Chapter 3: AC and DC Motors - DC Motors: General Principles of Operation

Basic Components

Two basic circuits are in any DC motor: the armature (the device that rotates, sometimes referred to as a rotor) and the field (the stationary part, sometimes referred to as a stator). The two components magnetically interact with one another to produce rotation in the armature. We will take a closer look at each of the parts and how they interact.

Figure 3-1 indicates a very simplistic view of the basic parts of the DC motor.

As seen in Figure 3-1, the armature and the field are two separate circuits and are physically next to each other to promote magnetic interaction.

The armature has an integral part, called a commutator. The commutator acts as an electrical switch, always switching the polarity of the magnetic flux to ensure that a "repelling" force taking place. The armature rotates as a result of the repelling motion created by the magnetic flux of the armature, in opposition to the magnetic flux created by the field winding.

The physical connection of voltage to the armature is accomplished by a device called brushes. Brushes are made of a carbon material that is in constant contact with the armature's commutator plates. The brushes are typically spring-loaded to provide constant pressure of the brush to the commutator plates. Figure 3-2 indicates how the armature and field windings are electrically connected.

As seen in Figure 3-2, the leads are brought out away from the windings, and usually are terminated in a conduit box. I_F indicates a field winding connection and I_A indicates an armature connection. (Note: "I" indicates current, meaning "intensity of current.")

The armature device would look like that indicated in Figure 3-3.

The windings are fitted inside slots in the armature. These slots are created by a series of iron "laminations" epoxied together into a long, narrow unit. These slots are actually skewed to allow for smooth rotational action at very low speeds. (Magnetic flux has a tendency to "jump" from field to field. When that occurs, a jerking motion is the result. With the windings at an angle, the magnetic interaction between the armature and field winding is dampened, and the jerking motion is greatly reduced.)

Many manufacturers actually have lengthwise holes around the inside center of the armature. This allows additional cooling air to flow through the armature, reducing overheating. Brushes contact the commutator, which is slightly smaller in diameter, compared with the main body of the device (right side of photo).

There are many coils (windings) around the armature to allow for maximum generation of torque. The polarity of the armature coils must be reversed at the precise time to ensure that the repelling action continues. This action is called commutation and takes place when properly aligned brushes are contacting the commutator.

Special windings called commutation windings are installed between the magnetic poles to straighten the magnetic field flowing through the armature. If these windings were not installed, a distortion or bending of the magnetic flux would occur, and reduced motor torque would be the result. Figure 3-4 indicates the location of commutation windings.

As brushes wear from constant contact with the commutator plates, arcing occurs. Arcing can be reduced by using commutation windings, but some arcing does occur. To reduce arcing, which causes degraded performance, brush replacement is required. The replacement is part of any routine preventative maintenance (PM) program.

The field winding unit is constructed in much the same way, with iron laminations making up the bulk of the device. Windings are inserted lengthwise around the windings. Iron laminations tend to increase the strength of the magnetic flux. Figure 3-5 indicates the typical construction of a motor field winding unit.

There are additional windings that are installed to the magnetic poles of the field windings. These windings are called compensation windings and tend to smooth the field flux across the pole. Without the compensation windings, the left side of the north pole would become saturated because of additional magnetic fields generated by the armature. Figure 3-6 indicates the location of these windings.

At this point, it should be noted that there is another type of DC motor that uses permanent magnets instead of field windings. These types of motors, designated "PM" DC motors, do not need a separate exciter or power supply to generate the field magnetic flux. Only a power supply for the armature is needed. If armature supply voltage is available, the PM DC motor includes all the necessary magnetic features to produce shaft rotation.

Figure 3-7 indicates the relationship of the main parts of a DC motor. These parts may look slightly different, depending on manufacturer, but all DC motors will have these components.

Basic Components

Two basic circuits are in any DC motor: the armature (the device that rotates, sometimes referred to as a rotor) and the field (the stationary part, sometimes referred to as a stator). The two components magnetically interact with one another to produce rotation in the armature. We will take a closer look at each of the parts and how they interact.

Figure 3-1 indicates a very simplistic view of the basic parts of the DC motor.

As seen in Figure 3-1, the armature and the field are two separate circuits and are physically next to each other to promote magnetic interaction.

The armature has an integral part, called a commutator. The commutator acts as an electrical switch, always switching the polarity of the magnetic flux to ensure that a "repelling" force taking place. The armature rotates as a result of the repelling motion created by the magnetic flux of the armature, in opposition to the magnetic flux created by the field winding.

The physical connection of voltage to the armature is accomplished by a device called brushes. Brushes are made of a carbon material that is in constant contact with the armature's commutator plates. The brushes are typically spring-loaded to provide constant pressure of the brush...