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What does PWM mean ?

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Rooftop

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Re: pwm

Sometime i confused about it too.
PWM = Pulse Width Modulation right? so i think it's about duty cycle and i tried to control my servo with it and it works fine.
But my friend make same duty cycle and adjust the frequency and it works fine too and he called it PWM(adjusting the frequency)
Can someone answer me how it exactly works?Thanks

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waseem

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Re: pwm

Hi!
don't be confused. you are doing it the right way. Whenever we talk about pulse width modulation, it means a wave with a constant frequency and variable duty cycle. The duty cycle can also be changed by changing the frequency,but that is not the right way.
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silvio

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Re: pwm

Rooftop said:
But my friend make same duty cycle and adjust the frequency and it works fine too and he called it PWM(adjusting the frequency)
Can someone answer me how it exactly works?

If you read carefully (especialy the text below the figure 2) the suggested web page then you shoul figure out easily if your friend is doing the right way.

Figure 2 shows a simple circuit that could be driven using PWM. In the figure, a 9 V battery powers an incandescent lightbulb. If we closed the switch connecting the battery and lamp for 50 ms, the bulb would receive 9 V during that interval. If we then opened the switch for the next 50 ms, the bulb would receive 0 V. If we repeat this cycle 10 times a second, the bulb will be lit as though it were connected to a 4.5 V battery (50% of 9 V). We say that the duty cycle is 50% and the modulating frequency is 10 Hz.

PWMFigure2.gif

Figure 2. A simple PWM circuit

Most loads, inductive and capacitative alike, require a much higher modulating frequency than 10 Hz. Imagine that our lamp was switched on for five seconds, then off for five seconds, then on again. The duty cycle would still be 50%, but the bulb would appear brightly lit for the first five seconds and off for the next. In order for the bulb to see a voltage of 4.5 volts, the cycle period must be short relative to the load's response time to a change in the switch state. To achieve the desired effect of a dimmer (but always lit) lamp, it is necessary to increase the modulating frequency. The same is true in other applications of PWM. Common modulating frequencies range from 1 kHz to 200 kHz.
 

Rooftop

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Re: pwm

thanks for the repply but i still have one question.

It's right about the freuquency but why i have different result from doing 1khz duty cycle 50% and 5khz duty cycle 50%?
that's not changing duty cycle but the frequency. if we can just adjust the frequency why we still called it PWM(Pulse Width Modulation) . Sorry i'm not offensive here... but i just want to know the truth because a lot of people often adjusting the frequency than duty cycle.
Thanks for your help and really sorry if my words is too rude .


Best reggards
 

Nimer

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Re: pwm

hi
pwm is a method of controlling the amount and rate of energy (not power) transfer from source(grid or battery) to load (motor) .

normally the rate of power transfer is chosen to be constant and this is achieved by keeping the frequency constant , if you choose not ,its okay but the panelty is that harmonics and motor torque pulsaton will increase in aparent manner .

on the other hand the amount of energy transfered is controlled by the duty cycle , this method does not produce the harmonics , this what makes it better than the aprevious method .


both methods are may and are some times used to control motors in whats called variable frequency variable voltage control VFVVC.




regards
 

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