4. Resultados
4.1.2. Efecto de la dieta hipercolesterolémica sobre los niveles de lípidos en sangre
At present, voltage source converters are mostly used in electrical drives. These converters utilize capacitors in the DC-link to store temporarily electrical energy. Switching the power electronic devices allows the DC voltage to be modulated which can result in a variable voltage and frequency waveform. The purpose of the modulator is to generate the required switching signals for these switching devices.
There are various ways of modulation. The overview of a modulation method is listed in Figure 2-13. Multilevel Modulation strategies are mainly divided into Synchronous and Asynchronous Modulation[25]. The division is based on the value of index called frequency modulation index.
𝑚𝑓 =𝑓𝑓𝑐𝑠 . Here 𝑚𝑓 is called frequency modulation index, 𝑓𝑐 is called carrier frequency and 𝑓𝑠
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Figure 2-13 Modulation strategies[24].
The modulation strategies are as shown in Figure 2-13. Modulation strategies are divided into two major areas which are as follows:
a) Asynchronous Modulation:
When stator frequency is very low, it impacts frequency modulation to be greater than 20. There will be high pulse number. In such scenario, zero crossing between carrier wave and reference signal is not synchronized even if it results in unequal number of pulses in positive and negative half cycle. For such high pulse number, difference of few pulse numbers between positive and negative half cycle does not make huge impact hence the effect of sub-harmonic component is minimal. Such kind of modulation in which zero crossings between the carrier wave and reference signal are not synchronized is called asynchronous modulation.
There are two types of asynchronous modulation.
i) Space Vector PWM
ii) Carrier Based PWM
Among these two type Carrier Based is discussed here. • Carrier Based PWM :
In Carrier Based PWM, generally there is reference wave which is compared with carrier wave. The switching pulses are determined by the result of the comparison. If reference wave is greater in magnitude than the carrier wave then pulse is ON otherwise OFF. Carrier Based PWM is further divided according to the sampling of the reference wave. They are
a) Naturally sampled PWM b) Regularly Sampled PWM
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Naturally sampled Pulse Width Modulation:
This is the classical method where analog circuitry like amplifier is used for comparison of carrier wave and reference wave. The reference voltage then change continuously within the triangular period (carrier wave) as in Figure 2-14. If continuously changing reference wave is greater than triangular wave, the switching pulse is ON otherwise OFF.
Figure 2-14 Naturally sampled PWM[23].
Regular sampled Pulse Width Modulation:
A method which is easier to implement in digital form is the regular sampled PWM. In this case the reference voltage is sampled at top and bottom of the triangular wave and then kept constant until next sample. This sampled reference wave is compared with the magnitude of triangular wave and switching occurs. Similar to Naturally Sampled PWM, the pulse is ON if sampled reference wave is greater than triangular wave and OFF for the opposite case. This method is further divided into:
• Symmetrical Regular sampled PWM • Asymmetrical Regular sampled PWM
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• Symmetrical Regular sampled PWM: The reference voltage is only sampled at the top of the triangular wave and kept constant within the complete triangular period.
• Asymmetrical Regular sampled PWM: The reference voltage is sampled both at the top and bottom of the triangular wave and kept constant within half the triangular period.
Figure 2-15 Symmetric and asymmetric regular sampled PWM.
The reference wave is stator frequency, which keep on changing. There will be change in the carrier wave as well if frequency modulation is kept constant. As we know the formula for the frequency modulation,
𝑚𝑓 =𝑓𝑓𝑐𝑠
where 𝑚𝑓 is called frequency modulation index, 𝑓𝑐 is called carrier frequency (or switching frequency) and 𝑓𝑠 is called reference fundamental frequency.
𝑓𝑐 = 𝑚𝑓∗ 𝑓𝑠
For, the application in motor, for the variable speed drive, the stator frequency (or fundamental frequency 𝑓𝑠 ) may vary. For particular mf, 𝑓𝑐 varies proportional to 𝑓𝑠. However to keep the
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decrease when stator frequency is increased or vice versa as seen in Figure 2-16. In order to keep carrier frequency constant, there is change in the ratio (mf) in the different ranges of fc.
Figure 2-16 Carrier based PWM [23].
b) Synchronous Modulation:
For small values of frequency modulation (20 or less), the number of pulse are lower hence even the difference in pulse number in positive and negative half cycle by one unit will generate sub harmonics. To prevent from such scenario, the carrier waveform signal and the control signal should be synchronized to each other. That means the zero crossing of carrier wave (triangular) and reference wave (sinusoidal) should coincide in order to prevent from sub harmonic. This type of modulation is called synchronous modulation. As a result of this, number of pulses in positive half cycle is equal to that in negative half cycle. The average switching frequency is fc /2. This type of modulation is important for low pulse number
whose violation can result in sub-harmonics in motor voltage. Synchronous Modulation is further divided into two parts called fundamental frequency synchronous PWM and Program PWM.
i) Fundamental frequency Synchronous PWM:
ii) Program Modulation PWM:
Among these two, this thesis deals with the second type. • Program Modulation PWM
This method pre-calculates optimal switching angles within a period of the fundamental period 1/fs to minimize the harmonic distortion. But these switching angles are not formed by carrier
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tables in hardware. This type of modulation is termed as Program Modulation. Since this is also synchronous modulation the number of pulses in positive and negative half cycle are equal. This is explained in detail in section below.