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PWM PWM control Duty 95% 50% 5% cycle Supply voltage Load A simple comparator with a sawtooth carrier can turn a sinusoidal command into a pulse-width modulated output. In general, the larger the command signal, the wider the pulse. Pulse-width modulation The output of a PWM amplifier is either zero or tied to the supply voltage, holding losses to a minimum. This specification defines the intended operation of a fan that implements the Pulse Width Modulation (PWM) control signal on the 4-wire fan interface. The introduction of 4 wire PWM controlled fans is a means to reduce the overall system acoustics. The expectation is a 4 wire.
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PWM is a technique that is used to reduce the overall harmonic distortion (THD) in a load current. It uses a pulse wave in rectangular/square form that results in a variable average waveform value f(t), after its pulse width has been modulated. The time period for modulation is given by T. Therefore, waveform average value is given by
$$bar{y}=frac{1}{T}int_{0}^{T}fleft ( t right )dt$$Sinusoidal Pulse Width Modulation
In a simple source voltage inverter, the switches can be turned ON and OFF as needed. During each cycle, the switch is turned on or off once. This results in a square waveform. However, if the switch is turned on for a number of times, a harmonic profile that is improved waveform is obtained.
The sinusoidal PWM waveform is obtained by comparing the desired modulated waveform with a triangular waveform of high frequency. Regardless of whether the voltage of the signal is smaller or larger than that of the carrier waveform, the resulting output voltage of the DC bus is either negative or positive.
The sinusoidal amplitude is given as Am and that of the carrier triangle is give as Ac. For sinusoidal PWM, the modulating index m is given by Am/Ac.
Modified Sinusoidal Waveform PWM
A modified sinusoidal PWM waveform is used for power control and optimization of the power factor. The main concept is to shift current delayed on the grid to the voltage grid by modifying the PWM converter. Consequently, there is an improvement in the efficiency of power as well as optimization in power factor.
Multiple PWM
The multiple PWM has numerous outputs that are not the same in value but the time period over which they are produced is constant for all outputs. Inverters with PWM are able to operate at high voltage output.
The waveform below is a sinusoidal wave produced by a multiple PWM
Voltage and Harmonic Control
A periodic waveform that has frequency, which is a multiple integral of the fundamental power with frequency of 60Hz is known as a harmonic. Total harmonic distortion (THD) on the other hand refers to the total contribution of all the harmonic current frequencies.
Harmonics are characterized by the pulse that represent the number of rectifiers used in a given circuit. It is calculated as follows −
$$h=left ( ntimes P right )+1 quad or quad -1$$Where n − is an integer 1, 2, 3, 4….n
P − Number of rectifiers
It is summarized in the table below −
Harmonic | Frequency |
1st | 60 Hz |
2nd | 120 Hz |
3rd | 180Hz |
4th | 240Hz |
5th . . 49th | 300Hz . . 2940Hz |
Harmonics have an impact on the voltage and current output and can be reduced using isolation transformers, line reactors, redesign of power systems and harmonic filters.
Series Resonant Inverter
A resonant inverter is an electrical inverter whose operation is based on oscillation of resonant current. Here, the switching device and the resonanting component are connected in series to each other. As a result of the natural features of the circuit, the current passing through the switching device drops to zero.
This type of inverter yields a sinusoidal waveform at very high frequencies in the range of 20kHz-100kHz. It is therefore, most suitable for applications that demand a fixed output such as induction heating and flourescent lighting. It is usually small in size because its switching frequency is high.
A resonant inverter has numerous configurations and thus it is categorized into two groups −
- Those with unidirectional switches
- Those with bidirectional switches
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Pulse Width Modulation or PWM is a common technique used to vary the width of the pulses in a pulse-train. PWM has many applications such as controlling servos and speed controllers, limiting the effective power of motors and LEDs.
Basic Principle of PWM
Pulse width modulation is basically, a square wave with a varying high and low time. A basic PWM signal is shown in the following figure.
There are various terms associated with PWM −
- On-Time − Duration of time signal is high.
- Off-Time − Duration of time signal is low.
- Period − It is represented as the sum of on-time and off-time of PWM signal.
- Duty Cycle − It is represented as the percentage of time signal that remains on during the period of the PWM signal.
Period
As shown in the figure, Ton denotes the on-time and Toff denotes the off-time of signal. Period is the sum of both on and off times and is calculated as shown in the following equation −
$$T_{total} = T_{on}+T_{off}$$Duty Cycle
Duty cycle is calculated as the on-time of the period of time. Using the period calculated above, duty cycle is calculated as −
$$D = frac{T_{on}}{T_{on}+T_{off}} = frac{T_{on}}{T_{total}}$$analogWrite() Function
The analogWrite() function writes an analog value (PWM wave) to a pin. It can be used to light a LED at varying brightness or drive a motor at various speeds. After a call of the analogWrite() function, the pin will generate a steady square wave of the specified duty cycle until the next call to analogWrite() or a call to digitalRead() or digitalWrite() on the same pin. The frequency of the PWM signal on most pins is approximately 490 Hz. On the Uno and similar boards, pins 5 and 6 have a frequency of approximately 980 Hz. Pins 3 and 11 on the Leonardo also run at 980 Hz.
On most Arduino boards (those with the ATmega168 or ATmega328), this function works on pins 3, 5, 6, 9, 10, and 11. On the Arduino Mega, it works on pins 2 - 13 and 44 - 46. Older Arduino boards with an ATmega8 only support analogWrite() on pins 9, 10, and 11.
The Arduino Due supports analogWrite() on pins 2 through 13, and pins DAC0 and DAC1. Unlike the PWM pins, DAC0 and DAC1 are Digital to Analog converters, and act as true analog outputs.
You do not need to call pinMode() to set the pin as an output before calling analogWrite().
analogWrite() Function Syntax
value − the duty cycle: between 0 (always off) and 255 (always on).
Example