Question:
Which one of the following options is correct regarding the typical double-squirrel-cage structure used in induction motors?
Which one of the following options is correct regarding the typical double-squirrel-cage structure used in induction motors?
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At starting, high rotor frequency makes reactance dominate; the outer cage's naturally low reactance gives it lower impedance and hence more current than the inner cage.
Updated On: Jul 20, 2026
- At starting, a larger portion of the rotor current flows in the outer cage
- At starting, a larger portion of the rotor current flows in the inner cage
- At rated speed, most of the rotor current flows in the outer cage
- The purpose of the double-squirrel-cage structure is to lower the effective rotor resistance at starting
Show Solution
Verified By Collegedunia
The Correct Option is A
Solution and Explanation
Step 1: Recall the construction of the two cages.
The outer cage sits near the rotor surface, made of higher-resistance material with LOW leakage reactance (little surrounding iron). The inner cage sits deep in the rotor slots, made of lower-resistance material with HIGH leakage reactance (surrounded by much more iron).
Step 2: Compare impedances at starting.
At starting, the rotor frequency equals the full supply frequency (slip \(s=1\)), which is the highest rotor frequency the machine ever sees. Reactance (\(X=2\pi fL\)) is therefore at its largest possible value for both cages at this instant, and it dominates each cage's impedance far more than resistance does.
Step 3: Compute which cage has lower impedance at starting.
The outer cage has low reactance to begin with, so even at maximum rotor frequency its total impedance stays relatively small. The inner cage's reactance, already large by design, becomes very large at starting frequency, making its total impedance much bigger than the outer cage's.
Step 4: Apply current division.
Since the two cages are effectively in parallel (both linking the same air-gap flux), the cage with the smaller impedance carries the larger share of the total rotor current. At starting, this is the outer cage.
Step 5: Rule out the other options.
Option (B) claims the opposite, contradicted by Step 4. Option (C) is about running speed, not starting, where the roles reverse (at low slip and low rotor frequency, reactance becomes negligible for both cages, and the low-resistance inner cage then dominates and carries most of the current) so (C) is false as stated for starting behavior, and even at rated speed it is the inner, not outer, cage that carries most current. Option (D) misstates the purpose: the double cage does not lower the rotor resistance at starting, it effectively RAISES the starting resistance seen by the machine (by forcing more current through the high-resistance outer cage), which is precisely what improves starting torque.
Step 6: Final Answer.
\[ \boxed{\text{At starting, a larger portion of the rotor current flows in the outer cage}} \]
The outer cage sits near the rotor surface, made of higher-resistance material with LOW leakage reactance (little surrounding iron). The inner cage sits deep in the rotor slots, made of lower-resistance material with HIGH leakage reactance (surrounded by much more iron).
Step 2: Compare impedances at starting.
At starting, the rotor frequency equals the full supply frequency (slip \(s=1\)), which is the highest rotor frequency the machine ever sees. Reactance (\(X=2\pi fL\)) is therefore at its largest possible value for both cages at this instant, and it dominates each cage's impedance far more than resistance does.
Step 3: Compute which cage has lower impedance at starting.
The outer cage has low reactance to begin with, so even at maximum rotor frequency its total impedance stays relatively small. The inner cage's reactance, already large by design, becomes very large at starting frequency, making its total impedance much bigger than the outer cage's.
Step 4: Apply current division.
Since the two cages are effectively in parallel (both linking the same air-gap flux), the cage with the smaller impedance carries the larger share of the total rotor current. At starting, this is the outer cage.
Step 5: Rule out the other options.
Option (B) claims the opposite, contradicted by Step 4. Option (C) is about running speed, not starting, where the roles reverse (at low slip and low rotor frequency, reactance becomes negligible for both cages, and the low-resistance inner cage then dominates and carries most of the current) so (C) is false as stated for starting behavior, and even at rated speed it is the inner, not outer, cage that carries most current. Option (D) misstates the purpose: the double cage does not lower the rotor resistance at starting, it effectively RAISES the starting resistance seen by the machine (by forcing more current through the high-resistance outer cage), which is precisely what improves starting torque.
Step 6: Final Answer.
\[ \boxed{\text{At starting, a larger portion of the rotor current flows in the outer cage}} \]
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