Introduction
A standard squirrel cage induction motor has one major limitation: low starting torque. You can increase starting torque by increasing rotor resistance — but that kills efficiency during normal running. So how do you get high starting torque AND high running efficiency in the same motor?
The answer is the double cage induction motor — a clever design that automatically provides high resistance at startup and low resistance during running, without any external switching or control. In this article, we'll explain why it's needed, how it's constructed, and how the physics of frequency-dependent impedance makes it work.
Table of Contents
Why is Double Cage Needed?
In a standard squirrel cage motor, the starting torque is limited because the rotor resistance is low (designed for efficiency at running speed). The fundamental conflict is:
The ideal solution: A motor that automatically has high rotor resistance during starting and low rotor resistance during running — without any external switching.
The double cage induction motor achieves exactly this using the physics of frequency-dependent reactance.
Construction of Double Cage Induction Motor
The stator is identical to a standard induction motor. The difference is entirely in the rotor, which has two sets of conductor bars (cages):
Outer Cage
- Placed near the rotor surface (close to stator)
- Made of high-resistivity material (brass or bronze)
- Bars have small cross-sectional area
- Low leakage reactance (close to stator flux)
Inner Cage
- Placed deep inside the rotor (away from stator)
- Made of low-resistivity material (copper)
- Bars have large cross-sectional area
- High leakage reactance (embedded deep, more leakage flux)
A slit (air gap) separates the two cages to minimize mutual coupling between them. Both cages are short-circuited by common end rings.
Summary
Inner Cage: Low R, High XL
Working Principle
The key to understanding the double cage motor is this: reactance depends on frequency, but resistance doesn't.
Where s = slip. At starting, s = 1 (rotor frequency = supply frequency). At running, s ≈ 0.03–0.05 (rotor frequency is very low).
At Starting (s = 1, frotor = 50 Hz)
- Rotor frequency is high (50 Hz)
- Inner cage reactance XL is very high → its impedance is high → very little current flows through it
- Outer cage reactance is low (it's close to stator) → its impedance is dominated by resistance → most current flows through it
- Since outer cage has high resistance → high starting torque
At Running Speed (s ≈ 0.03, frotor ≈ 1.5 Hz)
- Rotor frequency is very low (1–2 Hz)
- Reactance of BOTH cages becomes negligible (XL ∝ f → nearly zero)
- Current division now depends only on resistance
- Inner cage has much lower resistance → most current flows through it
- Low resistance path → low I²R losses → high efficiency
The motor automatically switches from high-resistance operation (starting) to low-resistance operation (running) — purely through physics, no external control needed.
Torque-Slip Characteristics
The total torque of a double cage motor is the sum of torques produced by each cage independently:
Key observations from the torque-slip curve:
- At s = 1 (starting): outer cage dominates → high starting torque
- At low slip (running): inner cage dominates → high efficiency
- The resultant curve (sum of both) gives high starting torque AND good running performance
- Starting torque is typically 3–4 times the full-load torque (vs 1.5–2 times for single cage)
Single Cage vs Double Cage Induction Motor
Applications
Double cage induction motors are used where:
- High starting torque is needed without external starters
- Direct-on-line (DOL) starting of heavy loads
- Compressors, pumps, and crushers that start under load
- Conveyor systems with loaded belts
- Applications where induction motors must start against significant mechanical resistance
FAQs
Why doesn't the inner cage contribute at starting?
Because at starting, rotor frequency equals supply frequency (50 Hz). The inner cage has high leakage inductance, so its reactance (X = 2πfL) is very high at 50 Hz. This high impedance blocks current from flowing through it. Only the outer cage (low reactance) carries significant current.
What material is used for the outer cage?
Brass or bronze — materials with high resistivity (5–6 times that of copper). The inner cage uses copper or aluminum for low resistance.
Can the torque-slip characteristic be customized?
Yes. By choosing different resistance and reactance ratios for the two cages, designers can shape the torque-slip curve for specific applications — from high starting torque to smooth acceleration profiles.
What is the purpose of the slit between the two cages?
The air slit reduces magnetic coupling between the outer and inner cages, allowing them to operate more independently. It also increases the leakage reactance of the inner cage (which is desirable for the frequency-dependent current division to work effectively).
Is a double cage motor less efficient than a single cage motor?
Slightly. The outer cage (high resistance) always carries some current even during running, contributing to additional copper losses. However, the difference is small (1–3%) and is acceptable given the significant improvement in starting performance.
Conclusion
The double cage induction motor elegantly solves the starting torque vs running efficiency conflict by using two rotor cages with different resistance and reactance characteristics. At starting (high frequency), current flows through the high-resistance outer cage giving high torque. At running speed (low frequency), current shifts to the low-resistance inner cage giving high efficiency. This automatic transition — driven purely by the physics of frequency-dependent impedance — makes it ideal for direct-on-line starting of heavy loads.