--- title: "Slip ring vs squirrel cage induction motor" subject: "Electrical Machines I" chapter: 5 key: "semester_4/electrical_machines_i:slip-ring-vs-squirrel-cage" source: https://www.bctengineer.com/q/semester_4/electrical_machines_i/slip-ring-vs-squirrel-cage site: BCT Engineering --- # Slip ring vs squirrel cage induction motor Electrical Machines I · Chapter 5: Three-Phase Induction Machines - Asked in: 2081 Chaitra (new course), 4+2 marks - Video: [Differences In Slip ring & Squirrel cage Induction Motors](https://www.youtube.com/watch?v=WX6r_pL9Nxw) — Smile and Learn Abhiraj - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/electrical_machines_i/slip-ring-vs-squirrel-cage - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines_i/chapter-5 ## Question Mention the basic difference between slip ring and squirrel cage induction motor. Discuss their application and its characteristics. ## Short answer Squirrel-cage rotor has shorted bars, simple, low starting torque, high starting current; used for fans, pumps. Slip-ring rotor has 3-phase winding with slip rings, external resistance gives high starting torque and low current; used for cranes, hoists, mills. ## Answer Both are 3-phase induction motors with the same stator; they differ only in the **rotor**. A squirrel-cage rotor has short-circuited bars, while a slip-ring rotor has a 3-phase winding brought out through slip rings. #### Basic differences | Point | Squirrel cage | Slip ring (wound rotor) | |---|---|---| | Rotor winding | Bars shorted by end rings | 3-phase star winding | | Slip rings, brushes | Absent | Present | | External resistance | Cannot be added | Can be added in rotor | | Construction | Simple, robust | Complex | | Cost and maintenance | Low | High (brushes wear) | | Starting torque | Low to moderate | High (adjustable) | | Starting current | High (5–7 × FL) | Low with rotor resistance | | Speed control | Not from rotor | Possible by rotor resistance | | Efficiency, pf at running | Better | Slightly lower | #### Characteristics - **Squirrel cage:** low rotor resistance gives small full-load slip (2–5%), so it runs at nearly constant speed with good efficiency, but has low starting torque and high starting current. Double-cage and deep-bar designs improve starting torque. - **Slip ring:** adding rotor resistance shifts maximum torque towards starting ($s_m = R_2/X_2$), giving high starting torque with low starting current. In running the rings are shorted and it behaves like a cage motor. Speed can be reduced (with loss) by keeping resistance in. #### Applications - **Squirrel cage:** fans, blowers, pumps, lathes, drilling machines, compressors, conveyors — general constant-speed drives (about 90% of industrial motors). - **Slip ring:** cranes, hoists, lifts, elevators, crushers, large compressors, mills — loads needing high starting torque or some speed control. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the basic difference between the two motors? A. The stator B. The rotor construction C. Number of phases D. Supply frequency Answer: B. Both use the same stator; rotors differ. (Key point: Basic difference) 2. Which motor has high starting torque with low starting current? A. Squirrel cage B. Slip ring with external rotor resistance C. Neither D. Both equally Answer: B. Rotor resistance limits current and raises torque. (Key point: Starting) 3. Why does a cage motor run at nearly constant speed with good efficiency? A. Low rotor resistance gives small full-load slip (2-5%) B. High rotor resistance C. It has slip rings D. It has external resistance Answer: A. Low R2 means small slip and low copper loss. (Key point: Running characteristic) 4. Which load suits a slip-ring motor? A. Fans and blowers B. Cranes and hoists C. Small pumps D. Lathes Answer: B. Cranes need high starting torque or speed control. (Key point: Applications) ## Related in Electrical Machines I - [Slip ring motor max-torque slip and full-load output](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aslip-ring-380v-180v-60nm-starting) — Electrical Machines I, chapter 5, asked 4× - [Maximum torque and torque at 1430 rpm of slip ring motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aslip-ring-400v-200v-10nm-1430rpm) — Electrical Machines I, chapter 5, asked 3× - [Slip for maximum torque and full-load output of slip ring motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aslip-ring-400v-delta-star-190v) — Electrical Machines I, chapter 5, asked 2× - [Max-torque speed and efficiency of 400 V slip ring motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aslip-ring-400v-190v-600w-1380rpm) — Electrical Machines I, chapter 5, asked 2× - [Effect of five-times rotor resistance on 40 kW slip ring motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aslip-ring-40kw-rotor-resistance-5x) — Electrical Machines I, chapter 5 - [Max torque speed and Tmax/Tst of slip ring motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aslip-ring-0p4-4ohm-max-start) — Electrical Machines I, chapter 5 ## Related in other subjects - [Slip power recovery for slip ring induction motor](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Aslip-power-recovery-slip-ring-motor) — Utilization of Electrical Energy, chapter 3, asked 2× - [250 HP 1410 rpm slip ring motor (kw 0.955)](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Aslip-ring-250hp-kw0955) — Electric Machine Design, chapter 4, asked 6× - [200 kW 1420 rpm slip ring motor (star)](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Aslip-ring-200kw-star) — Electric Machine Design, chapter 4, asked 5× - [250 HP 4-pole slip ring motor](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Aslip-ring-250hp-4pole) — Electric Machine Design, chapter 4, asked 5×