# Harmonics

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 Author: johnbowens ID: 124964 Filename: Harmonics Updated: 2016-05-15 13:14:47 Tags: Harmonics JATC IBEW Folders: Description: Harmonics chapter NJATC, Transformers, 2009 Show Answers:

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1. Define a harmonic. NJATC, Transformers 2006, PP 99
A harmonic is a voltage or current at a frequency that is an integer whole number multiple (second, third, fourth,) etc. of the fundamental frequency.
2. What causes harmonics? NJATC, Transformers 2006, PP 99
Harmonics are caused by nonlinear loads that draw current in pulses, for only a portion of the cycle, resulting in a distorted waveform.
3. What can be used to measure the amount of voltage and current harmonics on a line? NJATC, Transformers 2006, PP 100
The power quality meter.
4. Odd-numbered harmonics add to each other or cancel each other out? NJATC, Transformers 2006, PP 101
Odd-numbered harmonics add together and increase their effect.
5. Odd-numbered harmonics do what regarding resistance and eddy currents? PP 101
Odd numbered harmonics have increased resistance losses and eddy current losses in transformers.
6. Even-numbered harmonics are generally what size and why? NJATC, Transformers 2006, PP 101
Even-numbered harmonics are generally fairly small because the tend to cancel each other out.
7. Even-numbered harmonics usually indicate what may be present in the transformer secondary winding and caused by what? NJATC, Transformers 2006, PP 101
Even-numbered harmonics usually indicate that a DC current may be present in the transformer secondary winding. The DC offset is typically caused by half wave rectification due to a failed rectifier.
8. A DC offset can cause a transformer to become what? NJATC, Transformers 2006, PP 101
A DC offset can cause a transformer to become saturated on alternate half-cycles and draw extremely high currents that can burn out the primary. Remember: a DC offset is caused by failed rectification due to a failed rectifier.
9. What is this symptom of a DC offset? NJATC, Transformers 2006, PP 101
These problems may be noticeable as a strong vibration and a very loud noise coming from the transformer core.
10. A DC offset equaling more than how much of a percent of the rated current can cause problems? NJATC, Transformers 2006, PP 101
1 percent.
11. What are Triplen harmonics? What type of loads draw these type of harmonics? NJATC, Transformers 2006, PP 101
Triplen harmonics are on multiples of the third harmonic 3rd, 9th, 15th etc. Only single phase loads generate triplen harmonics.
12. Which harmonic can cause problems such as overloading neutral conductors, telephone interference, and transformer overheating? NJATC, Transformers 2006, PP 101
• Triplen harmonics.
13. Which harmonic is the only harmonic that contributes to the problem of high neutral currents? NJATC, Transformers 2006, PP 101
• Only the triplen harmonics contribute to the problem of high neutral currents. Because of their lower current levels and higher frequencies, The ninth, 15th, and higher triplen harmonics generally distort the neutral current only slightly, and often do not have a significant effect on actual RMS neutral current.
14. Define phasor rotation. NJATC, Transformers 2006, PP 101
Phaseor rotation is the order in which waveforms from each phase (A, B, and C) cross zero.
15. What is harmonic sequence? NJATC, Transformers 2006, PP 101
Harmonic sequence is the phasor rotation with respect to the fundamental 60 Hz frequency.
16. Why is phase sequence of a Harmonic important? NJATC, Transformers 2006, PP 101
The sequence of a harmonic is important because it determines the affects a harmonic has on the operation of loads and components such as conductors within a power distribution system.
17. Positive sequence harmonics are what? name the numbers. NJATC, Transformers 2006, PP 101
• First, fourth, seventh, etc.
18. Positive sequence harmonics have what phase sequence NJATC, Transformers 2006, PP 101
Positive sequence harmonics have the same phase sequence as the fundamental harmonic.
19. Positive sequence harmonic rotates in what direction in regards to the fundamental in an induction motor? NJATC, Transformers 2006, PP 101
Positive sequence harmonics rotate in the same direction as the fundamental in an induction motor.
20. What is the numeric sequence of negative harmonics? NJATC, Transformers 2006, PP 102
Second, fifth, eighth, etc.
21. A negative sequence harmonic rotate in what direction from the fundamental in an induction motor? NJATC, Transformers 2006, PP 103
• A negative sequence harmonic rotates in the opposite direction from the fundamental and induction motor.
22. Does a negative sequence harmonic reduce the forward torque of a motor? NJATC, Transformers 2006, PP 103
• Yes.
23. What is the numerical order of zero sequence harmonics? NJATC, Transformers 2006, PP 103
Third, sixth, ninth, etc.
24. Zero sequence harmonics cannot cancel but can add together where? NJATC, Transformers 2006, PP 103

What are significant contributors to current on neutral wires? NJATC, Transformers 2006, PP 103
Zero sequence harmonics do not cancel each other, but can add together in the nuetral conductor of three phase, 4-wire systems.

Single phase devices with rectifier power supplies, such as fluorescent lighting with electronic ballasts, computers, copiers, and other similar electronic devices, are significant contributors to current on neutral wires.
25. Define total harmonic distortion. NJATC, Transformers 2006, PP 103
• Total harmonic distortion is the amount of harmonics on A line compared to the fundamental frequency of 60 Hz.

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