Before an internal combustion engine can start running and generate power on its own, it requires an external source to make it start running. This work is done by a starter motor. Starter motor provides a certain degree of momentum till the engine generates enough torque in the power stroke to overcome the resistance during suction, compression and exhaust strokes. It requires a large amount of force to start an engine.
STARTER MOTOR DESIGN:
Most of the vehicles today use a reduction gear type starter motor.
Starter Solenoid:
Starter solenoid has 2 main functions:
The solenoid housing consists of a solenoid switch or armature which is movable. There are 2 solenoid windings, named as pull-in winding and hold-in winding. The solenoid switch moves towards the solenoid core, pressed against the return spring. The movable solenoid switch is connected to a contact plate at one end, pressed against the contact spring. The contact plate when comes in contact with the contact switches, completes the starter primary circuit.
Solenoid Switch
The solenoid coil consists of a pull-in winding and a hold-in winding. The magnetic force in the pull-in winding pulls the armature towards the solenoid core. The magnetic pull in the pull-in winding should be high enough to close the air gap between armature and the core. Once the air gap is closed enough, the magnetic force in the pull-in winding is sufficient to hold the armature, and thus the contact plate comes in contact with the contact switches and the primary circuit is completed.
Stator Housing:
The stator housing consists of a 6 pole permanent magnet which acts as a stator. The armature rotates inside the housing as the rotor. The drive from armature is not directly given to the pinion. A reduction gear assembly having 3 planetary gears takes the drive from the armature. The reduction gears provide high starting torque required to start the engine.
4 pole stator
The current is transferred from the solenoid switch to the armature via four carbon brushes (two positive and two negative). The armature has a laminated core which is press fitted into the armature shaft. The laminated core has slots along the circumference in which copper wire is fitted. The copper wires are connected to each other in a specific pattern and are welded to the commutator plate. The entire setup is known as armature winding.
The current flows through the carbon brushes which are in sliding contact with the commutator. Due to the rotation of the armature, the current is passed to the individual commutator plates in sequence.
The circuit in the armature winding is arranged in such a way that the current flowing in the copper wires adjacent to the north poles of the magnetic field created by permanent magnets, always flow towards the pinion. Whereas the current in the wires adjacent to south poles, flow in opposite direction (towards the commutator). Since the armature is rotating, the commutator reverses the flow in wires as different commutator plates come in contact with the carbon brushes in a sequence.
The flow of current through the armature which is placed in a magnetic field produces a force which rotates the armature. The commutator maintains the torque in the armature. The rotational force or torque is proportional to the current flowing through the armature, the strength of the magnetic field, length of the laminated core, and the diameter of the armature.
Reduction Gear Assembly:
In a direct drive mechanism, the drive from the armature is directly given to the one way clutch and pinion assembly. In case of cold starting, the engine requires more torque to be started. Therefore, the starter motor size has to be increased to meet the high torque demand.
A reduction gear assembly can achieve higher torque without having to increase the size of the starter motor. A planetary gear assembly acts as the reduction gear. Gear ratios can be varied from 3.4:1 to 6:1 based on the torque requirements. Warm starting engines can start on higher transmission ratio, and cold starting engines require lower transmission ratio.
The planetary gear assembly has a sun gear (drive gear) which is attached to the armature shaft. It has 3 planet gears engaged between the outer gear with internal teeth and sun gear. The outer gear transfers the drive to the pinion.
Overrunning Clutch:
Overrunning clutch is also known as one-way clutch. It is positioned between the armature and pinion. Its task is to disengage the pinion from the pinion drive shaft as soon as the ring gear starts rotating at a higher speed compared to the pinion. Therefore, one way clutches prevent the armature from over acceleration once the engine has started.
Roller type overrunning clutch is commonly used for commercial vehicle application. It has a clutch shell with roller race. The roller race has cylindrical rollers which are pressed into a constricted space with the help of springs. The clutch shell drives the pinion due to a helical linkage between the shell and pinion shaft.
When the overrunning clutch is at rest, the cylindrical rollers are pressed into the constricted space between the roller race and clutch shell. As a result of this, the rollers are jammed and thus the drive from the armature is given to the pinion. The pinion is forced to rotate when the rollers are jammed.
As soon as the ring gear starts to rotate at a higher speed, it will make the pinion to overrun. At this time, the rollers are pushed to the broader side of the roller race due to friction between pinion shaft and the rollers. Now the pinion is disengaged from the pinion shaft.
STARTER MOTOR OPERATION:
The starter motor starts an engine by engaging its pinion gear with the ring gear, which in turn is meshed with the flywheel. In a typical starter motor, the pinion gear has 10 teeth. The ring gear has around 130 teeth.
When the ignition key is pressed, the ignition switch completes the circuit and current from the battery flows to the starter motor solenoid. Magnetic field is created in the solenoid winding due to electromagnetism. The solenoid winding pulls the solenoid armature, thereby operating the engaging lever which engages the pinion with the ring gear.
Under ideal circumstances, the teeth of the pinion would mesh with the teeth of the ring gear. But in most cases, the teeth of both the gears would collide with each other as they try to engage. In this case, the engine cannot be started as there would be no power delivered to the flywheel to rotate the crankshaft. The solenoid armature won’t be pulled in further and the circuit remains incomplete for the current to flow to the armature.
Typically the above solution can be solved by pushing the vehicle forward and this would change the position of the ring gear as the rotation of wheels will make the differential gears to rotate, then the propeller, then the transmission, then the crankshaft and finally rotating the flywheel and ring gear. The engagement is tried again by starting the ignition.
The above solution requires human effort and consumes crucial time. Manufacturers came up with a simple solution to add a meshing spring between the pinion and engaging lever. In this case, when the teeth of both gears collide, the solenoid armature will continue to move in as the meshing spring is compressed by the lever. The circuit is completed and the current flows through the armature. The armature starts rotating due to the rotational force created when a current carrying conductor is placed in a magnetic field. The drive from the armature is given to the pinion, which when rotates brings the pinion teeth to align with the gap in the ring gear teeth and they finally mesh and start the engine.
Use of a reduction gear would increase the torque and the overall size of the starter motor can be kept smaller. The overrunning clutch makes sure that the pinion disengages with the pinion drive shaft once the pinion starts overrunning.
STARTER MOTOR DESIGN:
Most of the vehicles today use a reduction gear type starter motor.
Starter Solenoid:
Starter solenoid has 2 main functions:
- To move the pinion gear outwards so that it can mesh with the ring gear
- To complete the starter motor’s primary electric circuit
The solenoid housing consists of a solenoid switch or armature which is movable. There are 2 solenoid windings, named as pull-in winding and hold-in winding. The solenoid switch moves towards the solenoid core, pressed against the return spring. The movable solenoid switch is connected to a contact plate at one end, pressed against the contact spring. The contact plate when comes in contact with the contact switches, completes the starter primary circuit.
Solenoid Switch
The solenoid coil consists of a pull-in winding and a hold-in winding. The magnetic force in the pull-in winding pulls the armature towards the solenoid core. The magnetic pull in the pull-in winding should be high enough to close the air gap between armature and the core. Once the air gap is closed enough, the magnetic force in the pull-in winding is sufficient to hold the armature, and thus the contact plate comes in contact with the contact switches and the primary circuit is completed.
Stator Housing:
The stator housing consists of a 6 pole permanent magnet which acts as a stator. The armature rotates inside the housing as the rotor. The drive from armature is not directly given to the pinion. A reduction gear assembly having 3 planetary gears takes the drive from the armature. The reduction gears provide high starting torque required to start the engine.
4 pole stator
The current is transferred from the solenoid switch to the armature via four carbon brushes (two positive and two negative). The armature has a laminated core which is press fitted into the armature shaft. The laminated core has slots along the circumference in which copper wire is fitted. The copper wires are connected to each other in a specific pattern and are welded to the commutator plate. The entire setup is known as armature winding.
The current flows through the carbon brushes which are in sliding contact with the commutator. Due to the rotation of the armature, the current is passed to the individual commutator plates in sequence.
The circuit in the armature winding is arranged in such a way that the current flowing in the copper wires adjacent to the north poles of the magnetic field created by permanent magnets, always flow towards the pinion. Whereas the current in the wires adjacent to south poles, flow in opposite direction (towards the commutator). Since the armature is rotating, the commutator reverses the flow in wires as different commutator plates come in contact with the carbon brushes in a sequence.
The flow of current through the armature which is placed in a magnetic field produces a force which rotates the armature. The commutator maintains the torque in the armature. The rotational force or torque is proportional to the current flowing through the armature, the strength of the magnetic field, length of the laminated core, and the diameter of the armature.
Reduction Gear Assembly:
In a direct drive mechanism, the drive from the armature is directly given to the one way clutch and pinion assembly. In case of cold starting, the engine requires more torque to be started. Therefore, the starter motor size has to be increased to meet the high torque demand.
A reduction gear assembly can achieve higher torque without having to increase the size of the starter motor. A planetary gear assembly acts as the reduction gear. Gear ratios can be varied from 3.4:1 to 6:1 based on the torque requirements. Warm starting engines can start on higher transmission ratio, and cold starting engines require lower transmission ratio.
The planetary gear assembly has a sun gear (drive gear) which is attached to the armature shaft. It has 3 planet gears engaged between the outer gear with internal teeth and sun gear. The outer gear transfers the drive to the pinion.
Overrunning Clutch:
Overrunning clutch is also known as one-way clutch. It is positioned between the armature and pinion. Its task is to disengage the pinion from the pinion drive shaft as soon as the ring gear starts rotating at a higher speed compared to the pinion. Therefore, one way clutches prevent the armature from over acceleration once the engine has started.
Roller type overrunning clutch is commonly used for commercial vehicle application. It has a clutch shell with roller race. The roller race has cylindrical rollers which are pressed into a constricted space with the help of springs. The clutch shell drives the pinion due to a helical linkage between the shell and pinion shaft.
When the overrunning clutch is at rest, the cylindrical rollers are pressed into the constricted space between the roller race and clutch shell. As a result of this, the rollers are jammed and thus the drive from the armature is given to the pinion. The pinion is forced to rotate when the rollers are jammed.
As soon as the ring gear starts to rotate at a higher speed, it will make the pinion to overrun. At this time, the rollers are pushed to the broader side of the roller race due to friction between pinion shaft and the rollers. Now the pinion is disengaged from the pinion shaft.
STARTER MOTOR OPERATION:
The starter motor starts an engine by engaging its pinion gear with the ring gear, which in turn is meshed with the flywheel. In a typical starter motor, the pinion gear has 10 teeth. The ring gear has around 130 teeth.
When the ignition key is pressed, the ignition switch completes the circuit and current from the battery flows to the starter motor solenoid. Magnetic field is created in the solenoid winding due to electromagnetism. The solenoid winding pulls the solenoid armature, thereby operating the engaging lever which engages the pinion with the ring gear.
Under ideal circumstances, the teeth of the pinion would mesh with the teeth of the ring gear. But in most cases, the teeth of both the gears would collide with each other as they try to engage. In this case, the engine cannot be started as there would be no power delivered to the flywheel to rotate the crankshaft. The solenoid armature won’t be pulled in further and the circuit remains incomplete for the current to flow to the armature.
Typically the above solution can be solved by pushing the vehicle forward and this would change the position of the ring gear as the rotation of wheels will make the differential gears to rotate, then the propeller, then the transmission, then the crankshaft and finally rotating the flywheel and ring gear. The engagement is tried again by starting the ignition.
The above solution requires human effort and consumes crucial time. Manufacturers came up with a simple solution to add a meshing spring between the pinion and engaging lever. In this case, when the teeth of both gears collide, the solenoid armature will continue to move in as the meshing spring is compressed by the lever. The circuit is completed and the current flows through the armature. The armature starts rotating due to the rotational force created when a current carrying conductor is placed in a magnetic field. The drive from the armature is given to the pinion, which when rotates brings the pinion teeth to align with the gap in the ring gear teeth and they finally mesh and start the engine.
Use of a reduction gear would increase the torque and the overall size of the starter motor can be kept smaller. The overrunning clutch makes sure that the pinion disengages with the pinion drive shaft once the pinion starts overrunning.
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