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Exam 13: Harmonics

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Question 1

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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)

E) A) and D)
F) None of the above

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Question 2

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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

E) All of the above
F) B) and D)

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Question 3

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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.

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.

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.

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 ____.

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.

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.

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.

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

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.

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

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

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

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

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

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.

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

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