Understanding Torque and H-Bridges for a DC Motor

Joined Jul 18, 2013
26,319
If you don't have a 12v supply of adequate current, stick it across an automotive battery, if it fails to run at ~3krpm and has considerable torque , then there is something wrong!
Max.

BarryBozeman

Joined Apr 1, 2016
41
You mean you can connect 12V to it and it doesn't run?
Or that you've never connected it directly to 12V?
If not I suggest you try that.

It doesn't run when connected directly to 12v.
It does run when the 12v is passed through a voltage regulator, but not well.
It runs well when I use a PWM circuit, but the PWM circuit is very noisy.

Joined Jul 18, 2013
26,319
It doesn't run when connected directly to 12v.
.
Post #21 will confirm it in short order!
Max.

shortbus

Joined Sep 30, 2009
9,764
While the PWM does "work", the frequency bleed is a problem.
It runs well when I use a PWM circuit, but the PWM circuit is very noisy.
What frequency "bleed"? If you can hear the PWM you need to increase it's frequency to one that's out of hearing range.

BarryBozeman

Joined Apr 1, 2016
41
Well, I'm using a PWM circuit based on the attached (using an LM339, along with a CD4050 and a TIP120)...

I've experimented with changing values for R1 and C1.
If R1=100k and C1=47pf, the circuit was still quite audible at slower motor rotations. Changing R1 to 470k stops it from working altogether.

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crutschow

Joined Mar 14, 2008
31,146
I've experimented with changing values for R1 and C1.
Why?
If R1=100k and C1=47pf, the circuit was still quite audible at slower motor rotations.
That should give a PWM frequency of about 100kHz.
If you are hearing anything then it must be some sub-harmonics of that.
Changing R1 to 470k stops it from working altogether.
470k to to large a resistance for that circuit.
If you want to reduce the frequency, increase the value of C1.

BarryBozeman

Joined Apr 1, 2016
41
I was told here that ultrasonic PWM is the only way to drive a motor quietly enough to use with audio equipment.
My suspicion is that anything with a square wave is going to cause some difficulty.

shortbus

Joined Sep 30, 2009
9,764
I was told here that ultrasonic PWM is the only way to drive a motor quietly enough to use with audio equipment.
So now the goal post has been moved? Audio equipment is far different than human hearing. Most people can't hear PWM in a motor when the frequency is over around 16kHz. And most motor PWM circuits use 20kHz as a top number.

BarryBozeman

Joined Apr 1, 2016
41
The first post says: "I can't use PWM for this application because it involves audio circuitry and the oscillation is too loud. "

I'm sorry if that was not stated clearly enough.

I've been trying to solve the same problem for almost a year now - I'm not sure how to phrase it any better.

crutschow

Joined Mar 14, 2008
31,146
My suspicion is that anything with a square wave is going to cause some difficulty.
You might try a series inductor and capacitor to ground to smooth the current going to the motor.
Do you have an oscilloscope to look at the waveforms?