Chapter 6:
Electric Motors
arrange commutator segments to match the number of coils so that you have the force on each of these coils acting in unison with the force on all the other coils.
DC Motors in the Real World Now it’s time you met real-world DC motors—their construction, definitions, and efficiency. Let’s start by looking at their components.
Armature
The armature is the main current-carrying part of a motor that normally rotates (brushless motors tend to blur this distinction) and produces torque via the action of current flow in its coils. It also holds the coils in place, and provides a low reluctance path to the flux. (Reluctance is defined as (H 3 1)/4 and measured in ampere-turns per lines of flux.) The armature usually consists of a shaft surrounded by laminated sheet steel pieces called the armature core. The laminations reduce eddy current losses; steel is replaced by more efficient metals in newer designs. There are grooves or slots parallel to the shaft around the outside of the core; the sides of the coils are placed into these slots. The coils (each with many turns of wire) are placed so that one side is under the north pole and the other is under the south pole; adjacent coils are placed in adjacent slots, as shown at the bottom of Figure 6-2. The end of one coil is connected to the beginning of the next coil so that the total force then becomes the sum of the forces generated on each coil.
Commutator
The commutator is the smart part of the motor that permits constant rotation by reversing the direction of current in the windings each time they reach the minimum flux point. This piece is basically a switch. It commutates the voltage from one polarity to the opposite. Since the motor rotor is spinning and has momentum, the switching process repeats itself in a pre-ordained manner. The alternating magnetic poles continue to provide the push to overcome losses (friction, windage, and heating) to reach a terminal speed. Under load, the motor behaves a bit differently, but the load causes more current to be drawn. Physically, it’s a part of the armature (typically located near one end of the shaft) that appears as a ring split into segments surrounding the shaft. These segments are insulated from one another and the shaft.
Field Poles
In the real world, electromagnets (recall your toolbox nail with a few turns of insulated copper wire wrapped around it) are customarily used instead of the permanent magnets you saw in Figure 6-1 and Figure 6-2. (Permanent magnet motors are, in fact, used, and you’ll be formally introduced to them and their advantages later in this section.) In a real motor the lines of flux are produced by an electromagnet created by winding turns of wire around its poles or pole pieces. A pole is normally built up of laminated sheet steel pieces, which reduce eddy current losses; as with armatures, steel has been replaced by more efficient metals in the newer models. The pole pieces are usually curved where they surround the armature to produce a more uniform magnetic field. The turns of copper wire around the poles are called the field windings.
139
