Deck 20: Harmonics

Question 1

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

Answer 1

There are several steps that can be followed in determining whether there is a problem with harmonics. One step is to do a survey of the equipment. This is especially important in determining whether there is a problem with harmonics in a single-phase system.
1. Make an equipment check. Personal computers, printers, and fluorescent lights with electronic ballast are known to produce harmonics. Any piece of equipment that draws current in pulses can produce harmonics.
2. Review maintenance records to see whether there have been problems with circuit breakers tripping for no apparent reason.
3. Check transformers for overheating. If the cooling vents are unobstructed and the transformer is operating excessively hot, harmonics could be the problem. Check transformer currents with an ammeter capable of indicating a true rms current value. Make sure that the voltage and current ratings of the transformer have not been exceeded.

Question 2

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

Answer 2

C

Question 3

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

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

Answer 3

D

Question 4

What are some devices that are responsible for harmonics?

Question 5

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 6

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

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

Question 7

What problems do triplens pose for neutral bus ducts?

Question 8

THDF stands for ____.

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

Question 9

What problems are caused by harmonics in circuit breakers?

Question 10

Harmonics called ____ have a zero sequence.

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

Question 11

Harmonics are produced by loads that draw current in ____.

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

Question 12

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

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

Question 13

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

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

Question 14

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 15

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 16

How are harmonics classified?

Question 17

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 18

Transformers are typically designed for ____-hertz operation.

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

Question 19

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

Question 20

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