Video Transcript
What type of current does a
commutator generator produce?
To answer this question, you’ll
need to understand the basic construction and operation of a commutator
generator. Like most electric generators, the
key component of a commutator generator that actually produces the electric current
is a loop of wire rotating around inside a magnetic field. As the loop rotates, the magnetic
flux through the loop changes, which induces a potential difference.
Now, for our particular kind of
generator, the loop is connected to a commutator, which is essentially a ring split
into two-halves. The commutator connects the loop of
wire to an external circuit through conductive brushes. And we use these brushes because
they can slide freely past the commutator allowing the loop of wire to rotate
without twisting up the external circuit but still maintaining electrical
contact.
When we connect external components
like this lamp across the brushes, the external components, the commutator, and the
loop of wire form a complete circuit, allowing the potential difference induced in
the loop to drive current through the external components. And it is this current that we’re
asked about in the question. To understand the current produced
by this generator, we’re going to draw the loop at three successive times. We will also draw the commutator at
those times as well, so we can see how this circuit is connected. Note that in our diagram, we have
colored one side of the commutator magenta and the other side yellow. And we have colored one of the
brushes yellow and one of the brushes magenta. This is so that we can keep track
of which brush is connected to which side of the commutator.
Here is our first diagram. On the right is the loop of wire
viewed from the side, so we can see how many magnetic field lines it is
crossing. On the left, we show the connection
of the commutators and the brushes. And we can see that at this time,
the magenta brush is connected to the yellow portion of the commutator and the
yellow brush is connected to the magenta portion of the commutator. This first picture actually shows
us the same instant in time as the initial diagram that we drew.
Since the loop is rotating
clockwise about its center from this angle, at a later time, the loop will be fully
perpendicular to the magnetic field. And at a still later time, the loop
will be tilted in opposite direction to what it was at the original time. At this second time, neither brush
is connected to a conducting portion of the commutator. And at the third time, the magenta
brush is connected to the magenta portion of the commutator and the yellow brush is
connected to the yellow portion of the commutator.
This leads us to our first
important observation. Between initial time and the final
time, the brushes and the commutator have swapped connections. In a direct-current circuit,
reversing electrical connections also reverses the direction of the current. This is what would happen, for
example, if we reverse the orientation of a battery. Now, this generator is not a
direct-current circuit, but we should keep this idea of reversing current in mind
when we make our next observation.
At the first time picture, the loop
of wire is crossing two of the magnetic field lines we have drawn. At the second time picture, the
loop is crossing four of the magnetic field lines we have drawn. So between the first and second
times, the flux through the loop has increased. Counting the same way, we see that
at the third time, the loop is again only crossing two magnetic field lines. So the flux has decreased between
the second time and the third time.
Now we recall that a changing flux
results in a potential difference, which is how the generator produces power. But we also recall that the sign of
the potential difference depends on the sign of the changing flux. Now the sign of the potential
difference determines which direction current will be driven. So if the generator is connected to
an external circuit, when the flux is increasing, the current in the loop will have
one direction. And when the flux is decreasing,
the current in the loop will have the opposite direction. But this means that between the
first and last times shown, the direction of current in the loop is reversed.
What we have then is that the
current in the loop reverses between our two times, but so does the connection to
the external circuit. Now remember that reversing the
connection reverses the direction of current. But the direction of current is
already reversed, so we are reversing a reversed current, which means that when the
connection to the commutator switches orientation, the direction of current in the
external circuit stays the same.
Indeed, this is the function of a
commutator. A commutator ensures that even
though the direction of current in the loop of the generator is changing, the
direction of current in the external circuit is constant.
The last observation we need to
make is that the magnitude of the current is not constant. And this makes sense. As the loop rotates in the magnetic
field, the amount of change in flux is not a constant quantity. So the amount of induced current is
not constant. In fact, the current in the loop of
wire itself is an alternating current that cycles back and forth between a maximum
magnitude in one direction and the same magnitude but in the opposite direction. Because the current in the loop is
an alternating current — that is, it changes magnitude — but the commutator makes
the direction of the current constant in the external circuit, this generator
produces rectified alternating current — that is, current that varies in magnitude
like alternating current but only has one direction.
