Question Video: Identifying the Current Produced in a Commutator Generator | Nagwa Question Video: Identifying the Current Produced in a Commutator Generator | Nagwa

Question Video: Identifying the Current Produced in a Commutator Generator Physics • Third Year of Secondary School

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What type of current does a commutator generator produce?

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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.

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