Squirrel Cage Motor MCQ Quiz in தமிழ் - Objective Question with Answer for Squirrel Cage Motor - இலவச PDF ஐப் பதிவிறக்கவும்

Starting of 3ϕ squirrel cage induction motor

• Star/Delta starters are probably the most common reduced voltage starters.
• They are used in an attempt to reduce the start current applied to the motor during start as a means of reducing the disturbances and interference on the electrical supply.

Explanation:

The rotor current in a 3ϕ induction motor is given by:

\(I_2 = {V_2\over \sqrt{(X_2)^2+({R_2 \over s})^2}}\)

For stable operation of a 3ϕ induction motor, it must operate in a low slip region ( s << 1).

If s << 1, then \( {R_2 \over s}>> {X_2}\), therefore neglecting X₂

\(I_2 = {sV_2\over R_2}\)

From the above expression, the rotor current is directly proportional to the slip.

If the load (I₂) increases, then the slip also increases.

The rotor power factor is given by:

\(cosϕ = {R_2\over \sqrt{({R_2\over s})^2+X_2} }\)

The motor is operating in the low slip region, so neglecting X₂.

cosϕ is directly proportional to the slip (s) and the slip (s) is directly proportional to the load (I₂)

So, as the load increases power factor also increases. As the power factor increases power factor angle decreases.

Depending upon the type of rotor used the three-phase induction motor is classified as:

1. Squirrel Cage Induction Motor
2. Slip Ring Induction Motor or Wound Rotor Induction Motor or Phase Wound Induction Motor

 

Squirrel Cage Induction Motor:

• A 3 phase squirrel cage induction motor is a type of three-phase induction motor which functions based on the principle of electromagnetism.
• It is called a ‘squirrel cage’ motor because the rotor inside of it – known as a ‘squirrel cage rotor’ – looks like a squirrel cage.
• This rotor is a cylinder of steel laminations, with highly conductive metal (typically aluminum or copper) embedded into its surface.
• When an alternating current is run through the stator windings, a rotating magnetic field is produced.
• This induces a current in the rotor winding, which produces its own magnetic field.
• The interaction of the magnetic fields produced by the stator and rotor windings produces a torque on the squirrel cage rotor.
• One big advantage of a squirrel cage motor is how easily you can change its speed-torque characteristics.
• This can be done by simply adjusting the shape of the bars in the rotor.
• Squirrel cage induction motors are used a lot in the industry – as they are reliable, self-starting, and easy to adjust.

 

​Slip Ring Induction Motor or Wound Rotor Induction Motor or Phase Wound Induction Motor:

• A wound-rotor motor, also known as slip ring-rotor motor, is a type of induction motor where the rotor windings are connected through slip rings to external resistance.
• Adjusting the resistance allows control of the speed/torque characteristic of the motor.
• Wound-rotor motors can be started with low inrush current, by inserting high resistance into the rotor circuit; as the motor accelerates, the resistance can be decreased.
• The construction of the slip ring induction motor is quite different compared to other induction motors.
• Slip rings Induction motor provides some advantages like provides high starting torque, low starting current and it improves the power factor.
• We can add external variable resistance to the rotor of this type of motor. So, we are able to control the speed of this type of motor easily.
• A wound-rotor motor can be used in several forms of adjustable-speed drive.

Skewing:

• ​In the squirrel cage induction motor, the rotor slots in the lamination or rotor core are not made parallel to the rotor shaft. 
• A slight angle is maintained between the rotor slots and the rotor shaft due to some operational advantages. This is called rotor skewing.
• As the rotor slot of the induction motor is skewed through some angle so that the bars lie under alternate harmonic poles of the same polarity.​
• In squirrel-cage induction motors, the rotor slots are usually given slight skew in order to eliminate magnetic locking between the stator and rotor and to reduce magnetic helm.

​
The functions of skewed rotor slots in induction motor:

• The skewed rotor slot increases the length of the copper bar thereby increasing the resistance of the rotor bars hence starting torque of the machine can be improved and also the starting current is drawn by the machine can be reduced.
• It makes the air gap flux distribution uniform thereby reducing the harmonic torque produced by the machine.
• As harmonic torque is reduced, the cogging or magnetic locking phenomenon due to harmonic torque can also be reduced. Here the magnetic locking tendency occurs when rotor teeth remain directly under stator teeth thus they might be magnetically locked.
• And we can also eliminate a particular harmonic by selecting a proper skew angle.
• The humming noise can be reduced.
• To eliminate the nth harmonic the skew angle required is α = 3600/n electrical degrees = 720 / (n × P) mechanical degrees where P is the number of poles.

Squirrel cage induction motors do not require brushes for transferring power. Unlike brushed motors, induction motors are brushless machines, which is one of the advantages of this type of motor. The absence of brushes reduces the wear and maintenance required, making them more reliable and suitable for various applications. The power transfer in a squirrel cage induction motor occurs through electromagnetic induction between the stator and the rotor, without the need for physical contact like brushes.

Squirrel Cage Induction Motor:

• 3 phase squirrel cage induction motor is a type of 3-ϕ induction motor based on the principle of electromagnetism.
• It is called a ‘squirrel cage’ motor because the rotor inside of it looks like a squirrel cage and is known as a squirrel cage rotor.
• This rotor is a cylinder of steel laminations, with highly conductive metal (typically aluminum or copper) embedded into its surface.
• It is also known as short-circuited rotor type motor. 
• It doesn't need a slip ring and brush assembly. 
• Airgap between the stator and rotor is uniform.
• External resistance cannot be introduced in the rotor circuit.

​

• When an alternating current is run through the stator windings, a rotating magnetic field is produced.
• This induces a current in the rotor winding, that produces its own magnetic field.
• The interaction of the magnetic fields produced by the stator and rotor windings produces torque on the squirrel cage rotor.
• One big advantage of a squirrel cage motor is easily it can change its speed-torque characteristics.
• This can be done by simply adjusting the shape of the bars in the rotor.
• Squirrel cage induction motors are mainly used in the industry. Because they are reliable, self-starting, and easy to adjust.

 

Construction:

Various parts of the squirrel cage induction motor are

• Stator
• Rotor
• Fan
• Bearing

Stator:

• It consists of a 3 phase winding with a core and metal housing.
• Windings are such placed that they are electrically and mechanically 120o apart from in space.
• The winding is mounted on the laminated iron core to provide a low reluctance path for generated flux by AC currents.
• The air gap between the stator and rotor is uniform and symmetrical.

 

Rotor:

• It is the part of the motor which will be in a rotation to give mechanical output for a given amount of electrical energy.
• The rated output of the motor is mentioned on the nameplate in horsepower.
• It consists of a shaft, short-circuited copper/aluminum bars, and a core.
• The rotor is made of non-insulated copper conductors in rod or bar formats & short-circuited with end rings. So it is also called a short-circuited rotor.
• Therefore in the case of the squirrel cage induction motor, it is not possible to add any external resistance in series with the rotor circuit for starting purposes.
• The rotor core is laminated to avoid power loss from eddy currents and hysteresis.
• It does not require a slip ring and brush assembly. 
• Conductors are skewed to prevent cogging during starting operation and give a better transformation ratio between stator and rotor.

 

Fan: The fan is attached to the backside of the rotor to provide heat exchange, and hence it maintains the temperature of the motor under a limit.

Bearings: Bearings are provided as the base for rotor motion, and the bearings keep the smooth rotation of the motor.

Important Points 

Slip ring induction motor	Squirrel cage induction motor
It has a slip ring type rotor	It has a squirrel cage type rotor
Cylindrical laminated core with parallel slots and each slot consist one bar	The slots of the rotor are not parallel but are skewed
Construction is complicated	Construction is simple
We can add external resistance to the rotor	The rotor bar is permanently shorted at the end of the ring; thus, it is not possible to add any external resistance
The rotor resistance starter can be used	Rotor resistance starter cannot be used
Starting torque is high	Starting torque is low
Brushes are present	Brushes are absent
The air gap between the stator and rotor is non-uniform.	The air gap between the stator and rotor is uniform.
Frequent maintenance is required	Less maintenance required
The power factor is low	Power factor is high
Speed control is possible	Speed control is not possible

 

In this method, the number of poles of the motor is changed by changing the number of stator windings. The synchronous speed of an induction motor is inversely proportional to the number of poles. By increasing the number of poles, the synchronous speed decreases and vice versa.

Since the no. of poles is not given therefore, considering a standard 2-pole motor running at 60 Hz, which has a synchronous speed of 3600 RPM.

Explanation:

An AC motor's standard speed is determined by the frequency of the AC power supply and the number of poles in the motor. The formula to calculate the synchronous speed (in RPM) of an AC motor is:

Synchronous Speed (Ns) = (120 × Frequency) / Number of Poles

Here, the frequency is given as 60 Hz. The most common AC motors have 2, 4, 6, or 8 poles. Using the formula:

Ns = (120 × 60) / 2

Ns = 7200 / 2

Ns = 3600 RPM

This means a 2-pole motor running at 60 Hz will have a synchronous speed of 3600 RPM.

Explanation:

Applications of Three-Phase Squirrel Cage Induction Motors

Introduction: Three-phase squirrel cage induction motors are widely used in various industrial and commercial applications due to their robust construction, simplicity, and efficiency. These motors operate on the principle of electromagnetic induction and are characterized by their ability to handle heavy loads effectively, making them suitable for applications involving continuous operation and high reliability.

Correct Option Analysis:

The correct option is:

Option 3: I and III only correct

Explanation:

Let us analyze the statements provided:

• Statement I: Driving escalators and elevators.
• Statement III: Operating centrifugal pumps in industrial plants.

Statement I - Driving escalators and elevators: Three-phase squirrel cage induction motors are commonly used for driving escalators and elevators. These applications require high torque and reliability to ensure smooth and safe operation. The robust design and ability to handle variable loads make these motors an ideal choice for such applications. Additionally, these motors can be easily controlled using variable frequency drives (VFDs) to adjust the speed and torque as needed for elevators and escalators.

Statement III - Operating centrifugal pumps in industrial plants: Centrifugal pumps are widely used in industries for pumping fluids such as water, chemicals, and other liquids. Three-phase squirrel cage induction motors are preferred for driving these pumps due to their ability to deliver consistent performance under varying load conditions. Their high efficiency and reliability ensure continuous operation, which is critical in industrial settings.

Hence, both Statement I and Statement III correctly describe applications of three-phase squirrel cage induction motors, making Option 3 the correct choice.

Important Information

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

• Statement II - Powering household refrigerators: Household refrigerators typically use single-phase induction motors rather than three-phase squirrel cage induction motors. Single-phase motors are more suitable for low-power applications like refrigerators, where the required power and torque are comparatively lower than industrial applications.
• Statement IV - Used in electric vehicles for propulsion: Electric vehicles generally use specialized motors such as permanent magnet synchronous motors (PMSMs) or brushless DC motors (BLDCs) for propulsion due to their high efficiency, compact design, and ability to deliver precise torque control. Three-phase squirrel cage induction motors are not commonly used in electric vehicles.

Conclusion:

Three-phase squirrel cage induction motors are versatile and widely used in applications requiring high reliability, efficiency, and the ability to handle heavy loads. While they are ideal for driving escalators, elevators, and centrifugal pumps in industrial plants, they are not suitable for powering household refrigerators or electric vehicles. Understanding the specific requirements of different applications helps in selecting the appropriate motor type, ensuring optimal performance and efficiency.

The rotor conductors in a squirrel cage induction motor are short-circuited via end rings. This creates a low-resistance path for the rotor current, which allows the motor to start and run smoothly.

The rotor conductors are typically made of copper or aluminum bars that are embedded in the rotor laminations. The end rings are made of copper or aluminum and are placed at the ends of the rotor. The end rings short-circuit the rotor conductors, creating a continuous loop for the rotor current to flow through.

I_sc= 30 A

Starting current when it work at 300V

I_st = 30 × 3 = 90 A

Full load current \({{\rm{I}}_{\rm{f}}} = \frac{{\rm{P}}}{{\sqrt 3 {\rm{V\;cos\theta }} \times {\rm{\eta }}}}\)

\(= \frac{{15 \times {{10}^3}}}{{\sqrt 3 \times 300 \times 9 \times 0.8}}\) = 40.09 A

∴ Ratio of starting current to full load current

\(= \frac{{90}}{{40.09}} = 2.244\)