Question 1

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

Accepted Answer

The formula provided is not the correct formula for determining the transformer harmonic derating factor (THDF). THDF calculations typically involve analyzing the harmonic content of the load current and its impact on the transformer, not simply a function of RMS and peak phase currents.

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.

Accepted Answer

Two ampere measurements must be made to determine the derating factor for a transformer using the method recommended by the Computer & Business Equipment Manufacturers Association.

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.

Accepted Answer

The "name" of a harmonic refers to its order, such as second, third, fourth, etc., relative to the fundamental frequency.

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 ____.

Accepted Answer

When all phases have equal current flow, the phasor summation of the currents is zero, which means that the neutral current will be zero. This is because the harmonics currents are equal and opposite in the three-phase system and will cancel out when summed.

Question 5

Harmonics cause an AC sine wave to become ____.

Accepted Answer

Harmonics cause an AC sine wave to become distorted. Harmonics are additional sinusoidal waveforms that are multiples of the fundamental frequency. When these harmonics are added to the original sine wave, it can cause distortion in the waveform, resulting in a non-sinusoidal shape.

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.

Accepted Answer

Harmonic currents cause a distortion of the RMS (Root Mean Square) value, which is the effective value of an AC waveform. This distortion can cause the breaker to trip when it should not, or not to trip when it should, leading to potential damage or dangerous situations in the electrical system.

Question 7

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

Accepted Answer

Harmonic currents are typically reflected in the delta primary winding of transformers. This is because the delta connection allows for the circulation of harmonic currents within the winding itself, without passing them on to the connected power system. This can lead to overheating and additional losses within the transformer.

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.

Accepted Answer

Triplens are odd harmonics that do not have a phasor rotation but can cause neutral current overload in a three-phase four-wire system, leading to equipment damage and power loss.

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.

Accepted Answer

Harmonic currents can cause overheating and reduced efficiency in emergency and standby generators, just like 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.

Accepted Answer

The phasor rotation of a negative harmonic is opposite that of the fundamental frequency, and this tends to weaken the rotating magnetic field of an induction motor, leading to a decrease in torque produced.

Question 11

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

Accepted Answer

Pulse width modulation (PWM) is a technique used to regulate the output voltage by varying the time a transistor conducts as compared to the time it is turned off. By adjusting the on-time of the transistor, the average voltage delivered to the load can be controlled. This is commonly used in applications such as motor control, LED dimming, and power electronics.

Question 12

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

Accepted Answer

A negative sequence harmonic rotates in the opposite direction to that of the fundamental frequency in an induction motor.

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.

Accepted Answer

Triplen harmonics (3rd, 9th, 15th, etc.) are additive in the neutral conductor and can create a circulating current that is greater than the phase currents, leading to an overloaded neutral buss. This is a common issue in three-phase systems with nonlinear loads, such as computers and adjustable speed drives.

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.

Accepted Answer

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

Question 15

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

Accepted Answer

Thermal-magnetic circuit breakers use a bimetallic trip mechanism that bends due to the heat produced by the circuit current, causing the breaker to trip. Therefore, the correct choice is A. None of the other options are correct.

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.

Accepted Answer

To determine the ratio of the two measurements, you should divide the value indicated by the true RMS ammeter by the value indicated by the average ammeter, but the correct interpretation of the ratio depends on the context and what you are trying to compare or analyze, not just a straightforward division for direct comparison without understanding the nature of the signals being measured.

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.

Accepted Answer

Pulling extra neutral conductors can help manage harmonics by reducing the potential for neutral overload, which is a common issue with harmonic currents. The ideal situation often involves ensuring that neutral conductors can handle the increased harmonic currents, rather than relying on a shared neutral which can become overloaded due to the additive nature of harmonic currents in the neutral wire.

Question 18

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

Accepted Answer

Triplens are actually the odd multiples of the third harmonic, specifically 3rd, 9th, 15th, etc., not 5th, 7th, 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.

Accepted Answer

Harmonics on single-phase power lines are typically caused by non-linear loads such as computer power supplies, electronic ballasts in fluorescent lights, and triac light dimmers, which distort the electrical current waveform.

Question 20

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

Accepted Answer

Electrical panels and buss ducts are designed to carry currents that operate at the standard frequency of 60 Hz in the United States. However, in some other countries, such as Japan, the frequency is 50 Hz.

Quiz 13: Harmonics