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PWM frequency limit for DC motor speed control

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savvej

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PS.note : Question is pertaining dc motor speed and direction control using 8086 uP ,8255,8254 and JK flip flop

According to our Microprocessor textbook,it is said that while using pwm for dc motor speed control,the frequency of the output should be in the range of 60Hz to 1000Hz.
What is it which is causing the limit for the frequency of pwm?

Just to jist the pwm method described in our text bk:
Consider the H bridge,2 inputs say A and B.
when we input A=1,B=0 , motor rotates in one direction(say forward)
when we input A=0,B=1,motor rotates in the reverse direction.
Now we connect A and B to Q and Q' of the flip flop.
//////////////////////////////////////////////////////////////////////////
You may skip this as it just explains how the PWM is generated
///////////////////////////////////////////////////////////////////////////
To the Clr and Preset inputs we connect the 2 timer outputs(counter1 and counter0) from the timer ic 8254.
The timer operates in a mode called rate generator mode,in which supposing that if we load a count of 'n' ,it will be high for 'n-1' clock cyles and then go low for 1 cyle and this constitutes a pulse.Now these pulses are generated continuously once we start the timer.
Now ,what we do is ,say that we connect Clr pin to Cnt1,and start it.Cnt0 output is phase shifted by starting it at a later time.Now what we get at the output of flip-flop is pwm.
/////////////////////////////////////////////////////////////////////////////
so by changing the duty cycle of PWM,motor can be run in
forward fast---forward ---forward slow---stop--reverse slow---reverse--reverse fast
<-----------------------------------------|------------------------------------>
As in we are changing the ratio reverse and forward duration of the motor and thus effective running motor in forward,stop or reverse direction..
===============================================================


Now what I have been using while working microcontrollers in dc motor conntrol,is we use L293D H brige driver IC and used to give just 1 PWM input (In case of driving 1 Motor) to its enable input controlling its speed and direction(using A and B inputs) which I think it better than what is mentioned in the textbook as it gives independent control.



PS.The textbook is followed is The Intel Microprocessor by Barry Brey
 
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jpanhalt

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According to our Microprocessor textbook,it is said that while using pwm for dc motor speed control,the frequency of the output should be in the range of 60Hz to 1000Hz.
What is it which is causing the limit for the frequency of pwm?

There is no such limit in PWM motor control. There have been lengthy debates about the interplay of inductance and frequency, but the bottom line is that there are many motor controllers out there operating at much higher frequencies than 1000 Hz. One reason to use a higher frequency is to avoid the annoying whine of lower frequencies.

I do not have that book available. When was it published? Can you copy and post the paragraph in question?

John

@Bill,

Thanks for the note about PM. I didn't notice it was off. It is now set to on.

John
 
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QuietMan

Member
I have an article about PWM.



You are welcome.

Bill Marsden
 
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savvej

Member
@jpanhalt

see the highlighted text in pic 1

and pic 2 is frontpage of the book.
 

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savvej

Member
just in case if the highted text s not clear,it says:
"It is important to keep this operating frequency below 1000Hz ,but above 60Hz"
and it s a PHI publication(In india) for Pearson Education ,of the year 2006.
 
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jpanhalt

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Is it referring to a specific motor? 2006 should be modern. Certainly, many PWM's DC motor controls were in the 2400 to 20K Hz range. I'm stumped, unless it is with reference to a specific motor and application.

John
 

savvej

Member
regaring the input fed to the H bridge:

@Quietman : I went through the Pwm link u sent

Just a verfication:quoting from the link
"While basically accurate, this schematic of an H-Bridge has one serious flaw, it is possible while transitioning between the MOSFETs that both transistors on top and bottom will be on simultaneously, and will take the full brunt of what the power supply can provide. This condition is referred to as shoot through, and can happen with any type of transistor used in a H-Bridge. If the power supply is powerful enough the transistors will not survive. It is handled by using drivers in front of the transistors that allow one to turn off before allowing the other to turn on."


Could you just elaborate on this "shoot through" phenomenon and explain which drivers are used to avoid it?


For the ckt daigram given in our text:
In giving the Pwm input to the H bridge we are using ic- 7406(hex invertor).
Is it for the same purpose of avoiding this "shoot through"?
 

misterT

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According to our Microprocessor textbook,it is said that while using pwm for dc motor speed control,the frequency of the output should be in the range of 60Hz to 1000Hz.
What is it which is causing the limit for the frequency of pwm?

For a DC motor there is no upper limit for the PWM frequency, the higher the better. Higher PWM frequencies results in smaller current ripple in the motor windings and more efficient operation of the motor. Sometimes people lower their PWM frequency because they can't get the motor turning with small duty cycles, but that is just poorly designed control.

The upper limit (1000Hz) in your application probably comes from the limitations of the control electronics. Either the H-Bridge can't switch faster or the PWM can't be generated faster (with desired resolution).
 
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misterT

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and regarding 7406 function? is it for avoiding shoot through effect?

No, they are just buffers for the signals Q and Q' from the flip-flop. There are full-bridge and half-bridge driver ICs that have the shoot-through protection integrated.
 

misterT

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could u just tell the specs of the driver ic?

ir2304 is a simple half-bridge MOSFET driver with internal deadtime, but you can't use it to drive transistors. I think you don't have to worry about shoot-through if you use the h-bridge shown in the schematic you posted (the one with inverters, transistors and flip-flop).
 
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