Direct-Current Motor Principles

Magnetic lines of force flow from the north pole to the south pole

FIGURE. Magnetic lines of force flow from the north pole to the south pole.

DC motors use the interaction of magnetic fields to convert the electrical energy into mechanical energy. Magnetic lines of force flow from the north pole to the south pole of a magnet. If a current-carrying conductor is placed within the magnetic field, two fields will be present. On the left side of the conductor, the lines of force are in the same direction. This will concentrate the flux density of the lines of force on the left side. This will produce a strong magnetic field because the two fields will reinforce each other. The lines of force oppose each other on the right side of the conductor. This results in a weaker magnetic field. The conductor will tend to move from the strong field to the weak field. This principle is used to convert electrical energy into mechanical energy in a starter motor by electromagnetism.

Interaction of two magnetic fields

FIGURE. Interaction of two magnetic fields.

Conductor movement in a magnetic field

FIGURE. Conductor movement in a magnetic field.

A simple electromagnet-style starter motor is shown. The inside windings are called the armature. The armature is the moveable component of the motor that consists of a conductor wound around a laminated iron core. It is used to create a magnetic field. The armature rotates within the stationary outside windings, called the field coils, which has windings coiled around pole shoes. Field coils are heavy copper wire wrapped around an iron core to form an electromagnet. Pole shoes are made of high-magnetic permeability material to help concentrate and direct the lines of force in the field assembly.

Simple electromagnetic motor

FIGURE. Simple electromagnetic motor.

When current is applied to the field coils and the armature, both produce magnetic flux lines. The direction of the windings will place the left pole at a south polarity and the right side at a north polarity. Hie lines of force move from north to south in the field. In the armature, the flux lines circle in one direction on one side of the loop and in the opposite direction on the other side. Current will now set up a magnetic field around the loop of wire, which will interact with the north and south fields and put a turning force on the loop. This force will cause the loop to turn in the direction of the weaker field. However, the armature is limited in how far it is able to turn. When the armature is halfway between the shoe poles, the fields balance one another. The point at which the fields are balanced is referred to as the static neutral point.

Field coil wound around a pole shoe

FIGURE. Field coil wound around a pole shoe.

Rotation of the conductor is in the direction of the weaker field

FIGURE. Rotation of the conductor is in the direction of the weaker field.

For the armature to continue rotating, the current flow in the loop must be reversed. To accomplish this, a split-ring commutator is in contact with the ends of the armature loops. The commutator is a series of conducting segments located around one end of the armature. Current enters and exits the armature through a set of brushes that slide over the commutator’s sections. Brushes are electrically conductive sliding contacts, usually made of copper and carbon. As the brushes pass over one section of the commutator to another, the current flow in the armature is reversed. The position of the magnetic fields are the same. However, the direction of current flow through the loop has been reversed. This will continue until the current flow is turned off.

Starter armature

FIGURE. Starter armature.

Starter and solenoid components

FIGURE. Starter and solenoid components.


Lamination construction of a typical motor armature

FIGURE. Lamination construction of a typical motor armature.

The armature is constructed with a laminated core made of several thin iron stampings that are placed next to each other. Laminated construction is used because, in a solid iron core, the magnetic fields would generate eddy currents. These are counter voltages induced in a core. They cause heat to build up in the core and waste energy. By using laminated construction, eddy currents in the core are minimized.

The slots on the outside diameter of the laminations hold the armature windings. The windings loop around the core and are connected to the commutator. Each commutator segment is insulated from the adjacent segments. A typical armature can have more than 30 commutator segments.

A steel shaft is fitted into the center hole of the core laminations. The commutator is insulated from the shaft.

Lap winding diagram

FIGURE. Lap winding diagram.

Two basic winding patterns are used in the armature: lap winding and wave winding. In the lap winding, the two ends of the winding are connected to adjacent commutator segments. In this pattern, the wires passing under a pole field have their current flowing in the same direction.

Wave-wound armature

FIGURE. Wave-wound armature.

In the wave-winding pattern, each end of the winding connects to commutator segments that are 90 or 180 degrees apart. In this pattern design, some windings will have no current flow at certain positions of armature rotation. This occurs because the segment ends of the winding loop are in contact with brushes that have the same polarity. The wave-winding pattern is the most commonly used due to its lower resistance.

Field Coils

The field coils are electromagnets constructed of wire ribbons or coils wound around a pole shoe. The pole shoes are constructed of heavy iron. The field coils are attached to the inside of the starter housing. Most starter motors use four field coils. The iron pole shoes and the iron starter housing work together to increase and concentrate the field strength of the field coils.

Field coils mounted to the inside of starter housing

FIGURE. Field coils mounted to the inside of starter housing.

When current flows through the field coils, strong stationary electromagnetic fields are created. The fields have a north and south magnetic polarity based on the direction the windings are wound around the pole shoes. The polarity of the field coils alternate to produce opposing magnetic fields.

Magnetic fields in a 4-pole starter motor

FIGURE. Magnetic fields in a 4-pole starter motor.

In any DC motor, there are three methods of connecting the field coils to the armature: in series, in Darallel (shunt), and a comDound connection that uses both series and shunt coils.