting**: as S3, but each cycle has a significant starting period. Applications: lifts, machine tools. **S5 Intermittent periodic with starting and electric braking**: start, load, electrical braking, rest. Applications: rolling-mill auxiliaries, centrifuges. ```text S5: start load brake rest P | /----\ | / \_______ +-----------------> t ``` **S6 Continuous duty with intermittent periodic loading**: load and no-load periods, no rest. ```text P |__ __ __ | |~~| |~~| |~~ (no-load, motor running) +-----------------> t ``` Applications: pressing, drilling, punching machines. **S7 Continuous duty with starting and braking**: start, load, electric braking repeatedly, no rest period. Applications: reversing rolling mills. **S8 Continuous duty with periodic speed changes**: load at one speed then at another speed, no rest. Applications: multi-speed machine tools, pole-changing motor drives. #### Summary | Class | Rest period? | Starting/braking important? | Example | |---|---|---|---| | S1 | No | No | Fan, pump | | S2 | Long | No | Valve, gate | | S3 | Yes | No | Crane | | S4 | Yes | Starting | Lift | | S5 | Yes | Starting + braking | Mill auxiliary | | S6 | No (no-load) | No | Press | | S7 | No | Starting + braking | Reversing mill | | S8 | No | Speed change | Multi-speed tool | For variable duty, the motor is sized by the **rms (equivalent) torque or power** over a cycle and checked for maximum torque. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is a motor rating specified for a duty class? A. Temperature rise depends on how the load varies with time B. Voltage depends on duty C. Speed of rotation is fixed by duty D. Duty decides motor colour Answer: A. Temperature rise depends on the duty (continuous, short, periodic, starting, braking), so the rating is given for a duty class. (Key point: Why rating depends on duty) 2. How is the cyclic duration factor (CDF) for S3 defined? A. t_off/(t_on + t_off) B. t_on/(t_on + t_off) x 100 % C. t_on x t_off D. Starting time/braking time Answer: B. CDF is the on-load time divided by the whole cycle time, in percent (15, 25, 40, 60 %). (Key point: Cyclic duration factor) 3. What is the main difference between S6 and S3? A. S6 has no-load running periods instead of rest, so the motor never stops B. S6 has long rest periods C. S6 has electric braking D. S6 is only for short time Answer: A. S6 is continuous duty with intermittent loading: load and no-load periods with no rest, whereas S3 has rest periods. (Key point: S6 vs S3) 4. In S2 short-time duty, what happens to temperature during the load period? A. It reaches the final steady temperature B. It never reaches the final temperature, then cools to ambient in rest C. It stays at ambient D. It falls continuously Answer: B. The load period is too short to reach final temperature, and the rest lets the motor cool to ambient. 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