Re: PWM Back Emf

Yes, thank you very much. That helps a lot. I misunderstood the
reason for the current sense resistor. I thought its purpose was to
detect current changes. I can see the how it all works now.


--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jim Register
<jtregister@...> wrote:

>
> Frank,
>
> The current sense resistor lets you get at the back emf of the

motor

> while power is applied, rather than when the motor is coasting.
>
> First, model the motor as a generator (back emf) in series with a
> resistor (the motor's internal resistance). Now, add the current

sense

> resistor from the motor to ground. We'll call the junction

between the

> motor and the current sense resistor "A".
> The voltage at point "A" = (applied voltage - back emf) x voltage
> divider ratio.
>
> To cancel out the applied voltage, add a second voltage divider,

with

> the same ratio, connected from the applied voltage to ground.

We'll

> call the output of this voltage divider "B". If you use an opamp

and

> subtract voltage "A" from "B" with a gain equal to the voltage

divider

> ratio, the output is back emf, all by itself. You will want a

trimpot

> in this second voltage divider so you can set the ratio

precisely. I

> just applied a small voltage to the motor while holding the shaft

from

> rotating and adjusted the trimpot for 0 volts out of the opamp.
>
> The times I have done this, the drive amps were analog, not PWM -

I have

> not dealt with sampling the back emf signal at a particular time.

I've

> only done this with small motors - 1 amp drive or less.
>
> Hope this helps,
> Jim
>
> On Sun, 2007-01-14 at 03:47 +0000, Frank wrote:
> > That doesn't really help me understand. It sounds like if I try

to

> > modify the speed by changing the PWM, it won't; it will return

to

> > preset speed. I think I am going to have to put a circuit

together

> > and try and understand it.
> >
> > Thanks for trying to explain it.
> >
> >
> >
> > --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Mariss Freimanis"
> > <mariss92705@> wrote:
> > >
> > > That's where the trimpot comes in. Too much and the motor

speeds up

> > > with increasing load. Too little and it slows down. Just right

and

> > the
> > > speed doesn't change at all. Kind of like Goldilocks, the 3

bears

> > and
> > > the beds, the chairs, the porridge or whatever.:-)
> > >
> > > Mariss
> > >
>