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PWM doubt

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blibala

New Member
is the speed of the motor is affected by the PWM frequecy or is it solely affected by the duty cycle of the PWM pulse?
or higher frequency makes motor spins faster and vice versa?
 

stevez

Active Member
I'll share what I learned from others and from my own research.

PWM is generally used on DC motors. I suppose that it could work on an AC supply to a universal motor.

I understand that the frequency of pulses is limited by heating and other problems.

As the pulse goes from very short (nearly "off") to very long (almost "on" continuously) the total power applied varies as if a constant DC voltage were varied from 0% to 100%. How the RPM varies would seem to be somewhat dependent on the load and the motor itself.
 

blibala

New Member
is the speed of the motor determined by the frequency?
i knew that the speed is determined by the duty cycle of the pulse.then wat about the frequency?
 

bmcculla

New Member
Only the duty cycle should affect the speed of the motor. In an ideal PWM system Frequency has no effect on the speed of a motor. However, as you increase the frequency the switching time of your transistors starts to have an effect on their power dissipation.

Transistors have the lowest resistance when they are fully on. When you first apply voltage to the gate it takes a while to charge the gate capacitor to turn on the transistor fully. Durring this turn on time the transistor has a higher resistance and so dissipates more power. The faster you switch your PWM the more time overall the transistors spend in the less efficient turning on or turning off state.

That said you should have no problem with frequencies well into the kHz as long as the component driving the transistor gate supplies a decent amount of current. There are MOSFET driver ICs designed to turn on and off mosfets very quickly if you are worried.

Hope this helps

Brent
 

stevez

Active Member
The bits and pieces that I gleaned from my readings suggested that the load itself might be impacted by frequency. The authors/designers were justifying their choices of frequency (somewhere around 400 Hz or so) explaining that higher might be better except that heating of the motor would result.
 

blibala

New Member
can i use power transistor TIP120? can it handle frequency more than kHz?
or can i use relay?can i switch on and off faster?
which one is better?
coz i decided to use PIC to generate PWM pulse to drive the transtistor.is it convenient?
 

bmcculla

New Member
Stevez- I think you are right. The inductance of the motor will start to work like a filter at higher frequencies disipating power. I don't think this effect will be a problem for most applications though. A grat deal of motor control applications opperate above 20kHz to get rid of the anoying buzz that is caused at lower frequencies.

blibala - I don't se any reason why the TIP120 wouldn't work into the kHz range. There isn't any real reason to go much above 20kHz though; just go high enough so the motor doesn't buzz.

Brent
 

Faiyaz

New Member
The average voltage Due to PWM is given by the integration of the total amplitude of pulses. Therefore although the frequency will not effect the o/p , it does effect the accuracy of the signal o/p.
 

AMSA84

New Member
What determines the speed of the motor is the amount of power is delivered to the DC motor. If the duty cycle is at 10% the ON time will be very small, so the power delivered to the motor is low. If the duty cycle is about 90% there will be more power delivered to the DC. I had that doubt to x)

Best regards.
 

Noggin

Member
This might go without saying, but I've seen schemes where frequency is adjusted to drive motors. The on pulse width time was not adjusted but the frequency was adjusted to speed up or slow down the arrival of the pulses. Of course, its the changing duty cycle the speeds up and slows down the motors, but one not familiar with PWM and/or driving DC motors might mistake this for the frequency controlling the motor speed.

In any case, a DC motor's speed is generally proportional to the voltage across its leads. Check the datasheet to see what the rated voltage of the motor is. If it is rated for 2.5 to 7v, I think common electrical knowledge (is there such a thing?) would alert someone to not drive it above 7v, but they might not worry about using 1.5v. In my experiences, this can also be bad. The lower voltage doesn't get the motor up to speed (especially under load) which can greatly increase the current through the motor, which will overheat it and cause premature failure.
 
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