Deck 13: Harmonics

Question 1

The formula for determining the transformer harmonic derating factor is: THDF = (1.414 x RMS phase current) + Instantaneous peak phase current.

Answer 1

False

Question 2

The most practical and straightforward method for determining the derating factor for a transformer is recommended by the Computer & Business Equipment Manufacturers Association. To use this method, ____ ampere measurement(s) must be made.

A) one
B) two
C) three
D) four

Answer 2

B

Question 3

Harmonics are classified by name, frequency, and sequence. The ____ refers to whether the harmonic is the second, third, fourth, and so on of the fundamental frequency.

A) name
B) frequency
C) sequence
D) alternate frequency

Answer 3

A

Question 4

A general procedure for dealing with harmonics is as follows: In a three-phase four-wire system, the 60 Hz part of the neutral current can be reduced by balancing the current on the phase conductors. If all phases have equal current flow, the neutral current will be ____.

A) zero
B) less than the current on the phase conductors
C) the same as the current on the phase conductors
D) greater than the current on the phase conductors

Question 5

Harmonics cause an AC sine wave to become ____.

A) regular
B) distorted
C) almost flat
D) none of the above (harmonics do not affect AC sine waves)

Question 6

Harmonic currents cause a distortion of the ____ value, which can cause the breaker to trip when it should not, or not to trip when it should.

A) RMS
B) SMF
C) RMF
D) BSF

Question 7

Harmonic currents are reflected in the ____ winding where they circulate and cause overheating.

A) wye primary
B) wye secondary
C) delta primary
D) delta secondary

Question 8

Although ____ do not have a phasor rotation, they can cause a great deal of trouble in a three-phase four-wire system, such as a 208/120 volt or 480/277 volt system.

A) capacitors
B) harmonics
C) transistors
D) triplens

Question 9

Transformers are not the only electrical component to be affected by harmonic currents. Emergency and standby generators can be affected in the same way as transformers.

Question 10

Since the phasor rotation of a negative harmonic is opposite that of the fundamental frequency, it tends to weaken the rotating magnetic field of an induction motor, causing it to produce less torque.

Question 11

____ modulation regulates the output voltage by varying the time a transistor conducts as compared to the time it is turned off.

A) Pulsating
B) Compulsory
C) Pulse width
D) Deep

Question 12

In an induction motor, a negative sequence harmonic rotates in the same direction as the fundamental frequency.

Question 13

Since triplen harmonics can cause the neutral current to be higher than the phase current, it is possible for the neutral buss to become overloaded.

Question 14

Harmonics are voltages or currents that operate at a frequency that is a multiple of the fundamental power frequency. If the fundamental power frequency is 60 Hz, for example, the second harmonic would be 120 Hz, and the third harmonic would be 180 Hz, and so on.

Question 15

Thermal-magnetic circuit breakers use a ____ trip mechanism, which is sensitive to the heat produced by the circuit current.

A) bimetallic
B) simple
C) convex
D) copper

Question 16

In a given problem, it is assumed that the true RMS ammeter indicates a value of 36.8 amperes and the average ammeter indicates a value of 24.8 amperes. To determine the ratio of the two measurements, divide the true RMS value by the average value.

Question 17

A general procedure for dealing with harmonics is as follows: Pull extra neutral conductors. The ideal situation is to only use a shared neutral.

Question 18

Triplens are the odd multiples of the third harmonic-3rd, 5th, 7th, 9th, etc..

Question 19

Harmonics on ____ power lines are generally caused by devices such as computer power supplies, electronic ballasts in fluorescent lights, triac light dimmers, and so on.

A) single-phase
B) dual-phase
C) three-phase
D) regular

Question 20

Electrical panels and buss ducts are designed to carry currents that operate at 90 Hz.