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Arduino FreeRTOS Mutex Examples

In this section, we will explain using an example to demonstrate mutexes in Arduino using freeRTOSThere are two freeRTOS Mutex examples in this tutorial, the first example demands some hardware (LCD) While the second does not need any hardware, you can try out both if you have the resources.

In the last tutorial, we considered in detail: semaphores and mutexes and we also established the difference between binary semaphores and mutexes.
Just for a review:Recall that a mutex is a locking mechanism that implements the take and gives functionality in itself, unlike a binary semaphore. See this tutorial if you have not before continuing.
Example 1: Protecting the LCD Resource Using Mutex in freeRTOS
Program Description

In this program, we Demonstrated the use a 16x2 LCD display to implement a mutex.

The LiquidCrystal library works with all LCD displays that are compatible with the Hitachi HD44780 driver. you can usually tell them by their 16-pin interface.

THE CIRCUIT

* LCD RS pin to digital…

Field Flashing of a Generator and Excitation Mechanism

This is part of my Egbin Thermal Power plant series and it is really interesting getting to understand how field flashing and excitation of the generator takes place. I am creating this content tailored towards a steam turbine generator. The concept is valid for all kinds of generators.

Alright, let me first introduce what field flashing is and then we will look at how it is done in a typical thermal plant.

The concept of Field Flashing


For electricity to be induced in a coil, according to the Faraday's Law of Electromagnetic induction, there are two requirements: The coil/conductor and the varying magnetic field. Such that whenever the coil is placed in a varying magnetic field, an emf (Electro Motive force) is induced in the coil.


Usually, in small dc motors, this magnetic field is generated by permanent magnets, however, in large generators and generating stations, an electromagnet (a soft iron core possessing a  magnetic field by the passage of electric current through it) is used.

The reason for using an electromagnet instead of a permanent magnet as you can intuitively suggest is:

  • For control: Since EMF induced depends on the flux density, altering the magnetic flux can alter the output voltage and power but the permanent magnet does not provide us with this functionality

  • To circumvent the degrading properties of permanent magnet with time, and heat. The permanent magnet is known to lose magnetism with time and increasing temperature

  • Permanent magnets cannot provide a strong magnetic flux needed for commercial power generation.

The rotor of the generator is made up field windings: turns of insulated conductors that generate the magnetic field as electric current passes through it.

When the rotation begins, there is no induced emf in the stator armature, because the residual magnetic field in the rotor is not enough to generate a reasonable output voltage. 

At higher peripheral speed up to 3000rpm for synchronous generators, DC current is now introduced into the rotor winding for a period of 3-5secs, this causes the rotor to be magnetized. This is known as exciting the field winding.

The rotation of the rotor now possesses a magnetic field which induces an emf in the static armature of the generator, thus voltage output is established.  This is done in open-circuit, i.e. no load connected to the generator's output.

For field flashing, the DC current supplied to the field windings can be:

  1. From a DC battery source

  2. From a DC motor

Generator Excitation Mechanism


As output voltage is established, it is tapped using the Generator's excitation system and sent back to the rotor field to sustain the excitation indefinitely.

The excitation system for a typical power generator in a power station consists of:

  • The SCR Transformer

  • The Thyristors

  • The AVR

  • Current and Voltage Transformers

The SCR Transformer: This is a dry-type transformer that steps down the high voltage output from the generator to a low voltage (Say 16kv/660V) for rectification. It  is sent to the thyristor stack to perform rectification.

The Thyristors: This is a power semiconductor electronic device that is used for rectification.

The AVR (Automatic Voltage Regulator): regulates the output voltage of the generator through the following means.

  • The output voltage is monitored through potential transformers (Voltage Transformers)

  • The output current is monitored through current transformers

  • The value of this output current and voltage is used to control the excitation, thus preventing under-excitation and over-excitation and their associated problems.

  • Excitation is controlled by controlling the firing angle of the thyristors.

  • The firing angle of the thyristors determines the output voltage from the thyristor rectifier that will be sent to the rotor windings through the generator slip rings.

Note: The voltage output from the thyristor rectifier is not a pure DC, thus it needs to be purified using a filter circuit.

Voltage and Current Transformer: These are instrument transformers that are used to perform control, metering and instrumentation. A current Transformer  (CT) is connected in series to the output of the generator and possesses a highly accurate and precise current ratio (eg steps down 1000A to 1A) being an instrument transformer, it presents a negligible load to the supply being measured. While a voltage transformer is connected in parallel to the output of the supply.

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