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WorksheetsW1 Quiz Basic Concept of Electrical Machine
Total questions: 25
Worksheet time: 14mins
According to Faraday's law of electromagnetic induction, an electromotive force (EMF) is induced in a conductor whenever it:
Is placed in a static magnetic field.
Carries a constant current
Cuts magnetic flux.
Is made of copper.
A conductor is moving through a uniform magnetic field. To maximize the induced EMF, the conductor should move:
Parallel to the magnetic field lines.
Perpendicular to the magnetic field lines.
At a 45-degree angle to the field lines.
In a circular path within the field.
What is the primary purpose of laminating the iron core in a transformer or electrical machine?
To increase the magnetic flux.
To reduce eddy current loss.
To increase the core's mechanical strength.
To simplify the construction process.
A simple DC motor and a simple DC generator are constructed identically. Analyze what determines whether the machine acts as a motor or a generator.
The direction of the magnetic field.
The number of turns in the armature winding.
Whether the input is electrical energy or mechanical energy.
The type of carbon brushes used.
In a purely inductive single-phase AC circuit, what is the phase relationship between the voltage and the current?
Voltage and current are in phase.
Voltage lags the current by 90 degrees.
Voltage leads the current by 90 degrees.
Voltage leads the current by 120 degrees.
A single-phase circuit supplied with 240 V AC has a load that draws 10 A. The real power consumed is 1.92 kW. Calculate the power factor.
1.0
0.9
0.8
0.7
A single-phase motor is connected to a 230 V supply. A technician measures that the voltage leads the current. What can be concluded about the load?.
It is a purely resistive load
It is a predominantly inductive load.
It is a predominantly capacitive load.
The measurement is incorrect.
A circuit contains a resistor and an unknown component. When connected to an AC source, the current lags the voltage by 30 degrees. The unknown component must be:
A capacitor.
A resistor.
An inductor.
A second power source.
Analyze the consequence of connecting a device rated for 120 V, 60 Hz to a 240 V, 60 Hz supply.
The device will operate more efficiently.
The device will draw half the current and work normally.
The device will likely be damaged due to excessive current.
The device will not turn on.
What is the standard phase displacement in degrees between any two phases in a balanced three-phase system?
90 degrees
120 degrees
180 degrees
360 degrees
What is a key advantage of transmitting power using a three-phase system compared to a single-phase system for the same amount of power?
It requires thicker conductors.
It results in lower transmission voltage.
It requires less conductor material overall.
It has a lower power factor.
In a balanced three-phase system, what is the instantaneous sum of the three phase currents at any point in time?
Equal to the peak phase current.
Three times the RMS phase current.
Equal to the line current.
Zero.
If one of the three fuses supplying a running three-phase induction motor blows, the motor will:
Immediately stop
Continue to run, but may overheat due to drawing higher current in the remaining two phases.
Speed up because the load is reduced.
Reverse its direction of rotation.
In a balanced star-connected system, the relationship between line voltage (VL) and phase voltage (Vph) is:
VL=Vph
VL=Sqrt(3)x Vph
Vph=Sqrt(3)x VL
VL=3×Vph
In a balanced delta-connected system, the relationship between line current (IL) and phase current (Iph) is:
IL=Iph
Iph=Sqrt(3) x IL
IL=Sqrt(3) x Iph
IL=3×Iph
A three-phase load is connected in a star configuration to a 415 V supply. What is the voltage across each phase of the load?
415 V
719 V
240 V
138 V
A delta-connected motor draws a line current of 34.6 A. What is the current flowing through each motor winding (phase current)?
60 A
34.6 A
20 A
11.5 A
A technician measures the voltage between a line and the neutral point of a star-connected system as 230 V. What would be the expected voltage measurement between any two lines?
460 V
400 V
230 V
133 V
The unit for Real Power is the Watt (W). What is the unit for Reactive Power?
Volt-Ampere (VA)
Watt-Hour (Wh)
Volt-Ampere Reactive (VAR)
Joule (J)
Power factor is correctly defined as the ratio of:
Real Power to Reactive Power
Apparent Power to Real Power
Real Power to Apparent Power
Reactive Power to Apparent Power
A 3-phase, 415 V motor draws a line current of 25 A at a power factor of 0.8 lagging. Calculate the real power drawn by the motor.
18 kW
14.4 kW
10.4 kW
8.3 kW
A three-phase load consumes 15 kW of real power from a 400 V supply. A clamp meter reads a line current of 27 A. Analyze the power factor of the load.
The power factor is leading.
The power factor is approximately 0.8
The power factor is unity.
The power factor is approximately 0.5
Analyze a purely resistive three-phase load (like a bank of heaters) connected in a star configuration. What would be its power factor?
0 lagging
0 leading
Unity (1.0)
0.5 lagging
2. In a generator, what is the input and output?
Electrical to mechanical
Electrical to thermal
Mechanical to magnetic
Mechanical to electrical
16. What happens to the magnetic field of an electromagnet when current stops?
It stays permanent
It becomes stronger
It reverses
It disappears
