Electrical Machines and Electronic Devices Flashcards (80 cards)

What does Ohm's law state about voltage and current at constant temperature?

Voltage is directly proportional to current through a resistor.

What does Kirchhoff's current law say about currents at a node?

The algebraic sum of currents entering a node is zero.

What does Kirchhoff's voltage law state about voltages around a closed path?

The algebraic sum of all voltages around a closed path is zero.

How is electric current defined mathematically?

Electric current is the time rate of change of charge, $i=\frac{dq}{dt}$.

What is the first step in nodal analysis?

Selecting a reference node.

What does mesh analysis assign to each mesh?

Mesh currents.

What does Thevenin’s theorem replace a linear bilateral network with?

A single voltage source in series with a single impedance.

What does Norton’s theorem replace a linear bilateral network with?

A single current source in parallel with a single impedance.

When is maximum power transferred to a load impedance?

When the load impedance is the complex conjugate of the source impedance.

What does impedance represent in an AC circuit?

Opposition to the flow of alternating current.

What is resonance in an AC circuit?

When applied voltage and current are in phase with unity power factor.

What is true about reactances in series resonance?

Inductive and capacitive reactances are equal and opposite.

What condition defines parallel resonance in AC circuits?

Imaginary part of total admittance is zero and power factor is unity.

What is the formula for real power in AC circuits?

$P=EI\cos\theta$

What is the phase voltage in a balanced three-phase star connection?

Line voltage divided by $\sqrt{3}$.

Name one safety step before wiring in electrical installation.

Disconnect the supply before wiring.

How does a transformer operate to induce EMF in the secondary coil?

By mutual induction between inductively coupled coils with AC flux linking both coils.

What is the EMF equation for the primary coil of a transformer?

$E_1=4.44N_1fB_mA$

What is the EMF equation for the secondary coil of a transformer?

$E_2=4.44N_2fB_mA$

How does the speed of a synchronous motor compare to load changes?

It runs at synchronous speed independently of load.

How does the speed of an induction motor change with load?

It runs below synchronous speed and decreases as load increases.

Why is a single-phase induction motor not self-starting?

Its stator winding produces alternating flux along one axis, not a revolving magnetic field.

What is the formula for electromagnetic force on a current-carrying conductor?

$F=BIl$

What does a DC motor convert direct-current electrical energy into?

Mechanical energy.

What rule gives the direction of force on a current-carrying conductor in a magnetic field?

Fleming’s left-hand rule.

What does B represent in the formula $F = BIl$?

Magnetic flux density in weber per square metre.

How does current flow in a basic DC motor to cause rotation?

From source through brushes and commutator into the loop.

What causes the loop in a DC motor to rotate?

Interaction of the loop’s magnetic field with the permanent-magnet field.

What defines a single-phase induction motor?

An AC motor with single-phase stator supply and squirrel-cage rotor induced by alternating stator flux.

What happens when a single-phase induction motor is energized?

Stator produces alternating flux that induces current in rotor bars, creating rotor flux.

What are the main methods to make a single-phase induction motor self-starting?

Split-phase, capacitor-start, capacitor-start capacitor-run, permanent-split-capacitor, and shaded-pole operations.

How does a shaded-pole motor produce a rotating flux?

Alternating flux induces current in shaded coil causing shaded-pole flux to lag unshaded-pole flux.

What energizes the field winding in a separately excited DC generator?

A separate external voltage source.

From where does a self-excited DC generator obtain its field-winding excitation?

From the generator’s own output.

How is the field winding connected in a series DC generator?

In series with the armature and load.

How is the field winding connected in a shunt DC generator?

In parallel with the armature.

In a short-shunt compound generator, where is the shunt field connected?

Across the armature.

In a long-shunt compound generator, across what is the shunt field connected?

Across both the armature and series field.

What are the three key characteristics of a DC shunt motor?

Torque versus armature current, speed versus armature current, and speed versus torque.

What is the formula relating armature torque in a DC motor?

Armature torque is proportional to the product of flux and armature current: $T_a \propto \Phi I_a$.

Why is the torque-versus-armature-current characteristic a straight line in a DC shunt motor?

Because the shunt-field flux is approximately constant.

How does the speed of a DC shunt motor change as load increases?

It decreases only slightly as load increases.

What is applied to the low-voltage side in the transformer open-circuit test?

Rated-frequency voltage is gradually applied.

Which side of the transformer is open during the open-circuit test?

The high-voltage side is open.

What readings are recorded during the transformer open-circuit test?

Voltmeter, ammeter, and wattmeter readings are recorded.

What is short-circuited in the transformer short-circuit test?

The low-voltage side is short-circuited.

What is applied to the high-voltage side in the transformer short-circuit test?

Rated-frequency voltage is gradually applied.

What does the wattmeter reading represent in the transformer short-circuit test?

It approximates full-load copper loss.

What is a transformer?

A static device transferring electrical energy by electromagnetic induction without changing frequency while changing voltage and current.

What does a DC generator convert mechanical energy into?

Electrical energy.

How is the field winding connected in a DC shunt motor?

In parallel with the armature.

Name three main constructional parts of a DC machine.

Yoke, poles and pole shoes, field winding.

What role does the commutator play in a DC generator?

It reverses armature connections to produce unidirectional current at terminals.

What is the formula for induced RMS emf per phase in a synchronous generator?

$E_{ph} = 4.44 K_c K_d f \Phi T_{ph}$

What defines an induction motor in terms of speed and components?

It is an asynchronous motor that does not run at synchronous speed and has a stationary stator and rotating rotor.

What components does the stator of a three-phase induction motor contain?

An outer frame, laminated silicon-steel core, and three-phase windings.

How does a squirrel-cage rotor differ from a phase-wound rotor?

Squirrel-cage uses uninsulated bars short-circuited by end rings; phase-wound uses insulated windings with slip rings and external resistance.

What is Zener breakdown and its use?

It occurs in a reverse-biased junction and provides a constant reference voltage.

What is peak inverse voltage in a diode?

The maximum reverse voltage a diode can withstand before Zener breakdown or damage.

How does an ideal diode differ from a practical diode in resistance?

An ideal diode has zero forward resistance and infinite reverse resistance; a practical diode does not.

What happens to the depletion-region width in forward bias of a PN junction?

It decreases due to recombination of electrons and holes near the junction.

How does forward bias affect majority-carrier flow across a PN junction?

It allows a heavy majority-carrier flow across the junction.

What distinguishes a network from a circuit?

A circuit contains at least one closed path, unlike a network.

How is the resistance of a 1000 W, 240 V electric iron calculated?

Resistance is $R=\frac{V^2}{P}=57.6\,\Omega$.

How is the current of a 1000 W, 240 V electric iron calculated?

Current is $I=\frac{V}{R}=4.166\,\mathrm{A}$.

How does a mesh differ from a loop in a network?

A mesh contains no other loop within it.

How is complex power expressed in AC circuits?

$S=P+jQ$ with real power as real part and reactive power as imaginary part.

What does the commutator do in a DC motor?

It converts alternating EMF into unidirectional EMF.

Why are single-phase motors widely used in homes and offices?

Because single-phase supply is common and these motors are reliable, inexpensive, simple, and easy to repair.

When is the auxiliary winding disconnected in a split-phase motor?

At approximately 75–80% of synchronous speed by a centrifugal switch.

What field windings does a compound generator have?

Both shunt and series field windings.

What connection types may a compound generator use?

Short-shunt or long-shunt connection.

Why should a DC shunt motor not be started at full load?

Starting a heavy load requires a very large armature current.

What are the principal parts of a transformer?

Core, primary winding, secondary winding, and enclosure.

What is a DC machine capable of operating as without construction changes?

Either a DC generator or a DC motor.

What two functions does the yoke serve in a DC machine?

Provides mechanical strength and carries magnetic flux.

What is the armature core made of and why?

Laminated cylindrical rotor to reduce eddy-current losses.

What effects does skewing squirrel-cage rotor conductors have?

It reduces humming, smooths torque, reduces magnetic locking, and increases rotor resistance.

Which diode type has lower reverse saturation current at similar power and current?

Silicon diodes have lower reverse saturation currents than germanium diodes.

How does increasing temperature affect the forward characteristics of a diode?

They become more nearly ideal as reverse saturation current increases.