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frequency and duty cycle relationship

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mexx

mexx

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hi folks

just when i was trying to control my dc motor by a pwm signal, i simply couldn't figure out the ideal frequency at which dc motors should be controlled..
and hows the frequency related to the speed of motor b'coz i guess its duty cycle which decides the speed at which the motors is moving.

when i gave 50% 38khz..motor was moving fine at a voltage supply of 10 volts
but when i gave 25khz ...it moved and stopped aftr sometime..
what is the reason?

thanks in advance
 

Please explain what circuit are you using to produce the PWM signal and what drives your motor.

And what you motor is!

Without details, fishing for an unknown fish in the ocean is TOO much. :D
 

i am using a h-bridge driver circuit and its geared dc motor .
there's no specification mentioned on it but when i gave a supply of 10volts it took 26mA.

although i am using 16f877 to control the motor, i am generating PWM by software and not using the pwm modules provided in the micro.
the driver circuit is L298N.
just one modification, instead of fast recoverydiodes mentioned in datasheet i am uding standard IN4007 across the motors

thats it.
 

Generally Half bridge circuits outputs will be desabled due to over currents etc. just check that out.

100 to 1000 Hz is generally fine. 25 or 35K is too much even thogh it works. As there is no sufficient time to swich on/switch off the outputs of half bridges and lot of transients can damage the motor/circuit.

Cheers
 

In many applications a concern is to keep the frequency out of the audible range.

Found many examples of actual circuits with 20 KHz.
 

The bridge you use is specified to work at 25kHz and up to 40kHz. Of course you can operate it at lower frequencies but it will become audible... Keep in mind when you increase the frequency the commutation losses also increases. That is why you are limited at 40k.

I don't think that your problem is related to the fact that you lowered the frequency... I would search elsewhere.

Using rectifier diodes (1N4xxx) in such a circuit isn't a very good idea. These diodes are very slow to turn on, thus they won't conduct the current accumulated in the motor properly. This will lead to EMI and transients problems and in the worst case your bridge can be destroyed by voltage spikes.

Hope this helps,

CyBeRbLaK
 

I am actually new to this topic. Right now I am working on a project which controls DC motors with H-bridges. H-briges are internal to micro. Now I am driving with a PWM of frquency 100Hz. Motor works fine. But I am able here sound. I was thinking that sound is due to physical movements.

Will sound disappears, If I increase the frequency of PWM above 20Khz? Is there any other factors to consider?

the spec says the maximum Highside and Lowside switching frequency of the H-bridge is 25Khz.

Thanks in advance
Cheers
 

I'm so strange to the term half bridge. I know only H-bridge which is used in order to reverse the direction of a DC motor.

When using PWM, I always use 4K - 5K of frequency. There's no transparent explanation how to choose the correct frequency. But f out of audible frequency band is ok (i used to try 20K, it's OK)
 

Idlebrain,

The bridge itself won't emit sound if you use frequencies above ~16kHz (at 16kHz you'll still hear a high-pitched whine).

Usually 20kHz is a nice figure.
 

I usually use 500 Hz or so, I've got no problems with noise.

When I first began, the motor stop because I undersized the transistor (or didn't protect it properly) and it burned up. This is easy to check, send DC to the H-bridge to turn it fully on. If nothing happens, you broke something. You can also connect the motor to DC to see if it still turns.
 

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