Find and read the page
on difference
amplifiers.
The striped thing
between the motor and
the dc generator is a
shaft which couples them
together mechanically.
The motor drives the
generator (tachometer)
via this shaft.
The set speed control
provides a dc voltage,
say 12 volts for maximum
speed and zero for
stationary. This could
be a potentiometer
providing any voltage in
a range from zero to +12
volts.
The difference amplifier
will amplify any
difference between its
two input voltages.
If the motor is
stationary and the speed
control is moved from
zero to half speed then,
since the tachometer is
not rotating and not
producing an output
voltage, there will a
difference in voltages
at the two inputs of the
difference amplifier.
Therefore there will be
an output voltage from
the amplifier.
Since this voltage is
not high enough in value
to drive the motor, it
is increased in
amplitude by the dc
amplifier.
A dc amplifier is a
special type of
amplifier which can
increase dc voltages.
For example, 10 volts dc
in could give 100 volts
dc out.
This amplified dc powers
the dc motor which
begins to increase its
speed of rotation.
This in turn rotates the
tacho which produces a
voltage proportional to
speed.
As the tacho voltage
increases it will
eventually reach the
same value as the "set
speed" voltage.
At this point there will
be no output from the
difference amplifier and
dc amplifier. The motor
is up to the correct
speed.
However, since the motor
is no longer powered by
the dc amplifier its
speed will start to
fall. But the tacho output
voltage will start to
fall, and there will
again be a difference
between the two input
voltages to the
difference amplifier. This will produce an
output from the
difference amplifier and
dc amplifier which will
power the motor and
correct this drop in
speed.
In a poorly designed
system this drop in
speed and its correction
can cause "hunting", a
regular variation in
speed. |