Slip ring Induction Motor, How it works?

Summary

The video script delves into the workings of induction motors, highlighting the difference between squirrel cage and slip ring rotor designs. It explains how squirrel cage motors, with their low starting torque, are unsuitable for applications requiring high initial force, such as in hoists. In contrast, slip ring induction motors are favored for their ability to generate high starting torque, achieved by reducing the phase difference between induced EMF and current through external resistance. This makes them ideal for heavy-duty industrial applications like elevators and cranes.

Takeaways

• 🌟 Induction motors are a staple in the industrial world for many decades due to their reliability and performance.
• 🔧 There are two types of rotors in induction motors: slip ring rotors, used in lifting hoists for their high starting torque, and squirrel cage rotors, which are simpler and used in most other applications.
• 🚫 Squirrel cage induction motors have a low starting torque, which can be problematic for certain applications that require a high initial force.
• 🔄 The working of an induction motor involves a rotating magnetic field (RMF) produced by the stator winding, which induces an electromotive force (EMF) in the rotor bars, leading to motion.
• 🔧 Faraday's law of electromagnetic induction and Lorentz's law are fundamental to understanding how induction motors generate motion.
• 🌀 The concept of inductance is crucial in induction motors, as it influences the phase difference between the applied voltage and the current, affecting motor performance.
• 🔌 In a simple circuit with a resistor and an inductor, the phase difference between the applied voltage and the current is due to inductive reactance, which increases with frequency.
• 🔄 The rotor's resistance and inductive reactance cause a phase-lag phenomenon, which affects the torque produced by the motor.
• 🔑 Induction motors produce maximum torque when the current induced in the rotor is near the maximum magnetic flux, aligning with the 'maximum torque condition'.
• 🔄 At startup, the high frequency of the induced EMF in the rotor due to zero speed results in a large phase difference, leading to the low starting torque of normal induction motors.
• 🔄 Slip ring induction motors overcome the low starting torque issue by using external resistance to reduce the phase difference between the induced EMF and the current, allowing for high torque even at startup.
• 📈 The use of external resistance in slip ring motors at startup helps the current induced to approach the maximum torque condition, resulting in higher starting torque compared to squirrel cage motors.
• 🏗️ Slip ring induction motors are particularly important in applications requiring high starting torque, such as elevators, cranes, hoists, and industrial machinery like printing presses.

Q & A

What is the main difference between squirrel cage and slip ring rotors in induction motors?

The main difference lies in the rotor construction. Squirrel cage rotors are simpler, with bars short-circuited by end rings, while slip ring rotors use three windings and can incorporate external resistance to reduce phase difference and produce high starting torque.

Why do induction motors with squirrel cage rotors produce low starting torque?

At startup, the rotor speed is zero, leading to a high rate of magnetic field cutting through the rotor, which results in a high frequency of induced EMF and a large phase difference, causing low starting torque.

How does the slip ring induction motor overcome the issue of low starting torque?

The slip ring induction motor uses external resistance connected to the rotor windings via slip rings. By increasing the resistance at startup, it reduces the phase difference, allowing the current induced to approach the maximum torque condition.

What is the significance of inductance in the operation of an induction motor?

Inductance, along with resistance in the rotor, causes a phase difference between the applied voltage and the current. This phase difference affects the torque produced by the motor, with maximum torque occurring when the induced current aligns with the maximum magnetic flux.

How does the slip ring induction motor adjust the phase difference during startup?

By increasing the external resistance connected to the rotor windings at startup, the slip ring induction motor reduces the phase difference between the induced EMF and the current, enabling it to produce high torque from the beginning.

What is the role of Faraday's law of electromagnetic induction in the operation of an induction motor?

Faraday's law states that an electromotive force is induced in a conductor when it cuts through a magnetic field. In an induction motor, this law is responsible for inducing EMF in the rotor bars, which in turn generates the current that causes the rotor to turn.

What is Lorentz's law, and how does it relate to the operation of an induction motor?

Lorentz's law describes the force experienced by a current-carrying conductor placed in a magnetic field. In an induction motor, this force acts on the current in the rotor bars, causing the rotor to rotate.

Why is the slip ring induction motor preferred for applications requiring high starting torque?

Slip ring induction motors are preferred because they can produce high starting torque by reducing the phase difference between the induced EMF and the current through the use of external resistance, making them suitable for applications like elevators, cranes, and hoists.

What are some advantages of slip ring induction motors besides high starting torque?

Besides high starting torque, slip ring induction motors offer the ability to control speed and torque more precisely through external resistance adjustments, making them versatile for various industrial applications.

What are the disadvantages of slip ring induction motors compared to squirrel cage motors?

While the script does not explicitly mention the disadvantages, slip ring induction motors are generally more complex, more expensive to manufacture and maintain due to the presence of slip rings and brushes, and may have lower efficiency due to additional resistance.

How does the concept of maximum torque condition relate to the performance of an induction motor?

The maximum torque condition is when the induced current in the rotor is near the maximum magnetic flux. An induction motor produces the most torque under this condition, which is crucial for understanding motor performance and optimizing its operation.

Outlines

00:00

🔧 Induction Motor Rotor Designs and Starting Torque Issues

This paragraph introduces the two types of rotors used in induction motors: the slip ring rotor and the squirrel cage rotor. It explains that squirrel cage motors, which are common in many applications, produce a low starting torque that can be problematic for certain uses. In contrast, slip ring induction motors are highlighted as a solution to this issue, as they generate high starting torque. The video will explore why these two designs exist and how they function. The explanation begins with the working principle of a squirrel cage induction motor, detailing the generation of a rotating magnetic field and the induction of electromotive force (EMF) in the rotor bars. It also touches on the concept of inductance, phase difference, and how these factors affect the torque produced by the motor, particularly at startup when the rotor speed is zero and the phase difference is high, leading to low starting torque.

05:05

🔄 Slip Ring Induction Motors: Overcoming Torque Limitations

The second paragraph delves into the slip ring induction motor's unique rotor construction, which includes three windings instead of bars, aimed at reducing the phase difference and thus increasing the starting torque. It explains how the use of external resistance connected to the rotor windings via slip rings can decrease the phase difference between the induced EMF and the current flow. By increasing the resistance at startup, the slip ring motor can produce high torque from the beginning. The paragraph also compares the starting torque of slip ring motors to squirrel cage motors through graphs, demonstrating the superior performance of slip ring motors. Additionally, it mentions the advantages of slip ring induction motors, their applications in heavy-duty industrial equipment like elevators, cranes, hoists, and printing presses, and acknowledges their disadvantages. The paragraph concludes with a call to support the creators on Patreon and to subscribe to their channel.

Keywords

💡Induction motors

Induction motors are a type of electric motor that use electromagnetic induction to produce a rotating magnetic field, which in turn causes the rotor to turn. They are crucial in the industrial world and are the main focus of the video's discussion. The script mentions two types of induction motors, the squirrel cage and the slip ring induction motors, highlighting their different applications and designs.

💡Slip ring rotor

A slip ring rotor is a type of rotor used in induction motors, specifically in applications requiring high starting torque, such as lifting hoists. The slip ring allows for the connection of external circuits to the rotor, enabling the adjustment of the rotor's electrical properties. The script explains that slip ring induction motors are used to overcome the low starting torque issue found in squirrel cage motors.

💡Squirrel cage rotor

The squirrel cage rotor is a simpler design used in most induction motors, consisting of conductive bars short-circuited by end rings. This design produces a low starting torque, which can be problematic for certain applications. The script contrasts the squirrel cage rotor with the slip ring rotor to illustrate the differences in torque production.

💡Starting torque

Starting torque refers to the torque produced by a motor at the moment it starts. It is a critical parameter for applications that require immediate and significant force upon startup. The script explains that squirrel cage motors produce low starting torque, while slip ring motors are designed to produce high starting torque, making them suitable for applications like hoists.

💡Rotating magnetic field (RMF)

A rotating magnetic field is created when a three-phase AC supply is connected to the stator winding of an induction motor. This field interacts with the rotor, inducing an electromotive force (EMF) in the rotor bars. The script describes how the RMF is essential for the operation of induction motors and its role in generating torque.

💡Faraday's law of electromagnetic induction

Faraday's law states that a change in the magnetic field within a coil of wire induces an electromotive force (EMF) in the coil. In the context of the video, this law explains how an EMF is induced in the rotor bars of an induction motor when the RMF cuts through them, leading to the generation of current and ultimately torque.

💡Lorentz's law

Lorentz's law describes the force experienced by a current-carrying conductor placed in a magnetic field. According to the script, this law is fundamental to understanding how the forces on the different rotor bars collectively make the rotor turn in an induction motor.

💡Inductance

Inductance is a property of an electrical conductor, such as a coil, that opposes changes in the electric current flowing through it. The script uses the concept of inductance to explain the phase difference between the applied voltage and the current in an induction motor, which affects the motor's torque production.

💡Inductive reactance

Inductive reactance is the opposition to the flow of alternating current (AC) in an inductor. The script mentions that inductive reactance, along with resistance, causes a phase difference in the rotor, affecting the torque production of an induction motor. The slip ring motor uses external resistance to reduce this phase difference.

💡Maximum torque condition

The maximum torque condition in an induction motor occurs when the maximum current induced on the rotor is near to the maximum magnetic flux. The script explains that this condition is crucial for achieving the highest possible torque and that slip ring motors are designed to meet this condition more effectively than squirrel cage motors.

💡External resistance

External resistance is used in slip ring induction motors to adjust the phase difference between the induced EMF and the current in the rotor. By increasing the resistance, the script explains, the phase difference can be reduced, allowing the motor to produce high torque at startup, which is a key advantage of slip ring motors.

Highlights

Induction motors have been a staple in the industrial world for many decades.

Two different rotor designs exist for induction motors: slip ring rotor and squirrel cage rotor.

Slip ring induction motors are used in lifting hoists due to their high starting torque.

Squirrel cage motors produce very low starting torque, causing problems in some applications.

The working principles of squirrel cage and slip ring induction motors are the same, but rotor construction differs.

Squirrel cage rotors have bars short-circuited by end rings, while slip ring rotors use three windings.

A rotating magnetic field (RMF) is produced in the air gap between stator and rotor when AC supply is connected.

Faraday's law of electromagnetic induction causes EMF to be induced in the rotor bars.

Lorentz's law explains the force experienced by current-carrying conductors in a magnetic field.

Inductance is a key concept in understanding induction motors, involving resistors and inductors in series with AC voltage.

Inductive reactance causes a phase difference between applied voltage and current in an AC circuit.

Higher frequency results in greater inductive reactance and phase difference.

An induction motor produces maximum torque when rotor current is near the maximum magnetic flux.

At startup, the high frequency of induced EMF in a squirrel cage motor causes a high phase difference and low starting torque.

Slip ring induction motors use external resistance to reduce phase difference and produce high starting torque.

Slip ring motors are advantageous in applications like elevators, cranes, hoists, and printing presses.

The video explores the differences between squirrel cage and slip ring rotors in induction motors and their practical applications.

Transcripts

- [Instructor] Induction motors have been ruling

the industrial world for many decades.

In the induction motors used in lifting hoist,

you will see a type of rotor called a slip ring rotor

whereas in most of the other applications,

you will see a simpler squirrel cage type of rotor.

Why are there are two different designs

of rotor construction for induction motors?

We will explore this in the video.

Normal induction motors or squirrel cage type motors

produce a very low starting torque

and for some applications,

this low starting torque will cause huge problems.

It is in these circumstances that slip ring induction motors

are used as they produce high starting torque.

Let's see this in detail.

First, let's have a look at the way

a squirrel cage induction motor works.

When a three-phase AC supply is connected

to the stator winding, it produces a rotating magnetic field

in the air gap between the stator and rotor.

This RMF cuts the rotor bars.

According to Faraday's law of electromagnetic induction,

an electromotive force is induced in the bars.

Because the rotor bars are short circuited by the end rings,

this induced EMF generates a current

to flow through the rotor bars.

According to Lorentz's law,

when a current carrying conductor is placed

in a magnetic field, it will experience force.

You can see the force distribution on the different bars

at a particular moment in time.

These collective forces make the rotor turn.

This explanation of the way an induction motor works

won't be complete without an understanding

of the concept of inductance.

To understand what inductance is,

let's consider a simple circuit.

The circuit is a combination of a resistor

and an inductor in series which is connected

to the AC sinusoidal voltage.

Let us connect a phase angle meter to circuit

to measure the phase difference

between the applied voltage and the current.

You can see that the current flowing through the circuit

is not in phase with the applied voltage.

This is because of the presence

of an inductive reactance in the circuit.

The higher the frequency of the electricity,

the greater will be the inductive reactance

and the phase difference.

A higher resistance value reduces this phase difference.

The exact same thing is also happening in the rotor.

The rotor is a combination of resistance

and inductive reactance.

Due to the same phase-lag phenomenon,

if the maximum EMF is on one bar,

then the maximum current will be on another bar.

Now, here is one interesting fact about induction motors.

An induction motor produces maximum torque

when the maximum current induced on the rotor

is near to the maximum magnetic flux.

This fact is clear from the comparison of these two visuals.

Let us call it the maximum torque condition.

Throughout this video, please keep this fact in mind.

As the current induced does not meet

the maximum torque condition,

this will definitely reduce the amount

of torque produced by the induction motor.

This phase difference will be high as the motor starts.

Let's see why.

At startup, the rotor speed is zero.

Due to this, the magnetic field will cut through the rotor

at a very high rate and the frequency of the induced EMF

will be high.

This leads to high phase difference

which causes the very low starting torque

of a normal induction motor.

To overcome this problem,

the slip ring induction motor comes into the picture.

The working principles and stator construction

of the slip ring induction motor are exactly the same

as that of a squirrel cage motor.

However, the rotor construction of the slip ring motor

is quite interesting.

Instead of bars, in this motor three windings are used.

This construction of the rotor

is aimed at reducing the phase difference.

Let's see how squirrel cage rotor does it.

For ease of understanding,

instead of the current 24 slots winding,

let's use a 12 slot winding.

Here again, the RMF induces EMF across the terminals

of the windings.

Let's join the winding ends in a star connection

and again assume that the inductive reactance is zero.

The current flow established in the winding

will be as shown.

However, in practice, the current flow

will be lagging behind the induced EMF.

Here again, the maximum torque condition is not met.

In the slip ring induction motor,

there is an option to reduce this EMF

current phase difference by use of external resistance.

The other ends of the coils are connected

to an external resistance via the slip rings.

We saw in the simple circuit that by increasing

the resistance value, we can decrease the phase difference.

As the slip ring induction motor starts,

the external resistance value is increased.

This reduces the phase difference angle

and the current induced

approaches the maximum torque condition.

This way, slip ring induction motors

will be able to produce high torque

even as they are starting.

These graphs clearly show the higher starting torque

produced by slip ring motors in comparison

to squirrel cage motors.

Apart from the high starting torque,

it also has some other advantages

and although slip ring induction motors

have some disadvantages, they play a very important role

in elevators, cranes, hoists, and in industrial uses,

such as printing presses.

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