Which of the following is NOT a valid advantage of skewing in squirrel cage induction motor?
It increases starting torque
Understanding Skewing in Squirrel Cage Induction Motors
Skewing is a construction technique used in squirrel cage induction motors. It involves positioning the rotor bars at an angle relative to the rotor axis, instead of keeping them parallel. This angle is typically equal to one or two stator slots.
This design modification has several important effects on the motor's performance characteristics. Let's examine the potential advantages listed in the options and see which one is not a valid benefit of skewing.
Analyzing the Effects of Skewing
Skewing influences the motor's operation in several ways:
- Reduced Harmonics: Skewing helps to reduce the harmonic content of the air gap flux. This reduction in harmonics minimizes unwanted vibrations and noise, often referred to as motor 'hum'.
- Preventing Cogging: Cogging occurs when the rotor locks into a position relative to the stator slots due to the magnetic attraction between them, especially at low speeds or during starting. Skewing distributes the magnetic forces more evenly along the rotor, preventing this locking effect and allowing the motor to start smoothly.
- More Uniform Torque: The interaction between the stator and rotor magnetic fields produces torque. Without skewing, this torque can fluctuate slightly as the rotor bars pass the stator slots. Skewing smooths out these fluctuations, resulting in a more uniform and stable torque production.
- Effect on Starting Torque: While skewing offers several advantages, it has a drawback concerning starting torque. Skewing effectively increases the length of the rotor bars relative to the flux lines they interact with, which can slightly increase the rotor resistance and decrease the effective reactance. However, the primary effect is a reduction in the magnitude of the induced voltage and thus the current at standstill (high slip), leading to a reduction in the starting torque.
Evaluating the Options for Skewing Advantages
Based on the analysis above, let's look at each option:
-
It reduces motor ‘hum’ during operation.
This is a valid advantage of skewing as it reduces harmonics and vibrations.
-
It prevents cogging.
This is a valid advantage of skewing as it helps avoid magnetic locking between rotor and stator slots.
-
It increases starting torque.
This is NOT a valid advantage. Skewing actually tends to decrease the starting torque of the motor.
-
It produces more uniform torque.
This is a valid advantage of skewing as it smooths out torque variations.
The question asks which of the options is NOT a valid advantage of skewing in squirrel cage induction motors. Based on our evaluation, increasing starting torque is not a benefit of skewing; rather, it is a disadvantage or a consequence of skewing.
Conclusion on Skewing Advantages
Skewing is a common feature in squirrel cage induction motors designed to improve performance characteristics such as reducing noise and vibration (hum), preventing cogging, and ensuring more uniform torque. However, it comes at the cost of a slightly reduced starting torque.
| Effect | Is it an Advantage? |
|---|---|
| Reduces motor 'hum' | Yes |
| Prevents cogging | Yes |
| Increases starting torque | No (It decreases starting torque) |
| Produces more uniform torque | Yes |
Therefore, the statement that skewing increases starting torque is not a valid advantage.
Revision Table: Squirrel Cage Motor Skewing
| Aspect | Effect of Skewing | Category (Advantage/Drawback) |
|---|---|---|
| Motor Noise ('Hum') | Reduced | Advantage |
| Cogging Phenomenon | Prevented | Advantage |
| Torque Uniformity | Increased uniformity | Advantage |
| Starting Torque | Reduced | Drawback |
Additional Information: Squirrel Cage Induction Motor Design
The squirrel cage induction motor is widely used due to its simple, rugged construction and reliability. The rotor consists of a core with slots, and conductors (bars) are placed in these slots, short-circuited at both ends by end rings. The design of these rotor bars and slots, including whether they are skewed or not, significantly impacts the motor's operating characteristics.
Beyond skewing, other design features like the shape of the rotor slots (e.g., round, rectangular, trapezoidal, or deep bar) also influence parameters like starting torque and efficiency. Deep bar rotors, for instance, are used to improve starting torque through skin effect.
Understanding these design choices is crucial for selecting the right motor for a specific application, balancing factors like starting performance, running efficiency, cost, and noise levels.
Important Questions from Three Phase Induction Motor
The stator of a 3-phase induction motor has 4 slots per pole phase. If the supply frequency is 50 Hz, calculate the number of stator poles produced and the total number of slots on the stator, respectively.
In an induction motor for a value slip(s) = 0, the torque (T) is
When an induction machine is allowed to run above synchronous speed, then this characteristic exactly matches which of the following options?
A 3-phase. 6-pole, 50 Hz, squirrel cage induction motor is running at a slip of 5%. The speed of stator magnetic field to rotor magnetic field and speed of rotor with respect to stator magnetic field are
The stator of a 3-phase induction motor has 2 slots per pole per phase. If supply frequency is 50 Hz, then calculate the number of stator poles and total number of slots on the stator.
