From: "Dwight K. Elvey" 
To: atm{at}shore.net
cc: JBHillman{at}ev1.net
Reply-To: "Dwight K. Elvey" 


>From: "Jerry B. Hillman" 
>
>Hi Dwight,
>I read with interest your post concerning the feedback issue on the DC
>motor.  As far as I know, I started this thread with the post you read about
>the motor singing.
>I appreciate the discription concerning feedback, IR (internal resistance?)
>and EMF (electromagnetic frequency?) but I am not an electronics engineer.
>You went over my head right away. I don't have any testing equipment and
>don't own, or know how to use an ohm meter.
>This isn't to say I am not willing to learn.  I just have to learn all this
>at the pace my high school education and intelligence forces me to.
>Thanks again for the information.  I can see that I have a lot to learn
>still.
>Clear skies, Jerry
>
>

Hi Jerry
 I thought is was about a DC motor.
 EMF stands for electro-motive force. I know that doesn't
mean much either. I'll put it this way, a DC motor can be a generator as
well as a motor. If you had perfect motor, the rotation rate of the motor
would exactly follow the voltage you put in. You know, 1 volt is 100 RPM
and 2 volts is 200 RPM. One would then ask that if the motor was connected
to a large fly wheel and suddenly removed the power source from the motor,
what would be the voltage on the motor cause by the generator effect ( call
'back EMF' )? It would be exactly equal to the voltage of the power supply
used to spin up the motor, in our perfect motor.
 Now, lets talk about delivering some actual work with the
motor. In this case, the torque of the perfect motor would be proportional
to the current flow into the motor. If our supply could supply infinite
current to the motor, we'd not be able to change the speed of our perfect
motor with any load.
 Well, we don't have perfect motors. There is the perfect
part of the motor that still makes the back EMF ( the spin of the motor
generates a voltage that is proportional to the RPM of the motor ) but we
have this additional part caused by the resistive loss of the motor.
Luckily, the resistive part is relatively constant. At really high RPM's,
the brushes floats a little and increase the effective resistance but as a
hole, it is real close to that that is measured at zero speed. This means
there are two parts to a motor. There is the perfect part that generates
the back EMF and the resistive part that is proportional to the load.
 If we know what the voltage drop is across the resistive
part, we can add it to the voltage we use to drive the motor and then any
input control voltage will equal the back EMF. This means that the input
voltage will be proportional to the RPM, and compensate for load.
 We can determine the voltage across the internal resistance
by knowing the resistance ( by measurement ) and using an external series
sense resistor to measure the current. Knowing the current is proportional
to the voltage across both the internal and external resistor, we can use
that in a feedback loop to drive the motor( Ohms law ).
 I hope this makes a little more sense. This can all be done
with simple Op-Amps.
 There is one more factor that needs
to be dealt with and that is the inertia of the motor. If we don't restrict
the frequency response of the amplifier, it will tend to over compensate
and oscillate. We can experiment with the real motor and try various
frequency roll-offs to get the system stable.
Dwight

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