Test: Induction Machine - 2 - Electrical Engineering (EE) MCQ

The frequency of the induced emf in an induction motor is lesser than the supply frequency.

Explanation:
In an induction motor, the stator windings are connected to the supply, and the rotor windings are short-circuited. When a three-phase supply is given to the stator windings, a rotating magnetic field is produced, which induces a voltage in the rotor windings.

Here's a detailed explanation of why the frequency of the induced emf is lesser than the supply frequency:

1. Slip: The difference between the synchronous speed of the rotating magnetic field and the actual speed of the rotor is known as slip. The slip is necessary for the production of torque in an induction motor.

2. Rotor Frequency: The rotor frequency is given by the formula: fr = s * fs, where fr is the rotor frequency, s is the slip, and fs is the supply frequency. As the slip is always less than 1, the rotor frequency will always be less than the supply frequency.

3. Rotor Voltage: The induced emf in the rotor windings is directly proportional to the rotor frequency. Since the rotor frequency is less than the supply frequency, the induced emf in the rotor windings will also be lesser than the supply frequency.

4. Rotor Current: The rotor current is inversely proportional to the rotor frequency. As the rotor frequency is lesser than the supply frequency, the rotor current will be higher than the supply current.

5. Torque Production: The torque produced in an induction motor is directly proportional to the rotor current. As the rotor current is higher than the supply current, the torque produced will be higher.

Therefore, the frequency of the induced emf in an induction motor is lesser than the supply frequency.

Introduction:

In an induction motor, the air gap flux density is an important parameter that affects the performance and characteristics of the motor. It refers to the magnetic field strength in the air gap between the stator and rotor of the motor.

Reasons for keeping the air gap flux density low:

There are several reasons why the air gap flux density is usually kept low in an induction motor:

1. Reduce losses:

- Low air gap flux density helps in reducing core losses, such as hysteresis and eddy current losses, in the motor.
- This leads to improved efficiency as less energy is wasted in the form of heat.

2. Prevent saturation:

- Keeping the air gap flux density low helps in preventing the magnetic saturation of the motor's core.
- Saturation occurs when the magnetic field strength exceeds a certain limit, leading to a decrease in efficiency and an increase in losses.

3. Control heating:

- By maintaining a low air gap flux density, the motor's temperature rise can be controlled effectively.
- High flux density can result in increased heating, which may lead to insulation degradation and reduced motor life.

4. Improve power factor:

- Low air gap flux density helps in improving the power factor of the induction motor.
- Power factor is a measure of how effectively the motor utilizes the supplied electrical power.
- By reducing the air gap flux density, the motor's reactive power consumption can be minimized, leading to a better power factor.

5. Enhance efficiency:

- Low air gap flux density contributes to improved motor efficiency.
- This is because lower flux density reduces the core losses and allows the motor to operate more efficiently.

6. Cost considerations:

- While reducing the air gap flux density may lead to a slight increase in the size of the motor, it helps in reducing the overall machine cost.
- Lower flux density results in smaller core size and lower material requirements, leading to cost savings.

Conclusion:

To summarize, the air gap flux density is usually kept low in an induction motor to reduce losses, prevent saturation, control heating, improve power factor, enhance efficiency, and minimize machine cost. These factors collectively contribute to the optimal performance and economic operation of the motor.

Explanation:
The rotor slots in a squirrel-cage induction motor are slightly skewed for the following reasons:

1. Reduce magnetic hum: Skewing the rotor slots helps in reducing the magnetic hum produced by the motor during operation. When the slots are skewed, the harmonics in the magnetic field are distributed more evenly, resulting in reduced noise or hum.

2. Prevent rotor locking: Skewing the rotor slots also helps in preventing the rotor from getting locked or stuck in a particular position. As the rotor rotates, the skewed slots prevent the rotor bars from aligning perfectly with the stator's magnetic field, reducing the chances of rotor locking.

3. Improve motor performance: Skewing the rotor slots improves the overall performance of the induction motor by reducing the cogging torque and torque pulsations. This results in smoother motor operation and improved efficiency.

4. Enhance motor stability: Skewing the rotor slots improves the stability of the motor by reducing the tendency of the rotor to vibrate or oscillate during operation. This helps in extending the motor's lifespan and reducing maintenance requirements.

5. Economize on copper: While not the primary reason for skewing rotor slots, it is worth mentioning that slight skewing can help in optimizing the distribution of copper in the rotor bars, resulting in reduced copper losses and improved efficiency.

In summary, skewing the rotor slots in a squirrel-cage induction motor helps in reducing magnetic hum, preventing rotor locking, improving motor performance and stability, and optimizing the copper distribution.

The frame of an induction motor is usually made of cast iron.

Explanation:

Cast iron is the most commonly used material for the frame of an induction motor. Here's why:

- Durable and strong: Cast iron is known for its high strength and durability, making it ideal for supporting the various components of an induction motor and withstanding the mechanical stresses during operation.

- Good heat dissipation: Induction motors generate a significant amount of heat during operation. Cast iron has excellent heat dissipation properties, allowing it to effectively dissipate heat and keep the motor cool.

- Magnetic properties: Cast iron has good magnetic properties, which are important for the functioning of the motor. It helps in reducing magnetic losses and improving the efficiency of the motor.

- Cost-effective: Cast iron is relatively inexpensive compared to other materials like aluminum or bronze, making it a cost-effective choice for motor frames.

- Machinability: Cast iron is easy to machine, allowing for precise manufacturing of the motor frame and ensuring proper alignment of the components.

- Protection against environmental factors: Cast iron has good resistance against corrosion and environmental factors like moisture and dust, providing protection to the internal components of the motor.

In summary, cast iron is the preferred choice for the frame of an induction motor due to its strength, heat dissipation properties, magnetic properties, cost-effectiveness, machinability, and protection against environmental factors.

Explanation:

The rotor of an induction motor never runs at synchronous speed because of the following reasons:

1. Relative Speed:
- The induction motor operates on the principle of electromagnetic induction.
- In an induction motor, the stator produces a rotating magnetic field, and the rotor is subjected to this rotating magnetic field.
- The rotor needs to have a relative speed with respect to the rotating magnetic field in order to induce currents and produce torque.
- If the rotor runs at synchronous speed, there will be no relative speed between the rotating flux and the rotor, resulting in no torque production.

2. Slip:
- The difference between the synchronous speed and the actual rotor speed is known as slip.
- Slip is necessary for the rotor to have a relative speed with respect to the rotating magnetic field.
- If the rotor runs at synchronous speed, the slip will be zero, and there will be no relative speed, resulting in no torque production.

3. Torque Production:
- Torque in an induction motor is produced due to the interaction between the rotating magnetic field and the induced currents in the rotor.
- The magnitude of torque production is directly proportional to the slip.
- When the slip is zero (rotor running at synchronous speed), there will be no induced currents in the rotor, and hence, no torque production.

Therefore, the correct answer is option D: zero and hence, torque will be zero.

Explanation:

A wound rotor motor, also known as a slip ring motor, is mainly used in applications where speed control is required. Here's why:

• Speed Control: The wound rotor motor allows for precise control of the motor's speed by adjusting the resistance in the rotor circuit. This makes it suitable for applications where variable speeds are required, such as in industrial machinery or conveyor systems.


• Starting Torque: The wound rotor motor typically provides high starting torque, which is beneficial in applications where a higher torque is needed to overcome the inertia of the load during startup.


• Cost: While the wound rotor motor offers advantages in terms of speed control and starting torque, it is generally more expensive compared to other types of motors. Therefore, it may not be the preferred choice in applications where cost is a major factor.


• Rotor Resistance: The wound rotor motor allows for the adjustment of rotor resistance during running, which can be useful in applications that require higher rotor resistance for specific operating conditions.

In summary, a wound rotor motor is mainly used in applications where speed control is required, as it offers the ability to adjust the rotor resistance and provides high starting torque.

DC Motor Characteristics

A DC motor is an electrical machine that converts electrical energy into mechanical energy. It operates based on the interaction between the magnetic field produced by the stator and the current-carrying conductors on the rotor. There are several types of DC motors, each with its own characteristics.

3-Phase Induction Motor

A 3-phase induction motor is a type of AC motor commonly used in industrial applications. It operates on the principle of electromagnetic induction, where a rotating magnetic field is produced by the stator windings, which induces currents in the rotor windings, causing the rotor to rotate. The key characteristics of a 3-phase induction motor include:

1. High Starting Torque: The 3-phase induction motor has a high starting torque, allowing it to start and accelerate heavy loads efficiently.

2. Simple Design and Construction: The motor has a simple and robust design, making it reliable and suitable for various applications.

3. Good Speed Regulation: The motor has good speed regulation, meaning it maintains a relatively constant speed even with varying loads.

4. Low Maintenance: The absence of brushes and commutators in an induction motor results in minimal maintenance requirements.

DC Motor Characteristics Similar to a 3-Phase Induction Motor

Among the various types of DC motors, the characteristic that is similar to a 3-phase induction motor is the DC shunt motor. This type of motor shares some similarities with the 3-phase induction motor, including:

1. Speed Regulation: Like the 3-phase induction motor, a DC shunt motor also has good speed regulation, meaning it can maintain a relatively constant speed regardless of the load.

2. Simple Design: Both motors have a relatively simple design, making them reliable and suitable for a wide range of applications.

3. High Starting Torque: While not as high as the induction motor, a DC shunt motor can provide a decent starting torque, allowing it to start and accelerate loads effectively.

4. Low Maintenance: Similar to the induction motor, a DC shunt motor also has low maintenance requirements due to the absence of brushes and commutators.

Therefore, the DC shunt motor is the correct answer as it shares similar characteristics with the 3-phase induction motor.

Slip Ring Induction Motors vs Squirrel Cage Motors: Why are Slip Ring Induction Motors less extensively used?

Introduction:
Slip ring induction motors and squirrel cage motors are two common types of induction motors used in various applications. While both have their advantages and disadvantages, slip ring induction motors are generally less extensively used compared to squirrel cage motors. There are several reasons for this.

1. Requirement of Slip Rings on the Rotor Circuit:
- Slip ring induction motors require slip rings on the rotor circuit. Slip rings are conductive rings that make electrical connections with brushes, allowing the rotor windings to be externally connected.
- This additional component adds complexity and cost to the motor design and operation.
- The presence of slip rings also increases the maintenance requirements of the motor, as they can wear out over time and require replacement or repair.

2. Rotor Windings are Generally Y-Connected:
- In slip ring induction motors, the rotor windings are generally Y-connected, meaning they are connected in a star configuration.
- This Y-connection allows for a higher starting torque and better control of the motor's speed.
- However, it also adds complexity to the motor design and requires additional connections and components.

3. Cost and Maintenance:
- Slip ring induction motors are generally more expensive to manufacture compared to squirrel cage motors.
- The presence of slip rings and the Y-connected rotor windings contribute to the higher cost.
- Additionally, slip ring induction motors require greater maintenance due to the presence of slip rings, which can wear out or become damaged over time.
- The need for regular maintenance and potential repair costs make these motors less attractive for many applications.

Conclusion:
In conclusion, slip ring induction motors are less extensively used than squirrel cage motors due to the requirement of slip rings on the rotor circuit, the Y-connected rotor windings, and the associated cost and maintenance requirements. While slip ring induction motors have certain advantages such as higher starting torque and speed control, these factors contribute to their reduced popularity in many applications.

Explanation:

When a squirrel-cage induction motor operates under no-load, the following statements hold true:

A: Rotor induced e.m.f. is low
- When the motor operates under no-load, the rotor current is minimal, which leads to a low rotor induced electromotive force (e.m.f.).

B: Rotor current is low
- Under no-load conditions, the motor does not have to produce significant torque, so the rotor current is low.

C: Power factor is low
- In a squirrel-cage induction motor, the power factor is typically low under no-load conditions. This is because the power factor is determined by the ratio of the real power (measured in watts) to the apparent power (measured in volt-amperes). Since there is no real power consumption under no-load, the power factor is low.

However, the statement that is not valid when a squirrel-cage induction motor operates under no-load is:

D: Slip is low
- Slip is defined as the difference between the synchronous speed and the rotor speed, divided by the synchronous speed. Under no-load conditions, the rotor speed approaches the synchronous speed, which means the slip is close to zero, not low.

Therefore, the correct answer is D: Slip is low.

The torque T produced by a three-phase induction motor is proportional to the square of the applied voltage V. This relationship is expressed by the formula:

T∝V²

If the supply voltage V is increased two times (i.e., doubled), then the torque T will be affected as follows:

T∝(2V)²

This indicates that the torque will increase four times. Thus, the correct answer is:

1. increased four times

Correct ans is A .
efficiency of motor is = Nr/Ns*100
Nr =1350
Ns= 1500
efficiency = 1350/1500*100=90

Explanation:
When the supply frequency of a three-phase induction motor is increased, its synchronous speed increases. This can be understood through the following points:

1. Relationship between Supply Frequency and Synchronous Speed:
- The synchronous speed of an induction motor is directly proportional to the supply frequency.
- The formula to calculate synchronous speed is given by: Synchronous Speed (Ns) = (120 x Frequency) / Number of Poles.
- As the supply frequency increases, the synchronous speed also increases.

2. Effect of Supply Frequency on Rotor Speed:
- The rotor speed of an induction motor is slightly less than the synchronous speed.
- The difference between the rotor speed and synchronous speed is known as slip.
- When the supply frequency is increased, the rotor speed also increases, thereby reducing the slip.

3. Effect of Supply Frequency on Motor Performance:
- Increasing the supply frequency increases the synchronous speed, which in turn increases the motor's overall speed.
- This can be advantageous in applications where higher speeds are required.
- However, it is important to note that increasing the supply frequency beyond the motor's design limits can result in overheating and damage to the motor.

Conclusion:
- When the supply frequency of a three-phase induction motor is increased, its synchronous speed increases.
- This increase in synchronous speed can have both advantages and limitations, depending on the specific application and the motor's design limits.

Double Squirrel Cage Induction Motor: Outer Cage Winding Characteristics

The outer cage winding in a double squirrel cage induction motor is characterized by the following:

1. High Resistance:
- The outer cage winding has a relatively high resistance compared to the inner cage winding.
- This higher resistance allows for a higher starting torque and lower starting current.
- The high resistance helps to limit the starting current and prevent excessive current draw during motor startup.

2. High Leakage Inductance:
- The outer cage winding also has a higher leakage inductance compared to the inner cage winding.
- The higher leakage inductance contributes to a more stable and efficient motor operation.
- It helps to reduce harmonic currents and improve power factor.

3. Lower Reactance:
- The reactance of the outer cage winding is lower than that of the inner cage winding.
- This lower reactance allows for better control of the motor's speed and torque characteristics.
- It helps to improve the motor's efficiency and performance.

4. Lower Rotor Resistance:
- In comparison to the rotor resistance of the inner cage winding, the rotor resistance of the outer cage winding is lower.
- This lower rotor resistance contributes to a more balanced torque distribution between the outer and inner cage windings.
- It helps to improve the motor's starting and running performance.

Therefore, the correct answer is C: high resistance. The outer cage winding in a double squirrel cage induction motor is characterized by a higher resistance compared to the inner cage winding.

Starting Methods for Very Large 3-Phase Induction Motors

There are various methods used to start very large 3-phase induction motors (> 25 h.p.), and these include:

1. Direct On Line (DOL) Starting:
- In this method, the motor is directly connected to the power supply.
- It is the simplest and most common method used for smaller motors.
- However, it is not suitable for very large motors due to the high starting current, which can cause voltage dips and lead to voltage instability in the power system.
- Therefore, DOL starting is not recommended for very large 3-phase induction motors.

2. Star-Delta Starting:
- This method involves initially connecting the motor windings in a star configuration during starting.
- After a predetermined time, the motor windings are switched to a delta configuration for normal operation.
- Star-delta starting reduces the starting current and torque, thereby minimizing the impact on the power system.
- It is commonly used for motors with medium to large power ratings.
- However, it is not suitable for very large motors (> 25 h.p.), as the starting torque may not be sufficient for heavy loads.

3. Autotransformer Starting:
- Autotransformer starting involves using an autotransformer to reduce the voltage supplied to the motor during starting.
- This reduces the starting current and torque, thereby minimizing the impact on the power system.
- Autotransformer starting is suitable for very large motors as it provides a smooth and controlled start.
- It is a cost-effective method compared to other starting methods like variable frequency drives (VFDs).
- Therefore, autotransformer starting is commonly used for very large 3-phase induction motors (> 25 h.p.).

4. None of the Above:
- This option implies that none of the mentioned starting methods (DOL, star-delta, autotransformer) are suitable for very large 3-phase induction motors.
- However, this is not the correct answer in this case, as autotransformer starting is indeed a suitable method for very large motors.

In conclusion, the correct answer is C: Autotransformer starting.

Leakage Flux in Induction Motor Compared to Transformer

The leakage flux in an induction motor and a transformer depends on the design and construction of the two devices. Let's compare the leakage flux in both:

1. Induction Motor:
- In an induction motor, the flux is produced by the stator winding and it is linked with both the stator and rotor windings.
- However, due to the air gap between the stator and rotor, some flux does not link with the rotor winding and is known as leakage flux.
- The leakage flux in an induction motor is higher compared to that in a transformer.

2. Transformer:
- In a transformer, the flux is produced by the primary winding and it is linked with the secondary winding.
- Since there is no air gap between the windings, the flux is fully linked and does not experience leakage.
- The leakage flux in a transformer is negligible.

Conclusion:
- Therefore, for the same kVA rating, the leakage flux in an induction motor is more than that in a transformer.
- This is because the presence of an air gap in the induction motor causes a significant portion of the flux to leak, while in a transformer the absence of an air gap ensures a minimal leakage flux.

Explanation:

In an induction motor, the number of poles refers to the number of magnetic poles present on the stator. The number of poles determines the motor's speed and the relationship between the number of poles and speed is given by the formula:

Speed (in RPM) = (120 * Frequency) / Number of Poles

Based on this formula, we can deduce the following:

1. Greater the number of poles, lesser the speed: As the number of poles increases, the speed of the motor decreases. This is because the numerator of the formula remains constant (120 * Frequency), while the denominator (Number of Poles) increases. Therefore, the overall speed decreases.

2. Lesser the speed, lesser the frequency: Since the speed is inversely proportional to the number of poles, a decrease in speed signifies a decrease in frequency. This is because the frequency remains constant, and the speed is determined by the number of poles.

Therefore, the correct answer is A: Lesser the speed.

The no-load speed of an induction motor depends upon the supply frequency and the number of its poles. Let's break down the explanation into headings and bullet points for better understanding:

1. Supply Frequency:
- The supply frequency, represented in hertz (Hz), refers to the number of cycles per second.
- The no-load speed of an induction motor is directly proportional to the supply frequency.
- As the supply frequency increases, the no-load speed of the motor also increases.

2. Number of Poles:
- The number of poles in an induction motor refers to the number of magnetic poles present in the stator.
- The no-load speed of an induction motor is inversely proportional to the number of poles.
- As the number of poles increases, the no-load speed of the motor decreases.

3. Maximum Flux/Phase:
- The maximum flux per phase is not directly related to the no-load speed of an induction motor.
- The maximum flux per phase determines the motor's torque production and power factor but not its speed.

4. Final Answer:
- Based on the given options, the correct answer is option D, which states that the no-load speed of an induction motor depends only on the supply frequency (option A) and the number of poles (option B).

In summary, the no-load speed of an induction motor is determined by the supply frequency and the number of poles. The supply frequency directly affects the speed, while the number of poles has an inverse relationship with the speed. The maximum flux per phase does not directly impact the no-load speed of the motor.

Operation of an Induction Motor

An induction motor operates based on the principle of mutual induction. Mutual induction refers to the production of an electromotive force (emf) in one coil due to the change in current in a neighboring coil.

Here is a detailed explanation of the operation of an induction motor:

1. Stator
- The stator of an induction motor consists of a stationary coil or winding, which is connected to an alternating current (AC) power supply.
- When the AC power supply is switched on, it generates a rotating magnetic field in the stator winding.

2. Rotor
- The rotor of an induction motor consists of a set of conductive bars or coils arranged in a cylindrical shape.
- The rotor is not connected to any external power source and relies on the rotating magnetic field produced by the stator for its operation.

3. Induced Voltage
- As the rotating magnetic field of the stator cuts across the conductive bars or coils of the rotor, it induces a voltage in the rotor.
- According to Faraday's law of electromagnetic induction, the induced voltage in the rotor causes a current to flow through the rotor conductors.

4. Rotating Magnetic Field
- The current flowing in the rotor conductors creates a magnetic field around them.
- The interaction between the rotating magnetic field of the stator and the magnetic field generated by the rotor causes the rotor to rotate.

5. Slip
- The speed of the rotating magnetic field produced by the stator is called synchronous speed.
- The actual speed at which the rotor rotates is slightly less than the synchronous speed and is known as slip.
- The slip allows the rotor to maintain a relative speed with respect to the rotating magnetic field, which enables torque production.

6. Torque Production
- The interaction between the rotating magnetic field and the magnetic field generated by the rotor produces a torque in the rotor.
- This torque allows the rotor to overcome inertia and start rotating.
- The rotor continues to rotate as long as the stator is supplied with AC power.

In conclusion, the operation of an induction motor is based on mutual induction, where the rotating magnetic field produced by the stator induces an emf in the rotor, resulting in its rotation.

Explanation:

An induction motor operates based on the principle of electromagnetic induction. When an alternating current is supplied to the stator windings of the motor, it creates a rotating magnetic field. This rotating magnetic field induces an emf (electromotive force) in the rotor windings, which in turn causes the rotor to rotate.

The frequency of the induced emf in an induction motor is determined by the speed of the rotor. As the rotor rotates at a speed lower than the synchronous speed (the speed of the rotating magnetic field), the frequency of the induced emf in the rotor windings is lower than the supply frequency.

Here's a more detailed explanation:

- The stator windings are connected to an alternating current power supply, which typically has a fixed frequency (e.g., 50 Hz or 60 Hz).
- The stator windings create a rotating magnetic field that interacts with the rotor windings.
- The rotor windings are short-circuited, which allows the induced emf to flow through them.
- The rotating magnetic field induces an emf in the rotor windings, which creates a current in the rotor circuit.
- The frequency of the induced emf is determined by the relative speed between the rotating magnetic field and the rotor.
- The rotor speed is lower than the synchronous speed, which means that the frequency of the induced emf is lower than the supply frequency.

In conclusion, the frequency of the induced emf in an induction motor is lesser than the supply frequency. Therefore, option C is the correct answer.

Explanation:
- The power factor of an induction motor is the ratio of its true power (kW) to its apparent power (kVA).
- At no load, the true power of the motor is zero because there is no mechanical load on the motor.
- The apparent power of the motor is equal to the product of the motor's rated voltage (V) and its rated current (A).
- Therefore, at no load, the power factor of an induction motor is zero because the true power is zero and the apparent power is non-zero.
- Option D: 0, is the correct answer.

Explanation:

The frequency of the induced emf in an induction motor depends on the relative speed between the rotating magnetic field produced by the stator and the rotor. Here's a detailed explanation:

1. Supply Frequency: The supply frequency is the frequency of the alternating current (AC) power source that is connected to the motor.

2. Rotating Magnetic Field: The stator of the induction motor produces a rotating magnetic field with a frequency equal to the supply frequency.

3. Slip: The relative speed between the rotating magnetic field and the rotor is known as slip. Slip is determined by the load on the motor and the design of the motor.

4. Induced Emf: The rotor of the induction motor is made of conducting bars. When the rotating magnetic field cuts across these bars, an emf is induced in them. This induced emf is known as the rotor emf.

5. Frequency of Induced Emf: The frequency of the induced emf in the rotor is directly proportional to the slip.

6. Slip and Frequency: As the slip increases, the frequency of the induced emf decreases. This is because the rotor is trying to "catch up" with the rotating magnetic field, resulting in a smaller relative speed between them.

7. Lower Frequency: Therefore, the frequency of the induced emf in the rotor is always lower than the supply frequency. It is important to note that the magnitude of the induced emf can be higher or lower depending on the slip and other factors.

Conclusion: The frequency of the induced emf in an induction motor is always less than the supply frequency. Therefore, the correct answer is C: Lesser than the supply frequency.

Explanation:

A capacitor start single phase induction motor is a type of motor that uses a capacitor to provide the starting torque. The power factor of a motor is a measure of how effectively it converts electrical power into mechanical power.

In the case of a capacitor start single phase induction motor, the power factor is determined by the combination of the motor's design and its load characteristics.

The power factor of a capacitor start single phase induction motor is typically between 0.6 lagging and 0.8 leading. This means that the motor consumes reactive power (lagging power) and may also generate reactive power (leading power) depending on the load.

In this case, the correct answer is option D: 0.6 lagging. This means that the motor consumes reactive power and has a lagging power factor of 0.6.

It's important to note that the power factor can vary depending on the specific motor design and load conditions. Therefore, the given answer is a generalization and may not apply to all capacitor start single phase induction motors.

All other parts except brushes and commutator are same in AC machine when outer looks are only taken in consideration. Commutator is used only in DC machine for providing mechanical rectification and not in AC machine.

• A slip ring is an electromechanical device that allows the transmission of power and electrical signals from a stationary to a rotating structure.
• It is used in case of AC supply, where  AC is required.
• With slip ring, the voltage in the external circuit varies like a sine wave and the current alternates the direction.
• Phosphor bronze is an alloy of copper, tin, and phosphorus. The tin increases the corrosion resistance and strength of the alloy. The phosphorus increases the wear resistance and stiffness of the alloy.
• Phosphorous bronze provide good electrical conductivity and low thermal conductivity
• Since slip ring induction motors used to accelerate heavy loads over a  long period of time and to withstand excessive heat phosphorous bronze slip is used.