motor produces a sound with PWM...doesn't work

[Rans]

Hello,
I used PWM with PIC16F877A to control a geared DC motor. MikroC was used for programming. I used an ULN2003 IC to drive the motor. Whenever I connect a LED or checked using the voltmeter, my program is working properly. However, as I connect the DC motor the motor produces an audible noise and attempting to rotate(but it doesn't rotate). The noise is high when the duty cycle is low. I used capacitors(0.1uF,1uF,47uF,etc) in parallel with the motor. When the capacitor value is high, the problem is partially ok. But still produces the noise slightly. However, if capacitors are used my timing would be different!
Kan anybody help me with this???

PS: I've checked all the supply voltages and pin connections, they are ok. I've changed ICs and checked. I'm pretty sure about the firmware as well.

PS: One of my motors was damaged running on this condition!!(now itz actual rpm is less than that I bought it newly!!) . Now i'm testing with a cheap motor.

 

[nandhu015]

Post your circuit so that answering will be easier.

What is the rating of your motor?

Nandhu

 

[Rans]

This is the cct:

https://www.electro-tech-online.com...ers/2270d1086534455-pic-l293d-circuit_193.jpg


The motor is 12 V /80 rpm motor... I directly tried with 6 V and the required current was about 250 mA (max current across L293D is 600 mA!!)

 

[himrwt]

it will be more clear if you post your circuit and algo. but from symptoms it seems that you are 'accelerating' and 'braking' motors in your pwm instead of 'exciting' and 'leaving'.

during your 'brake' period back emf must be generating which stops any motion produced in motors. this causes vibrations and subsequent noises.
as impedance of motor windings remains low at stationary position, high current passes through them and cause burn-out. this explains low rpm and damaging of motors.

with your current circuit increasing your duty cycle may help. i believe duty cycle of 80% and above should help. also increase in frequency along with capacitors across motor terminals may help.

i prefer using l293 with dc motors for pwm. with l293 you can toggle enable pins, which removes short circuit condition. this way you can avoid 'back-emf'.

hope that be helpful. if you have any doubts feel free to ask.

himanshu