PHEP 17 Flashcards by Anita Dickson

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What is another name for effective values?

Root mean square values (RMS)

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Does an AC amp produces heat at the same rate as a DC amp?

Yes

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What effect does self induced voltage have on applied voltage?

It opposes it and reduces effectiveness

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What variables determine inductive reactance?

-2π constant -frequency (f) -inductance (L) XL=2πfL

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What are the 2 factors that limit current in an inductive circuit?

inductive reactance (XL) and resistance( R)

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What are the units of capacity?

Farrads

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What is power factor?

Ratio of true power to apparent power PF=TP/AP or PF=CosΘ

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AC Meters and instuments are designed to read __________ votage and currents.

effective

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The effective values of voltage and current are equal to the maximum values multiplied by?

0.707 Eeff=0.707 x Emax or Emax=1.414 x Eeff

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The 3 factors that limit the flow of current in an AC circuit are?

Resistance 2. Inductive Reactance 3.Capacitive Reactance

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In a pure resistance circuit, voltage and current are?

in Phase

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In an AC circuit, what represents total Opposition to current and what is the symbol?

Impedance, Z

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What is the symbol for inductive reactance?

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Whats used to determine inductive reactance?

The fequencey and the amount of inductance in a circuit

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What does ELI & ICE represent?

ELI=voltage leads current (inductance) ICE=current leads voltage (capacitive) The letters represent the symbols for voltage, current, inductance & capacitance

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Capacitive reactance varies inversely with?

The frequency and the size of the capacitor.

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What does VARs stand for?

Volt-Ampere-Reactive

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The power factor is equal to the cosine of the?

Angle of phase between voltage and current

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What is the measurement of the primary harmonic?

60hz

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What increments do harmonics go by?

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How can you reduce the third harmonic?

By connecting a set of windings in a transformer in delta

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Inductive reactance varies _______ with fequency.

Directly

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Capacitive reactance varies ______ with frequency.

Inversely

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Resonance is when?

Inductive reactance and capacitive reactance in a circuit cancels each other out and leaves only resistance

A 60-cycle AC current reverses its direction ________ times per second.

120

Covert 39.8 microfarads to Farads

0.0000398 (10-6)

The formula for Capacitive Reactance is?

XC=1/2πfC

In an AC circuit containing 12 ohms of pure resistance ans 12 ohms of inductive reactance. The resulting current is now ____ out of phase and _____.

45°; Lagging

In an AC circuit containing 12 ohms of resistance and 12 ohms of capacitance. The resulting current would be ______ out of phase and ________

45°; leading

The formula for Inductive Reactance is?

Xl=2πfL

Self induced voltage opposes _______ voltage and therefore its effectiveness. This type of opposition is called _____?

applied; inductive reactance

In an AC curcuit containing inductive reactance, the current is said to be ________ _______ the applied voltage

90°; lagging

VARs stands for?

Volts-Ampere-Reactive

The length of the Vector is proportional to the ________ of the given quantity

Magnitude

Which of the following best defines a cycle? A) one Alternation B) Two Alternations C) Three Alternations D) Four Alternations

B) Two Alternations (one in each direction)

How Many Times Larger is the Maximum voltage value than the effective voltage value ? A) 1.414 B) 1.111 C) 0.707 D) 0.637

A) 1.414 (IMAX=1.414 x I effective) Ieff=0.707xIMax 1/0.707 x Ieff=0.707xIMax/0.707 Imax =1/0.707 x Ieff Imax = 1.414 x Ieff

An AC voltmeter indicates that the effective voltage between two conductors in a cable is 440 volts. How many times during a cycle is the insulation subjected to maximum voltage stress? A) 4 B) 3 C) 2 D) 1

C) 2

The magnitude of a vector is denoted by the? A) Time Axis B) Length of line C) Length of line drawn to scale D) Length of time axis drawn to scale

C) Length of line drawn to scale

Direction of a simple vector? A) Cannot be determined B) will indicate the reference point C) Is indicated by the magnitude D) Is indicated by the arrow at the end of the line

D) Is indicated by the arrow at the end of the line

In the mathematical development of the sine wave the sines of the angles greater than 180 degrees corresponds with those in the top half of the circle except that? A) the have a plus sign B) They have a plus sign and must be plotted above the baseline C) they have a minus sign D) they have a minus sign and must be plotted below the base line

D) They have a minus sign and must be plotted below the base line

The effective value of an AC voltage or current of a sine waveform is defined in terms of the equivalent DC? A) heating effect B) voltage effect C) cooling effect D) current effect

A) Heating effect

The root means square value is also known as the? A) average value B) effective value C) peak value D) instantaneous value

B) Effective value

Most AC voltmeters and ammeters are calibrated in? A) RMS values B) Average value C) Peak values D) Instantaneous value

A) RMS Values

Positive rotation of a vector is? A) Clockwise B) Counterclockwise C) either clockwise or counterclockwise D) rstablished by the frequency

B) Counterclockwise

The maximum value of a single sine wave of AC voltage is 141.4 volts. What is the effective value? A) 63.7 volts B) 70.7 volts C) 99.97 volts D) 141.4 volts

C) 99.97 volts Eeff = 0.707 x Emax Eeff = 0.707 x 141.4 volts

When frequency is increased in an AC inductive circuit, the current flow will? A) Increase because of greater inductive voltage B) Increase because of less inductive voltage C) Decrease because of greater inductive voltage D) Decrease because of less inductive voltage

C) Decrease because of greater inductive voltage

The unit of measurement for reactive power is the? A) volt-amp VA B) volt-amp-wattage VAW C) volt-amp-reactive VAR D) volt-amp-kilo-watt KVAW

C) Volt-Amp-Reactive VAR

The prime factors that determine inductive reactance are? A) frequency and current B) frequency and induced voltage C) Inductance and frequency D) Inductance and voltage

C) Inductance and frequency

What are the prime factors affecting capacitive reactance? A) frequency and capacitance B) frequency and inductance C) frequency and resistance D) non of the above

A) frequency (f) and capacitance ( C) XC=1/2πfC

What is impedance? A) current divided by voltage in an AC circuit B) Vector sum of voltage drop in an AC circuit C) resistance of an AC circuit D) opposition to current flow in an AC circuit created by resistance and reactances

D) Opposition to current flow in an AC circuit created by resistance and reactances

In the formula for capacitive reactance, the symbol C represents capacitance in? A) farads B) microfarads C) micromicrofarads D) ohms

A) Farads XC=1/2πfC

The phase angle between current and voltage in a circuit containing both resistance and inductive reactance is? A) 0 because XL and R are equal B) greater than 0° but less than 90° C) a constance 45° D) 90° at all times

B) greater than 0° but less than 90°

What is the phase relationship between current and voltage in a pure capacitive circuit? A) no phase angle B) current lags voltage by 90° C) voltage leads current by 90° D) voltage lags current by 90°

D) voltage lags current by 90° (ICE)

Voltage and current are considered to be out of phase with each other in a pure inductive (L) circuit by what amount? A) current leads voltage by 90° B) current lags voltage by 90° C) current leads voltage by 180° D) current lags voltage by 180°

B) current lags voltage by 90° (ELI)

The vector sum of XC and R in a series capacitive-resistive circuit is the total? A) opposition to voltage in a circuit B) capacitive reactance in a circuit C) resistance in a circuit D) opposition to current flow in a circuit

D) Opposition to current flow in a circuit (resistance to current flow)

Power Factor is the? A) amount of apparent power in an AC circuit B) amount of apparent power plus real power C) ratio on true (or real) power to apparent power D) efficiency of a circuit expressed as an angle

C) ratio of true (or real) power to apparent power PF=TP/AP

The unit of measurement for inductive reactance is the? A) ohm B) volt C) ampere D) Henry

A) ohm

A 39.8 microfarad capacitor is connected in series with a 20 ohm resistor. The applied voltage across this circuit is 134 volts and the requency is 100 cps. What is the capacitive reactance at this frequency? A) less than one ohm B) 20 ohms C) 40 ohms D) greater than 41 ohms

C) 40 ohms XC = 1/2πfC XC = 1/2π100x0.0000398 XC = 1/0.025 microfarad = 10-6 39.8 microfarads = 0.0000398

If the true power in a circuit is 500 watts, the apparent power is 707 volt-amperes and the cosine of the phase angle between them is 0.717. The power factor would be? A) less than one percent B) 7.07 percent C) 70.7 percent D) none of these

C) 70.7 percent PF=TP/VA PF=500w / 707va PF=0.7072 or 70.72%

PHEP 17 Flashcards

(60 cards)