2-3
current flowing through it from an external source. This current causes a magnetic field to be produced.
This field is indicated by the dotted line through the loops. The loop (armature) field is both attracted and
repelled by the field from the field poles. Since the current through the loop goes around in the direction
of the arrows, the north pole of the armature is at the upper left, and the south pole of the armature is at
the lower right, as shown in figure 2-2, (view A). Of course, as the loop (armature) turns, these magnetic
poles turn with it. Now, as shown in the illustrations, the north armature pole is repelled from the north
field pole and attracted to the right by the south field pole. Likewise, the south armature pole is repelled
from the south field pole and is attracted to the left by the north field pole. This action causes the armature
to turn in a clockwise direction, as shown in figure 2-2 (view B).
After the loop has turned far enough so that its north pole is exactly opposite the south field pole, the
brushes advance to the next segments. This changes the direction of current flow through the armature
loop. Also, it changes the polarity of the armature field, as shown in figure 2-2 (view C). The magnetic
fields again repel and attract each other, and the armature continues to turn.
In this simple motor, the momentum of the rotating armature carries the armature past the position
where the unlike poles are exactly lined up. However, if these fields are exactly lined up when the
armature current is turned on, there is no momentum to start the armature moving. In this case, the motor
would not rotate. It would be necessary to give a motor like this a spin to start it. This disadvantage does
not exist when there are more turns on the armature, because there is more than one armature field. No
two armature fields could be exactly aligned with the field from the field poles at the same time.
Q1. What factors determine the direction of rotation in a dc motor?
Q2. The right-hand rule for motors is used to find the relationship between what motor
characteristics?
Q3. What are the differences between the components of a dc generator and a dc motor?
COUNTER EMF
While a dc motor is running, it acts somewhat like a dc generator. There is a magnetic field from the
field poles, and a loop of wire is turning and cutting this magnetic field. For the moment, disregard the
fact that there is current flowing through the loop of wire from the battery. As the loop sides cut the
magnetic field, a voltage is induced in them, the same as it was in the loop sides of the dc generator. This
induced voltage causes current to flow in the loop.
Now, consider the relative direction between this current and the current that causes the motor to run.
First, check the direction the current flows as a result of the generator action taking place (view A of fig.
2-2). (Apply the left-hand rule for generators which was discussed in the last chapter.) Using the left
hand, hold it so that the forefinger points in the direction of the magnetic field (north to south) and the
thumb points in the direction that the black side of the armature moves (up). Your middle finger then
points out of the paper (toward you), showing the direction of current flow caused by the generator action
in the black half of the armature. This is in the direction opposite to that of the battery current. Since this
generator-action voltage is opposite that of the battery, it is called "counter emf." (The letters emf stand
for electromotive force, which is another name for voltage.) The two currents are flowing in opposite
directions. This proves that the battery voltage and the counter emf are opposite in polarity.
At the beginning of this discussion, we disregarded armature current while explaining how counter
emf was generated. Then, we showed that normal armature current flowed opposite to the current created
by the counter emf. We talked about two opposite currents that flow at the same time. However, this is a
