Magnetic Fields and AC Circuits Quiz

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Questions and Answers

What is the required maximum flux in the core of a transformer with a primary voltage of 3,000 V and a flux density of 1.2 Wb/m2?

1.0 Wb

0.6 Wb

0.8 Wb

0.4 Wb (correct)

For the 25-kVA transformer with a 2000/200 V rating, what is the efficiency at half full load if the iron loss is 350 W and the copper loss is 400 W?

88.2 %

90.5 %

92.5 %

91.3 % (correct)

In a transformer delivering 400 kVA at a power factor of 0.8 lagging, what is the percentage voltage drop if the percentage resistance is 2.5% and percentage reactance is 5%?

8.0 %

5.0 %

3.2 %

6.5 % (correct)

How many turns would the primary have if a single-phase transformer operates at a primary voltage of 1500 V with 250 turns on the secondary winding?

200 turns Signup and view all the answers

What defines the term 'absolute permeability'?

A constant value representing the ability of a vacuum to support magnetic fields. Signup and view all the answers

What is the no-load current drawn by the primary if the iron loss is 2 watt/kg and the density of iron is 7.8 gram/cm3?

0.4 A Signup and view all the answers

In an A.C. circuit, what is the relationship between current and power in a purely inductive circuit?

Current leads power by 90 degrees. Signup and view all the answers

Which characteristic is associated with series resonance?

Inductive reactance equals capacitive reactance. Signup and view all the answers

What is the primary function of armature reaction in an alternator?

To produce a magnetic field that opposes the original field. Signup and view all the answers

What is the effect of a high power factor on voltage regulation in alternators?

It decreases the voltage regulation. Signup and view all the answers

What is one of the primary purposes of conducting open circuit and short circuit tests on an alternator?

To determine losses and efficiency at rated load. Signup and view all the answers

What is the significance of voltage regulation in a transformer?

It indicates the transformer's response to load changes. Signup and view all the answers

In a star-connected three-phase transformer, how is the percent regulation calculated for varying power factors?

Considering both reactance and resistance at different load conditions. Signup and view all the answers

Study Notes

Magnetic Fields & Permeability

Permeability is a material's ability to support the formation of a magnetic field within itself.
Absolute permeability is the measure of the permeability of a vacuum.
Relative permeability is the ratio of a material's permeability to the permeability of a vacuum.

Magnetic Circuit Basics

Flux (Φ) refers to the total magnetic field lines passing through a given area.
MMF (Magnetomotive Force) is the force that creates a magnetic field, measured in Ampere-Turns (AT).
Reluctance (S) is the opposition to magnetic flux, analogous to resistance in an electrical circuit.
The relationship between these is analogous to Ohm's Law in electrical circuits: Φ = MMF / S

AC Circuits

Resistive circuit - Current and voltage are in phase. Power is P = I²R.
Inductive circuit - Current lags behind voltage by 90 degrees. Power is P = I²XL.
Capacitive circuit - Current leads voltage by 90 degrees. Power is P = I²XC.

Series Circuits

R-L circuit - Voltage leads the current.
R-C circuit - Voltage lags behind the current.
R-L-C circuit - Impedance depends on the values of R, L, and C, and the frequency of the AC supply.

Resonance

Series Resonance occurs when the inductive and capacitive reactances in a circuit are equal and opposite.
Properties of series resonance:
- The impedance of the circuit is minimum.
- The current in the circuit is maximum.
- The voltage across the inductor and capacitor are equal and opposite.

Transformers

Working Principle: Based on the principle of electromagnetic induction.
Construction: Consists of a core made of laminated iron, with two windings: primary & secondary.
Operation - Voltage is induced in the secondary winding due to the changing magnetic flux in the core.
No-load operation: Primary winding draws a small current (magnetizing current), creating a magnetic field.
On-load operation: Secondary winding's current affects the magnetic field, leading to a voltage drop on the primary side.
Equivalent circuit: Represents the transformer's behavior using resistance, reactance, and a magnetizing branch.

Alternators

Working Principle: Based on Faraday's law of electromagnetic induction.
Construction: Consists of a stator (stationary part) and a rotor (rotating part).
Operation: Rotor's magnetic field cuts the stator windings, inducing an alternating voltage.

Alternator Types

Salient pole alternators: Have large, prominent poles, suitable for low speeds.
Non-salient pole alternators: Have smooth rotor, suitable for high speeds.

Armature Windings

Full-pitch windings: Each coil spans 180 electrical degrees, resulting in a sinusoidal EMF waveform.
Fractional-pitch windings: Each coil spans less than 180 electrical degrees, resulting in a less sinusoidal waveform.

Armature Reaction

The armature current's magnetic field interacts with the rotor's magnetic field, creating an effect called armature reaction.
Effects of power factor (PF) on armature reaction:
- Lagging PF: Armature reaction weakens the main field.
- Leading PF: Armature reaction strengthens the main field.

Transformer and Alternator Testing

Open Circuit (OC) Test: Measures the transformer's core losses.
Short Circuit (SC) Test: Measures the transformer's copper losses.

Voltage Regulation

Definition: The change in voltage at full load compared to no-load.
Methods of voltage regulation:
- Field excitation adjustment
- Voltage regulators

Hunting

Definition: An undesired oscillation in speed and torque caused by load fluctuations or system instability.
Prevention:
- Improved damping mechanisms
- Automatic voltage regulators

Three-Phase Transformers

Connection Types:
- Star (Y) connection
- Delta (Δ) connection

Parallel Operation of Transformers

Necessity: To handle increased load demands.
Conditions for parallel operation: Same voltage rating
- Same phase angle
- Same polarity

Transformer Efficiency

Definition: Output power divided by input power.
Factors affecting efficiency:
- Core losses
- Copper losses

Transformer Voltage Regulation

Definition: The change in voltage at full load compared to no-load.
Calculation: VR = (VNL - VFL) / VFL * 100%

Vector Grouping

Purpose: To enable parallel operation of transformers by ensuring phase compatibility.
Types:
- Star-Star (Yy0)
- Delta-Delta (Dd0)
- Star-Delta (Yd11)

Transformer Maintenance

Yearly/Half-yearly:
- Inspection of insulation
- Oil testing and replacement
- Cleaning and inspection of cooling system

Effect of Excitation Change

Increased excitation: Increases the magnetic field strength, leading to increased voltage and current.
Decreased excitation: Decreases the magnetic field strength, leading to decreased voltage and current.

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Description

Test your understanding of key concepts related to magnetic fields, permeability, and AC circuits. This quiz covers topics like magnetic flux, MMF, and the behavior of resistive, inductive, and capacitive circuits. Challenge your knowledge and reinforce your learning!