2-4
bit oversimplified, as you may already suspect. Actually, only one current flows. Because the counter emf
can never become as large as the applied voltage, and because they are of opposite polarity as we have
seen, the counter emf effectively cancels part of the armature voltage. The single current that flows is
armature current, but it is greatly reduced because of the counter emf.
In a dc motor, there is always a counter emf developed. This counter emf cannot be equal to or
greater than the applied battery voltage; if it were, the motor would not run. The counter emf is always a
little less. However, the counter emf opposes the applied voltage enough to keep the armature current
from the battery to a fairly low value. If there were no such thing as counter emf, much more current
would flow through the armature, and the motor would run much faster. However, there is no way to
avoid the counter emf.
Q4. What causes counter emf in a dc motor?
Q5. What motor characteristic is affected by counter emf?
MOTOR LOADS
Motors are used to turn mechanical devices, such as water pumps, grinding wheels, fan blades, and
circular saws. For example, when a motor is turning a water pump, the water pump is the load. The water
pump is the mechanical device that the motor must move. This is the definition of a motor load.
As with electrical loads, the mechanical load connected to a dc motor affects many electrical
quantities. Such things as the power drawn from the line, amount of current, speed, efficiency, etc., are all
partially controlled by the size of the load. The physical and electrical characteristics of the motor must be
matched to the requirements of the load if the work is to be done without the possibility of damage to
either the load or the motor.
Q6. What is the load on a dc motor?
PRACTICAL DC MOTORS
As you have seen, dc motors are electrically identical to dc generators. In fact, the same dc machine
may be driven mechanically to generate a voltage, or it may be driven electrically to move a mechanical
load. While this is not normally done, it does point out the similarities between the two machines. These
similarities will be used in the remainder of this chapter to introduce you to practical dc motors. You will
immediately recognize series, shunt, and compound types of motors as being directly related to their
generator counterparts.
SERIES DC MOTOR
In a series dc motor, the field is connected in series with the armature. The field is wound with a few
turns of large wire, because it must carry full armature current. The circuit for a series dc motor is shown
in figure 2-3.
