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.
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.
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 a starter motor. However, shunt motors may be found as wiper motors, power window motors, power seat motors, and so on.
FIGURE. A compound motor uses both series and shunt coils.
In a compound motor most of the field coils are connected to the armature in series and one field coil is connected in parallel with the battery and the armature. This configuration allows the compound motor to develop good starting torque and constant operating speeds. The field coil that is shunt wound is used to limit the speed of the starter motor. Also, on Ford’s positive engagement starters, the shunt coil is used to engage the starter drive. This is possible because the shunt coil is energized as soon as battery voltage is sent to the starter.
Permanent Magnet Motors
FIGURE. A permanent magnet motor has only an armature circuit, as the field is created by strong permanent magnets.
Most newer vehicles have starter motors that use permanent magnets in place of the field coils. These motors are also used in many different applications. When a permanent magnet is used instead of coils, there is no field circuit in the motor. By eliminating this circuit, potential electrical problems are also eliminated, such as field-to-housing shorts. Another advantage to using permanent magnets is weight savings; the weight of a typical starter motor is reduced by 50%. Most permanent magnet starters are gear-reduction-type starters.
Multiple permanent magnets are positioned in the housing around the armature. These permanent magnets are an alloy of boron, neodymium, and iron. The field strength of these magnets is much greater than typical permanent magnets. The operation of these motors is the same as other electric motors, except there is no field circuit or windings.
FIGURE. Components of a brushless DC motor. The hall-effect sensor is used to determine rotor position.
The brushless motor uses a permanent magnet rotor and electromagnet field windings. Since the motor design is brushless, the potential for arcing is decreased and longer service life is expected. In addition, arcing can cause electromagnetic interference (EMI) that can adversely affect electronic systems. High output brushless DC motors are used in some HEV-drive vehicles.
FIGURE. Brushless motor used by Honda in some of their HEVs.
Control of the stator is by an electronic circuit that switches the current flow as needed to keep the rotor turning. Power transistors that are wired as “H” gates reverse current flow according to the position of the rotor. Motor speed can be controlled by PWM of the driver circuits. Rotor position is usually monitored by the use of Hall-effect sensors. However, rotor position can also be determined by monitoring the CEMF that is present in stator windings that are not energized.