If you are driving a transistor connected to ground, it means the motor
has one lead to VCC, than it seems to be an under driving  situation.
This should be the reason why your voltmeter shows the same voltage no
matter which pwm signal you use. The transistor is not switching
completely to ground.  Why don't you show a simple schematic about this
driving design?

Wagner

soon lee wrote:
>
> thanks for the infor
> the wave that i see is not that noisy there are pulse that synchronics with
> the pwm with the max at supply voltage and min at about  0.9 Vss
> no matter the duty cycle min is about 0.9Vss
>
> 5v
>  ___       ___
> |      |      |      |    PWM
> |      |___|      |
> 0v
> Vss(output)
>  ___        ___         ___
> |      |___ |      |____|     |    0.9Vss
> |
> |
> |
> 0v
> so anyone have any idea what when wrong??
> thanks
>
> regards
>
> ----- Original Message -----
> From: Wagner Lipnharski <wagner@USTR.NET>
> To: <PICLIST@MITVMA.MIT.EDU>
> Sent: Wednesday, November 24, 1999 10:08 PM
> Subject: Re: PWM speed control[OT]
>
> > Measuring Voltage is not the ideal way to analyze a PWM waveform.
> >
> > The old analog voltmeters could do it in someway because the needle
> > shows you the Voltage average by the mechanical inertia.
> >
> > The actual electronic digital voltmeters can not. They are terrible to
> > measure square waves and all, even if set to AC mode. They use an ADC to
> > convert instantaneous signals to digital and then they show it at the
> > LCD display, so by doing this sometimes they got the positive side,
> > sometimes not, and it is not a regular thing, so they display crazy
> > levels.
> >
> > Without the motor you are measuring right the digital switched voltage
> > (if your meter does it correctly).
> >
> > WITH the motor you are measuring also the motor EMF when the PWM is off,
> > so in general terms you can not do it safely.  Try to scope the signal,
> > and you will find out how messy is the signal over the motor; there are
> > ringings, spurious signals and all.
> >
> > Wagner
> >
> > > soon lee wrote:
> > >
> > > Hi people
> > >
> > > I have a problem here hope you can help me
> > > ok I am actually using a L298 H-bridge to control the motor
> > > the EN pin is tied to a PWM signal and by using the different in duty
> > > cycle to
> > > get the change in speed.
> > >
> > > the problem is that when I measure the output at the driver side
> > > without the motor, the value that I get is proportional to the duty
> > > cycle every thing is perfect but when I connect it to the motor... the
> > > voltage does not defer much. the value for 30% to 90% only change
> > > about 1V.so any one have any idea how to solve this. is it I didn't
> > > use the correct frequency??
> > >
> > > the value I am using are
> > > PWM     62.5Hz
> > > voltage    30V
> > >
> > > thanks
> > > regards
> > >