EE Diagnostic Exam on Alternators

Questions and Answers

What is the full load copper loss of a 10 kVA, 2400/240 V transformer with a primary winding resistance of 6 Ω and a secondary resistance of 0.06 Ω?

201 W (correct)

105 W

176 W

208 W

What is the operating speed in rpm required for a 6-pole generator to develop an emf of 120 V, if it develops 100 V at 1500 rpm?

1800 rpm (correct)

1250 rpm

1500 rpm

1042 rpm

For a conductor with an area of 40,000 cm², what is the diameter in inches?

0.50 (correct)

0.20

0.60

0.40

What resistance should be connected in series with 4 uF capacitance to achieve a time constant of 2 s?

200 kΩ Signup and view all the answers

What is the equivalent capacitance of two identical capacitors, each with capacitance C, connected in parallel and the combination in series with a third identical capacitor?

2C/3 Signup and view all the answers

What is the power that a 220 V, 75 W bulb will draw when connected to a 110 V source?

9.375 W Signup and view all the answers

What is the inductance of a cylindrical core that is 50 cm long, has a cross section of 5 cm2, and is wound with 500 turns of wire with a permeability of 2000?

0.628 H Signup and view all the answers

What is the upper cut-off frequency of a series LC circuit with an inductance of 200 uH and a capacitance of 300 pF, given a Q of 60?

655 kHz Signup and view all the answers

If the ratio of the two wattmeter readings in a balanced 3-phase system is 2:1, what is the power factor of the system?

0.8 Signup and view all the answers

Given that a nichrome wire is 1 m long with a 1 x 10-6 m2 cross-sectional area and carries a current of 4 A under 2 V, what is the resistivity of the nichrome wire?

4 x 10-7 Ω-m Signup and view all the answers

Study Notes

Electrical Concepts & Calculations

A 3-phase alternator rated 50 MVA, 22 kV has a synchronous reactance of 5 pu. When translated to a base of 100 MVA and 220 kV, its pu value needs calculation.
A bulb rated 220 V, 75 W connected to a 110 V source will draw a specific power calculated using the formula ( P = \frac{V^2}{R} ).
Inductance of a cylindrical core can be calculated using parameters such as length, cross-sectional area, number of turns, and core permeability.
For a series LC circuit with a defined Q factor, the upper cut-off frequency can be determined based on inductance and capacitance values.

Power & Efficiency in Electrical Systems

In a balanced 3-phase system, the ratio of two wattmeter readings can indicate the power factor for various loading conditions.
The performance of transformers can be evaluated through equivalent resistance, reactance, and voltage regulation, considering load characteristics.
The copper loss and core loss in transformers can indicate the loading efficiency and can be analyzed to find max efficiency conditions.

Capacitors and Reactance

The voltage across capacitors connected in series can be calculated by evaluating their individual capacitances and the applied voltage.
Reactance in RLC circuits affects power consumption, which can be assessed from known impedance and voltage supply.

Motor Operations

The operational speed of induction motors in relation to synchronous speed accounting for slip can be calculated using generator frequency.
The addition of synchronous motors affects the power factor of existing loads, necessitating a specific calculation to achieve the desired improvement.

Energy Storage & Inductance

Energy stored in an inductor is linked to its current through ( E = \frac{1}{2}Li^2 ) and can be influenced by time constants in circuits.
Calculating self-inductance in coupled coils involves understanding series aiding and opposing configurations.

Electromagnetic Theory

The magnetic field generated around current-carrying conductors can be determined at specific distances, aiding in electromagnetic field calculations.
The concept of charge and electric potential forms a basis for understanding electric fields from point charges.

Current and Resistance Calculations

Series resistive circuits with combined resistor loads can determine the resulting current flow under various configurations.
Electric heat generation and energy conversion rates can be measured from circuit parameters and resistor values.

General Electrical Principles

The relationship between power, current, and voltage forms critical calculations in both DC and AC circuits.
Understanding capacitive and inductive reactances and their impact on overall circuit performance is vital in electrical engineering applications.

Transformer and Voltage Regulation

Transformers' behavior under various loads can dictate efficiency and performance, particularly focusing on voltage drops and regulation percentages.
Both energy and power factor optimizations are crucial for maximizing system performance in power distribution networks.