Future developments in starting systems
5.3.1 Starter-motor control
Solenoid switches
The current supply to the starting motor is always a significant amount (hundreds of amps) and this must be controlled via a special switching arrangement commonly known as a starter solenoid. The main purpose of the solenoid is to switch the large current to the starter motor by using a smaller ‘control’ current and this means the starter motor cabling can be kept as short as possible between the starter motor and the battery. This helps to reduce unnecessary electrical resistance in the supply circuit and ensures that maximum power can be delivered to the starter with minimal losses due to resistance and cable heating.
A simple solenoid arrangement is shown in Figure 5.22.
This is a single coil/single stage solenoid typical of the type used with inertia-type starter motors. In this application the only function is to switch the electrical supply to the motor. Effectively the solenoid is a heavy- duty relay controlled via a signal wire from the ignition switch.
For pre-engaged type starters the solenoid is built into the starter motor and performs two functions:
● it moves the drive pinion/clutch assembly outwards
to engage with the engine flywheel ring gear
● it switches the current flowing into the motor armature.
A typical design of a pre-engaged solenoid is shown in Figure 5.23.
The solenoid core protrudes into the solenoid coil from one side, while the plunger protrudes from the
Figure 5.22 Starter-motor solenoid
The starter motor has a series or series-parallel field winding which gives it the ideal characteristic to generate maximum torque at zero speed. Modern starter motors use powerful permanent magnet fields and as such are lighter and smaller
Most starter motors are pre-engaged. They use a solenoid to fully engage the starter motor pinion with the flywheel teeth before the motor is energised and torque transmitted to the engine Many modern starter motors use small high-speed motors and reduction gear sets. This allows the construction of smaller, lighter starter motors The starter motor pinion is fitted with a one-way clutch to prevent the engine from driving the motor as, if this happened, the motor would be destroyed due to over-speeding of the armature The starter motor drives the engine via the flywheel ring gear with a 10:1 gear ratio
K ey P oints K ey P oints
other side. The distance between the core and the plunger represents the total travel of the plunger. The solenoid windings, core and plunger form the magnetic circuit. The electrical wiring arrangement is shown in Figure 5.24.
When the solenoid is energised, the magnetic field draws the plunger into the coil. This movement is utilised to first move the pinion into engagement (via a lever) and then to close the solenoid contacts (switch). Two windings are generally employed and these are known as ‘pull-in’ and ‘holding’ windings. The pull-in winding has its earth return path via the motor armature (see
starter operation is complete. This arrangement reduces thermal stresses in the solenoid yet allows sufficient magnetic forces to be generated for the required functionality. It also reduces the overall drain on the battery as the pull-in winding is low resistance and can draw up to 50 A in some cases.
Power supply and cables
The power supply to the starting motor has a significant effect on its performance. The cable arrangement must be as short as possible and dimensioned appropriately to minimise volt drop to avoid cable heating and power loss. Generally the starter motor duty cycle is very short, but in exceptional circumstances the current flowing may be high for longer periods and hence the supply cable and switching components must be rated to deal with this operation mode without suffering damage.
Another factor to be considered is temperature as elevated temperatures will increase resistance in the cables. To determine the permissible temperature rise a transient current density of 30 A/mm2 is taken together
with the short-circuit current for the starter motor and these factors dictate the required cross-section of the power cable.
qw = Ik J
where:
qw = Cable cross section (mm
2)
Ik = Short-circuit current of the starter motor (A) J = Current density (taken as 30 A/mm2).
The electric circuit of the starter motor is shown schematically in Figure 5.25.
Figure 5.23 Pre-engaged solenoid
Figure 5.24) whereas the holding winding has a direct earth connection via the body. When the solenoid is initially energised current flows through both windings and this generates the strong magnetic field needed to overcome the forces required to engage the pinion fully, and to provide sufficient field strength to overcome the large air gap between the plunger and the coil.
As the plunger travels into the magnetic field the air gap reduces and thus the field strength increases. Once the plunger reaches its final position, closing the solenoid contacts, the field strength required to maintain this position is much less. Therefore, due to the fact that the earth path for the pull-in winding is via the motor armature, once the solenoid contacts close, the pull-in winding is effectively short-circuited (thus switched off). The field strength of the holding winding alone is sufficient to hold the plunger in position until
1 Battery 2 Starter motor 3 Ignition/starter switch 4 Solenoid switch 4a Pull-in winding 4b Hold-in winding
Figure 5.24 Electrical wiring arrangement
1 Solenoid armature 2 Pull-in winding 3 Hold-in winding 4 Solenoid core 5 Contact spring 6 Switch contacts 7 Electrical connection 8 Switch contact 9 Armature shaft (split) 10 Return spring
Electrical circuits 115
Figure 5.25 Schematic of the electric circuit of a starter motor
Ri Internal impedance of battery Rs Starter-motor impedance Rperm Power-cable impedance UL No-load voltage of battery UK Battery-terminal voltage Us Starter-motor voltage Ubr Voltage drop across brushes Uind Induced voltage in armature winding
116 Starting-motor systems Fundamentals of Motor Vehicle Technology: Book 3
The voltage at the battery terminals is the open circuit voltage minus the voltage drop due to the battery’s internal resistance. The voltage available to the starter motor is further reduced by the voltage drop in the cabling. The actual voltage at the commutator is further reduced by voltage drop at the brush contact face, which is approximately 1.2 V per brush pair (irrespective of current) totalling 2.4 V for positive and negative brushes. In addition, the armature itself has resistance, and also a back-emf is generated when the armature rotates in the magnetic field.
The main technical parameters that affect the performance of the starting system which can be checked and improved are:
● battery – well charged, good condition, low internal resistance
● cabling – appropriately sized and minimum length
to reduce voltage drop
● starter motor itself – good condition of solenoid contacts and brushes to minimise volt drop.
The starter-motor solenoid has two functions. It acts as a relay to switch the high current from the battery to the motor when required. In addition, for a pre-engaged type starter, it provides the force to move the pinion into mesh with the flywheel The cabling between the battery and starter motor must very thick and as short as possible to minimise power losses due to voltage drop.
The solenoid employs separate pull-in and holding windings. This reduces current draw and prevents excessive heating yet still provides the necessary magnetic force K ey P oints K ey P oints
Starter-motor technology has improved with the introduction of smaller, lighter motors with greater power density, and these improvements in technology have made possible the installation of starter motors which can perform reliably over the life of the engine even under the most extreme conditions. The biggest problem is that once the engine is running, the starter motor is dead weight. By combining the starter motor with the generator as a single electrical machine considerable weight savings can be made. This technology is under development and is known as ISG or integrated starter-generator. There are additional benefits to be gained with such a system, for example energy recovery (during braking or overrun conditions) and energy boosting (where a short-term power boost for the engine is needed).
A typical starter-generator arrangement is shown in Figure 5.26.
This includes the flywheel/cutch assembly. The motor itself is an AC multi-phase machine with an
5.4
FUTURE DEVELOPMENTS IN STARTING SYSTEMS
Figure 5.26 Typical starter-generator arrangement
inverter drive system that can operate in motor or absorb mode to generate torque for starting, or absorb torque for regeneration.
ISG systems are discussed in more detail in FMVT: