Introduction to harmonic distortion
Harmonic distortion (THD) refers to harmful interference at various multiples of the original frequency.
When amplifying a frequency signal of 1kHZ, the 2nd harmonic of 2kHZ and 3kHZ and many higher harmonics will be generated. Theoretically, the smaller this value, the lower the distortion.
Because the amplifier is not ideal enough, the output signal not only contains the amplified input components, but also adds some new frequency components (harmonics) that are 2 times, 3 times, 4 times or even higher than the original signal. ), causing the output waveform to be distorted.
This kind of distortion caused by harmonics is called harmonic distortion.
Harmonic distortion analysis
Total harmonic distortion refers to the extra harmonic components in the output signal that are more than the input signal due to nonlinear components when the audio signal source passes through the power amplifier. .
Harmonic distortion is caused by the system not being completely linear. We use the ratio of the root mean square of the newly added total harmonic component to the original effective value of the signal to express it.
For example, when an amplifier outputs 10V of 1000Hz and adds Lv of 2000Hz, there will be 10% second harmonic distortion.
The sum of all additional harmonic levels is called total harmonic distortion.
Generally speaking, the total harmonic distortion at the frequency of 1000Hz is the smallest, so many products use the distortion at this frequency as its indicator.
But total harmonic distortion is related to frequency, so the U.S. Federal Trade Commission stipulated in 1974 that total harmonic distortion must be measured within the full audio range of 20 to 20,000 Hz, and the maximum power of the amplifier must be Measured under the condition that the load is 8 ohm loudspeaker and the total harmonic distortion is less than 1%.
The minimum requirements for total harmonic distortion stipulated by the International Electrotechnical Commission are: 0.5% for the preamplifier, and less than or equal to 0.7% for the combined amplifier, but in fact it can be achieved below 0.1%: FM stereo tuner Less than or equal to 1.5%, in fact it can be less than 0.5%; the laser record player can even be less than 0.01%.
1. Electric power field
In the electric power field, the ratio of the root mean square value of each harmonic to the root mean square value of the fundamental wave is called the harmonic content of that harmonic.
The ratio of the sum of the squares of the root mean square values ??of all harmonics to the root mean square value of the fundamental wave is called total harmonic distortion.
Harmonic distortion is usually equal to total harmonic distortion.
2. Audio field
Total harmonic distortion refers to the extra harmonic components in the output signal that are more than the input signal due to nonlinear components when the audio signal source passes through the power amplifier. .
Harmonic distortion is caused by the system not being completely linear. We use the ratio of the root mean square of the newly added total harmonic component to the original effective value of the signal to express it.
For example, when an amplifier outputs 10V at 1000Hz and adds 1v at 2000Hz, there will be 10% second harmonic distortion.
The sum of all additional harmonic levels is called total harmonic distortion.
Generally speaking, the total harmonic distortion at the frequency of 1000Hz is the smallest, so many products use the distortion at this frequency as its indicator.
But total harmonic distortion is related to frequency, so the U.S. Federal Trade Commission stipulated in 1974 that total harmonic distortion must be measured within the full audio range of 20 to 20,000 Hz, and the maximum power of the amplifier must be Measured under the condition that the load is 8 ohm loudspeaker and the total harmonic distortion is less than 1%.
The minimum requirements for total harmonic distortion stipulated by the International Electrotechnical Commission are: 0.5% for the preamplifier, and less than or equal to 0.7% for the combined amplifier, but in fact it can be achieved below 0.1%: FM stereo tuner Less than or equal to 1.5%, in fact it can be less than 0.5%; the laser record player can even be less than 0.01%.
Since the current method of measuring distortion is a single sine wave, it cannot reflect the entire picture of the amplifier.
The actual music signal is a variety of complex waves with different rates, including dynamic indicators such as rate conversion and transient response.
Therefore, high-quality amplifiers sometimes also indicate parameters such as intermodulation distortion, transient distortion, and transient intermodulation distortion.
(l) Intermodulation distortion (IMD): The simple harmonic signal waves of 125Hz and 1kHz output by the intermodulation distortion meter are input into the amplifier being measured at an amplitude of 4:1. The intermodulation distortion coefficient is measured on the load.
(2) Transient distortion (TIM): After the square wave signal is input to the amplifier, its output waveform envelope is expressed by its ability to maintain its envelope.
If the slew rate of the amplifier is not enough, the square wave signal will be deformed and cause transient distortion.
Mainly reflected in rapid musical mutation signals, such as percussion instruments, pianos, xylophones, etc. If the transient distortion is large, the crisp music will become ambiguous.
(3) Transient intermodulation distortion: Mix the 3.15kHz square wave signal and the 15kHz sine wave signal at a peak amplitude ratio of 4:1. After being amplified, all intermodulation distortion products are newly added. RMS value as a percentage of the original sine amplitude.
If the amplifier uses deep large-loop negative feedback, the transient intermodulation distortion is generally larger, which specifically reflects the sound being sluggish, stiff, and without a sense of presence; on the contrary, the sound is smooth, delicate, and natural.
Harmonic Distortion Control
In a water treatment plant, connecting distorted equipment to a backup generator can create interference currents.
You can first adopt a temporary solution and then use an active harmonic filter.
Variable frequency drive (VFD) has many advantages in regulating the speed of key motors and optimizing power consumption, but it can also easily cause harmonic distortion in adjacent power distribution systems.
The power distribution system can absorb some distortion, but when the VFD is directly connected to the generator-driven circuit, the interference generated may also affect the reliability of the operation.
Usually, water treatment plants are equipped with VFDs, ozone generators, and other loads that can cause harmonic distortion.
Most factories are also equipped with emergency backup generators to provide power to important equipment in case the external power supply stops or is abnormal.
Reverse osmosis desalination plants like this are always concerned about the long-term effects of harmonics and whether their 938kVA backup generators can operate reliably when large variable frequency pumps cause harmonic distortion.
The factory workers have been worried that the generator may malfunction if the emergency operation time is extended.
To determine the extent of the fault, operators compiled harmonic measurements, compared distortion levels during normal use and when using a backup generator, and used engineering analysis to evaluate harmonic suppression techniques based on the measured data.
Schneider Electric detects harmonic distortion at the incoming terminals of factory switchboards.
The test equipment is a portable circuit monitor capable of measuring more than 200 power system parameters.
The measurement of harmonic distortion adopts a sampling rate of 512 points per cycle to ensure the accuracy of the 250th harmonic.
Load tests showed that under the circumstances, the plant's backup generator was operating at close to 53% of its rated load.
The load peaks during the test were 403kW and 431kVA.
The average RMS current in the test shows the impact of different machine operations.
During the test, the voltage dropped slightly when using a generator for power supply, but it was able to maintain an acceptable steady-state voltage range - 100% of 480V, whether in normal use or with generator power supply. ~103%.
The voltage imbalance is also less than 1%, which is within the acceptable range.
Load testing shows that the power factor is the lowest when the load is minimal and the power factor is the highest when all equipment is running at the same time.
Due to the inherent power factor improvement characteristics of harmonics, it is difficult for conventional harmonic filters to attenuate the harmonics on this circuit.
Load tests also show that the voltage distortion amplitude on the 480V bus can reach a maximum peak value of approximately 6.5% during normal use and approximately 10% when using a backup generator (as shown in the harmonic distortion diagram) .
When a 60hp unit is operated with a 250hp unit, the current distortion is reduced due to the cancellation effect produced by the combination of the ΔY transformer and the linear reactor.
The total current demand distortion coefficient TDD (total demand distortion) is obtained by dividing 80% of the generator's rated current by the harmonic current, or directly taking 900amps.
The IEEE519-1992 standard "Recommended Procedures and Requirements for Harmonic Control in Power Systems" provides some guidelines on "what degree of harmonic distortion is acceptable".
Initially, this standard was used as a recommended procedure to provide reference for power users and their customers; now, this standard has been popularized and used by a large number of factories and enterprises as a guiding document for measuring harmonic currents in existing equipment.
Tests conducted on normal power usage at the desalination plant and on backup generator power showed that key power system parameters, including voltage calibration and imbalance and current imbalance, were within acceptable limits.
Although the degree of harmonic distortion is not serious enough to have a significant impact on the normal production operation of the factory, further reducing harmonics is still a task that cannot be ignored.
Operators are concerned about the long-term effects of harmonics. Since the harmonics when using standby generators often exceed the harmonic range specified by the IEEE519-1992 standard, they are even more worried about the extension of emergency operation time. Can the generator continue to work stably and reliably?
In addition, harmonic reduction technology can also extend the service life of equipment and enhance system reliability.
The harmonic limits table shows test results compared to the IEEE519-1992 standard harmonic limits commonly used for power generating equipment.
As demonstrated, the online measured data exceeds this limit.
In addition, Schneider Electric also conducted computer simulations of different wave suppression technologies.
Schneider Electric can estimate the reduction of harmonic current under different situations through harmonic simulation.
As mentioned earlier, the harmonic elimination effect of the 5th and 7th harmonic currents has been achieved when the 250hp equipment and the 60hp equipment work simultaneously.
The system requires the current of the 5th harmonic to be reduced by 27% and the current of the 7th harmonic to be reduced by 16% respectively. However, this causes the total RMS current to increase by 19%.
The worst case scenario is the highest harmonic peak, which occurs when only 250hp equipment is operating.
In general, there are 4 solutions:
1. Bypass a ΔY isolation transformer - every 250hp equipment has a ΔY isolation transformer.
Bypassing one of the ΔY isolation transformers can achieve good results, that is, reducing the amount of harmonic current distortion.
Coupled with the above-mentioned method of using 250hp and 60hp equipment at the same time, the 5th and 7th harmonic currents are weakened.
While the 5th and 7th harmonic currents in the bypass loop remain unchanged, there will be additional cancellation once one or two more 250hp units are run.
However, this technology is only suitable for temporary modifications where a more effective solution is not available.
2. Replace a ΔY isolation transformer - A more effective harmonic reduction technique is to replace one of the ΔY isolation transformers with a ΔY interleaved winding transformer instead of bypassing it.
Because the harmonic current does not move through the ΔY interleaved winding transformer, such an improvement also enhances the elimination effect of the 5th and 7th order currents.
This approach also preserves the positive effects of harmonic attenuation.
3. Passive harmonic filter - a 5th order passive harmonic filter can be installed in the 480V main circuit, but since the passive harmonic filter will also increase the basic power factor, this This method is not practical.
Since the power factor of the equipment is already very high (up to 94% at full load), the system cannot tolerate more load without the factory achieving state-of-the-art power factor.
4. Active harmonic filter - The factory's best solution is to reduce the harmonic current by installing an active filter on the 480V main circuit.
The active filter can measure the amount of harmonic current required by the load and cause a 180° phase shift in the current.
This method can greatly reduce the degree of harmonic distortion and is usually used in situations where harmonic limits must be strictly adhered to.
In addition, Schneider Electric recommends the installation of on-site power monitoring equipment that can track equipment performance, voltage quality, interference and expense in harmonic distortion.
Transitional and Permanent Solutions
The factory reduces the level of harmonic distortion by temporarily bypassing the isolation transformer, thereby increasing harmonic cancellation.
This unconventional approach helped the plant survive the peak summer season.
The crew then installed active harmonic filters on the main switchboard.
This device finally eliminates the effects of harmonic distortion.
The measurement results after installing the active filter show that the current distortion is less than 8% and the voltage distortion is less than 2%.
A brief discussion on harmonic distortion
As early as the 1930s, F.H. Brittain’s eleven test items for speaker evaluation included harmonic distortion. In the 1950s, the speaker proposed by L.L. Brenek It is among the eight most important features in every electroacoustic test system today.
From BK systems worth hundreds of thousands to domestic test systems worth several thousand yuan, they are all regarded as important measurement objects.
It can be seen that harmonic distortion has always been a very important parameter in the electroacoustic world.
Harmonic distortion: When a sinusoidal signal with a fundamental frequency of f is input to a speaker, the speaker output, in addition to f, is produced by the nonlinear distortion of the speaker, and is an integer multiple of f. Components: 2f.3fnf, we call it harmonic distortion.
Harmonic distortion is divided into three categories, and the ones we commonly use are THD (TOTALHARMONICDISTORTION) total harmonic distortion, several harmonic distortions (HARMONICDISTORTION) and characteristic total harmonic distortion (also in actual measurements) It will be subdivided into even-order harmonic distortion, odd-order harmonic distortion and SUB-harmonics). Their respective characteristics are specified as: the ratio of the effective value of the total harmonic sound pressure generated by the distortion to the effective value of the total output sound pressure Pt; The ratio of the effective value of the harmonic sound pressure produced by distortion to the effective value of the total output sound pressure Pt; the ratio of the effective value of the total harmonic sound pressure produced by distortion to the average characteristic sound pressure Pm.
In the classification of distortion, it is classified as nonlinear distortion of the speaker.
To treat harmonic distortion, we can use the existentialism of Sartre, a famous French philosophy master, to look at it! Harmonic distortion exists objectively! Now we take the cone speaker as an example: at low frequencies or during large amplitude movements, the support system composed of the speaker's ring and elastic wave (centering support piece) no longer conforms to the linear Hooke's law (or Hooke's law). Law) For example, when performing a pure tone test on a speaker, the "pop" sound produced by the edge of the folded ring is commonly known as "beating". This is an extreme manifestation of nonlinearity; when the speaker with a folded edge of the cloth has a large amplitude (near fo, Not all loudspeakers have the maximum amplitude of vibration at fo). When moving, we can clearly see the distortion of the edge of the cloth.
Damping glue is often "applied" to the fabric edge folding ring. Damping glue is divided into "oil-based" and "water-based". "Oil-based" is mostly used on PA speakers, but in high-end factory products We can often see transparent and shiny water-based damping glue "applied" on the rubber folding ring. Generally, the "applied" glue does not exceed 1/2 of the folding ring, but this kind of glue application method and amount of glue are difficult to control; In order to improve the rubber folding ring (mostly NBR? nitrile-butadienerubber rubber is now used), the shape of the folding ring is often processed. However, China has not done enough in this aspect. In the domestic design of speaker monomers, the quality of the folding ring is often emphasized. Compliance ignores another quantity of the ring and the damping of the rubber. Although the centralized parameter system is often used to analyze cone speakers, that is especially at low frequencies, but do we all regard cone speakers as subwoofers and basses? Woolen cloth? At this time, the harmonic distortion has a subtle relationship with Qm (mechanical quality factor) in the Thille-SmallDepartments.
The non-uniformity of the magnetic induction density between the washer (upper conductive plate) and the iron support of the T-iron along the axial direction (the vibration direction of the voice coil) is another cause of harmonic distortion.
The method commonly used in China now is to use symmetrical magnetic circuits to improve it (as shown in the figure). On the bass unit, those who know it have done so, but have never seen it. Think about it carefully. The reason is all "money causes trouble"! Denmark's PELESS adds an aluminum ring to the inside of the monomer to improve it! (As shown in the picture) Of course, this method is the easiest to discover.
The frequency is the increase of BL (magnetic force coefficient).
From a perspective, harmonic distortion at mid-to-high frequencies and harmonic distortion at low frequencies are determined by two different quantities. In cone speakers, harmonic distortion exists objectively. It can only be improved, not eliminated.
To solve the harmonic distortion of cone loudspeakers, we must adopt an alternative sound generation principle, but this is a long way to go for the entire industry. Distortion is just like measurement error. It depends on the foreign manufacturers. The advertisement "HALCRO------the amplifier with the lowest distortion in the world", but in China I saw a product advertisement of a certain brand of Chinese company: "Completely solve the speaker intermodulation distortion!" I fell in love, I laughed, this is Misleading and fooling the Chinese people further demonstrates the company's irresponsibility and ignorance!
The objective existence of harmonic distortion is "reasonable".
The results of the objective test are consistent with the subjective feeling. From the analysis of the hearing mechanism of the human ear, the human ear can only distinguish the first six to seven harmonics (harmonics), and it is very difficult to distinguish the harmonics above the sixth. It is difficult to separate them from each other perceptually, because after the sixth order of harmonics, the two corresponding areas where two adjacent harmonics fall on the basilar membrane of the human ear are close to each other and cover within a critical band. It is difficult to Separate them from each other in feel.
However, the impact of high-order harmonics on sound quality cannot be ignored. Through experiments, it was found that abnormal noise comes from high-order harmonics.
According to the "order" of harmonic distortion, it can be divided into "soft distortion" and "hard distortion".
But for speakers, "even-order" harmonic distortion and "odd-order" harmonic distortion are more instructive in improving sound quality, especially in terms of listening.
It’s “contribution” to the popularity of tube amps among audiophiles.
From the perspective of music acoustics, the fundamental frequency of an instrument does not sound harmonious with respect to each harmonic. If there are more harmonic harmonic components in the music, the tone will be rich, pure and pleasant, and it will not sound harmonious. If there are too many harmonics, the music will be rough and harsh. Odd harmonics above the seventh order will make the sound rough and harsh.
HALCRO------The lowest distortion amplifier in the world
The distortion of the speaker in the mid-crossing frequency band is mainly caused by the nonlinearity of the magnetic circuit (core). In order to eliminate The nonlinear distortion caused by the iron core is currently often used in a structure called "linear magnetic circuit". The characteristic of this magnetic circuit structure is that the top of the iron core is made into a concave shape so that it faces the magnetic conductive plate. The part of the voice coil is close to the magnetic saturation state due to the reduction of the core cross-sectional area. At this time, the voice coil is equivalent to an air-core coil, thus avoiding the influence of the core and reducing nonlinear distortion.
When the voice coil makes a long stroke movement.
The voice coil wire on the voice coil jumps out of the uniform area of ??the air gap half magnetic field, so that the electromechanical conversion coefficient BT cannot maintain a constant electrodynamic effect F=BTI and the linear relationship is destroyed, resulting in nonlinear distortion.
To improve the distortion caused by this reason, two methods are generally used: one is to use a short voice coil, and the other is to use a long voice coil. The so-called short voice coil means that the length of the voice coil is made smaller than The thickness of the magnetically conductive plate is small, as shown in the figure, so that the voice coil does not jump out of the uniform area of ??the magnetic field during vibration, thereby avoiding nonlinear distortion.
This method causes cost submission and is not commonly used. The so-called long voice coil refers to the length of the voice coil, which is made longer than the thickness of the washer, so that the voice coil interacts with all the magnetic components during vibration. Through phase coupling, (including uniform area and non-uniform area), the average magnetic induction density B remains generally constant to avoid nonlinear distortion, but this method will inevitably cause the speaker to have a thicker sound under the same DC resistance. The sensitivity of the coil wire decreases because you increase the rotation amplitude of the voice coil and the mass vibration of the voice coil increases. BT, your magnetic gap becomes larger due to the thickening of the voice coil wire, B becomes smaller, and B2T2MD.