For 4th Semester Polytechnic EE Students
Written by Garima Kanwar | Blog: Rajasthan Polytechnic
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Course Code EE 4001
Course Title Fundamentals of Power Electronics
5. Industrial Control Circuits
Industrial control circuits are essential for automating and controlling various processes in industrial settings. These circuits use power electronic devices like SCRs (Silicon Controlled Rectifiers), TRIACs, and other thyristors to manage high power efficiently while offering control over electrical systems. Below are the applications of industrial control circuits that are common in many industries.
5.1 Applications of Industrial Control Circuits
5.1.1 Burglar’s Alarm System
Application: A burglar’s alarm system detects unauthorized entry into a building or property and triggers an alarm.
Working Principle:
- The system typically uses sensors like magnetic reed switches, infrared sensors, or motion detectors.
- When the sensor detects an intrusion, it closes or opens a circuit.
- The change in circuit status is fed to a controller (which may include SCRs or TRIACs for controlling higher power devices like sirens or lights).
- The controller triggers an alarm (e.g., loud siren, flashing lights) and may send notifications to a monitoring center.
Control Components:
- TRIAC or SCR: These devices may be used to switch on/off higher loads such as the siren or lights in response to the signal from the sensor.
5.1.2 Battery Charger Using SCR
Application: SCRs can be used in battery chargers to control the charging process efficiently.
Working Principle:
- AC voltage is supplied to the battery charger circuit, and the SCR is used to control the charging process.
- The firing angle of the SCR is adjusted to provide the correct amount of voltage to the battery.
- A lower firing angle will provide more power to the battery.
- As the battery reaches full charge, the SCR can be triggered to reduce the charging voltage to prevent overcharging.
Control Components:
- SCR: Controls the charging current by adjusting the firing angle.
- Voltage/Current Control: Feedback from the battery may be used to adjust the SCR's firing angle to regulate charging.
5.1.3 Emergency Light System
Application: An emergency light system provides backup lighting during power failures or emergencies.
Working Principle:
- In normal operation, the lights are powered by the main AC supply.
- When the power supply fails, the system switches over to a backup power source (such as a battery).
- SCRs may be used to switch between the main power supply and the backup power source seamlessly.
- A controller detects the failure and sends a signal to the SCR to turn on the backup power and keep the emergency lights operating.
Control Components:
- SCR: Used for smooth switching between power sources.
- Battery: Provides the backup power when the main AC supply is unavailable.
5.1.4 Temperature Controller Using SCR
Application: SCR-based temperature controllers are used to control heating elements in systems like ovens, furnaces, or water heaters.
Working Principle:
- A temperature sensor (like a thermistor or RTD) measures the temperature of the system.
- The sensor sends the temperature signal to a controller, which compares it with the setpoint.
- If the temperature is below the setpoint, the controller triggers the SCR to allow current through the heating element.
- The firing angle of the SCR is adjusted to control the power delivered to the heating element.
- At lower firing angles, more power is delivered to the heater.
- At higher firing angles, less power is delivered, helping to regulate the temperature.
Control Components:
- SCR: Controls the power delivered to the heating element.
- Temperature Sensor: Provides feedback to the controller.
5.1.5 Illumination Control / Fan Speed Control Using TRIAC
Application: TRIACs are widely used for controlling the brightness of lights (illumination control) and the speed of fans.
Working Principle:
- Illumination Control:
- TRIAC is used to control the power delivered to incandescent lights or fluorescent lights.
- By adjusting the firing angle of the TRIAC, the amount of power supplied to the light is controlled, thus adjusting the brightness.
- Fan Speed Control:
- The TRIAC regulates the voltage applied to an AC fan motor. By adjusting the firing angle, the average voltage supplied to the motor is reduced or increased, which in turn controls the speed of the fan.
Control Components:
- TRIAC: Acts as a switch to control the power delivered to the load.
- Potentiometer or Controller: Used to adjust the firing angle and thus control the speed of the fan or brightness of the light.
5.1.6 SMPS (Switched-Mode Power Supply)
Application: An SMPS is a power supply unit that efficiently converts electrical power to the required voltage and current levels for a device, using high-frequency switching techniques.
Working Principle:
- An SMPS uses switching elements like MOSFETs, SCRs, or TRIACs to control the flow of energy to the load.
- The input AC voltage is first rectified to DC, and then the voltage is converted to the desired level using high-frequency switching.
- Feedback loops are used to regulate the output voltage by adjusting the duty cycle of the switching element.
Control Components:
- Switching Elements (SCRs, MOSFETs, TRIACs): Control the energy delivered to the output.
- Inductors and Capacitors: Smooth out the ripple and maintain stable output voltage.
Conclusion:
Industrial control circuits using devices like SCRs, TRIACs, and SMPS are crucial for automating processes and providing efficient control over power delivery in various applications. These circuits provide the flexibility and precision needed for controlling heating, lighting, motor speed, charging, and more. They help optimize energy use and improve the safety and reliability of systems like alarm systems, battery chargers, temperature controllers, and emergency lighting systems.
Important Questions to Practice:
- How does a burglar’s alarm system utilize SCRs for control?
- Explain the working of a battery charger using SCR and how it regulates the charging current.
- Describe the principle behind emergency light systems and the role of SCRs in switching the power source.
- How does an SCR-based temperature controller regulate the heating element power?
- What is the working principle of a TRIAC-based illumination control circuit and a fan speed control circuit?
- How do SMPS circuits function, and what role do SCRs and TRIACs play in these circuits?
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