Magnetic Permeability, Induction, and Automotive Applications
Magnetic Permeability
Magnetic permeability is the ability of a material to attract and repel lines of force.
- Ferromagnetic Permeability: Its value far exceeds 1. It concentrates lines of force in materials like iron.
- Paramagnetic Permeability: Its permeability is approximately 1 (they behave like a vacuum). Examples include exam paper and plastic.
- Diamagnetic Permeability: Its permeability is less than 1. The material repels the field lines.
Hysteresis
When a ferromagnetic material, upon which a magnetic field has been acting, ceases to be under the influence of that field, it does not completely lose its magnetism; it retains a certain residual magnetism. To demagnetize it, one would need to apply a magnetic field opposite to the initial one. In the drawing, the body becomes magnetized until it reaches its peak. If we stop magnetizing it, it does not return to its origin but to another value. To return it to its original value, a current is induced in the opposite direction.
Saturation
Saturation is the moment when a permeable body can no longer accept more lines of force. When a body is slightly magnetized, it’s much easier for lines of force to pass through. When it’s highly magnetized, it reaches saturation and strongly resists the passage of lines of force.
Autoinduction
Autoinduction is the creation of an electric field in a body; its flow varies depending on its intensity. The variation of the field creates an electromotive force that opposes the intensity.
Mutual Induction
Mutual induction occurs when two coils are in close proximity, inducing a magnetic flux on each other.
Starter Motor
Mechanical Function
The fork displaces the pinion, which engages the flywheel. This is assisted by the spring. Once the engine starts, the Bendix mechanism retracts the pinion to prevent the armature from overspeeding.
Electrical Function
When the starter is engaged, current flows from the battery to the holding coil, creating a magnetic field that attracts the plunger. This allows current to flow to the induction coil, causing the armature and pinion to rotate.
Components
- Input Positive (50)
- Contactor Coil
- Return Spring
- Plunger
- Fork
- Contact Terminals
- Collector
- Armature (Induced)
- Induction Coil
- Spring
- Ring
- Freewheel
- Pinion Gear
- Top of Pinion Hub
Alternator Excitation Current
“Connected” State
The Zener diode (ZD) breakdown voltage has not been reached, meaning no current flows through the Zener diode branch towards the base of transistor T1. T1 is cut off. With T1 cut off, current flows from the excitation diodes through terminal D+ and resistor R6 to the base of transistor T2, making it conductive. T2’s conductivity connects terminal DF and the base of T3. This makes T3 conductive, like T2. Transistors T2 and T3 are configured as a Darlington pair, forming the regulator’s power stage. Excitation current (Iexc) flows through T3 and the field winding, increasing during the connection time and causing an increase in alternator voltage. Simultaneously, the voltage across the transistor increases towards its theoretical value. If the actual alternator voltage exceeds the theoretical value, the system enters the regulation state.
“Disconnected” State
Current flows from D+ through resistors R1 and R2 in the branch containing the Zener diode to the base of transistor T1, making it conductive. Consequently, the voltage at the base of T2 drops practically to zero relative to the emitter, and both T2 and T3 are cut off as the power stage. The excitation current circuit is interrupted, the excitation current decreases, and the alternator voltage drops. As the voltage falls below the nominal value, the Zener diode returns to its non-conducting state, and the power stage reconnects the current. Disrupting the excitation current causes a voltage peak due to the inductance in the field winding (stored magnetic energy). This peak could destroy transistors T2 and T3 if not prevented by connecting the “extinguisher diode” D3 in parallel with the excitation winding. The extinguisher diode takes over the exciting current during the interruption, preventing the occurrence of the voltage peak. The Zener diode becomes conductive when the breakdown voltage is reached.