Dc Motor Field Winding Designs

A series-wound starter motor
The ⚡ field windings ⚡ and armature of the DC motor can be wired in various ways. The motor design is referenced by the method these two components are wired together. In addition, many motors are using permanent magnet fields. Also, many newer motors are designed to be brushless. Series-Wound Motors Most starter motors are series-wound with current flowing first to the field windings, then to the brushes, through the commutator and the armature winding contacting the brushes at that time, then through the grounded brushes back to the battery source. This design permits all of the current that passes through the field coils to also pass through the armature. FIGURE. A series-wound starter motor. A series-wound motor will develop its maximum torque output at the time of initial start. As the motor speed increases, the torque output of the motor will decrease. This decrease of torque output is the result of counter electromotive force (CEMF) caused by self-induction. Since a starter motor has a wire loop rotating within a magnetic field, it will generate an electrical voltage as it spins. This induced voltage will be opposite the battery voltage that is pushing the current through the starter motor. The faster the armature spins, the greater the amount of induced voltage that is generated. This results in less current flow through the starter from the battery as the armature spins faster. Figure shows the relationship between starter motor speed and CEMF. Notice that, at 0 (zero) rpm, CEMF is also at 0 (zero). At this time, maximum current flow from the battery through the starter motor will be possible. As the motor spins faster, CEMF increases and current decreases. Since current decreases, the amount of rotating force (torque) also decreases. FIGURE. Graph illustrating the relationship between CEMF, starter motor speed, and current draw. As speed increases so does CEMF, reducing current draw and torque. Shunt-Wound Motors FIGURE. A shunt-wound (parallel) starter motor. Electric motors, or shunt motors, have the field windings wired in parallel across the armature. Shunt means there is more than one path for current to flow. A shunt-wound field is used to limit the speed that the motor can turn. A shunt motor does not decrease in its torque output as speeds increase. This is because the CEMF produced in the armature does not decrease the field coil strength. Due to a shunt motor's inability to produce high torque, it is not typically used as...

Read

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. FIGURE. Interaction of two magnetic fields. 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. 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....

Read

Starting Systems and Motor Designs

Major components of the starting system. The solid line represents the starting (cranking) circuit and the dashed line indicates the starter control circuit
The internal combustion engine must be rotated before it will run under its own power. The starting system is a combination of mechanical and electrical parts that work together to start the engine. The starting system is designed to change the electrical energy, which is being stored in the battery, into mechanical energy. To accomplish this conversion, a starter or cranking motor is used. The starting system includes the following components: Battery. Cable and wires. Ignition switch. Starter solenoid or relay. Starter motor. Starter drive and flywheel ring gear. Starter safety switch. FIGURE. Major components of the starting system. The solid line represents the starting (cranking) circuit and the dashed line indicates the starter control circuit. Components in a simplified cranking system circuit are shown. This chapter examines both this circuit and the fundamentals of electric motor operation.

Read

12