LEARNING OUTCOME 3

Magnetism

Magnetism is a fundamental physical phenomenon caused by the motion of electric charges. It is a force that attracts or repels certain materials and creates magnetic fields.

Principles of Magnetism

1. Magnetic Fields:

• Moving electric charges create magnetic fields. These fields exert a force on other moving charges or magnetic materials.
• Magnetic fields are visualized as lines of force, which form closed loops around the source of the magnetic field.

2. Magnetic Poles:

• Magnets have two poles: a north pole and a south pole.
• Like poles repel each other (north-north or south-south), and unlike poles attract each other (north-south).

3. Magnetic Materials:

• Certain materials, like iron, nickel, and cobalt, are strongly attracted to magnets. These are called ferromagnetic materials.
• Other materials are weakly attracted or repelled by magnets.

4. Electromagnetism Connection:

• Magnetism and electricity are intimately connected. Moving electric charges create magnetic fields, and changing magnetic fields create electric fields.

Concepts of Magnetism

Permanent Magnetism:

• This type of magnetism is exhibited by materials that retain their magnetic properties even after the external magnetic field is removed.
• These materials have atoms with aligned electron spins, creating a net magnetic field.
• Examples include naturally occurring magnets like lodestone and artificially magnetized materials like steel.

Electromagnetism:

• This is the magnetism produced by an electric current.
• When an electric current flows through a wire, it creates a magnetic field around the wire.
• The strength of the magnetic field is proportional to the current.
• An electromagnet consists of a coil of wire wrapped around a ferromagnetic core. When current flows through the coil, it creates a strong magnetic field.
• Electromagnets are used in various applications, such as electric motors, generators, and solenoids.

Electromagnetic Induction:

• This is the phenomenon where a changing magnetic field induces an electric current in a conductor.
• If a conductor is moved through a magnetic field or if a magnetic field changes around a conductor, a voltage (electromotive force or EMF) is induced in the conductor.
• This principle is the basis for electric generators, transformers, and many other electrical devices.
• Essentially, if a magnetic field lines "cut" across a conductor, or if a conductor "cuts" across magnetic field lines, then a voltage is induced within that conductor.

Generator Principle (Electromagnetic Induction)

The generator principle is based on Faraday's law of electromagnetic induction. This law states that:

• A changing magnetic field induces an electromotive force (EMF) or voltage in a conductor.
• The magnitude of the induced EMF is proportional to the rate of change of the magnetic flux linking the conductor.

In simpler terms, when a conductor moves through a magnetic field or when a magnetic field changes around a conductor, a voltage is generated.

Construction and Operating Principle of Generators

Construction:

• Magnetic Field: A source of magnetic field, either permanent magnets or electromagnets (field windings).
• Conductors (Armature): Coils of wire that move through the magnetic field.
• Mechanical Motion: A means of rotating the armature, such as a turbine, engine, or hand crank.
• Slip Rings or Commutator: Components that connect the rotating armature to the external circuit.

Operating Principle:

• Mechanical energy is used to rotate the armature within the magnetic field.
• As the conductors of the armature move through the magnetic field, a voltage is induced in them.
• The induced voltage causes current to flow through the external circuit.
• The direction of the induced current is determined by Fleming's right hand rule.

Principle of Operation of AC and DC Generators

AC Generators (Alternators):

• Operation:
  • The armature rotates within a stationary magnetic field.
  • The induced voltage in the armature changes polarity with each half rotation, producing an alternating current (AC).
  • Slip rings are used to connect the rotating armature to the external circuit, allowing the AC to flow.
  • The frequency of the AC is determined by the speed of rotation and the number of poles in the generator.
• Explanation: Because the coil is constantly moving through changing magnetic fields, the current produced is constantly changing direction.

DC Generators (Dynamos):

• Operation:
  • The armature rotates within a stationary magnetic field.
  • A commutator is used to convert the AC generated in the armature into DC.
  • Commutation: The commutator is a split ring that reverses the connections between the armature and the external circuit every half rotation. This ensures that the current flowing through the external circuit is always in the same direction.
  • Polarization: In older DC generators, residual magnetism in the field windings is essential for starting the generation process. This residual magnetism initiates a small current, which strengthens the field windings, leading to increased output. Modern generators utilize permanent magnets, or electronic controls to provide the initial field.
• Explanation: The commutator is the key component that transforms the AC produced in the armature into DC. It acts as a mechanical rectifier.
• DC generator ripple: Due to the method of commutation, DC generators produce a ripple voltage. Modern DC generators, or alternators that are rectified to DC, use electronic regulation to smooth out the ripple.

Motor Principle

The motor principle is based on the interaction between a magnetic field and a current-carrying conductor. When a current-carrying conductor is placed in a magnetic field, it experiences a force. This force causes the conductor to move.

This principle is the inverse of the generator principle.

• Fleming's Left-Hand Rule: This rule helps determine the direction of the force on a conductor in a magnetic field. The thumb, forefinger, and middle finger of the left hand are extended at right angles to each other. The forefinger represents the magnetic field, the middle finger represents the current, and the thumb represents the motion (force).

Construction and Operation Principle of Motors

Construction:

• Magnetic Field: A source of magnetic field, either permanent magnets or electromagnets (field windings).
• Armature (Rotor): A rotating component consisting of coils of wire.
• Commutator: A split ring that reverses the current in the armature coils, ensuring continuous rotation.
• Brushes: Stationary contacts that connect the external circuit to the commutator.
• Housing: Provides mechanical support and protection.

Operation Principle:

• When current flows through the armature coils, a magnetic field is created around the coils.
• This magnetic field interacts with the magnetic field of the permanent magnets or field windings, creating a force on the coils.
• The commutator and brushes reverse the current in the coils every half rotation, ensuring that the force on the coils is always in the same direction, causing continuous rotation.

Types of DC Motors

Series Wound Motor:

• Construction: The field windings are connected in series with the armature.
• Operation:
  • The same current flows through both the field windings and the armature.
  • The magnetic field strength and torque are proportional to the armature current.
  • The motor has high starting torque but low speed regulation.
  • As load decreases, the motor speed increases dramatically, and can cause damage to the motor.
• Application: Used in applications requiring high starting torque, such as starter motors, cranes, and electric tools.

Parallel Wound (Shunt) Motor:

• Construction: The field windings are connected in parallel (shunt) with the armature.
• Operation:
  • The field current is independent of the armature current.
  • The motor has relatively constant speed under varying loads.
  • The starting torque is moderate.
• Application: Used in applications requiring constant speed, such as machine tools, fans, and pumps.

Compound Wound Motor:

• Construction: The motor has both series and parallel field windings.
• Operation:
  • The motor combines the characteristics of series and parallel wound motors.
  • The series field windings provide high starting torque, while the parallel field windings provide better speed regulation.
  • The degree of series and parallel winding combination determines the operating characteristics.
• Types:
  • Cumulative compound: Series field aids shunt field, providing increased starting torque.
  • Differential compound: Series field opposes shunt field, providing very flat speed torque curve.
• Application: Used in applications requiring a balance of starting torque and speed regulation, such as elevators, conveyors, and printing presses.

Characteristics of Motor Circuit Types

The way a motor is wired into a circuit significantly impacts its performance characteristics, such as starting torque, speed regulation, and efficiency. We'll focus on DC motor circuits, as they demonstrate these principles most clearly.

1. Series Wound Motor Circuits

• Characteristics:
  • High Starting Torque: Due to the armature current flowing through the field windings, the magnetic field is strong at low speeds, resulting in high torque.
  • Variable Speed: Speed varies significantly with load. As load decreases, speed increases dramatically, potentially leading to dangerous overspeed.
  • Low Speed Regulation: Speed changes significantly with load variations.
  • High Current Draw at Start: The motor draws a large current when starting.
  • Application: Ideal for applications requiring high starting torque, such as:
    • Starter motors
    • Cranes and hoists
    • Electric tools
• Circuit Characteristics:
  • Field windings are in series with the armature.
  • Current is the same through the field and armature.
  • Voltage is divided between the field and armature.

2. Shunt (Parallel) Wound Motor Circuits

• Characteristics:
  • Constant Speed: Speed remains relatively constant under varying loads.
  • Moderate Starting Torque: Provides moderate starting torque.
  • Good Speed Regulation: Speed changes minimally with load variations.
  • Lower Starting Current: Draws less current during start-up compared to series motors.
  • Application: Suitable for applications requiring constant speed, such as:
    • Machine tools
    • Centrifugal pumps
    • Fans and blowers
• Circuit Characteristics:
  • Field windings are in parallel with the armature.
  • Voltage is the same across the field and armature.
  • Current is divided between the field and armature.

3. Compound Wound Motor Circuits

• Characteristics:
  • Combined Characteristics: Exhibits a blend of series and shunt motor characteristics.
  • High Starting Torque: Provides higher starting torque than shunt motors.
  • Improved Speed Regulation: Offers better speed regulation than series motors.
  • Variable Speed: Speed varies with load, but less than in series motors.
  • Application: Used in applications requiring a balance of starting torque and speed regulation, such as:
    • Elevators
    • Conveyors
    • Printing presses
• Circuit Characteristics:
  • Includes both series and shunt field windings.
  • Can be connected as cumulative compound (series field aids shunt field) or differential compound (series field opposes shunt field).
  • Cumulative compound provides high starting torque and moderate speed regulation.
  • Differential compound provides very flat speed torque curve.

4. AC Motor Circuits

• Induction Motor Circuits:
  • Characteristics:
    • Simple and robust construction.
    • Relatively constant speed.
    • Require starting methods to reduce high starting current.
    • Used in a wide range of industrial and domestic applications.
  • Circuit Characteristics:
    • Direct online starting (DOL) for small motors.
    • Star-delta starting, reduced voltage starters, or variable frequency drives (VFDs) for larger motors.
• Synchronous Motor Circuits:
  • Characteristics:
    • Operate at a constant synchronous speed.
    • Used in applications requiring precise speed control.
    • Require DC excitation for the rotor.
  • Circuit Characteristics:
    • DC excitation circuit for the rotor.
    • AC supply for the stator.
• Universal Motor Circuits:
  • Characteristics:
    • Can operate on both AC and DC.
    • High speed and high starting torque.
    • Used in portable tools and appliances.
  • Circuit Characteristics:
    • Series wound.
    • Brushes and commutator.

Starter Motors:

Starter motors are essential for initiating the combustion process in an internal combustion engine. They convert electrical energy from the battery into mechanical energy to crank the engine.

• General Construction:
  • Armature: A rotating component with windings.
  • Field Windings or Permanent Magnets: Create a magnetic field.
  • Commutator and Brushes: Transfer electrical current to the armature.
  • Solenoid: An electromagnetic switch that engages the starter drive.
  • Drive Mechanism: Engages the starter pinion gear with the engine's flywheel or flexplate.
  • Housing: Encloses and protects the components.

Types of Starter Motors

Pre-Engaged Starter Motor:

• Construction:
  • Uses a solenoid to shift the pinion gear into engagement with the flywheel before the motor starts to rotate.
  • Includes an overrunning clutch to prevent the engine from driving the starter motor after it starts.
• Operation:
  • When the ignition key is turned, the solenoid is energized.
  • The solenoid's plunger moves, shifting the pinion gear into mesh with the flywheel and closing the main contacts to supply current to the motor.
  • The motor rotates, cranking the engine.
  • Once the engine starts, the overrunning clutch disengages the pinion gear.
• Explanation: This design provides a positive and reliable engagement, reducing the risk of gear damage.

Gear Reduction Starter Motor:

• Construction:
  • Incorporates a gear reduction mechanism between the armature and the pinion gear.
  • This mechanism increases the torque output of the motor.
  • Often uses permanent magnets instead of field windings.
• Operation:
  • The motor rotates at a higher speed, and the gear reduction mechanism reduces the speed and increases the torque applied to the flywheel.
  • This allows for a smaller and lighter motor to crank larger engines.
• Explanation: Gear reduction starters are more efficient and powerful than direct-drive starters of the same size.

Axial Starter Motor:

• Construction:
  • The armature and field windings are arranged axially, meaning they are aligned along the same axis.
  • This design is compact and lightweight.
  • Often uses permanent magnets.
• Operation:
  • The axial arrangement allows for a shorter and more efficient magnetic flux path, resulting in higher torque output.
  • The solenoid and drive mechanism operate similarly to a pre engaged starter.
• Explanation: Axial starters are commonly used in modern vehicles due to their compact size and high performance.

Coaxial Starter Motor:

• Construction:
  • The solenoid and drive mechanism are arranged coaxially with the armature. Meaning that the solenoid and the armature share the same centre axis.
  • This design results in a very compact and integrated unit.
• Operation:
  • The coaxial arrangement allows for a very efficient transfer of force from the solenoid to the drive mechanism.
  • The solenoid's movement directly engages the pinion gear with the flywheel.
• Explanation: Coaxial starters are designed for space-constrained applications.

Inertia Starter Motor:

• Construction:
  • Uses a Bendix drive, which relies on inertia to engage the pinion gear.
  • The pinion gear is mounted on a threaded shaft.
• Operation:
  • When the motor starts, the inertia of the pinion gear causes it to move along the threaded shaft and engage with the flywheel.
  • When the engine starts, the flywheel overruns the pinion gear, causing it to disengage.
• Explanation: Inertia starters are older designs and are less common in modern vehicles. They are relatively simple but can be prone to engagement problems.

Operating principles of starter

The operating principles of starter motor drives revolve around engaging the starter motor's pinion gear with the engine's flywheel or flex plate to initiate cranking, and then disengaging it once the engine starts. Here's a breakdown of the key principles:

1. Engagement:

• Solenoid Activation (Pre-Engaged and Coaxial):
  • When the ignition key is turned to the start position, the starter solenoid is energized.
  • The solenoid's electromagnetic force pulls a plunger, which performs two primary functions:
    • Mechanical Engagement: The plunger pushes the pinion gear forward, engaging it with the teeth of the flywheel or flex plate.
    • Electrical Connection: The plunger closes heavy-duty electrical contacts, allowing high current to flow from the battery to the starter motor.
• Inertia Engagement (Bendix Drive):
  • In older inertia-type starters, the Bendix drive mechanism uses the inertia of the pinion gear itself.
  • When the starter motor begins to rotate, the inertia of the pinion gear causes it to move along a spiral thread on the armature shaft, engaging with the flywheel.
  • This system relies on the sudden start of the starter motor to cause the pinion gear to slide forward.

2. Torque Transfer:

• Once the pinion gear is engaged, the starter motor's rotational force is transferred to the flywheel or flex plate.
• This rotational force cranks the engine, causing the pistons to move and the air-fuel mixture to be compressed.

3. Disengagement:

• Overrunning Clutch:
  • Modern starter motors utilize an overrunning clutch (also known as a one-way clutch or freewheel).
  • This clutch allows the pinion gear to rotate in one direction (when driven by the starter motor) but prevents it from rotating in the opposite direction (when driven by the engine).
  • When the engine starts and its speed exceeds the starter motor's speed, the overrunning clutch disengages the pinion gear, preventing the engine from driving the starter motor, which could cause severe damage.
• Inertia Disengagement (Bendix Drive):
  • In Bendix drive systems, when the engine starts and its speed exceeds the starter motor's speed, the pinion gear is forced to rotate faster than the spiral thread on the armature shaft.
  • This causes the pinion gear to move back along the thread and disengage from the flywheel.
• Solenoid Retraction:
  • When the ignition key is released from the start position, the solenoid is de-energized.
  • A return spring pushes the plunger back to its original position, disengaging the pinion gear and disconnecting the electrical connection to the starter motor.

Key Considerations:

• Gear Ratio: The gear ratio between the starter motor's pinion gear and the flywheel or flex plate is crucial for providing sufficient torque to crank the engine.
• Overrunning Clutch Function: A properly functioning overrunning clutch is essential for preventing damage to the starter motor and flywheel.
• Solenoid Operation: The solenoid must operate reliably to ensure proper engagement and disengagement.
• Electrical Connections: Clean and secure electrical connections are vital for delivering sufficient current to the starter motor.

Change-Over Relay (Also Known as a SPDT Relay)

Function:

• A change-over relay is a Single Pole Double Throw (SPDT) relay. It has one common terminal and two output terminals.
• Its purpose is to switch between two different circuits or functions based on the relay's state (energized or de-energized).
• In a starter system, it might be used to switch between different power sources or to control different stages of the starting process.

Operation:

• When the relay is de-energized, the common terminal is connected to one of the output terminals (normally closed, NC).
• When the relay is energized, the common terminal switches and connects to the other output terminal (normally open, NO).
• In a starting system, this could be used to:
  • Switch power from a low-current control circuit to a high-current starter solenoid circuit.
  • Disable certain accessories during engine cranking to conserve battery power.
  • Switch between a primary and secondary power source for the starting system.

Example:

• A change-over relay could be used to disable the vehicle's headlights while the starter motor is engaged, thus providing more power to the starter.

Start Repeating Relay (Also Known as a Cranking Relay)

Function:

• A start repeating relay is designed to allow repeated attempts to start the engine, especially in situations where the engine fails to start on the first try.
• It is often used in conjunction with automatic starting systems or remote start systems.
• It is used to prevent the starter motor from being engaged for an excessive amount of time.

Operation:

• When the start signal is received, the relay energizes, engaging the starter motor.
• If the engine fails to start within a predetermined time, the relay de energizes, stopping the starter motor.
• After a short delay, the relay can be energized again, allowing another attempt to start the engine.
• This cycle can repeat for a set number of attempts or until the engine starts.
• This protects the starter motor from overheating.

Example:

• In a remote start system, a start repeating relay would allow the system to attempt to start the engine multiple times if it stalls or fails to ignite.

Double Starting Relay (Also Known as a Starter Interlock Relay)

Function:

• A double starting relay, or starter interlock relay, is primarily a safety device.
• It prevents the starter motor from engaging while the engine is already running, preventing damage to the starter and flywheel.
• It also can be used to prevent the starter from engaging unless certain conditions are met, such as the vehicle being in park, or neutral.

Operation:

• The relay uses a signal from the engine's RPM or oil pressure sensor to determine if the engine is running.
• If the engine is running, the relay prevents the starter circuit from being energized, even if the ignition key is turned to the start position.
• It can also use signals from the transmission to only allow starting in park or neutral.
• If the engine is not running and the necessary conditions are met, the relay allows the starter circuit to be energized.

Example:

• A double starting relay prevents a driver from accidentally engaging the starter motor while the engine is running, which could damage the starter and flywheel teeth.
• It also prevents a vehicle from starting while in drive.

Keyless start systems

Keyless start systems, also known as push-button start systems, have become increasingly common in modern vehicles. They offer convenience by eliminating the need to physically insert and turn a key. Here's how they operate:

1. Key Fob and Radio Frequency Identification (RFID):

• The system relies on a key fob, which contains a small transmitter.
• The fob emits a low-frequency radio signal that communicates with the vehicle's electronic control unit (ECU).
• The vehicle has antennas that detect the fob's signal.
• When the fob is within a certain range of the vehicle (typically a few feet), the vehicle's system recognizes its presence.
• This communication uses RFID technology, which allows for secure authentication.

2. Authentication and Authorization:

• Once the vehicle detects the fob's signal, it initiates an authentication process.
• The vehicle and the fob exchange encrypted codes to verify that the fob is authorized to start the vehicle.
• If the authentication is successful, the vehicle unlocks the steering column (if equipped) and enables the start button.
• Some systems also use rolling codes, which change with each use, to further enhance security.

3. Start Button Operation:

• After successful authentication, the driver can press the start button to start the engine.
• The start button typically sends a signal to the ECU, which then engages the starter motor.
• The ECU controls the starting process, including the engagement of the starter motor and the timing of the ignition and fuel injection systems.
• Some vehicles require the driver to press the brake pedal or clutch pedal simultaneously with the start button for safety reasons.
• The system can also control other vehicle functions, such as unlocking the doors, adjusting the seats, and setting the climate control.

4. Automatic Shutdown and Security:

• Some keyless start systems have an automatic shutdown feature that turns off the engine after a certain period of idling.
• This feature is designed to prevent the engine from running unnecessarily, especially if the driver forgets to turn it off.
• The system also includes security features to prevent unauthorized starting.
• If the fob is not present or if the authentication fails, the vehicle will not start.
• Many keyless start systems also have an immobilizer function, which prevents the engine from starting even if the ignition is hot wired.

5. Comfort and Convenience Features:

• Many keyless start systems are integrated with other comfort and convenience features.
• For example, some systems allow the driver to remotely start the engine from a distance.
• Some systems also offer personalized settings, such as seat and mirror positions, which are automatically adjusted when the driver enters the vehicle.
• Some systems will also lock the doors automatically when the key fob leaves a certain range of the vehicle.