Single Phase Motor and Special Machines MCQ Quiz - Objective Question with Answer for Single Phase Motor and Special Machines - Download Free PDF

Voltage Application at Startup for Small Capacity Motors (Up to 3 Hp):

Correct Option: Full normal voltage can be applied at startup for small capacity motors (up to 3 Hp).

Explanation:

Small capacity motors, such as those up to 3 Hp, are designed to handle the application of full normal voltage at startup without adverse effects. These motors typically have robust designs and lower power ratings, which allow them to start directly with full voltage. This method of starting is known as "direct-on-line" (DOL) starting.

When full normal voltage is applied directly to the motor at startup, the motor achieves its rated speed almost instantly, resulting in quicker operation. This technique is suitable for smaller motors because they require relatively low starting current and do not pose a significant risk to the electrical system or the motor itself.

Advantages of Applying Full Normal Voltage:

• Simplicity: Direct application of full voltage eliminates the need for complex starting mechanisms, making it a simple and cost-effective method.
• Quick Startup: The motor reaches its rated speed rapidly, reducing the time needed for startup and enhancing operational efficiency.
• Compact Design: Small capacity motors are designed to withstand the initial surge of current during startup, ensuring reliable operation.
• Cost-Effective: No additional equipment, such as soft starters or variable frequency drives, is required, reducing overall costs.

Disadvantages:

• Although this method is suitable for small motors, applying full normal voltage to larger motors can cause excessive current draw, leading to potential damage to the motor and the electrical system.
• Direct-on-line starting may result in a mechanical shock to the connected equipment, which could lead to wear and tear over time.

Important Information

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

Option 1: Zero voltage.

This option is incorrect because applying zero voltage at startup would prevent the motor from starting entirely. Motors require a certain level of electrical energy to overcome inertia and initiate rotation. Without voltage, the motor remains stationary and cannot perform any operation.

Option 2: Double the normal voltage.

Applying double the normal voltage to a motor at startup is highly unsafe and impractical. Excessive voltage can lead to overheating, insulation breakdown, and permanent damage to the motor. Additionally, the electrical system may experience significant stress, causing potential hazards such as short circuits or equipment failure.

Option 3: Half the normal voltage.

This option involves applying reduced voltage at startup, which is not recommended for small capacity motors. While reduced voltage methods, such as soft starters or star-delta starters, are used for larger motors to limit inrush current, small motors are designed to handle full normal voltage without requiring such techniques. Applying half voltage may result in slower startup, lower torque, and inefficient operation.

Option 4: Full normal voltage.

This is the correct option. As explained above, small capacity motors (up to 3 Hp) are designed to start directly with full normal voltage, ensuring quick and efficient operation without compromising the motor's performance or safety.

Conclusion:

For small capacity motors, the application of full normal voltage at startup is the most suitable approach. It ensures simplicity, cost-effectiveness, and efficient operation. Other options, such as zero voltage, double voltage, or half voltage, are either impractical or unsuitable for these motors. Understanding the characteristics of small motors and their ability to handle direct-on-line starting is crucial for selecting the appropriate starting method.

Synchronous Reluctance Motor

Definition: A Synchronous Reluctance Motor (SynRM) is a type of electric motor that operates using the principle of reluctance torque. It does not require windings or permanent magnets on the rotor, making it a simple and cost-effective motor design. The rotor in a synchronous reluctance motor is specifically designed to have a high degree of anisotropy, meaning it has different magnetic reluctance in different directions.

Flux Barriers and Their Purpose:

In the rotor of a synchronous reluctance motor, "flux barriers" are strategically placed to guide the magnetic flux. These barriers are essentially non-magnetic regions that prevent the magnetic flux from passing through certain parts of the rotor, thereby creating a preferred path for the flux. This anisotropy is key to the operation of the motor, as it enables the generation of reluctance torque.

The flux barriers in a synchronous reluctance motor are typically filled with air or non-magnetic material (Option 4). These materials have very low magnetic permeability, which ensures that magnetic flux is restricted from passing through them. By doing so, the rotor creates distinct magnetic paths with varying reluctances, thereby improving the motor's efficiency and torque production.

Advantages of Filling Flux Barriers with Air or Non-Magnetic Material:

• Cost-Effectiveness: Air or non-magnetic materials are inexpensive compared to other materials like ferrite or copper, reducing the overall cost of the motor.
• Lightweight: Using air or lightweight non-magnetic materials helps keep the rotor's weight low, improving dynamic performance and reducing inertia.
• Improved Torque Characteristics: The anisotropic design created by these barriers enhances the motor's ability to generate reluctance torque efficiently.
• Thermal Stability: Non-magnetic materials generally exhibit good thermal properties, ensuring stable operation over a wide range of temperatures.

Applications: Synchronous reluctance motors are widely used in applications requiring high efficiency and cost-effectiveness, such as in industrial drives, pumps, and fans.

Analysis of Other Options

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

Option 1: Ferrite

Ferrite is a magnetic material commonly used in transformers, inductors, and magnetic cores. However, it is not suitable for filling flux barriers in a synchronous reluctance motor. The purpose of the flux barriers is to create regions of high reluctance (low permeability), which is not achievable with ferrite since it is a magnetic material. Using ferrite would defeat the purpose of creating anisotropy in the rotor, thereby reducing the motor's efficiency and torque production.

Option 2: Copper

Copper is an excellent conductor of electricity and is commonly used in windings and electrical connections. However, it is not appropriate for filling flux barriers in a synchronous reluctance motor. Copper does not have the non-magnetic properties required for creating high reluctance regions. Additionally, using copper would increase the motor's cost and weight unnecessarily, without providing any benefits to the flux barrier design.

Option 3: Laminated Steel

Laminated steel is used in the construction of motor cores to reduce eddy current losses. However, it is a magnetic material with high permeability, making it unsuitable for flux barriers in a synchronous reluctance motor. The purpose of the flux barriers is to restrict the magnetic flux, which cannot be achieved with laminated steel. Instead, laminated steel is typically used in the stator core of the motor, where magnetic flux needs to be guided effectively.

Conclusion:

The correct option for filling flux barriers in a synchronous reluctance motor is air or non-magnetic material (Option 4). This choice ensures the creation of high reluctance regions in the rotor, which is essential for efficient motor operation. The use of air or non-magnetic materials contributes to the motor's cost-effectiveness, lightweight design, and improved torque characteristics. Other options such as ferrite, copper, and laminated steel are not suitable for this purpose, as they do not meet the requirements for creating the necessary anisotropic properties in the rotor.

Torque Production in Reluctance Motors

Definition: Reluctance motors are a type of synchronous motor where the torque is produced due to the tendency of the rotor to align itself with the position of the minimum reluctance path in the magnetic field. These motors do not rely on permanent magnets or a DC excitation but instead utilize the inherent magnetic properties of the rotor material and its magnetic saliency.

Working Principle: The torque in reluctance motors is primarily generated due to magnetic saliency. Magnetic saliency refers to the difference in magnetic reluctance along different axes of the rotor. The rotor is designed with anisotropic properties, meaning it has different magnetic characteristics along different directions.

When a rotating magnetic field is produced by the stator, the rotor experiences a torque due to the alignment tendency with the axis of minimum reluctance. This alignment minimizes the reluctance of the magnetic circuit, and the rotor continues to rotate in sync with the stator field to maintain this alignment. This synchronous operation is the primary mechanism of torque generation in reluctance motors.

Correct Option Analysis:

The correct option is:

Option 3: Magnetic saliency

The torque in reluctance motors arises due to the difference in reluctance in the rotor's magnetic path. The rotor aligns itself to minimize the reluctance, and this alignment tendency is responsible for producing torque. This phenomenon, called magnetic saliency, is the defining characteristic of reluctance motors.

Advantages of Magnetic Saliency in Reluctance Motors:

• Eliminates the need for permanent magnets or external excitation, reducing manufacturing costs.
• Simple and robust construction of the rotor, with no windings or magnets.
• High efficiency due to minimal energy losses in the rotor.

Limitations:

• Requires precise rotor and stator design to achieve optimal performance.
• Lower power factor compared to permanent magnet synchronous motors (PMSMs).
• Higher torque ripple, which may require additional control strategies to minimize vibrations.

Important Information

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

Option 1: Permanent magnet alignment

This option is incorrect as reluctance motors do not use permanent magnets for torque generation. Unlike permanent magnet synchronous motors (PMSMs), the reluctance motor relies solely on the rotor's magnetic saliency and the stator's magnetic field for torque production. Permanent magnet alignment is not a factor in reluctance motors.

Option 2: Induction principles

This option is also incorrect. While induction motors use electromagnetic induction to produce torque, reluctance motors operate on a completely different principle. There is no induced current in the rotor of a reluctance motor; instead, the torque is generated due to the rotor's alignment with the stator's magnetic field based on magnetic saliency.

Option 4: Eddy current generation

This option is incorrect because eddy currents are not a significant source of torque in reluctance motors. Eddy currents are undesirable phenomena in most electrical machines as they lead to energy losses in the form of heat. Reluctance motors are designed to minimize eddy current losses, and torque generation is independent of eddy currents.

Option 5: No correct answer

This option is clearly incorrect as magnetic saliency (Option 3) is the definitive and scientifically accurate explanation for torque generation in reluctance motors.

Conclusion:

Torque in reluctance motors is primarily produced due to magnetic saliency, which is the tendency of the rotor to align itself with the minimum reluctance path in the magnetic field. This principle differentiates reluctance motors from other types of motors, such as permanent magnet synchronous motors and induction motors. Understanding the concept of magnetic saliency and its role in torque production is crucial for designing and utilizing reluctance motors effectively in various applications.

Universal Motors

Definition: A universal motor is a type of electric motor that can operate on either direct current (DC) or single-phase alternating current (AC). It is called a "universal" motor because of its ability to run on both types of current. These motors are known for their high speed and compact size, making them suitable for various portable and household applications.

Construction: Universal motors are typically series-wound motors, where the field winding is connected in series with the armature winding. This configuration provides high starting torque and allows the motor to operate efficiently on both DC and AC supplies.

Working Principle: The universal motor operates on the same principle as a DC series motor. When current flows through the field winding and the armature winding, it generates a magnetic field that interacts with the armature's magnetic field. This interaction produces torque, causing the rotor to rotate. The motor's ability to run on AC is due to the fact that the magnetic field in both the stator and rotor reverses simultaneously, ensuring consistent torque production.

Advantages:

• High speed: Universal motors can achieve very high speeds, often exceeding 10,000 RPM.
• Compact size: Their small size makes them ideal for portable and space-constrained applications.
• High starting torque: Universal motors can deliver significant torque at startup, making them suitable for applications requiring sudden acceleration.
• Versatility: The ability to operate on both AC and DC power sources adds to their flexibility.

Disadvantages:

• High noise levels: Due to their high-speed operation, universal motors can be noisy.
• Shorter lifespan: The brushes and commutator in universal motors experience wear and tear, leading to a shorter lifespan compared to brushless motors.
• Lower efficiency: These motors are less efficient than other types of motors, particularly at low speeds.

Applications:

• Vacuum cleaners
• Drills
• Mixers and blenders
• Hairdryers
• Portable tools

Correct Option Analysis:

The correct option is:

Option 2: Vacuum cleaners

Universal motors are widely used in vacuum cleaners due to their high speed, compact size, and ability to deliver high torque. These characteristics make them ideal for creating the suction required in vacuum cleaners. Additionally, their lightweight nature allows for the portability and ease of use that is essential in household appliances like vacuum cleaners.

Additional Information

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

Option 1: Refrigerators

Universal motors are not commonly used in refrigerators. Refrigerators typically use induction motors because they are more efficient, quieter, and have a longer lifespan. The compressor in a refrigerator operates at a relatively low speed, and induction motors are better suited for such applications.

Option 3: Ceiling fans

Ceiling fans generally use single-phase induction motors, specifically capacitor start and run motors. These motors are energy-efficient, quiet, and reliable, making them ideal for continuous operation over long periods. Universal motors are unsuitable for ceiling fans due to their high speed and noise levels.

Option 4: Air conditioners

Air conditioners use induction motors, particularly split-phase or three-phase induction motors, depending on the type and size of the unit. These motors are preferred for their efficiency, durability, and ability to handle the high loads associated with air conditioning systems. Universal motors are not used in air conditioners due to their lower efficiency and shorter lifespan.

Option 5: (Not provided in this context)

No additional option is mentioned, so no analysis is required for this point.

Conclusion:

Universal motors are highly versatile and are specifically suited for applications requiring high speed, compactness, and high starting torque. Among the given options, vacuum cleaners are the most appropriate application for universal motors due to their operational requirements. Other appliances like refrigerators, ceiling fans, and air conditioners rely on induction motors, which are more efficient and better suited to their functional needs.

Explanation:

Assertion (A) and Reason (R) Analysis:

Assertion (A): Detent torque is present in permanent magnet stepper motors even when unpowered.

Reason (R): Detent torque arises from the interaction between the rotor magnets and stator teeth.

Stepper motors are widely used in applications where precise positioning and control are required. Among the different types of stepper motors, permanent magnet stepper motors have a unique characteristic called "detent torque." Let us analyze the given statement and reason to determine the correctness of the options provided.

Detent Torque in Permanent Magnet Stepper Motors:

Detent torque is the torque exerted by a stepper motor when it is unpowered. It is a result of the magnetic interaction between the permanent magnets on the rotor and the ferromagnetic material of the stator teeth. This interaction creates a natural alignment between the rotor and the stator teeth, even when no electrical current is supplied to the motor. This alignment creates a resistive force that opposes any external attempt to move the rotor from its natural position, giving rise to detent torque.

Analysis of Assertion (A):

The statement asserts that detent torque is present in permanent magnet stepper motors even when unpowered. This is indeed true because detent torque is a characteristic feature of permanent magnet stepper motors, arising from the magnetic properties of the rotor and stator. It does not depend on the motor being powered or energized.

Analysis of Reason (R):

The reason states that detent torque arises from the interaction between the rotor magnets and stator teeth. This is also true. The rotor of a permanent magnet stepper motor is made of permanent magnets, while the stator has teeth made of ferromagnetic material. The magnetic interaction between these components creates the detent torque that holds the rotor in a specific position even when the motor is unpowered.

Relationship Between Assertion (A) and Reason (R):

Both the assertion and the reason are true, and the reason correctly explains the phenomenon described in the assertion. The detent torque is a direct consequence of the magnetic interaction between the rotor magnets and the stator teeth, as stated in the reason.

Correct Option Analysis:

The correct option is:

Option 4: Both A and R are true, and R explains A.

This option is correct because the assertion is a factual statement about detent torque in permanent magnet stepper motors, and the reason provides a valid and accurate explanation of the underlying cause of detent torque.

Additional Information

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

Option 1: Both A and R are true, but R does not explain A.

This option is incorrect because while both the assertion and the reason are true, the reason does indeed explain the assertion. Detent torque arises specifically due to the interaction between the rotor magnets and the stator teeth, as described in the reason.

Option 2: Both A and R are false.

This option is incorrect because both the assertion and the reason are true. Detent torque is a real phenomenon present in permanent magnet stepper motors, and it is caused by the magnetic interaction between the rotor and stator.

Option 3: A is false, but R is true.

This option is incorrect because the assertion is true. Detent torque is indeed present in permanent magnet stepper motors even when unpowered. Additionally, the reason is also true and accurately explains the assertion.

Conclusion:

Understanding the concept of detent torque in permanent magnet stepper motors is essential for applications requiring precise positioning and control. Detent torque is a result of the magnetic interaction between the rotor magnets and the stator teeth, and it is a defining characteristic of permanent magnet stepper motors. The correct option, therefore, is Option 4: Both A and R are true, and R explains A.

• The single-phase induction motor is not self-starting. Hence, it requires an auxiliary means or equipment to start the motor.
• Mechanical methods are impractical and, therefore the motor is started temporarily converting it into the two-phase motor.
• Commonly used starting methods for a ceiling fan is a permanent capacitor or single value capacitor motor.
• The permanent capacitor motor also has a cage rotor and the two windings named as main and auxiliary windings.
• The capacitor C is permanently connected in the circuit both at the starting and the running conditions.
• It is also called as a single value capacitor motor as the capacitor is always connected in the circuit.
• A paper capacitor is used in the permanent capacitor motor as an electrolytic capacitor cannot be used for the continuous running operation of the ceiling fan.
• The cost of the paper capacitor is higher, and the size is also large as compared to the electrolytic capacitor of the same ratings.

Unexcited single phase synchronous motor runs at constant speed equal to synchronous speed of revolving flux. They do not need a dc excitation for their rotors.

Capacitor start capacitor run 1-phase Induction motor:

• Capacitor ‘A’ is Running  Capacitor made up of oil type of low capacity. Manufactured for continuous duty cycle.
• Capacitor ‘B’ is Starting Capacitor made up of electrolytic type of high capacity. Manufactured for short duty cycle.
• Rating of starting capacitor is 10 to 15 times the running capacitor.
• Running winding also known as Main winding and starting winding as Auxilary winding.
• Initially, both capacitors are connected in parallel, the motor starts with high starting torque due to the phase difference created by starting winding and running winding.
• The starting torque is proportional to the sine of the angle between the two currents produced by the starting winding as well as running winding. Angle maintained at θ = 90° 

• In running condition After the motor has reached 75% full-load speed, the switch opens and only capacitor A (running capacitor) remains in the starting winding circuit.

Advatnages:

• High starting torque .
• High efficiency and power factor.
• Noise free operation.

Disadvantages:

High cost

Applications:

• Motors are used to start heavy loads where frequent starting is required.
• Pumps, compressors, refrigerator, conveyors and machine tools.

Important Points

Types of Single Phase Induction Motors:

• Split-phase motor
• Capacitor start motor
• Capacitor start capacitor run motor
• Shaded pole motor

Concept:

Step angle of the hybrid stepper motor is given by

\(β = {{(N_s -N_r)× 360°} \over (N_s× N_r)}\)

Where N_s = Number of stator teeth

N_r = Number of rotor teeth

Calculation:

Given that, N_r = 60

N_s = 8 × 6 = 48

∴ stepping angle can be calculated as 

\(β = {{(60 -48)× 360°} \over (60× 48)}\)

β = 1.5°

• Single phase induction motor is used in household refrigerator.

Brushless DC motor (BLDC) motor

• A brushless DC motor is an electronically commuted DC motor that does not have brushes like conventional DC motors.
• The controller provides pulses of current to the motor windings which control the speed and torque of this motor.
• The working principle of a BLDC motor is based on Lorentz force law.
• The Lorentz force law states that whenever a current-carrying conductor is placed in a magnetic field it experiences a force. As a consequence of reaction force, the magnet will experience an equal and opposite force.
• With the brushed motor, rotation is achieved by controlling the magnetic fields generated by the coils on the rotor, while the magnetic field generated by the stationary magnets remains fixed.
• To change the rotation speed, you change the voltage for the coils.
• With a BLDC motor, it is the permanent magnet that rotates ( N and S ); rotation is achieved by changing the direction of the magnetic fields generated by the surrounding stationary coils. To control the rotation, you adjust the magnitude and direction of the current into these coils.

Concept:

A hybrid stepper motor is a combination of variable reluctance and permanent magnet-type motors. The rotor of a hybrid stepper motor is axially magnetized like a permanent magnet stepper motor, and the stator is electromagnetically energized like a variable reluctance stepper motor

The step angle of the stepper motor is defined as the angle traversed by the motor in one step.

Step angle of the hybrid stepper motor is given by
\(\beta =\frac{{N_r -N_s}}{{N_s\times N_r}}\times 360^\circ \)

Where N_s = Number of stator teeth 

N_r = Number of rotor teeth

Calculation:

Given that, N_r = 50

N_s =  8 x 5 = 40

∴ stepping angle can be calculated as 

\(\beta =\frac{{50 -40}}{{50\times 40}}\times 360^\circ \)

β = 1.8°

Universal motor: It is used in high-speed vacuum cleaners, vacuum cleaners, drink and food mixers, domestic sewing machine, mixer-grinder.

D.C. shunt motor: It is a constant speed motor and is used in centrifugal pumps, fans, blowers

3-phase synchronous motor: It is used for constant speed applications where speed is independent of the load over the operating range of the motor

Split Phase Induction Motor:

• The Split Phase Motor is also known as a Resistance Start Motor.
• It has a single cage rotor, and its stator has two windings known as main winding and starting winding.
• Both the windings are displaced 90 degrees in space.
• The main winding has very low resistance and a high inductive reactance whereas the starting winding has high resistance and low inductive reactance. 

• A resistor is connected in series with the auxiliary winding.
• The current in the two windings is not equal as a result the rotating field is not uniform.
• Hence, the starting torque is small, of the order of 1.5 to 2 times of the start, running torque.
• At the starting of the motor both the windings are connected in parallel.

The phasor diagram of the Split Phase Induction Motor is shown below.

• The current in the main winding (IM) lag behind the supply voltage V almost by the 90∘  
• The current in the auxiliary winding IA is approximately in phase with the line voltage.
• Thus, there exists a time difference between the currents of the two windings.
• The time phase difference ϕ is not 90 degrees, but of the order of 30 degrees.
• This phase difference is enough to produce a rotating magnetic field.

Concept:

The synchronous speed is given by

\({N_s} = \frac{{120f}}{P}\)

Where,

f is the supply frequency in Hz or C/s

P is the number of poles

The induction motor rotates at a speed (Nr) close but less than the synchronous speed.

Slip of an induction motor is given by,

\(s = \frac{{{N_s} - {N_r}}}{{{N_s}}}\)

Where,

Ns is the synchronous speed

and Nr is the rotor speed

Rotor current frequency for forward slip (fr) = sf

Rotor current frequency for backward slip (f_r') = (2 - s) f

The slip is negative when the rotor speed is more than the synchronous speed of the rotor field and is in the same direction.

Calculation:

Given - 

Number of poles (P) = 6

Frequency (f) = 50 Hz

Speed of the induction motor, N_r = 900 rpm

Speed of the rotating magnetic field is, 

\({N_s} = \frac{{120f}}{P} = \frac{{120 \times 50}}{{6}} = 1000\;rpm\)

Slip, \(s = \frac{{1000 - 900}}{{1000}} = 0.1 = 10\%\)

The two rotor current frequencies are

\(\begin{array}{l} sf = 0.1 \times 50 = 5\;Hz\\ \left( {2 - s} \right)f = 1.9 \times 50 = 95\;Hz \end{array}\)