Induction Motor Control and Braking Systems
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Questions and Answers

What component is used to minimize the ripples in the D.C output for the induction motor?

  • Thyristor bridge
  • L.C filter (correct)
  • Transformer
  • Slip ring

A.C must be left connected while feeding D.C to the stator of the induction motor.

False (B)

What is the purpose of the braking thyristor controller in the system?

To control the dynamic braking of the induction motor.

The DC output from the thyristor bridge is fed to the _______ of the induction motor.

<p>stator winding</p> Signup and view all the answers

Match the components with their functions:

<p>Thyristor bridge = Rectification of A.C to D.C L.C Filter = Filtering D.C output Dynamic braking controller = Regulating braking torque Slip ring = Variable resistance</p> Signup and view all the answers

Which of the following statements about the 3-phase slip ring induction motor is correct?

<p>It has the ability to control rotor resistance. (A)</p> Signup and view all the answers

What happens to the input A.C voltage when it is fed into the thyristor converter?

<p>It is stepped down to a lower voltage.</p> Signup and view all the answers

What is the value of the initial braking current calculated?

<p>107.44 A (B)</p> Signup and view all the answers

The thyristor converter must have a higher voltage rating when the input A.C voltage is stepped down.

<p>False (B)</p> Signup and view all the answers

What is the formula used to calculate the initial braking torque?

<p>T_p = (3 * I_braking * R) / w</p> Signup and view all the answers

The speed difference $N_{speed}$ when the speed is 940 r.p.m. is equal to ___ r.p.m.

<p>60</p> Signup and view all the answers

Match the following variables with their respective values:

<p>Pole pairs (P) = 3 Speed (N) = 1000 r.p.m. Initial braking torque (T_p) = 103.04 N-m Angular velocity (w) = 104.72 rad/sec</p> Signup and view all the answers

What occurs during regenerative braking?

<p>The induction motor operates as a generator (A)</p> Signup and view all the answers

During rheostatic braking, the stator winding is connected to an AC supply.

<p>False (B)</p> Signup and view all the answers

What happens when the rotor speed falls to synchronous speed during regenerative braking?

<p>Regenerative braking comes to an end.</p> Signup and view all the answers

Rheostatic braking involves disconnecting the _____ winding from the AC supply.

<p>stator</p> Signup and view all the answers

Match the braking methods with their descriptions:

<p>Rheostatic braking = Uses a DC source for stator winding Regenerative braking = Rotor runs above synchronous speed Dynamic braking = Involves controlling speed through resistors Mechanical braking = Involves physical friction to slow down</p> Signup and view all the answers

What is the relationship between rotor speed and synchronous speed during regenerative braking?

<p>The rotor speed must be greater than synchronous speed (C)</p> Signup and view all the answers

The power returned to the supply during regenerative braking is constant, regardless of rotor speed.

<p>False (B)</p> Signup and view all the answers

What does negative slip indicate in the context of an induction machine?

<p>The induction machine operates as an induction generator.</p> Signup and view all the answers

During regenerative braking, when the load forces the motor to run above synchronous speed, it is in the _____ quadrant.

<p>2nd</p> Signup and view all the answers

Which statement describes a benefit of using a current source inverter (CSI) over a voltage source inverter (VSI)?

<p>CSIs have inherently better protection against short circuits. (D)</p> Signup and view all the answers

CSIs have a wider speed range compared to VSIs.

<p>False (B)</p> Signup and view all the answers

What is the main reason the torque-slip relationship of a 3-phase induction motor needs to maintain constant air-gap flux?

<p>To provide high torque throughout the control range.</p> Signup and view all the answers

The ______ provides the rated motor torque and speed for an induction motor running at 1200 rpm.

<p>inverter frequency</p> Signup and view all the answers

Match the following components with their primary characteristics:

<p>CSI = Higher cost and weight VSI = Can feed multiple motors in parallel Induction Motor = Operates on slip speed Torque-slip relationship = Maintains torque when flux is constant</p> Signup and view all the answers

At what frequency is the motor operating when determining stator current and torque for rated slip speed?

<p>30 Hz (C)</p> Signup and view all the answers

A single CSI can be used to feed multiple motors in parallel.

<p>False (B)</p> Signup and view all the answers

What is the rated stator voltage for the Y-connected squirrel cage induction motor described?

<p>400V</p> Signup and view all the answers

The induction motor operates at a speed of ______ when powered with 50 Hz.

<p>1370 rpm</p> Signup and view all the answers

Which of the following is a consequence of using a large inductance in the DC link for CSI drives?

<p>Higher cost and weight (C)</p> Signup and view all the answers

What is the total equivalent impedance (Z) of the induction motor?

<p>24.7463 + j20.3543 (A)</p> Signup and view all the answers

The stator current at full load is 7.21 A.

<p>True (A)</p> Signup and view all the answers

What is the synchronous speed of the motor at 30 Hz?

<p>900 r.p.m.</p> Signup and view all the answers

The full load torque, T, of the induction motor is ______ N-m.

<p>22.73</p> Signup and view all the answers

Match the following components with their values:

<p>Full Load Stator Current = 7.21 A Full Load Rotor Current = 5.865 A Full Load Torque = 22.73 N-m Slip Speed = 130 r.p.m.</p> Signup and view all the answers

What is the full load rotor current of the induction motor?

<p>5.865 A (D)</p> Signup and view all the answers

The motor speed is 770 r.p.m. when the synchronous speed is 900 r.p.m.

<p>True (A)</p> Signup and view all the answers

What does the variable 's' refer to in the context of induction motors?

<p>Slip speed</p> Signup and view all the answers

The equation for the total equivalent impedance Z includes R and ______ components.

<p>jX</p> Signup and view all the answers

What is the formula to calculate synchronous speed in RPM for an induction motor?

<p>$N_s = 120f/P$ (B)</p> Signup and view all the answers

Flashcards

Regenerative Braking

A type of braking in an induction motor where the motor's kinetic energy is converted into electrical energy and fed back to the power source.

Plugging (Reverse Current Braking)

A braking method for an induction motor where the direction of the current in the stator winding is reversed, causing a braking torque.

Slip (Induction Motor)

The ratio of the rotor's speed to the synchronous speed of the rotating magnetic field.

Slip Speed

The difference between the synchronous speed and the rotor speed of an induction motor.

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Cage Induction Motor

A type of induction motor where the rotor is made of a solid cage-shaped structure, eliminating the need for brushes or slip rings.

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Thyristor Controller

A method of controlling the current in the stator winding of an induction motor using thyristors.

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LC Filter

An electrical circuit that smooths out the ripples in direct current (DC) by storing and releasing energy.

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Plugging

A type of braking that involves reversing the current in the stator winding of an induction motor, creating a braking torque.

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Dynamic (Rheostatic) Braking

This type of braking uses a DC source to excite the stator winding, producing a stationary magnetic field that opposes the rotor's motion.

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Negative Slip

Occurs in an induction motor when the load makes it spin faster than the synchronous speed, causing the motor to operate as a generator.

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Slip

The ratio of the slip speed to the synchronous speed, representing the motor's efficiency in converting electrical energy to mechanical energy.

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Synchronous Speed

The speed at which the rotating magnetic field in an induction motor rotates.

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Returned Power

The electrical energy returned to the power source during regenerative braking.

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Squirrel-Cage Motor

A type of induction motor where the rotor is a solid cage-shaped structure, eliminating the need for brushes or slip rings.

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Current Source Inverter (CSI)

A type of inverter that uses a controlled current source to drive an AC motor.

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Voltage Source Inverter (VSI)

A type of inverter that uses a controlled voltage source to drive an AC motor.

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Squirrel Cage Induction Motor

A type of AC motor where the rotor is made of a cage-shaped structure that eliminates the need for brushes or slip rings.

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Variable Frequency Drive (VFD)

The ability to change the speed of an electric motor by adjusting the frequency of the power supply.

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Constant Flux Control

The ability to maintain a constant magnetic field in an induction motor, regardless of speed or torque.

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Torque-Slip Relationship

The relationship between the torque produced by an induction motor and the slip.

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Plugging Torque

The torque produced by an induction motor when it is in the plugging braking mode. It is calculated by considering the plugging current, motor parameters, and the speed.

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Plugging Current

The current flowing through the motor windings when the motor is in plugging braking mode. It is significantly higher than the normal operating current.

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Equivalent Impedance (Z) of an Induction Motor

The total equivalent impedance (Z) of an induction motor, representing the combined resistance and reactance components.

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Resistance (R) in Induction Motor Impedance

The resistance component of the equivalent impedance, accounting for energy dissipation in the motor windings.

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Reactance (X) in Induction Motor Impedance

The reactance component of the equivalent impedance, representing the opposition to changes in current due to magnetic fields.

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Rotor Reactance (X2) in Induction Motor Impedance

The reactance component of the equivalent impedance, associated with the rotor winding.

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Full Load Stator Current

The full load stator current is the amount of current flowing through the stator windings when the motor operates at its full rated output.

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Full Load Rotor Current

The full load rotor current is the amount of current flowing through the rotor winding when the motor operates at its full rated output.

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Full Load Torque

Full load torque is the maximum rotational force that the motor can produce when operating at its full rated output.

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Study Notes

Topic: Electrical Drives

  • Electrical drives convert electrical energy to mechanical energy. They are used to control the motion of machinery.

Topic: Components of Electrical Drives

  • Load: The equipment or mechanism being driven.
  • Motor: The source of mechanical power.
  • Power modulator: Adjusts the power flow to the motor.
  • Control unit: Controls the power modulator to adjust the motor speed.
  • Source: The electrical supply source.

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Description

This quiz covers key concepts related to the control and braking systems of induction motors. It includes questions on components like thyristor converters, braking current calculations, and the functions of various motor parts. Test your understanding of these essential topics for effective motor operation.

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