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NCATT AET Volume 1 > 2-1) AC-Terms > Flashcards

2-1) AC-Terms Flashcards

1
Q

What is the difference between AC and DC?

A

AC (alternating current) switches polarity from positive to negative while DC (direct current) provides a constant source or positive or negative voltage.

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2
Q

What is the term for the time required for one cycle of AC to occur?

A

One period–the time for one full cycle from 0V to Peak Positive back to 0V then Peak Negative and then back to 0V

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3
Q

What is frequency? What is its unit of measurement?

A

Frequency is defined as the number of cycle of AC completed in one second.

Frequency is measured in Hertz.

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4
Q

What is the AC frequency used in most aircraft?

A

400 Hz

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5
Q

What is impedance (Z)?

A

Impedance is an electrical characteristic of a conductor when an alternating current is applied. Impedance is the total opposition to the flow of AC in an electrical circuit (resistance, capacitance and inductance).

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6
Q

What is the unit of impedance?

A

The value of impedance is expressed in Ohms Ω.

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7
Q

What determines the value of impedance?

A

Resistors, capacitors and inductors create resistance to current flow differently. The relationship of the three types of resistance in a circuit determines total impedance.

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8
Q

What is the resulting impedance when two signals are exactly in phase with each other?

A

Very low impedance

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9
Q

What is the resulting impedance when two signals are exactly 180 degrees out of phase with each other?

A

Very high impedance

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10
Q

What is reactance?

A

Reactance is the opposition to alternating current as a result of inductance or capacitance. Reactance arises from the presence of inductance and/or capacitance within a circuit.

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11
Q

What is the symbol for reactance?

A

X

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12
Q

What 2 values are required to calculate total impedance?

A

Reactance (X) and Resistance (Ω)

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13
Q

What is capacitive reactance?

A

Capacitive Reactance is the opposition to current flow in a circuit that is produced by a capacitor. The effect that capacitance has on an AC circuit is termed “capacitive reactance.”

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14
Q

What is the phase relationship of current and voltage in a capacitive circuit?

A

Current Leads Voltage

ICE

I (current) C (capacitance) E (voltage)

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15
Q

What is inductive reactance?

A

Inductive reactance is the opposition to current flow in a circuit that contains an inductor. The effect that inductance has on an AC circuit is termed inductive reactance.

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16
Q

What is the phase relationship of current and voltage in an inductive circuit?

A

Voltage Leads Current

ELI

E (voltage) L (Leads / Inductance) I (current)

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17
Q

What is the formula for capacitive reactance?

A

Xc = 1 / 2 ∏ f C

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18
Q

What is the formula for inductive reactance?

A

XL = 2 ∏ f C

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19
Q

What is the relationship between the formulas for capacitive reactance and inductive reactance?

A

They are reciprocals of one another. The formulas are very similar except they work in opposite ways. Capacitive reactance is the reciprocal of Inductive Reactance

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20
Q

What is the symbol for Capacitive Reactance?

A

XC

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21
Q

What is the symbol for Inductive Reactance?

A

XL

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22
Q

How many degrees of rotation produce on AC cycle?

A

360 degree

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23
Q

How does the movement of a conductor through a magnetic field correspond to the points on a sine wave?

A

When a conductor is cutting through no flux lines (at 0 or 180), the voltage is 0. When the conductor rotates through 90 or 270 degrees, the voltage peaks when the conductor cuts across the maximum number of flux lines.

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24
Q

What is Root Mean Square (RMS)?

A

RMS is the effective value of sine-wave alternating current.

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25
Q

Why is RMS value useful?

A

The value of an AC voltage is continually changing from zero up to the positive peak, through 0 to the negative peak and back to zero again. RMS value is the effective value of varying voltage or current. It is the equivalent stead DC (constant) value that gives the same effect.

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26
Q

Formula to calculate VRMS from VPEAK

A

VRMS = 0.707 x VPEAK

27
Q

Formula to calculate VPEAK from VRMS

A

VPEAK = 1.414 x VRMS

28
Q

What is effective voltage?

A

Effective voltage is the amount of AC that produces the same amount of heat as a corresponding value of DC. The effective voltage is referred to as the RMS value.

29
Q

What are phase angles?

A

Phase angles are the differences between two similarly varying quantities (sine-waves). A phase angle is a difference in angle between two sinusoidally varying quantities that have the same frequency.

30
Q

What is Power Factor?

A

This is the ratio of the actual power dissipated in an electrical system to the input power of volts multiplied by amp. The power factor is the ratio of power dissipated over input.

It is the ratio of true power to apparent power in an AC circuit.

31
Q

What is Power Factor?

A

This is the ratio of the actual power dissipated in an electrical system to the input power of volts multiplied by amp. The power factor is the ratio of power dissipated over input.

It is the ratio of true power to apparent power in an AC circuit.

32
Q

How is power factor (PF) calculated? What is the formula?

A

Power factor is equal to the cosine of the phase displacement between current and voltage.

Power Factor = cos θ

=True Power / Apparent Power

θ=Phase displacement

33
Q

What is True Power?

A

True Power is the actual power available in an AC circuit.

34
Q

How is True Power calculated?

A

True Power is the product of the circuit voltage and the current in phase with this voltage. Values are given in Watts.

Power (Watts) = I (amps) x E (volts)

35
Q

What is Reactive Power?

A

Reactive Power is the “phantom power” that is the result of the voltage and current used by the inductors and capacitors in an AC circuit. The resistance to current flow created by reactive components impedes current flow and therefore, they also use voltage and current. Current and voltage used in these components does not contribute to power used in the rest of the circuit.

36
Q

What is Apparent Power?

A

Apparent Power is the product of effective voltage and effective current that is expressed in Volt-Amps (instead of Watts) without reference to phase shift (if any) between voltage and current.

37
Q

What is the formula for Apparent Power?

A

Apparent Power = Eeff x Ieff

38
Q

What is the units of Reactive Power?

A

Volt-Ampere-Reactive (VAR)

39
Q

What are the units of Apparent Power?

A

Volt-Amperes (VA)

Not Watts!

40
Q

What is the formula for True Power?

A

True Power = Apparent Power x cos θ

=Eeff x Ieff x cos θ

41
Q

What is a Wye Wound? Where might this be found?

A

Windings are arranged in the shape of a Y. May be found on a three-phase generator or transformer. One end of each of the 3 windings is connected to form a common point.

42
Q

What is this?

A

Wye Wound found on a 3 phase generator or transformer with each end connected to a common ground point.

The Wye (Y-shaped) configuration, sometimes called a star winding, connects all of the windings to a central point (parallel circuits) and power is applied to the remaining end of each winding. They product 3 waveforms 120 degrees apart from each other. All 3 phases share a common ground or return line.

43
Q

What is this?

A

Delta Wound in the shape of a triangle. For example, on a 3-phase generator, motor or transformer, two of the phase windings are in series and, across the third phase winding.

The delta configuration connects the three windings to each other (series circuits) in a triangle-like circuit, and power is applied at each of the connections. This produces 3 distinct phases at the output on 3 separate grounds.

44
Q

Which is generally more efficient: delta or wye configuration?

A

Efficiency is greatly effected by a motor’s construction, but the wye winding is normally more efficient. A wye connection does not contain a closed loop in which parasitic currents can flow, preventing loss.

45
Q

What is polyphase?

A

This is a production of two or more phases of AC, or of two or more alternating voltages of the same frequency.

46
Q

What is the most common polyphase in aviation?

A

3 phases that are 120 degrees apart

47
Q

What is a rectifier?

A

A rectifier is an electronic device that converts AC voltage to a type of DC voltage.

48
Q

What are 3 types of rectifiers?

A

Half-Wave, Full Wave, Bridge

49
Q

What is the process of converting AC to DC called?

A

Rectification

50
Q

How does a half-wave rectifier work?

A

One diode only passes half of the AC wave along to the output when it is forward biased. When it is reverse biased, that half of the alternation is blocked.

51
Q

How does a full-wave rectifier work?

A

Two diodes share a common tap in the middle of a transformer. By using the center tap on the transformer, the two secondaries are 180 degrees out of phase with each other. Only one diode is forward biased at a time, but since the transformer output is 180 degrees out of phase with each other, there are no breaks in the rectifier output. The frequency output of a full-wave rectifier is 2x higher than the input on the transformer primary.

52
Q

What is an inverter?

A

An inverter is the opposite of a rectifier. An inverter converts DC power into AC power through a series of transformers, rectifiers and amplifiers. It cannot produce a perfect alternating sine waveform. It mostly acts like a switch turning on and off in various polarity.

53
Q

What are the two types of inverters? What are the differences between them?

A

Rotary and static. A rotary inverter is nothing more than a DC motor driving an AC generator. A static inverter produces an AC waveform using solid state devices.

54
Q

What is voltage drop?

A

The reduction in voltage in an electrical circuit between the source and the load.

55
Q

What is the problem with excessive voltage drop?

A

Excessive voltage drop will result in unsatisfactory operation of electrical equipment and represents energy wasted in the wiring system. Voltage drop can also cause damage to electric motors.

56
Q

Define Kirchhoff’s Current Law (KCL)

A

The algebraic sum of the current flow away from any point in an electrical circuit is equal to the sum of the current flowing into that point.

The current entering any junction is equal to the current leaving that junction.

57
Q

What is the principle of conservation of electric charge?

A

At any point in an electrical circuit that does not represent a capacitor plate; the sum of currents flowing toward that point is equal to the sum of currents flowing away from that point. Adopting the convention that every current flowing toward the point is positive and every current flowing away is negative.

58
Q

How does Kirchhoff’s Current Law relate to Ohm’s Law for current in a parallel circuit?

A

In a parallel circuit, the sum of all individual branch currents is equal to the total current from the power supply and is equal to the total current returning to the power supply.

59
Q

Define Kirchhoff’s Voltage Law (KVL)

A

The algebraic sum of all of the voltage drops in any close circuit is equal to zero. The total voltage used by all of the resistors will equal the total voltage applied.

60
Q

What are 4 other names for Kirchhoff’s Current Law?

A

Kirchhoff’s First Law, Kirchhoff’s Point Rule, Kirchhoff’s Junction Rule, Kirchhoff’s Nodal Rule

61
Q

What are 3 other names for Kirchhoff’s Voltage Law?

A

Kirchhoff’s Second Law, Kirchhoff’s Loop Rule, Kirchhoff’s Mesh Rule

62
Q

How might Kirchhoff’s Voltage Law be written in an equation?

A

The sum of all the voltages around the loop is equal to the applied (total) voltage.

VT=V1+V2+V3+V4

63
Q

How is a bridge circuit structured?

A

A bridge circuit contains four impedances or resistances that form a square. Two diagonally opposite corners are connected to an input device. The other two diagonally opposite corners are connected to an output device.

A bridge circuit is a type of electrical circuit in which the current in a conductor splits into two parallel paths and then recombines into a single conductor, thereby enclosing a loop. It was originally used for measurement purposes.

64
Q

What is the best known type of bridge circuit and what is it used for?

A

Wheatstone Bridge for measuring resistance

NCATT AET Volume 1 (21 decks)

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