Squirrel & Slip Ring Induction Motor Starting Methods

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Questions and Answers

What is the main cause of the crawling phenomenon in motors?

Excessive load on the motor

Improper alignment of the rotor windings

Slot harmonics interaction (correct)

Uneven air gap between stator and rotor

Which factor contributes to cogging phenomenon in motors using permanent magnets?

Imperfect alignment of the rotor magnets (correct)

Too high operating frequency

Excessive rotor speed

Inadequate stator winding

What is a key feature of doubly fed induction machines (DFIM)?

Wound rotor with direct connection to the grid

Single-phase winding for output

Two separate three-phase winding sets (correct)

Fixed speed operation without variable frequency input

How do doubly fed generators manage changes in wind turbine speed?

By feeding adjustable frequency AC power to the field windings Signup and view all the answers

What is the main advantage of using a doubly fed induction machine in wind turbines?

Reduced mechanical stress during wind gusts Signup and view all the answers

What is a primary disadvantage of the Stator Resistance Starting method?

It decreases the starting torque and increases acceleration time. Signup and view all the answers

Which starting method applies full line voltage to the motor as soon as it is started?

Direct On-Line Starting Signup and view all the answers

What happens during the Autotransformer Starting method when the motor achieves about 80% of its rated speed?

Full line voltage is applied across the motor terminals. Signup and view all the answers

What is the primary purpose of using external resistance in the Stator Resistance Starting method?

To limit the starting current and reduce voltage. Signup and view all the answers

Which of the following starting methods is suitable for motors up to 10 kW?

Direct On-Line Starting Signup and view all the answers

Which starting method involves a gradual removal of resistance from the circuit?

Stator Resistance Starting Signup and view all the answers

Which starting method utilizes a transformer to adjust the voltage supplied to the motor?

Autotransformer Starting Signup and view all the answers

What characteristic of the squirrel cage induction motor impacts the effectiveness of Direct On-Line starting?

Ability to handle high starting currents Signup and view all the answers

What is the main advantage of autotransformer starting?

Low power loss Signup and view all the answers

In which starting method does the motor initially connect in star and later switch to delta?

Star-delta starting Signup and view all the answers

What happens to the voltage in the star connection during the star-delta starting method?

It receives V/?3 volts Signup and view all the answers

Which starting method is primarily used for slip ring induction motors in industrial applications?

Rotor-resistance starting Signup and view all the answers

What is the purpose of using external resistance in rotor-resistance starting?

To reduce starting current Signup and view all the answers

Which component connects the rotor windings to external resistances in a slip ring induction motor?

Slip rings Signup and view all the answers

What type of motor is a slip ring induction motor categorized as?

Asynchronous motor Signup and view all the answers

How does the use of slip rings impact the rotor windings in a slip ring induction motor?

Allows for control over external resistance Signup and view all the answers

What happens to starting torque in the star-delta starting method due to reduced voltage during starting?

Decreases significantly Signup and view all the answers

What physical principle primarily explains the operation of a slip ring induction motor?

Faraday’s law of electromagnetic induction Signup and view all the answers

What is the primary function of slip rings in induction motors?

To improve efficiency by eliminating dangling wires. Signup and view all the answers

In the context of slip ring induction motors, what happens when external resistance is added?

It improves the power factor and starting torque. Signup and view all the answers

What is the expression for external resistance R when slip at pull-out torque Smax is given?

$R = r(S’max/Smax - 1)$ Signup and view all the answers

Which of the following describes a disadvantage of slip ring induction motors compared to squirrel cage motors?

Higher maintenance costs due to complex construction. Signup and view all the answers

In which scenario does regenerative braking occur in slip ring induction motors?

When the speed of the motor exceeds synchronous speed. Signup and view all the answers

Which method of speed control involves adjusting the duty cycle of the current flow?

Using thyristor circuits. Signup and view all the answers

How does adding external resistance during motor startup affect the current drawn?

It allows a higher current draw than normal. Signup and view all the answers

Which characteristic distinguishes a slip ring motor from a squirrel cage motor?

It has an external resistance circuit. Signup and view all the answers

Which type of braking utilizes the motor acting as a generator?

Regenerative braking. Signup and view all the answers

What defines the maximum torque produced by a slip ring induction motor?

The slip value at pull-out torque. Signup and view all the answers

What occurs to the torque of a motor during plugging?

The torque reverses. Signup and view all the answers

What is a primary disadvantage of dynamic braking in motors?

Power is wasted. Signup and view all the answers

What happens during single phasing in a three-phase induction motor?

Uneven current distribution occurs. Signup and view all the answers

Which of the following is NOT a cause of single phasing?

Low voltage supply Signup and view all the answers

What type of motor is particularly known for high starting torque?

High Torque Induction Motor Signup and view all the answers

What is the main effect of uneven current distribution due to single phasing?

Reduced efficiency and potential motor damage Signup and view all the answers

During crawling, what primarily causes the induction motor to show odd behavior at low speeds?

Asymmetry in the magnetic field Signup and view all the answers

What is typically the characteristic of cogging in an induction motor?

Jerky movements at startup. Signup and view all the answers

What condition is likely to lead to crawling in an induction motor?

Low speed operation Signup and view all the answers

To avoid cogging, what should be ensured regarding the slots of rotor and stator?

Different number of slots between rotor and stator. Signup and view all the answers

Study Notes

Squirrel Cage Induction Motor Starting Methods

Direct On-Line (D.O.L.) Starting: Connects the motor directly to the three-phase supply. Suitable up to 10kW, but draws high starting current (4-7 times rated).
Stator Resistance Starting: Reduces starting current by adding external resistance in series with each stator phase. Resistance is gradually removed as motor accelerates.
Autotransformer Starting: Uses an autotransformer to reduce starting voltage, limiting starting current. Suitable for larger motors (over 25 hp).
Star-Delta Starting: Starts the motor connected in star configuration, then switches to delta configuration when sufficient speed is reached. Suitable for medium-sized motors (up to 25 hp).

Slip Ring Induction Motor Starting Methods

Direct On-Line (D.O.L.) Starting: Same as for squirrel cage induction motor.
Stator Resistance Starting: Same as for squirrel cage induction motor.
Autotransformer Starting: Same as for squirrel cage induction motor.
Rotor Resistance Starting: Uses external variable resistors connected to the rotor circuit via slip rings. Resistance is reduced as the motor accelerates. This method is most commonly used for slip ring induction motors.

Slip Ring Induction Motor and Its Working

Rotor Construction: Wound with multiple windings connected to external resistance through slip rings, providing better control over torque and speed.
Working Principle: Based on Faraday’s law of electromagnetic induction; stator magnetic field induces current in the rotor windings, creating torque.
Slip Rings: Act as electromechanical interfaces, transmitting power and electrical signals from stationary to rotating components.
Slip: The difference between synchronous speed and rotor speed.
Torque Control: External resistance in the rotor circuit is adjusted to control torque and speed.

Difference Between Squirrel Cage and Slip Ring Induction Motor

Rotor Type: Squirrel cage has a simple, robust rotor; Slip ring has a wound rotor with slip rings and brushes.
Starting Torque: Slip ring motors have higher starting torque.
Starting Current: Slip ring motors have lower starting current.
Efficiency: Squirrel cage motors are typically more efficient.
Maintenance: Squirrel cage motors require less maintenance due to their simpler construction.
Cost: Squirrel cage motors are generally less expensive.

Electrical Braking Types

Regenerative Braking: Motor operates as a generator when speed exceeds synchronous speed, feeding energy back to the supply.
Plugging Type Braking: Reverses the supply terminals, creating a braking torque.
Dynamic Braking: Disconnects the motor from supply and connects it across a resistor, causing the motor to act as a generator and dissipate energy in the resistor.

High Torque IM (Induction Motor)

Induction motors are capable of producing high starting torque, especially when designed specifically for that purpose.

Single Phasing

Occurs when one phase of a three-phase supply loses power.
Causes include blown fuses, broken or loose connections, and problems with contactors or relays.
Effects: Uneven current distribution in the motor windings, leading to potential damage, reduced performance, noise, and vibration.

Crawling and Cogging of Induction Motor

Crawling: The motor runs at a very low speed due to uneven torque production caused by harmonics. It typically occurs at 1/7th of synchronous speed.
Cogging: The motor experiences jerky movements at startup due to the alignment of stator and rotor slots.

Doubly-Fed Induction Machines

These machines have independent excitation for both the stator and rotor windings, allowing for more flexible control of speed and torque.
Used in applications like wind turbines and adjustable speed drives.

Crawling Phenomenon

Occurs at low speeds when the motor operates below a certain percentage of its rated speed.
High core saturation causes uneven torque production leading to unusual motion.
Insufficient voltage impedes proper magnetic field setup causing inefficiency.
Interactions between stator and rotor slot harmonics lead to uneven torque production, resulting in crawling.

Cogging Phenomenon

Primarily occurs in motors using permanent magnets due to imperfect alignment of the magnets, creating resistance during startup.
Rotor design plays a crucial role, particularly in permanent magnet motors. When the number of rotor slots equals the number of stator slots, they align in a way that the stator slots get locked with the rotor slots, causing cogging.
Uneven air gaps can contribute to cogging.

Doubly Fed Induction Machines (DFIM)

DFIMs are wound rotor induction machines similar to traditional squirrel-cage machines.
DFIMs consist of two three-phase winding sets: a stator and a rotor.
By feeding adjustable frequency AC power to the field windings, the magnetic field can be adjusted, allowing for variations in motor or generator speed.

Doubly Fed Electrical Generators

Similar to AC generators but possess additional features allowing operation at speeds slightly above or below their synchronous speed.
This is advantageous for large variable speed wind turbines because wind speed can fluctuate.
A sudden wind gust on a wind turbine causes the blades to accelerate. However, a synchronous generator is locked to the grid frequency and cannot speed up leading to high forces in the mechanism.
Allowing the turbine to speed up immediately when hit by a wind gust reduces stress and helps convert wind energy into useful electricity.
One solution is to use an inverter to convert the generated frequency to DC and then back to AC at the desired output frequency. However, this method is expensive for large wind turbines.
Doubly Fed Generators are another solution.
Instead of a DC fed field winding and an armature winding, there are two three-phase windings, one stationary and one rotating, both connected to equipment outside the generator.
One winding is directly connected to the output and produces 3-phase AC power at the grid frequency.
The second winding is connected to 3-phase AC power at variable frequency.
This winding compensates for changes in turbine speed by adjusting frequency and phase using an AC to DC to AC converter.
The converter is bidirectional, allowing power to flow in either direction.

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Description

This quiz explores the various starting methods for squirrel cage and slip ring induction motors, including Direct On-Line, Stator Resistance, Autotransformer, and Star-Delta starting techniques. Understand the advantages and suitable applications of each method for different motor sizes. Test your knowledge on electrical engineering concepts related to motor starting methods.