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WorksheetsMIDTERM EXAM DC MACHINES
Total questions: 55
Worksheet time: 41mins
Which statement best describes a separately-excited DC generator?
Field winding is in series with the armature.
Field winding is connected to a separate external DC source.
Field winding is connected in parallel with the armature.
It always has no armature resistance.
The armature-circuit voltage equation for a generator is:
E=V-IaRa
V=E+IaRa
V=E-IaRa
E=IaRa-V
A series-wound generator characteristic typically shows:
Terminal voltage nearly constant with load.
Terminal voltage that increases with load (before saturation).
No field current at full load.
No dependence of generated emf on load current.
A compound-wound generator is used to:
Eliminate armature resistance.
Combine advantages of shunt and series fields to control voltage regulation.
Operate only at no-load.
Make the machine single-phase only.
Which loss is approximately proportional to load current in a DC machine?
Iron (core) loss
Friction and windage loss
Brush contact loss
All constant losses
For maximum efficiency of a DC generator (operating point), the variable loss should equal:
Zero.
The constant losses (iron + friction + windage + shunt copper).
The output power.
Twice the constant losses.
If armature resistance Ra increases, then for the same generated emf and load current, the terminal voltage will:
Increase.
Decrease.
Stay the same.
Become equal to generated emf.
Who first demonstrated a connection between electricity and magnetism by showing a compass deflection from a current-carrying conductor?
Michael Faraday
André Ampère
Hans Christian Orsted
James Clerk Maxwell
The magnetic field inside a long solenoid (or coil) is most similar to the field of:
an electric dipole
a bar magnet
a point charge
a permanent magnet with no poles
When a current-carrying conductor is placed at right angles to a magnetic field, the magnitude of the force on the conductor is given by:
F=BIL
F=BIL
F=BI/L
F=I/L/B
The unit tesla (T) for magnetic flux density BBB can be defined as the flux density that produces a force of 1 N on a 1 m conductor carrying 1 A (i.e. 1 T=1 N/(A·m)). This follows from:
B=F/IL
B=FIL
B=IL/F
none of the above
Fleming’s left-hand rule maps the directions of (first finger, second finger, thumb) to:
Current — Field — Motion
Field — Current — Motion
Motion — Current — Field
Field — Motion — Current
The force on a charged particle of charge Q moving perpendicular to a magnetic field B with speed v is:
F=Qv/B
F=QvB
F=QB/v
F=Q/Bv
Faraday’s Second Law states that the magnitude of induced e.m.f. is proportional to:
the number of turns only
the total magnetic flux linkage change per unit time
the rate of current increase
the magnetic field strength only
In a generator, the induced e.m.f. becomes maximum when:
the coil plane is parallel to the magnetic field
the coil plane is perpendicular to the magnetic field
the flux linkage is maximum
the current through the coil is zero
Lenz’s Law demonstrates the principle of:
Conservation of magnetic flux
Conservation of energy
Conservation of charge
Conservation of current
The induced e.m.f. in a straight conductor moving at an angle θ with the magnetic field is given by:
E = B l v cos θ
E = B l v sin θ
E = B v² l
E = B / l v sin θ
Which of the following devices does NOT rely on magnetism?
Computer disk drives
Tape recorders
VCR
Wooden spoon
What is the primary function of magnetic material in transformers and electric machinery?
To generate electricity
To dissipate heat
To shape and direct magnetic fields
To increase resistance
What is magnetic flux density defined as?
The total magnetic field produced by a source
The amount of flux passing through a perpendicular area
The resistance to magnetic flow
The force exerted by a magnetic field
The unit of magnetic flux is:
Tesla (T)
Ampere (A)
Weber (Wb)
Henry (H)
What is magnetomotive force (MMF) analogous to in an electrical circuit?
Current
Resistance
Voltage
Power
What is the effect on magnetic flux when two magnets are placed with similar poles adjacent to each other?
Attraction
Repulsion
No effect
Increase in magnetic field strength
What does a hysteresis loop represent?
The relationship between voltage and current
The lagging of flux density behind changes in magnetic field strength
The ideal magnetic behavior of a material
The energy stored in a magnetic field
What is the significance of the area of a hysteresis loop?
Represents the strength of the magnetic field
Indicates the material's permeability
Proportional to the hysteresis loss
Determines the saturation flux density
What is the permeability of free space (μ0)?
4π × 10⁻⁷ H/m
8.85 × 10⁻¹² F/m
1
Depends on the magnetic field strength
What is the relative permeability (μr) of a vacuum?
0
1
Approximately 100-250
Infinite
In the context of transformers and electric machinery, which statement best describes the role of magnetic materials?
Which of the following accurately describes the relationship between magnetic flux density (B) and magnetic field strength (H) in a material with relative permeability (μr)?
A) B is inversely proportional to H and μr
B) B is directly proportional to H but inversely proportional to μr
C) B is directly proportional to both H and μr
D) B is inversely proportional to H but directly proportional to μr
A magnetic circuit is comprised of sections with varying materials and dimensions. Which parameter remains constant throughout the entire series magnetic circuit, assuming no leakage?
Magnetic field strength (H)
Magnetic flux density (B)
Magnetic flux (Φ)
Reluctance (S)
Hysteresis loss in a ferromagnetic material is primarily attributed to:
Eddy current losses within the material.
The energy required to continually realign magnetic domains during AC magnetization.
The material's inherent resistance to magnetic flux.
Air gaps present within the magnetic circuit
What phenomenon describes the production of a potential difference across a conductor when exposed to a varying magnetic field?
Resistance
Electromagnetism
Electromagnetic Induction
Capacitance
Who is credited with the discovery of electromagnetic induction?
Isaac Newton
Albert Einstein
Michael Faraday
Nikola Tesla
Faraday's law of induction predicts how a magnetic field will interact with an electric circuit to produce what?
Resistance
Capacitance
Electromotive Force (EMF)
Inductance
What is the fundamental operating principle of transformers and inductors?
Resistance
Capacitance
Electromagnetic Induction
Ohm's Law
Which of the following is NOT a factor that affects the inductance of an inductor?
The number of turns of wire
The cross-sectional area of the coil
The presence of a magnetic core
The color of the wire insulation
What does Lenz's law state about the direction of an induced EMF?
It is in the same direction as the change in flux.
It opposes the change of flux responsible for inducing it.
It is perpendicular to the magnetic field.
What rule is used to determine the relative directions of magnetic field, motion, and induced EMF?
Ohm's Law
Kirchhoff's Law
Fleming's Right-Hand Rule
Thevenin's Theorem
What is the unit of inductance?
Ohm
Farad
Henry
Tesla
The energy stored in an inductor is given by:
V=IR
Q=CV
W=1/2LI²
P=VI
An air gap of 3 mm is introduced into a magnetic circuit. The effective area of the air gap is 9 cm². If the desired flux in the air gap is 0.5 x 10⁻⁴ Wb, calculate the MMF required to establish this flux in the air gap.
397.9 A
132.7 A
265.3 A
442.1 A
A conductor 0.2 meters long moves at a speed of 5 m/s perpendicular to a magnetic field with a flux density of 1.2 T. What is the induced EMF in the conductor?
0.24 V
1.2 V
0.12 V
0.6 V
A coil has an inductance of 4 H. If the current through the coil changes from 0 to 2 A in 0.5 seconds, what is the magnitude of the induced EMF?
2 V
4 V
8 V
16 V
A coil of 500 turns has a flux of 25 mWb linking with it when carrying a current of 8 A. What is the inductance of the coil?
1.56 H
3.125 H
0.16 H
160 H
An inductor of 6 H has a current of 4 A flowing through it. How much energy is stored in the magnetic field of the inductor?
12 Joules
24 Joules
48 Joules
96 Joules
The mutual inductance between two coils is 0.5 H. If the current in one coil changes at a rate of 10 A/s, what is the magnitude of the EMF induced in the second coil?
2 Volts
5 Volts
10 Volts
20 Volts
A magnetic pole face has a rectangular section with dimensions 150 mm by 80 mm. If the flux density is 9.0 mT, calculate the total flux emerging from the pole.
108 μWb
110 μWb
112 μWb
114 μWb
A coil of 500 turns is wound uniformly on a ring of non-magnetic material. The ring has a mean circumference of 50 cm and a uniform cross-sectional area of 5 cm². If the current in the coil is 8 A, calculate the magnetic field strength.
6000 A/m
6000 A/m
6000 A/m
7000 A/m
An 8-pole, wave-connected armature has 1200 conductors and runs at 600 rpm. If the flux per pole is 0.02 Wb, what is the generated e.m.f?
1400 V
960 V
480 V
1920 V
A 4-pole wave wound generator has 800 conductors and is running at 600 rev/min. If the flux per pole is 0.05 Wb, what is the generated voltage?
200 V
400 V
800 V
1600 V
A 2-pole machine has an armature with 300 conductors, wave-wound. If the machine runs at 600 rev/min and each conductor cuts 0.1 Wb, what is the generated EMF?
150 V
300 V
600 V
900 V
A generator develops an emf E=240 V and delivers an armature current Ia=50 A. The armature resistance is Ra=40 mΩ. Determine the terminal voltage V.
236 V
238 V
240 V
242 V
A generator is connected to a 50 Ω load and a current of 10 A flows. Armature resistance is 0.5 Ω. Determine (a) terminal voltage V and (b) generated emf E.
500 V, 502 V
500 V, 505 V
480 V, 488 V
490 V, 500 V
A shunt generator supplies 50 kW at 400 V through supply cables of resistance 0.2 Ω. Field winding resistance Rf=50 Ω; armature resistance Ra=0.05 Ω. Determine the generated emf E.
≈ 420.0 V
≈ 431.68 V
≈ 425.00 V
≈ 440.25 V
A short-shunt compound generator supplies 50 A. The field resistance Rf=30 Ω, series resistance Rse=0.03 Ω, and armature resistance Ra=0.05 Ω. Determine the generated emf E.
301.50 V
304.50 V
305.00 V
300 V
