Double Cage Motor MCQ Quiz - Objective Question with Answer for Double Cage Motor - Download Free PDF

Why is WRIG preferred over Squirrel Cage Induction Generator (SCIG) in some wind turbines?

Correct Option: Option 2 - WRIG allows external rotor resistance control, enabling better efficiency and power factor control.

Introduction:

The Wound Rotor Induction Generator (WRIG) and the Squirrel Cage Induction Generator (SCIG) are two common types of induction generators used in wind turbines. The choice between these two depends on several factors, including efficiency, controllability, maintenance requirements, and cost. While SCIGs are widely used due to their simplicity and robustness, WRIGs are often preferred in certain applications due to their enhanced controllability and adaptability to varying operational conditions.

Explanation of the Correct Option:

WRIGs have a unique design feature that sets them apart from SCIGs: their rotors are equipped with slip rings and external resistors. This configuration allows external rotor resistance control, which is key to their advantages in wind turbine applications.

1. External Rotor Resistance Control:

• In a WRIG, the rotor windings are connected to external resistors through slip rings. By adjusting the external resistance, the performance characteristics of the generator can be modified as per the operational requirements.
• This capability allows for better control of the generator's torque-speed characteristics, which is particularly important in wind turbines where wind speeds can vary significantly.

2. Efficiency and Power Factor Control:

• The external resistance control in WRIGs helps optimize the efficiency of the generator. By adjusting the resistance, energy losses can be minimized, and the generator can operate closer to its optimal efficiency point under varying wind conditions.
• Power factor control is another significant advantage. WRIGs can adjust their reactive power output by varying the external resistance, which helps improve the overall power factor of the wind turbine system. This is especially beneficial in grid-connected systems where maintaining a good power factor is essential for reducing losses and complying with grid codes.

3. Application in Wind Turbines:

• WRIGs are often used in variable-speed wind turbines, where their ability to adapt to changing wind conditions is a significant advantage. This adaptability is achieved through the external resistance control and the generator's inherent design flexibility.
• In contrast, SCIGs are typically limited to constant-speed applications due to their fixed rotor design, which does not allow for external resistance control.

4. Limitations of WRIGs:

• Despite their advantages, WRIGs have some drawbacks, such as higher maintenance requirements due to the presence of slip rings and brushes.
• However, in many cases, the benefits of enhanced controllability and efficiency outweigh these maintenance concerns, making WRIGs a preferred choice for specific wind turbine applications.

Important Information

To further understand the analysis, let’s evaluate the other options:

Option 1: WRIG does not require maintenance due to the absence of slip rings.

This option is incorrect because WRIGs use slip rings and brushes, which require periodic maintenance. In fact, the presence of slip rings is one of the main distinguishing features of WRIGs compared to SCIGs, which do not have slip rings and are therefore maintenance-free in this regard.

Option 3: WRIG works only with constant-speed wind turbines.

This statement is incorrect as WRIGs are particularly suited for variable-speed wind turbines. Their ability to control external rotor resistance makes them adaptable to varying wind speeds, unlike SCIGs, which are typically limited to constant-speed applications.

Option 4: WRIG has no moving parts, making it more durable.

This option is also incorrect because WRIGs, like all rotating electrical machines, have moving parts, including the rotor, slip rings, and brushes. The presence of these components means WRIGs are not as maintenance-free or durable as SCIGs, which have a simpler and more robust construction.

Option 5: Statement is missing or does not apply to WRIGs.

This option does not provide any valid information relevant to the comparison between WRIGs and SCIGs, and therefore, it does not contribute to the discussion.

Conclusion:

The key advantage of WRIGs over SCIGs lies in their ability to control external rotor resistance, which enables better efficiency and power factor control. This feature makes WRIGs highly suitable for variable-speed wind turbine applications, where adaptability to changing wind conditions is crucial. While WRIGs have higher maintenance requirements due to the presence of slip rings, their enhanced performance and controllability often justify their use in specific scenarios. Understanding the operational characteristics and limitations of different types of induction generators is essential for selecting the most appropriate technology for wind turbine applications.

The correct answer is Both I and II.

Key Points

• Squirrel Cage Induction Motor:
  • These motors are known for their simple and rugged construction.
  • The rotor consists of a cylindrical core with conductors placed in slots and short-circuited by end rings.
  • They have no brushes, slip rings, or commutators, which significantly reduces maintenance requirements.
  • The absence of these components means fewer moving parts that can wear out over time.
  • Hence, statement I is correct.
• Slip Ring Induction Motor:
  • These motors have a rotor winding connected to external resistors through slip rings.
  • The slip rings and brushes allow for external resistance to be added to the rotor circuit, which is useful for starting torque and speed control.
  • However, the presence of brushes and slip rings means that these motors require more maintenance compared to squirrel cage motors.
  • Brushes and slip rings are subject to wear and tear and need regular inspection and replacement.
  • Hence, statement II is correct.

Additional Information

• Squirrel Cage Induction Motor:
  • These motors are widely used in industrial applications due to their durability and low cost.
  • They are typically used in applications where constant speed is required.
  • Examples include fans, blowers, pumps, and conveyors.
• Slip Ring Induction Motor:
  • These motors are often used in applications requiring high starting torque and variable speed.
  • They are commonly found in cranes, elevators, and hoists where load conditions change frequently.
  • Adding external resistance to the rotor circuit can improve the starting torque and control the speed of the motor.

Double squirrel-cage induction motor:

• A double squirrel-cage induction motor has two parallel windings in the rotor
• The rotor of a double squirrel cage motor has two independent cages on the same rotor
• The figure below shows the cross-sectional diagram of a double squirrel cage rotor

• Bars of high resistance and low reactance are placed in the outer cage, and bars of low resistance and high reactance are placed in the inner cage
• The outer cage has a high ‘resistance to reactance ratio' whereas, the inner cage has a low 'resistance to reactance ratio'
• The outer cage has a smaller cross-sectional area than the lower cage.

Important Points for double cage induction motor:

• Higher starting torque
• Outer cage increases CU loss due to high resistance
• Inner cage reduces full load power factor
• Efficiency reduces
• Cost is higher

Application:

• Rolling Mills
• Cranes
• Compressors
• Conveyors
• Reciprocating pumps

Double squirrel-cage induction motor:

• A double squirrel-cage induction motor has two parallel windings in the rotor
• The rotor of a double squirrel cage motor has two independent cages on the same rotor
• The figure below shows the cross-sectional diagram of a double squirrel cage rotor

• Bars of high resistance and low reactance are placed in the outer cage, and bars of low resistance and high reactance are placed in the inner cage
• The outer cage has a high ‘resistance to reactance ratio' whereas, the inner cage has a low 'resistance to reactance ratio'
• The outer cage has a smaller cross-sectional area than the lower cage.

Important Points for double cage induction motor:

• Higher starting torque.
• Low starting current.
• Outer cage increases cu loss due to high resistance
• Inner cage reduces full load power factor
• Efficiency reduces
• Cost is higher

Application:

• Rolling Mills
• Cranes
• Compressors
• Conveyors
• Reciprocating pumps

Double squirrel-cage induction motor:

• A double squirrel-cage induction motor has two parallel windings in the rotor
• The rotor of a double squirrel cage motor has two independent cages on the same rotor
• The figure below shows the cross-sectional diagram of a double squirrel cage rotor

• Bars of high resistance and low reactance are placed in the outer cage, and bars of low resistance and high reactance are placed in the inner cage
• The outer cage has a high ‘resistance to reactance ratio' whereas, the inner cage has a low 'resistance to reactance ratio'
• The outer cage has a smaller cross-sectional area than the lower cage.

Important Points for double cage induction motor:

• Higher starting torque
• Outer cage increases CU loss due to high resistance
• Inner cage reduces full load power factor
• Efficiency reduces
• Cost is higher

Application:

• Rolling Mills
• Cranes
• Compressors
• Conveyors
• Reciprocating pumps

• A double squirrel-cage induction motor has two parallel windings in rotor
• Rotor of a double squirrel cage motor has two independent cages on the same rotor
• The figure below shows the cross-sectional diagram of a double squirrel cage rotor

• The outer cage has a smaller cross-sectional area than the lower cage.
• Bars of high resistance and low reactance are placed in the outer cage, and bars of low resistance and high reactance are placed in the inner cage
• The outer cage has high ‘resistance to reactance ratio' whereas, the inner cage has low 'resistance to reactance ratio'.
• Hence double cage rotor has low starting current and high starting torque, therefore, it is more suitable for direct on line starting

Double squirrel-cage induction motor:

• A double squirrel-cage induction motor has two parallel windings in the rotor
• The rotor of a double squirrel cage motor has two independent cages on the same rotor
• The figure below shows the cross-sectional diagram of a double squirrel cage rotor

• Bars of high resistance and low reactance are placed in the outer cage, and bars of low resistance and high reactance are placed in the inner cage
• The outer cage has a high ‘resistance to reactance ratio' whereas, the inner cage has a low 'resistance to reactance ratio'
• The outer cage has a smaller cross-sectional area than the lower cage.

Important Points for double cage induction motor:

• Higher starting torque
• Outer cage increases CU loss due to high resistance
• Inner cage reduces full load power factor
• Efficiency reduces
• Cost is higher

Application:

• Rolling Mills
• Cranes
• Compressors
• Conveyors
• Reciprocating pumps

Double squirrel-cage induction motor:

• A double squirrel-cage induction motor has two parallel windings in the rotor
• The rotor of a double squirrel cage motor has two independent cages on the same rotor
• The figure below shows the cross-sectional diagram of a double squirrel cage rotor

• Bars of high resistance and low reactance are placed in the outer cage, and bars of low resistance and high reactance are placed in the inner cage
• The outer cage has a high ‘resistance to reactance ratio' whereas, the inner cage has a low 'resistance to reactance ratio'
• The outer cage has a smaller cross-sectional area than the lower cage.

Important Points for double cage induction motor:

• Higher starting torque.
• Low starting current.
• Outer cage increases cu loss due to high resistance
• Inner cage reduces full load power factor
• Efficiency reduces
• Cost is higher

Application:

• Rolling Mills
• Cranes
• Compressors
• Conveyors
• Reciprocating pumps

Double squirrel-cage induction motor:

• A double squirrel-cage induction motor has two parallel windings in the rotor
• The rotor of a double squirrel cage motor has two independent cages on the same rotor
• The figure below shows the cross-sectional diagram of a double squirrel cage rotor

• Bars of high resistance and low reactance are placed in the outer cage, and bars of low resistance and high reactance are placed in the inner cage
• The outer cage has a high ‘resistance to reactance ratio' whereas, the inner cage has a low 'resistance to reactance ratio'
• The outer cage has a smaller cross-sectional area than the lower cage.

Important Points for double cage induction motor:

• Higher starting torque
• Outer cage increases CU loss due to high resistance
• Inner cage reduces full load power factor
• Efficiency reduces
• Cost is higher

Application:

• Rolling Mills
• Cranes
• Compressors
• Conveyors
• Reciprocating pumps

Starting torque develop by a three-phase induction motor is given by

\({T_{st}} = \frac{{180}}{{2\pi {N_s}}}\left( {\frac{{E_2^2{R_2}}}{{R_2^2 + X_2^2}}} \right)\)

Where R2 and X2 are the rotor resistance and inductive reactance at starting respectively, E2 is the rotor induced emf.

Double squirrel-cage induction motor:

• A double squirrel-cage induction motor has two parallel windings in the rotor
• The rotor of a double squirrel cage motor has two independent cages on the same rotor
• The figure below shows the cross-sectional diagram of a double squirrel cage rotor

• Bars of high resistance and low reactance are placed in the outer cage, and bars of low resistance and high reactance are placed in the inner cage
• The outer cage has a high ‘resistance to reactance ratio' whereas, the inner cage has a low 'resistance to reactance ratio'
• The outer cage has a smaller cross-sectional area than the lower cage.

Important Points for double cage induction motor:

• Higher starting torque
• Outer cage increases CU loss due to high resistance
• Inner cage reduces full load power factor
• Efficiency reduces
• Cost is higher

Application:

• Rolling Mills
• Cranes
• Compressors
• Conveyors
• Reciprocating pumps

• A double squirrel-cage induction motor has two parallel windings in rotor
• Rotor of a double squirrel cage motor has two independent cages on the same rotor
• The figure below shows the cross-sectional diagram of a double squirrel cage rotor

• The outer cage has a smaller cross-sectional area than the lower cage.
• Bars of high resistance and low reactance are placed in the outer cage, and bars of low resistance and high reactance are placed in the inner cage
• The outer cage has high ‘resistance to reactance ratio' whereas, the inner cage has low 'resistance to reactance ratio'.
• Hence double cage rotor has low starting current and high starting torque, therefore, it is more suitable for direct on line starting

Double squirrel-cage induction motor:

• A double squirrel-cage induction motor has two parallel windings in the rotor
• The rotor of a double squirrel cage motor has two independent cages on the same rotor
• The figure below shows the cross-sectional diagram of a double squirrel cage rotor

• Bars of high resistance and low reactance are placed in the outer cage, and bars of low resistance and high reactance are placed in the inner cage
• The outer cage has a high ‘resistance to reactance ratio' whereas, the inner cage has a low 'resistance to reactance ratio'
• The outer cage has a smaller cross-sectional area than the lower cage.

Important Points for double cage induction motor:

• Higher starting torque
• Outer cage increases CU loss due to high resistance
• Inner cage reduces full load power factor
• Efficiency reduces
• Cost is higher

Application:

• Rolling Mills
• Cranes
• Compressors
• Conveyors
• Reciprocating pumps

Double squirrel-cage induction motor:

• A double squirrel-cage induction motor has two parallel windings in the rotor
• The rotor of a double squirrel cage motor has two independent cages on the same rotor
• The figure below shows the cross-sectional diagram of a double squirrel cage rotor

• Bars of high resistance and low reactance are placed in the outer cage, and bars of low resistance and high reactance are placed in the inner cage
• The outer cage has a high ‘resistance to reactance ratio' whereas, the inner cage has a low 'resistance to reactance ratio'
• The outer cage has a smaller cross-sectional area than the lower cage.

Important Points for double cage induction motor:

• Higher starting torque
• Outer cage increases cu loss due to high resistance
• Inner cage reduces full load power factor
• Efficiency reduces
• Cost is higher

Application:

• Rolling Mills
• Cranes
• Compressors
• Conveyors
• Reciprocating pumps

• A double squirrel-cage induction motor has two parallel windings in rotor
• Rotor of a double squirrel cage motor has two independent cages on the same rotor
• The figure below shows the cross-sectional diagram of a double squirrel cage rotor

• The outer cage has a smaller cross-sectional area than the lower cage.
• Bars of high resistance and low reactance are placed in the outer cage, and bars of low resistance and high reactance are placed in the inner cage
• The outer cage has high ‘resistance to reactance ratio' whereas, the inner cage has low 'resistance to reactance ratio'.
• Hence double cage rotor has low starting current and high starting torque, therefore, it is more suitable for direct on line starting