Alternator Construction and Principles
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Questions and Answers

What happens to the terminal voltage of a DC shunt generator when the load increases?

  • The voltage remains constant regardless of load.
  • The terminal voltage increases create a larger inductive reactance.
  • The terminal voltage drops due to armature reaction and resistance. (correct)
  • The terminal voltage rises due to increased load.

Which of the following statements correctly explains the effect of the distribution factor on electrical winding?

  • Distributing the winding reduces mechanical strength.
  • Distributing the winding shapes the voltage wave closer to a sinusoidal function. (correct)
  • Distributing the winding increases the overall resistance.
  • Distributing the winding leads to unbalanced voltage throughout the system.

What is the consequence of a lower lagging power factor in an electrical circuit?

  • It causes a significant voltage rise in the system.
  • It has no effect on voltage levels in the circuit.
  • It results in an increase in the amount of current flowing.
  • It leads to a higher voltage drop across the circuit. (correct)

Before an AC generator can deliver electrical load, which condition must be satisfied?

<p>It must be brought up to synchronous speed by its prime mover. (C)</p> Signup and view all the answers

In a pure resistance circuit, how do current and voltage behave?

<p>They are in phase with each other. (D)</p> Signup and view all the answers

What occurs to the flux in a DC shunt generator when under load?

<p>The flux diminishes as a result of armature reaction. (B)</p> Signup and view all the answers

What is the relationship between current and voltage in a pure reactance circuit?

<p>The current lags the voltage by 90 degrees. (A)</p> Signup and view all the answers

How does a lower leading power factor affect voltage in a circuit?

<p>It leads to a voltage rise. (A)</p> Signup and view all the answers

What does the pitch factor (Kp) represent in an electrical machine?

<p>The reduction in electromotive force (emf) (A)</p> Signup and view all the answers

In concentrated winding, what characterizes the emfs in the coils?

<p>They are in phase (B)</p> Signup and view all the answers

How does capacitive load affect circuit current in relation to voltage?

<p>Current leads voltage (C)</p> Signup and view all the answers

What is the primary effect of a distributed winding compared to concentrated winding?

<p>Increased phase difference between emfs (D)</p> Signup and view all the answers

What does the distribution factor indicate in relation to generated voltage?

<p>It must be multiplied to find generated voltage (D)</p> Signup and view all the answers

Which load type corresponds with a unity power factor?

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

What happens to the terminal voltage behavior of alternators in the presence of a capacitive load?

<p>It will rise (D)</p> Signup and view all the answers

What factors influence the terminal voltage behavior of alternators?

<p>The magnitude of the load and actual overall power factor (C)</p> Signup and view all the answers

What is the primary condition for paralleling an alternator with an infinite bus-bar system?

<p>The incoming alternator's terminal voltage must match the bus-bar's voltage. (C)</p> Signup and view all the answers

Which of the following statements about synchronous motors is correct?

<p>They can develop a starting torque equal to their pull-out torque. (B)</p> Signup and view all the answers

What defines synchronous speed in an alternator?

<p>The speed at which the rotating magnetic field rotates. (B)</p> Signup and view all the answers

What describes full-load torque in an induction motor?

<p>Torque developed at its rated power output. (B)</p> Signup and view all the answers

Which of these is NOT a condition for paralleling an incoming alternator?

<p>The armature winding must span 180 electrical degrees. (A)</p> Signup and view all the answers

Which method is used for speed control of induction motors?

<p>Concatenation Method (A), Reduced-Voltage Starting Method (D)</p> Signup and view all the answers

What does voltage regulation refer to in the context of alternators?

<p>The percentage change in terminal voltage from no-load to full-load conditions. (D)</p> Signup and view all the answers

Which type of winding creates a rotating field at synchronous speed in synchronous motors?

<p>Whole-coiled winding (D)</p> Signup and view all the answers

What is the purpose of slip in an induction motor?

<p>To indicate the difference between actual rotor speed and synchronous speed. (A)</p> Signup and view all the answers

What is the primary principle behind an induction motor's operation?

<p>Magnetic induction and Lenz's Law. (D)</p> Signup and view all the answers

What characterizes the power factor operation in synchronous motors?

<p>They can operate under a wide range of power factors. (C)</p> Signup and view all the answers

What must be true about the phase of the incoming alternator voltage during synchronization?

<p>It must be in phase with the bus-bar's voltage. (D)</p> Signup and view all the answers

Which of the following statements accurately describes dynamic braking?

<p>It involves disconnecting the AC source quickly. (B)</p> Signup and view all the answers

Which method of starting involves interchanging two stator leads?

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

What is the purpose of slip rings in a wound-rotor induction motor?

<p>To provide an electrical connection to the rotor windings. (B)</p> Signup and view all the answers

What characterizes a squirrel-cage rotor?

<p>It consists of bars short-circuited at both ends. (C)</p> Signup and view all the answers

What is the primary characteristic of a synchronous motor in terms of speed?

<p>Runs at synchronous speed, which is constant (D)</p> Signup and view all the answers

Which type of rotor is typically used in lower-speed applications?

<p>Salient Pole Rotor (A)</p> Signup and view all the answers

Why is a synchronous motor described as 'not self-starting'?

<p>It needs to synchronize with the supply after reaching synchronous speed (A)</p> Signup and view all the answers

What is the function of the d.c. source in a synchronous motor once the rotor reaches synchronous speed?

<p>It creates poles that attract the stator poles (D)</p> Signup and view all the answers

What kind of applications are synchronous motors preferred for?

<p>Driving high-power loads at low speeds (D)</p> Signup and view all the answers

What determines the synchronous speed of a motor?

<p>The frequency of the AC power supply and the number of poles (A)</p> Signup and view all the answers

Which rotor construction is made by forcing molten conducting material into slots?

<p>Squirrel Cage Rotor (D)</p> Signup and view all the answers

What type of rotor is primarily used for speed control or high starting torque needs?

<p>Wound Rotor (C)</p> Signup and view all the answers

What is the synchronous speed in an induction motor primarily determined by?

<p>Supply frequency and number of poles in stator winding (D)</p> Signup and view all the answers

During the plugging period in an induction motor, what happens to the motor speed?

<p>The speed decreases as it works against the load (B)</p> Signup and view all the answers

What is the primary purpose of a transformer?

<p>To transform electric power from one circuit to another at the same frequency (D)</p> Signup and view all the answers

What effect does applying reverse torque to an induction motor have?

<p>It decreases the motor's speed (C)</p> Signup and view all the answers

What components are essential in the construction of a transformer?

<p>Two coils with mutual inductance and a laminated steel core (D)</p> Signup and view all the answers

How does a transformer adjust voltage?

<p>By using the turns ratio of its coils (B)</p> Signup and view all the answers

What characteristic about electricity in power lines makes transformers necessary?

<p>It can exceed 300,000 volts, which is too high for direct use (B)</p> Signup and view all the answers

What is a wound-rotor motor distinguished by?

<p>The rotor has insulated windings connected to slip rings (A)</p> Signup and view all the answers

Flashcards

Pitch Factor (Kp)

The factor that reduces the electromotive force (emf) in an electrical machine, accounting for how windings are distributed in slots.

Concentrated Winding

A winding where all coils for a phase are located in a single slot per pole.

Distributed Winding

A winding distributed across several slots per pole, with coils for a phase spread out.

Slot Angle (α)

The phase difference between coils in a distributed winding due to their placement in different slots.

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Distribution Factor (Kd)

The factor that accounts for the phase difference between coils in a distributed winding, reducing the calculated voltage.

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Capacitive Load

A load that charges and discharges energy, causing the current to lead the voltage.

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Inductive Load

A load that opposes changes in current, causing the current to lag behind the voltage.

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Resistive Load

A load that has no phase difference between current and voltage, meaning the current and voltage are in sync.

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Synchronizing Alternators

Connecting one generator to another or to a power grid, maintaining identical voltage, frequency, and phase sequence.

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Incoming Machine

An alternator that is being connected in parallel with a system.

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Running Machine

An alternator carrying a load or supplying power.

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Voltage Regulation

The difference in terminal voltage between no-load and full-load conditions, expressed as a percentage of full-load voltage.

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Salient Pole Rotor

A rotor used in low-speed alternators with projecting poles.

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Slip Rings

Metal rings mounted on the rotor shaft providing connection for DC excitation current to the field windings.

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Synchronous Speed

The speed at which the rotating magnetic field of the alternator rotates, determined by frequency and the number of poles.

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Whole-Coiled Winding

An armature winding where each coil spans 180 electrical degrees.

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Voltage Drop/Rise

The decrease or increase in voltage at the terminals of a generator when load changes.

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Constant Power Load

A load where the current is inversely proportional to the voltage (eg. LED lights).

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Short Circuit

A load where a large amount of current flows despite little change in voltage.

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Full-Load Torque

The torque an induction motor produces when running at its rated power output.

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Induction Motor

A type of AC motor where the rotor's motion is induced by a rotating magnetic field.

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Full-Voltage Starting

A method to start an induction motor by applying full voltage to the stator windings.

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Reduced-Voltage Starting

A method to start an induction motor by reducing the voltage applied to the stator.

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Plugging

A method to slow down an induction motor by reversing the phase sequence of the supply voltage.

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Revolving Magnetic Field

The rotating magnetic field produced by the interaction of polyphase currents in the stator windings.

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Slip

The difference between the theoretical synchronous speed and the actual rotor speed.

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Wound-Rotor Method

A method to control the speed of a wound-rotor induction motor by adjusting the resistance in the rotor circuit.

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Stator

The stationary part of an electric motor or generator, which contains the windings that produce a magnetic field.

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Rotor

The rotating part of an electric motor or generator.

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Non-Salient Pole Rotor (Cylindrical Rotor)

A smooth cylindrical rotor with slots for windings.

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Transformer

A static device that transfers electrical power from one circuit to another at the same frequency, changing the voltage level. It can increase or decrease the voltage.

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Voltage Transformation

The ratio between the number of turns in the primary and secondary windings of a transformer determines the voltage ratio.

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Torque

A twisting force that tends to produce rotation.

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Kinetic Energy

The energy possessed by an object due to its motion.

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Kinetic Energy Absorption

During plugging, the motor takes in energy from the moving load, reducing its speed.

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Study Notes

Alternators

  • Alternators are primary energy sources
  • Operate on electromagnetic induction principles
  • Constant speed operation is crucial
  • Rotor windings generate a magnetic field
  • Stator windings receive induced voltage

Alternator Construction

  • Rotor: Large electromagnet with field windings, supplied with DC through slip rings
  • Stator: Laminated steel with slots for armature windings.
  • Slip rings: Metal rings for DC connection to the field windings
  • Brushes: Graphite brushes on slip rings to deliver DC current

Advantages of Stationary Armature

  • Bracing is easier (high centrifugal forces)
  • Insulation is simpler (higher voltages)
  • Voltage output is easier (no brush drop)
  • Field exciter (low voltage, safely transferred)

Armature Windings

  • Symmetrically distributed in slots (3-phase)
  • Identical windings, spaced 120 electrical degrees apart
  • Full-pitch or fractional-pitch coils, affecting voltage waveform
  • Half-coiled: Twice as many turns, half as many coils (same phase voltage)
  • Whole-coiled: Half as many turns, twice as many coils(same phase voltage)

Frequency of Alternators

  • Frequency is directly related to rotational speed
  • Synchronous generators synchronize frequency with rotational speed

Losses in Alternators

  • Rotational: Friction, windage, ventilation, hysteresis, eddy currents
  • Electrical: Field winding, armature winding, brush contact
  • Stray losses: Eddy currents (armature copper conductors)

Load and Power Factor

  • Load changes affect voltage regulation (drop or rise)
  • Lagging power factor: Greater voltage drop
  • Leading power factor: Greater voltage rise

Additional Topics

  • Salient pole rotors: Used for low-speed machines with many poles
  • Non-salient pole (Cylindrical) rotors: Used for high-speed machines (turbine generators)

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Description

This quiz covers the fundamental principles and construction of alternators, including their operation based on electromagnetic induction. Explore the components like rotors, stators, and understand the advantages of stationary armatures. Test your knowledge on armature windings and their configurations.

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