To Mr. Darrel Taylor once again

[atwoz]

Again Darrel, I want to thanks you for another of your great extensions, this time the Multi_SPWM. It works great, I have a question tho.

Suppose I want to use those PWM's to drive a motor, where currents can be high. How can I step up the voltage? Using transistors maybe? A ULN2803A perhaps? What would you recommend? Maybe a npn with a pnp, dunno..

thanks in advance!

[Darrel Taylor]

Hi Art,

The question of low frequency PWM driving motors has come up before.
malc-c was trying to use it on a model train.

There was lot's of whining coming from the motor.
Didn't sound good at all.

I'm more of a software guy, so I really don't have much to offer on the hardware side.
But it seems that motors want a much higher frequency than Multi_SPWM can deliver.
<br>

[Acetronics2]

Hi, Art

you will find the whole "story" here:

http://www.picbasic.co.uk/forum/showthread.php?t=5687

and

http://www.picbasic.co.uk/forum/showthread.php?t=6205

in the end ... we used a 16F872/876 @ 4Mhz ... that allows a 245 Hz HPwm ... and you can use the TWO channels of the device, if needed .

an IRLZ 44 ( do not forget the flywheel diode !!! ) was used to drive power and a simple relay used for inverting polarity ...

May be MalcC will add here some "user infos" ...

Alain

[Charles Linquis]

I do a lot of motor speed control using PWM. Generally, you should choose a PWM frequency above human hearing (>20KHz). This guarantees that you won't create any growls, squeals or other noises.
Also, a lot of motors don't like the chopped DC that PWM produces (this is especially true of the brushless types). In that case, you need to provide a storage inductor and a filtering capacitor to smooth the output. By properly choosing the components, you can wind up with a signal to the motor that is a varying DC voltage with only tens of millivolts of ripple. The higher the PWM frequency, the smaller the inductor needs to be. I generally use 78KHz as the minimum frequency.
Generally, the topology you will want to use is called a "buck" regulator. An excellent tutorial, as well as component selection guides can be found on the National Semiconductor website.

I realize that such frequencies generally eliminates the possibility of using software PWM, but such an approach is necessary for serious use. As an example, one of my designs uses one PWM channel of an 18F8722, and smoothly controls a 25A at 48V brushless motor (over 1HP).

[Jnutzz]

Hello,

Could there be a way to adjust frequency on the fly?

Thanks
Jordan Adair

[mackrackit]

Hardware or software ?

[Jnutzz]

Hardware..

[mackrackit]

You will need to setup a couple of variables. One for Dutycycle and one for Frequency to control the Channel.


DC VAR BYTE 'Dutycycle
FQ VAR WORD 'Frequency

HPWM 1,DC,FQ

[Jnutzz]

I'm using multi_SPWM. I'm wondering if there is a way to change the frequency using a pot?

[Darrel Taylor]

Not the way it is written.

The frequency is calculated a compile-time and cannot be changed at run-time.

Of course, that doesn't mean it can't be done.
Just that you can't do it with that program.
<br>

[malc-c]

May be MalcC will add here some "user infos" ...

Alain

Not much to add, in that, whilst it worked there were issues, one of which later came to light later after some use in that the motor life was shortened some what (these small 5 pole motors apparently don't like pulsed supplies as it has a heating effect on the motor).