Side-by-side comparison
| Parameter | Squirrel Cage | Wound Rotor Induction Motor |
|---|---|---|
| Rotor Construction | Aluminum or copper bars short-circuited by end rings — no external connection | Three-phase wound winding with slip rings for external resistance connection |
| External Rotor Resistance | Not possible — rotor is short-circuited internally | Possible via slip rings — used to increase starting torque |
| Starting Torque | Moderate — star-delta or DOL starters used to limit current | High — external rotor resistance shifts torque-speed curve for max torque at standstill |
| Starting Current | 6–8 times rated current at DOL start | Reduced to 2–3 times rated using rotor resistance |
| Efficiency at Full Load | Higher — no slip ring losses, simpler rotor | Lower — slip ring and brush contact losses |
| Maintenance | Very low — no brushes, no slip rings | Higher — brushes wear, slip rings need periodic cleaning |
| Speed Control | Limited (frequency control via VFD) | Possible by varying external rotor resistance (but inefficient) |
| Application | Pumps, fans, compressors, conveyors, 90% of industrial induction motors | Cranes, hoists, ball mills, rolling mills needing high starting torque |
Key differences
Squirrel cage rotor resistance R2 is fixed and low, optimizing running efficiency but limiting starting torque to about 150% of rated. Wound rotor motors add external resistance R_ext via slip rings — at standstill, adding R_ext = R2_internal achieves maximum torque (condition for max torque: slip = R2/(X2) and R_ext shifts this slip to 1). A 100 kW wound rotor motor on a crane can produce 250% starting torque while drawing only 2.5× rated current. But brush wear and slip ring maintenance add annual cost; squirrel cage motors with a VFD (Variable Frequency Drive) now match the performance at higher efficiency for most applications.
When to use Squirrel Cage
Use a squirrel cage induction motor for all standard industrial drives where starting conditions are moderate — for example, a 37 kW, 415 V, 3-phase squirrel cage motor driving a centrifugal pump with a star-delta starter.
When to use Wound Rotor Induction Motor
Use a wound rotor induction motor when high starting torque against high-inertia loads is required and speed control by rotor resistance is acceptable — for example, a 150 kW wound rotor motor on an overhead travelling crane in a steel plant.
Recommendation
In exam problems and design selection, choose squirrel cage for 90% of applications — it is more efficient, requires less maintenance, and pairs with a VFD for speed control. Choose wound rotor only when the question specifically mentions high starting torque requirements or rotor resistance control.
Exam tip: Examiners test the condition for maximum torque in a wound rotor motor — state that maximum torque occurs at slip s_max = R2 / X2, and adding external rotor resistance R_ext shifts s_max toward 1, achieving maximum torque at starting.
Interview tip: Interviewers at industrial automation companies ask about the difference in starting behavior — explain that squirrel cage motors draw 6–8× rated current at DOL start while wound rotor motors reduce this to 2–3× by inserting external rotor resistance through slip rings.