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Problems when reducing speed of a motor using PIC 16f877 PWM

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eltonsy

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hi all,

i'm a mechanical engineer who is new to the embedded system world.

i was working on a feeder for some kind of machine. anyways i am using the pic as controller (pic 16f877) i used the PWM module to control a DC Motor speed and direction using an H-Bridge IC this is my prooblem "when ever i connect the motor to h-Bridge it works perfect at full speed but whenever i try to reduce the speed using the PIC PWM the motor produces a high noise and don't rotate.

Please Help me with this problem i've been stuck there.

Thanks
 

Re: motor and pic 16f877

Would you send more info!
 
Re: motor and pic 16f877

i am using an H bridge IC and i send to it the control signal from the microcontroller for the PWM.

the problem is when is when i do this the motor produces a noise and don't move
???
 

motor and pic 16f877

Hello eltonsy
Some question are in order....
When running at full speed you are still pulsing your H bridge? Or, you turn on just two transistors in the bridge and apply full voltage to motor?
Can you reverse rotation (at full speed at least) in your bridge?
What is your switching frequency? depending on motor size, something lower than - let's say - 2 kHz is too low.
Best regards
 

Re: motor and pic 16f877

thanks for the quick responce

i meant when i turn on the 2 transistors and apply full voltage i said this to confirm there is no problem with the H-Bridge. when i start pulsing the h bridge the motor start this noise and stop moving.

my frequence is 2.5 KH
my motor is 30V,2 A
please help me with this cause i have been stuck in here for a long time

thanks
 

motor and pic 16f877

eltonsy,

I think you should set a higher switching freq.
PWM works only if the motor inductance and freq response act as integrator. Your motor is small, so some 10 kHz may be a minimum switching frequency.
BTW, your motor is whistling at 2.5 kHz??
If you don't have idea, compare the sound with a signal from an audio generator.
If the whisting is at lower freq. you should review your PWM generation.
Hope this helps
Best regards
 

Re: motor and pic 16f877

The low frequency is one of possible problems, have you a scope, will be good if you see what exactly happen. Because if you start motor without load i think that he must be rotate, despite the low frequency.
If you have a scope, look the amplitude over the motor, if you dont have steep front may be the problem is in your H bridge. When you start the PWM if the bridge increase very fast temperature this can be a effect of the slant front. And ..... you have a diodes parallel to the bridge transistors ?
 

Re: motor and pic 16f877

Hi,
I found the same high pitched noise from a RC model motor. I was trying to drive it with PWM at approx 3kHz. Some of thing I investigated was the following,

1) the frequency of the PWM
If the frequency was to low the effective average DC voltage/current would be insufficient to create the torque required to overcome the rotor's inertia

2) As some other member said the frequency has to be picked such that the motor's equivalent circuit (resistance / inductor) functions as a integrator

A combination of the above solved the whineing problems i had, although I also blew a few of the H-Bridge IC's during motor transients, resulting from fast directional changing.

just my 2 cents,
cheers
slayer
 

Re: motor and pic 16f877

I have similar problem with small dc motor (under 5W) and one solution is to try with much lower frequency ~200Hz. It works fine for me.
I thinhk problem is in high inductance .
 

Re: motor and pic 16f877

Yes, high inductance is other problem, for this reason when using PWM current control in stepper motor the supply give much high voltage, for examples, in my 2 coordinate machine design i use 5 V stepper motors, but when i start in PWM i use 30 V power supply. This is a guarantee steep front, PWM i use to fix the current to nominal.
 

motor and pic 16f877

i need a pic 16f877 project. if any body have any idea then mail me
 
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