- Deep Bar Double Cage Induction Motor Definition: A deep bar double cage induction motor is defined as a motor that uses a double-layered rotor to enhance starting torque and efficiency.
- Starting Torque Improvement: The double cage rotor design improves starting torque by utilizing high resistance in the outer bars.
- Construction of Double Cage Rotor: The rotor has two layers; the outer layer with small cross-section bars and high resistance, and the inner layer with large cross-section bars and low resistance.
- Operational Principle: At standstill, the outer bars carry more current due to higher inductive reactance in the inner bars, shifting to inner bars as speed increases.
- Speed-Torque Characteristics: The motor achieves higher starting torque with higher rotor resistance at standstill, and efficient running torque with lower inductive reactance at full speed.
Generally in induction motor related operations, squirrel cage induction motors are widely used. The starting torque equation of an induction motor is given by
Where, R2 and X2 are the rotor resistance and inductive reactance at starting respectively, E2 is the rotor induced EMF and
Ns is the RPS speed of synchronous stator flux. Here in this equation the starting torque of induction motor Tsh is proportional to rotor resistance R2.
However, squirrel cage motors have low starting torque due to their low rotor resistance. To improve this, a double bar double cage rotor is used to increase rotor resistance.
The motive is to provide higher value of rotor resistance in such a manner that the rotor with its higher valued resistance provides higher torque and more efficiency.
Why Starting Torque is Poor in Squirrel Cage Induction Motor?
In squirrel cage motors, rotor resistance cannot be adjusted like in slip ring motors. The rotor’s low fixed resistance results in poor starting torque because the induced voltage and current have the same frequency as the supply at standstill, causing high inductive reactance. This leads to the current lagging significantly behind the voltage, reducing starting torque to only 1.5 times the full load torque. Despite high current, this low torque makes it unsuitable for high loads. A deep bar double cage motor provides higher starting torque.
Construction of Deep Bar Double Cage Induction Motor
In deep bar double cage rotor bars are there in two layers.
The outer layer contains bars with small cross-sections and high resistance, which are shorted at both ends. This results in low flux linkage and low inductance. The high resistance of the outer cage improves starting torque by providing a high resistance to inductive reactance ratio.
The inner layer has bars with large cross-sections and low resistance. These bars are embedded in iron, resulting in high flux linkage and high inductance. The resistance to inductive reactance ratio is low, making the inner layer efficient for running conditions.
Operational Principle Construction of Deep Bar Double Cage Induction Motor
At the stand still condition the inner and outer side bars get induced with voltage and current with the same frequency of the supply. Now the case is that the inductive reactance (XL = 2πfL) is offered more in the deep bars or inner side bars due to skin effect of the alternating quantity i.e. voltage and current. Hence the current tries to flow through the outer side rotor bars.
The outer side rotor offers more resistance but poor inductive reactance. The ultimate resistance is somewhat higher than the single bar rotor resistance. The higher valued rotor resistance results more torque to be developed at the starting. When the speed of the rotor of the deep bar double cage induction motor increases, the frequency of the induced EMF and current in the rotor gets gradually decreased. Hence the inductive reactance (XL) in the inner side bars or deep bars gets decreased and the current faces less inductive reactance and less resistance as a whole. Now no need for more torque because the rotor already has arrived to its full speed with running torque.
Speed Torque Characteristics of Deep Rotor IM
Where, R2 and X2 are the rotor resistance and inductive reactance at starting respectively, E2 is the rotor induced EMF and
Ns is the RPS speed of synchronous stator flux and S is the slip of the rotor speed. The above speed-torque graph shows that the higher valued resistance offers higher torque at the stand still condition and the max torque will be achieved at higher valued slip.
Comparison between Single Cage and Double Cage Motors
- A double cage rotor has low starting current and high starting torque. Therefore, it is more suitable for direct on line starting.
- Since effective rotor resistance of double cage motor is higher, there is larger rotor heating at the time of starting as compared to that of single cage rotor.
- The high resistance of the outer cage increases the resistance of double cage motor. So full load copper losses are increased and efficiency is decreased.
- The pull out torque of double cage motor is smaller than single cage motor.
- The cost of double cage motor is about 20-30 % more than that of single cage motor og same rating.
