Low-power Idle and Power-down Modes Applications Indoor applications no imp ambient light is present To measure speed of object moving at a very high speed Industry Tachometers Stepper motor; First of all, a stepper motor is a motor that converts electrical power into mechanical power. The main difference between them and all the other motors is the way they revolve. Unlike other motors, stepper motors does not continuously rotate! Instead, they rotate in steps from which they got the name. Each step is a fraction of a full circle. This fraction depends mostly from the mechanical parts of the motor, and from the driving method.

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When such loads are fed by inverters, its essential that output voltage of the inverters is so controlled as to fulfill the requirements of AC loads. There are various techniques to vary the inverter gain. The most efficient method of controlling the gain and output voltage is to incorporate pulse-width modulation PWM control within the inverters.

The carrier based PWM schemes used for multilevel inverters is one of the most straight forward methods of describing voltage source modulation realized by the intersection of a modulating signal Duty Cycle with triangular carrier wavefroms. The Alternative PWM strategies with differing phase relationships are: Alternate phase disposition APOD : Every carrier wave form is in out of phase with its neighbor carrier by degree. Phase Opposition Disposition POD : All carrier waveforms above zero reference are in phase and are degree out of phase with those below zero reference.

Phase Disposition PD : All carrier waveforms are in phase 1. A digital to analog converter DAC is then employed for converting them into their analog signal forms. Phase controlled converters when operated in the inverter mode are called line commutated inverters. But line commutated inverters require at the output terminals an existing AC supply which is used for their commutation. This means that line commutated inverters cant function as isolated AC voltage sources or as variable frequency generators with DC power at the input.

Therefore, voltage level, frequency and waveform on the AC side of the line commutated inverters cant be changed. On the other hand, force commutated inverters provide an independent AC output voltage of adjustable voltage and adjustable frequency and have therefore much wider application. A current source inverter is fed with adjustable current from a DC source of high impedance,i. In a CSI fed with stiff current source, output current waves are not affected by the load.

From view point of connections of semiconductor devices, inverters are classified as under Bridge Inverters Series Inverters Parallel Inverters Page 2 1. The unit provides up to 1.

An inverter converts the DC electricity from sources such as batteries, solar panels, or fuel cells to AC electricity. The electricity can be at any required voltage; in particular it can operate AC equipment designed for mains operation, or rectified to produce DC at any desired voltage. Grid tie inverters can feed energy back into the distribution network because they produce alternating current with the same wave shape and frequency as supplied by the distribution system.

They can also switch off automatically in the event of a blackout. Micro-inverters convert direct current from individual solar panels into alternating current for the electric grid.

One type of UPS uses batteries to store power and an inverter to supply AC power from the batteries when main power is not available. When main power is restored, a rectifier is used to supply DC power to recharge the batteries. It is widely used at domestic and commercial level in countries facing Power outages.

To do this, AC power is first rectified to provide DC power. The inverter then changes the DC power to high frequency AC power. At the receiving location, an inverter in a static inverter plant converts the power back to AC. An inverter provides the controlled power. In most cases, the variable-frequency drive includes a rectifier so that DC power for the inverter can be provided from main AC power.

Since an inverter is the key component, variablefrequency drives are sometimes called inverter drives or just inverters.

Various improvements in inverter technology are being developed specifically for electric vehicle applications. The output power can never exceed the input power, but efficiencies can be high, with a small proportion of the power dissipated as waste heat. No quantization of the samples normally occurs Fig. In principle the pulses may occupy the entire time between samples, but in most practical systems the pulse duration, known as the duty cycle, is limited to a fraction of the sampling interval.

Such a restriction creates the possibility of interleaving during one sample interval one or more pulses derived from other PAM systems in a process known as time-division multiplexing TDM. Figure 1 a Analog signal, s t. The pulse duration can be changed relative to fixed leading or trailing time edges or a fixed pulse center. To allow for time-division multiplexing, the maximum pulse duration may be limited to a fraction of the time between samples Fig.

In addition, the maximum time excursion of the pulses may be limited Fig. In these so-called binary digital systems, the analog-to-digital conversion process is extended by the additional step of coding, in which the amplitude of each pulse representing a quantized sample of s t is converted into a unique sequence of one or more pulses with just two possible amplitudes. The complete conversion process is known as pulse-code modulation. Assuming that the number of quantization levels is limited to 8, each level can be represented by a unique sequence of three two-valued pulses Figure 2 a Three successive quantized samples of an analog signal.

First, the design parameters of a PCM transmission system depend critically on the bandwidth of the original signal s t and the degree of fidelity required at the point of reconstruction, but are otherwise largely independent of the information content of s t. This fact creates the possibility of deploying generic transmission systems suitable for many types of information.

Second, the detection of the state of a two-state variable in a noisy environment is inherently simpler than the precise measurement of the amplitude, duration, or position of a pulse in which these quantities are not constrained. Third, the binary pulses propagating along a medium can be intercepted and decoded at a point where the accumulated distortion and attenuation are sufficiently low to assure high detection accuracy.

New pulses can then be generated and transmitted to the next such decoding point. This so-called process of repeatering significantly reduces the propagation of distortion and leads to a quality of transmission that is largely independent of distance.

Pulse-width modulation PWM of a signal or power source involves the modulation of its duty cycle, to either convey information over a communications channel or control the amount of power sent to a load. The entire control circuit can be digital, eliminating the need for digital-to-analog converters. Using digital control lines will reduce the susceptibility of your circuit to interference. Finally, motors may be able to operate at lower speeds if you control them with PWM. When you use an analog current to control a motor, it will not produce significant torque at low speeds.

The output voltage control can be obtained without any additional components. With this method, lower order harmonics can be eliminated or minimized Along with its output voltage control. As higher order harmonics can be filtered easily the higher order harmonics can be minimized. When a high frequency signal has amplitude varied in response to a lower frequency signal we have AM amplitude modulation.

When the signal frequency is varied in response to the modulating signal we have FM frequency modulation. These signals are used for radio modulation because the high frequency carrier signal is needs for efficient radiation of the signal.

When communication by pulses was introduced, the amplitude, frequency and pulse width become possible modulation options. In many power electronic converters where the output voltage can be one of two values the only option is modulation of average conduction time.

Figure Figure 3 Page 8 3. The advantage of linear processing for this application lies in the ease of de-modulation. The modulating signal can be recovered from the PWM by low pass filtering. For a single low frequency sine wave as modulating signal modulating the width of a fixed frequency fs pulse train the spectra is as shown in Fig 2. Clearly a low pass filter can extract the modulating component fm. Figure 4 3. As seen in Fig 2 the output signal goes high when the sine wave is higher than the saw tooth.

This is implemented using a comparator whose output voltage goes to logic HIGH when ne input is greater than the other. Other signals with straight edges can be used for modulation a rising ramp carrier will generate Figure 5 Page 9 3.

This is an easy scheme to implement using analog electronics but suffers the imprecision and drifts of all analog computation as well as having difficulties of generating multiple edges when the signal has even a small added noise. Many modulators are now implemented digitally but there is difficulty is computing the precise intercept of the modulating wave and the carrier.

Regular sampled PWM makes the width of the pulse proportional to the value of the modulating signal at the beginning of the carrier period. In Fig 5 the intercept of the sample values with the triangle determine the edges of the Pulses. For a saw tooth wave of frequency fs the samples are at 2fs. Figure 6 Regular sampled PWM 3. Once the sine wave reaches the peak of the triangle the pulses will be of maximum width and the modulation will then saturate. The Modulation depth is the ratio of the current signal to the case when saturation is just starting.

Thus sine wave of peak 1. The most efficient method of controlling the gain and output voltage is to incorporate pulse width modulation PWM control within the inverters. The commonly used techniques are I.

The PWM techniques listed above differ from each other in the harmonic content in their respective output voltages. Thus,choice of a particular PWM technique depends upon the permissible harmonic content in the inverter output voltage. The inverter topology remains the same for a single phase inverter and for a three phase inverter.

But now the devices are now switched ON and OFF several times within each half cycle to control the output voltage which has low harmonic content. Page 11 4. The gating signals are generated by comparing a rectangular reference signal of the amplitude Ar with triangular carrier wave of amplitude Ac,the frequency of the carrier wave determines the fundamental frequency of output voltage. By varying Ar from 0 to Ac,the pulse width can be varied from 0 to percent.

The ratio of Ar to Ac is the control variable and defined as the modulation index. The generation of gating signals for turning ON and OFF transistors by comparing a reference signal with a triangular carrier wave. The frequency Fc, determines the number of pulses per half cycle. The modulation index controls the output voltage. This type of modulation is also known as uniform pulse width modulation UPWM.

The distortion factor and lower order harmonics are reduced significantly. The gating signals are generated by comparing a sinusoidal reference signal with a triangular carrier wave of frequency Fc. The frequency of reference signal Fr ,determines the inverter output frequency and its peak amplitude Ar,controls the modulation index M,and rms output voltage VO.

The number of pulses per half cycle depends on carrier frequency. One of the most straightforward methods of describing voltage-source modulation is to illustrate the intersection of a modulating signal duty cycle with triangle waveforms.



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