I;m trying to get my PWM bi-directional motor control to work silently. The motor driver I have isn't working at its rated frequency. Cheap Amazon seller... I was playing around I was wondering if this analog device would work to drive a bi-directional motor. It seems too simple, so I'm guessing no.

 

Functionally it might work, but with poor performance from the motor and generate a huge amount of waste heat.
So your guess is mostly correct.

What is the PWM driver and what frequency is it supposed to operate?

 

A resistor in series with a motor limits its current. Its current is high when it starts and when it is loaded then it does not work properly with a series resistor. PWM produces full power for each pulse but it averages the voltage for a low speed with lots of torque.

 

I'm using this motor driver. HiLetgo BTS7960 43A High Power Motor Driver Module/Smart Car Driver Module for Arduino Current Limit https://www.amazon.com/dp/B00WSN98DC/ref=cm_sw_r_apan_i_SQSDWWN9D9SQY992SYZE?_encoding=UTF8&psc=1
The chips are rated at 25 kHz. One direction will run up to 4 kHz, the other direction will run up to 8 kHz. Measured correct input frequency at the controller inputs. But nothing out from the motor side. Cheap controller maybe.

 

So why not run it with a 4KHz PWM?

Bob

 

So why not run it with a 4KHz PWM?

Bob

It's very loud under a heavy load.

 

It's very loud under a heavy load.

You are hearing a 4kHz whine?

 

You are hearing a 4kHz whine?

Average human hearing is up to 20 khz

 

A Brushed-Motor is never going to be totally quiet, ( until You get into the BIG-Money ).
For quiet You need a BLDC-Motor, and it won't necessarily be all-that quiet either.
It also depends on your definition of "quiet".
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Got it running at 15.6 kHz. which is above both mine and my wife's range of hearing. I was entering the Arduino code incorrectly. No whine will full power at a dead stall.

 

When I was a teenager some "ultrasonic" burglar alarms were too loud. Now at my age of 76 my hearing aids produce the high audio frequencies. Maybe I should add woofers to my smoke detectors.

 

As it turns out, running at 15.6 kHz, the motor is too weak. I have been looking into solutions. I was wondering if I could place a ceramic, non-polarized, capacitor between the motor terminals to absorb the PWM generated frequency in the motor circuit. Then I could run it at 7.8 kHz. Would this work?

 

You do not want to short-circuit the PWM pulses with a capacitor which will overload the PWM source.
When the pulse widths are wide then they are almost the same as the power supply DC that might have a voltage that is too low for the amount of power you want.

 

You need a "Freewheeling-Diode" attached to the Motor-Terminals.
It must be able to handle the Full-Load-Amps of the Motor.

A small Capacitor, ( 0.1uf ), ( 100nf ), won't make it any quieter, but it might help to keep down
the normally generated RFI-hash, which could smoke parts.
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You need a "Freewheeling-Diode" attached to the Motor-Terminals.
It must be able to handle the Full-Load-Amps of the Motor.

A small Capacitor, ( 0.1uf ), ( 100nf ), won't make it any quieter, but it might help to keep down
the normally generated RFI-hash, which could smoke parts.

Freewheeling-Diode. Never heard of that before. But would it work with a bi-directional motor? I though diodes were one-way devices.

 

You do not want to short-circuit the PWM pulses with a capacitor which will overload the PWM source.
When the pulse widths are wide then they are almost the same as the power supply DC that might have a voltage that is too low for the amount of power you want.

Not sure I understand what you're saying. The 5v PWM source controls the H-bridge. What I'm trying to do is smooth out the 24v circuit that is controlled by that PWM signal.

 

Average human hearing is up to 20 khz

Mine isn't but that wasn't the purpose of my question.
I was asking if he heard a 4kHz whine, or something else.

 

What I'm trying to do is smooth out the 24v circuit that is controlled by that PWM signal.

Likely you will need go to a higher PWM frequency to reduce the audible noise.
You can't use a capacitor.

 

Likely you will need go to a higher PWM frequency to reduce the audible noise.
You can't use a capacitor.

Tried that. Can't hear it at 15.6 kHz but the motor is significantly weaker. Too weak actually.

 

Can't hear it at 15.6 kHz but the motor is significantly weaker. Too weak actually.

That's likely more to due to slow switching of the output transistors rather than any affect the frequency causes with the motor.