Deck 21: Harmonics

Question 1

Harmonics are produced by loads that draw current in .

A) a sine wave
B) a linear manner
C) a continuous manner
D) pulses

Answer 1

D

Question 2

How are harmonics classified?

Answer 2

Harmonics are classified by name, frequency, and sequence. The name refers to whether the harmonic is the second, third, fourth, and so on, of the fundamental frequency. The frequency refers to the operating frequency of the harmonic. The second harmonic operates at 120 hertz, the third at 180 hertz, the fourth at 240 hertz, and so on. The sequence refers to the phasor rotation with respect to the fundamental waveform. In an induction motor, a positive sequence harmonic would rotate in the same direction as the fundamental frequency. A negative sequence harmonic would rotate in the opposite direction of the fundamental frequency. Harmonics called triplens have a zero sequence. Triplens are the odd multiples of the third harmonic (3rd, 9th, 15th, 21st, and so on).

Question 3

A ratio of between measurements made with a true rms ammeter and an average value ammeter indicates no harmonic distortion.

A) 0.5
B) 0.75
C) 0.833
D) 1.0

Answer 3

D

Question 4

THDF stands for .

A) triplen harmonic derating factor
B) triplen harmonic desensitizing field
C) transformer honed resultant field
D) transformer harmonic derating factor

Question 5

Transformers are typically designed for -hertz operation.

A) 50
B) 60
C) 100
D) 120

Question 6

The instantaneous peak value of current is equal to the rms value multiplied by .

A) 0.651
B) 1.111
C) 1.414
D) 1.730

Question 7

Ammeters that respond to the average value convert the alternating current into direct current. This value must be increased by a factor of to change the average reading into the rms value for a sine wave current.

A) 0.651
B) 1.111
C) 1.414
D) 1.730

Question 8

Harmonics called have a zero sequence.

A) positives
B) negatives
C) doublens
D) triplens

Question 9

The phasor rotation of a negative harmonic causes an induction motor to produce .

A) positive feedback
B) a high magnetic field
C) less torque
D) more torque

Question 10

What problems do triplens pose for neutral bus ducts?

Question 11

Harmonic currents in the neutral conductor induce currents into the communication cable.

A) high-frequency
B) low-frequency
C) no new
D) constant

Question 12

What problems are caused by harmonics in circuit breakers?

Question 13

What steps should you follow when determining whether there are harmonic problems in single-phase systems?

Question 14

It is necessary to use an ammeter that responds to current when making a check for harmonics.

A) single-phase
B) rectified
C) true rms
D) average

Question 15

One way to deal with harmonic problems is to install a neutral conductor.

A) larger
B) smaller
C) common
D) triplen

Question 16

Harmonics are voltages or currents that operate at a frequency that is the fundamental power frequency.

A) a multiple of
B) a divisor of
C) double
D) within 10 Hz of

Question 17

A is used to smooth the pulsations caused by a bridge rectifier.

A) phasor
B) filter capacitor
C) transistor
D) diode

Question 18

To minimize interference, communication cables are run as close to the conductor as possible.

A) positive phase
B) negative phase
C) triplen
D) neutral

Question 19

The most accurate method for determining whether there is a harmonics problem is to use a(n) .

A) transformer
B) average value ammeter
C) true rms ammeter
D) harmonic analyzer

Question 20

What are some devices that are responsible for harmonics?