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Is it better to use PWM with DC Chopper to control DC motor?

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mostafa0020

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noise using pwm to control dc motor

Hi all,
Controlling DC motor with a DC chopper is a well known matter,but,using PWM technique with DC motor seems strange with me!!as we use it in AC systems to improve the harmonic behaviour of the output current"both load and network current waveform".we cost though some switching losses.but,what is the benefit of using PWM on a DC chopper!!??
 

pwm technique for controlling dc motors

If I'm not mistaken, DC chopper also controlled using PWM technique, right? Pls correct me if I'm wrong.
 

variable chopper using pwm

u r right
and that i have studied is that we use simple PWM to control single pulse width and freqeuncy of the AC chopper.
my question was about why,from drive circuit view,we control the width using PWM technique based on triangular or sawtooth carrier wave compared with a constant DC reference wave as known.which a modulation index..
the problem is out of the harmonics control as its corresponding on the AC choppers or inverters.
My question is about the driving wave generation.why PWM?
 

advantages of pwm technique in dc

These switching methods are much more efficient that putting a drop in series with the motor. This is sort of like the difference between a switching and an analog regulated power supply.

The other advantage of PWM is at the low end of the motor speed. The mechanical rotation speed is more linearly related to the width than the motor speed would be related to a low voltage. This effect was first observed in toy model railroads. The acceleration of the engines from a stop was not as realistic unless PWM was used.
 

pwm based dc chopper power supply

mostafa0020 said:
my question was about why,from drive circuit view,we control the width using PWM technique based on triangular or sawtooth carrier wave compared with a constant DC reference wave as known.which a modulation index.
This is probably one of the methods to generate variable-duty-cycle chopping waveform to power electrical motor. The triangular waveform can be symmetrical or asymmetrical.
Another way of producing 'chopping waveform' to control motor is 'bang-bang' and 'hysterisis' controls.

mostafa0020 said:
the problem is out of the harmonics control as its corresponding on the AC choppers or inverters.
That's why a filter is required if you wish to reduce the harmonics back to main. Usually the frequency of PWM used to control motor is FIXED. Let's say the frequency is fixed at Fo. The only variable, in this case, is the duty cycle. If we look into frequency (noise) spectrum, we will see there is a 'spike' with highest amplitude at that particular frequency. This 'spike', sometimes, might prevent the product from complying with EMC regulation. Some researchers have shown that, if the PWM frequency changed 'randomly', the energy of noise spectrum will be spread over a wide range of frequencies so that the highest peak is reduced.
 

pwm dc dc chopper

In a dc-dc converter, there is a fixed and not very regulated voltage (say the rectifier voltage) in one side, on the other side we want to have a regulated and variable dc voltage which for example we can use for controling a DC motor.
a chopper that chop the primary dc voltage to give a percentage of that voltage to the output or to regulate it, can be used with fix frequency and variable pulse width.
why fix frequency? because we design the magnetic element for a fix frequency and why variable pulse width? to control the percentage of transfered power (or simply voltage). that is PWM.
 

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