Hello there. I'm new here.

I'm gonna make an R/C project. Right now I'm testing the H-Bridge, so I have made an PWM based on 555.

There is here the first problem. When I start making a frequency variation through the potentiometer, just right DC motor start rolling there is a strange sound coming from the DC motor. I think it might be something related with the frequency and the DC motor (coils?).

I'd like to know how can I eliminate that sound.

 

The sound you hear may be the coils in the motor acting like miniature speakers. If you change the frequency, the noise should change. It is very common. Some people go high frequency (e.g., 20KHz); some just live with it.

John

 

Ok. I just thought about that. But imagine..

There is a contorl signal that will allow the PWM signal drive the transistors to make the car go fwd and backwd. If I want the car to ride kinda slow, wich frequency I'll need? Something like what, 2khz? But maybe you have are right, because I'm not experimenting the car at the ground, he's on air (supported).

But do you understand what I'm trying to say?

Best regards

 

It is the duty cycle, which controls the speed, not the frequency.

 

I find 2KHz quite annoying. At 20KHz, hearing response is fairly low (gone if you are old like me), but there are harmonics. Just make the frequency something you can live with and control speed with duty cycle, as already mentioned. John

 

The inductance of a motor reduces the current when the PWM frequency is high.
If you want to have full power then the frequency should be from 200Hz to about 4kHz.

__________________
Uncle $crooge

 

The sound could be generated in the motor because its being driven by a square wave instead of a sine wave. The square wave has multiple frequencies see FFT which will shake the coils and make noises. You may be able to dampen the effect if you put a capacitor across the motor.

 

That point has been argued many times. I think 4QD has a fairly balanced discussion of it and makes both high- and low-frequency drives.

Ultimately, it becomes a balance of linearity, efficiency, and noise. The OP did not say much about his motor.

If I were just building a toy to run around the house, I would opt for higher frequency to avoid the annoyance.

John

 

Ok. Thank you guys. There is another doubt:

I've noted that the frequency on the output of 555 (PWM mode) is diferent from the frequency at the motor input.

Can anyone tell me why? That is not supposed to be equal? The transistors are switching at the same frequency than the 555? Or what determines the frequency at the motor input is the ON time of the duty-cycle?

Best regards

 

You could smooth the current out a bit with a large capacitor right at the motor, similiar to the noise blocking caps that are on most DC motors. Small motor run capacitors may work as they're not usually polarized. I've heard some people suggest using back to back electrolytics as a non-polarized cap but I don't know how reliable this is.

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What you mean by "smooth the current out a bit with a large capacitor right at the motor"?

Please explain yourself.

Best regards!

 

As for the frequency shift, my first guess is that you are seeing an artifact of they way you are measuring it. Are you using an o'scope or something like a digital multimeter? Does the error change with duty cycle?

John

 

Well, I'm using a digital multimeter (but tomorrow I'll watch on a scope). The frequency at the motor input changes a little bit. Variantions ~400Hz.

So my doubt is that the frequency at the motor input is defined by the ON time of the duty-cycle?

Another question, how can you guys explain me why the output of the PWM 555 signal changes when we vary the potentiometer? I know that the "speed" of the motor is based on the ON time of the duty-cycle, but what I cannot understand is why the voltage changes. For example, 15% of the duty-cycle corresponds to 2.5V and 50% carresponds to 5.0V. Can you understand what I mean?

Best regards

 

Your voltmeter is showing a time averaged voltage, not peak to peak. That is, on DC, the sampling time includes several cycles. When you use the o'scope, you will see the peaks are fairly constant in amplitude. John

 

Ok then. Tomorrow I'll study the wave forms in every single part of the cricuitry and if doubt persist I'm gonna let you know.

Best regards