Lesson Notes By Weeks and Term v5 - Grade 11

Lesson Video

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Performance objectives

• Describe the function of control circuit components.
• Explain the operation of protection devices.
• Design a simple forward/reverse motor control circuit.
• Select appropriate protection devices for a given load.
• Troubleshoot common faults in control systems.

Lesson summary

This week, we delve into the crucial topic of control circuits and protection devices. Imagine a factory in Gauteng producing goods for the South African market or a renewable energy system installed in a rural community in KwaZulu-Natal. Both rely heavily on electrical systems that need to be controlled and protected. Control circuits are the brains behind the automation, enabling machines to perform tasks efficiently and safely. Protection devices act as the guardians, preventing damage from faults like overcurrents or short circuits.

Lesson notes

2.1 Control Circuit Components: Relays: Relays are electromechanical switches. A small current in the coil energizes an electromagnet, which then pulls an armature to open or close one or more sets of contacts. They are used to control a larger current with a smaller current. They provide isolation between the control circuit and the power circuit.

Example:* Using a relay to control a 230V motor with a 24V DC signal from a Programmable Logic Controller (PLC).

Types:* Electromechanical relays, Solid State Relays (SSRs)

Contactors: Contactors are heavy-duty relays designed to switch high-current loads, typically used for controlling motors, heating elements, and lighting circuits. They usually have multiple contacts (typically three for three-phase applications). They are essential for motor starters.

Example:* A contactor starting a 3-phase induction motor used in a water pump in a farm.

Key specifications:* Voltage rating, Current rating (Amperes), Horsepower (HP) rating.

Timers: Timers introduce a time delay into a control circuit. They are used for sequencing operations, controlling the duration of events, and providing automatic control.

Types:* On-delay timers, Off-delay timers.

Example:* Using an on-delay timer to start a second pump 5 seconds after the first pump starts to maintain water pressure.

Operation:* On-delay timers wait for a set time after power is applied before activating their contacts. Off-delay timers activate their contacts immediately when power is applied and maintain activation for a set time after power is removed.

Sensors: Sensors detect physical quantities (temperature, pressure, light, proximity) and convert them into electrical signals that can be used by the control circuit.

Types:* Proximity sensors, Temperature sensors (thermocouples, RTDs), Pressure sensors, Limit switches.

Example:* Using a proximity sensor to detect the presence of a product on a conveyor belt in a packaging plant to start the sealing process. 2.2 Protection Devices: Fuses: Fuses are overcurrent protection devices that contain a fusible link which melts and breaks the circuit when the current exceeds a certain value. They are a simple and inexpensive form of protection.

Types:* Cartridge fuses, Blade fuses, High Rupturing Capacity (HRC) fuses.

Selection Criteria:* Voltage rating, Current rating (amps), Breaking capacity (kA).

Example:* Protecting a domestic appliance like a kettle from overcurrent using a 13A fuse.

Circuit Breakers: Circuit breakers are electromechanical devices that automatically interrupt the circuit when a fault occurs (overcurrent or short circuit). They can be reset after the fault is cleared, unlike fuses.

Types:* Miniature Circuit Breakers (MCBs), Molded Case Circuit Breakers (MCCBs).

MCBs:* Used for residential and light commercial applications. They have lower current ratings and breaking capacities.

MCCBs:* Used for industrial and commercial applications. They have higher current ratings and breaking capacities.

Tripping Curves:* Understanding tripping curves (Time vs. Current) is crucial for selecting the appropriate circuit breaker. Types include B, C, and D, each suited for different load characteristics (e.g., inductive loads, resistive loads).

Example:* Using an MCB in a distribution board to protect a lighting circuit in a classroom.

Earth Leakage Protection (ELCB/RCCB): Earth leakage protection devices (Earth Leakage Circuit Breakers - ELCBs, Residual Current Circuit Breakers - RCCBs) detect small leakage currents to earth, which can be dangerous. They trip the circuit quickly to prevent electric shock.

Operation:* They measure the difference in current between the live and neutral conductors. If this difference exceeds a certain value (sensitivity, typically 30mA), the circuit breaker trips.

Importance:* Essential for protecting people from electric shock in homes, factories, and construction sites.

Example:* An RCCB installed in a distribution board to protect the occupants of a house from electrocution if there is a fault in an appliance that causes current to leak to earth. 2.3 Motor Control Circuit

Example: Forward/Reverse Control Let's design a simple motor control circuit for forward/reverse operation using relays and pushbuttons. This is relevant in various applications like conveyor belts or machine tools.

Components: 3-Phase Motor 3 Contactors (Forward Contactor, Reverse Contactor, Overload Contactor) 2 Pushbuttons (Forward Pushbutton, Reverse Pushbutton) 1 Stop Pushbutton Overload Relay Control Transformer (e.g., 230V to 24V) - to provide a safe control voltage.

Operation: Pressing the Forward Pushbutton energizes the Forward Contactor coil. The Forward Contactor's main contacts close, applying power to the motor in the forward direction. The contactor also has a normally open (NO) auxiliary contact that seals in the circuit, so the motor continues to run even after the pushbutton is released.