Essay: Control system – stabilise frequency of voltage and generate electricity on demand

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  • Control system - stabilise frequency of voltage and generate electricity on demand
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Design a control system that automatically stabilizes the frequency of the generated voltage at a certain amount, and to be able to generate electricity on demand

Introduction

Electricity consumption is rising on a global scale. This rise demands electricity generation whenever possible. Also, human feedback over the system can have many possible errors and is not always constant. So, the project assigned to our team will allow us to make electricity available for small scale generations, but the concept can be applied in large scales too. The main reason of doing this project is to combine the three basic components; the internal combustion engine (ICE), the synchronous generator and the speed control system.

This project has two main objectives; to design a control system that automatically stabilizes the frequency of the generated voltage at a certain amount, and to be able to generate electricity on demand. These two objectives will be beneficial in decreasing the number of staff members working in power stations in remote locations. Also, it can be used in places unreachable with the normal electricity network. These places can have their own automatically controlled power source.

Like any other project, there must be in-scope and out-of-scope items in order to measure the success of the planned project. As for this system, the generator must be synchronous, the output must be 60Hz with a ±2Hz error limit at full load and the rotation of the shaft of the (ICE) must be automatically-controlled. If possible, the generator should give a three-phase output and a 220V output.

Information was collected about the assigned project to build a clear idea about the terminology and fundamental engineering concepts that is related to it. After that, research about the main components of the project had been conducted to visualize how they will fit together and understand the physical components that depends on it to work.

Technical Objectives

The basic idea of this project is to have device that consumes chemical energy and converts it to heat and mechanical energy. This mechanical output will be used by another device to generate electrical energy. The frequency of the resultant current is monitored by a feedback system which will increase or decrease the speed of the ICE depending on the situation. Figure (1) shows a block diagram of main components of the whole system. For each block or component, there are different types of devices that can be used to achieve the wanted results.

i. Synchronous Generators

Electrical machines, in general, are made of two main electrical parts and two main mechanical parts. The electrical parts are the field and armature windings. The mechanical parts are the stator and rotor.

The field winding produces the main magnetic flux. This flux will affect the armature winding and produce emf on it. As for the mechanical structure, the rotor is the rotating part of the machine and the stator is the static (fixed) part. Electrical machines can a have different setups of these parts for different uses.

One of these setups is the synchronous generator. This type of machine has the field winding on the rotor and the armature winding on the stator. Field windings are excited using a DC power source via slip rings and generates magnetic fields. So, by rotating the rotor, the magnetic field will rotate and generate alternating emf across the armature winding on the stator. There are different structures of synchronous generators. The following are various classifications for it. [1]

Rotors of synchronous generators are classified as salient pole rotors and non-salient pole rotors. The following is a comparison between the two.

Salient pole rotors (see figure (2))

• are made of laminations made of steel;

• have large diameter and short axial length;

• have non-uniform air gaps;

• are generally used in lower speed electrical machines, usually around 100 RPM to 1500 RPM.

• have a flux distribution relatively less than the non-salient pole rotor, hence the generated emf waveform is not as smooth as cylindrical rotor.

Non-salient pole (cylindrical) rotors (see figure (3))

• are made of solid forged steel;

• have small diameter and long axial length;

• have uniform air gaps so the noise is less;

• are used in high speed electrical applications, usually around 1500 RPM to 3000 RPM.

• have a good flux distribution, hence gives a smoother emf waveform. [2]

Windings in the synchronous generator are classified as single-layer or double-layer windings. In the single-layer winding, one coil-side in each slot. In the double-layer winding, each slot have even number of coil-sides may be 2, 4 etc. Double-layer windings are more common and have better emf waveform in case of generators. Figure (4) shows schematics for these two types of windings.

Windings in the synchronous generator can also be classified as full-pitch and short-pitch winding. The pole pitch in any electrical machine is always equal to 180 electrical degrees and the coil pitch is the distance between the two coil-sides of a coil. Therefore, if the pole pitch is equal to coil pitch then the coil is termed a full-pitch coil, but in case the coil pitch is less than the pole pitch, then it is called short-pitch. Figure (5) shows the schematics for the full-pitch and short-pitch windings. [3]

Short-pitch windings are commonly used because the length required for the end connections of coils is less, which reduces the cost of copper. Also, they eliminate the unwanted harmonics, hence their waveforms of an induced emf is a smoother sinusoid.

All these structures and more should be kept in mind when choosing the synchronous generator for any project that require it, in order to get the assigned result effectively and economically.

The equivalent circuit of a synchronous generator can be simplified into a simple RL circuit with R representing the conductors resistance and L representing the reactance of them. The induced emf in the armature windings can be represented as an emf source. Figure (6) shows a simple generator equivalent circuit. [1]

ii. Sensors

Sensors (or sometimes called “transducers”) are simple, standalone devices that do a specific function. This function is mainly converting physical parameters (for example: movement, temperature, light…etc) into electrical signals. Figure (7) shows different types of sensors. [4]

How these devices are stimulated depends on the structure of the sensor. Table(1) shows the different types of stimulus to a sensor. [5]

Sensors can be classified based on the energy supplement or consumption into two kinds: Active Sensors that require power supply (e.g. the photoconductive cell) and Passive Sensors that do not require power supply (e.g. radiometers). [4]

In the case of this project, speed sensors are required to measure the angular velocity of the rotating shaft. Speed sensing can be achieved by a number of principles i.e. Variable Reluctance based, Hall Effect based, Eddy Current based, Pitot Tube based…etc. Each principle has its own pros and cons. Table (2) shows a brief description of some of these types. [6]

Sensor type Variable Reluctance Speed Sensors Hall Effect Speed Sensors Eddy Current Speed Sensors

Principle of Work Time-varying flux because of moving ferromagnetic material induces voltage Same as Variable Reluctance but only sensitive to flux’s magnitude, not the rate of change Measure the change in impedance of a coil due to Eddy Currents

Pros

• Passive (Do not require external power supply)

• Low Cost

• Light Weight

• Durable in harsh environments

• Can detect slow speeds.

• Ferrous or magnetic targets

• Directly provides digital output compatible with microcontrollers and PLC’s.

• Highly immune to electromagnetic interference induced failures

• Operates from -40ºC to 150ºC • Only sensitive to flux’s magnitude, not the rate of change

Cons

• Material must ferrous

• Definite lower limit on the speed of the target

• High cost of electronic circuitry • Near-zero speed response

• No magnetic drag

• Relatively large air gaps

• Ferrous on non-ferrous materials • Cannot be used with digital devices directly

Diagram

iii. Tachometer

Tachometers are instruments that measure the angular frequency of a rotating target. They work on the basis of receiving signals from the speed sensors and translating them to a display. These devices are usually connected to an analogue display in cars, trains, and many other places that require the monitoring of RPMs. Nowadays, tachometers have digital displays. Figure (8) shows an analogue tachometer and figure (9) shows a digital one. Also, the tachometer can have direct contact with the target or no-contact. [7]

iv. Internal Combustion Engines (ICE)

As mentioned in the background assignment that the internal combustion engine is one of the main parts of the project. Simply, the internal combustion engine (ICE) is an engine that have the ability to convert the chemical energy to mechanical energy through many processes inside the engine cylinders. The ICE has too many mechanical systems such as Air Intake & Exhaust System, Fuel Supply System, Cooling System…etc. For this reason, the ICE can have many different combination of components depending on what the objective is. Table (3) shows the many different types of the ICE by different criteria. [8]

Criteria Type

Type of fuel used Diesel Engine

Petrol (Gasoline) Engine

Number of strokes per cycle 2-Stroke

4-Stroke

Number of cylinders Single-Cylinder Engine

Multi-Cylinder Engine

Type of ignition Spark-Ignition Engine

Compression-Ignition Engine

Type of cooling system Air-Cooled

Water-Cooled

Position of cylinders Horizontal Engine

Vertical Engine

This report will mostly focus on the major electrical component of the ICE which is the Engine Control Unit (ECU).

The ECU is an electronic circuit that can control actuators (e.g. spark plugs, air control valves, the ignition coil…etc) in an internal combustion engine to ensure optimum engine performance. As the assigned function of the project is primarily controlling the ICE’s shaft’s angular frequency or RPM, hence controlling the frequency, controlling the ECU will be beneficial in achieving that function. The following points are some of the major tasks of the ECU which can be seen in figure (10). [9]

• Control of air/fuel ratio: Determining the amount of fuel injected inside combustion chamber based on a number of sensor readings.

• Control of ignition timing: Adjusting the exact timing of the spark of the spark plugs (ignition timing) to supply better power and economy.

• Control of idle speed: Controlling the Idle speed

• Control of variable valve timing: Controlling the time in the engine cycle at which the valves open

v. Microcontrollers

Microcontrollers are small, embedded systems that are dedicated to do a certain tasks pre-programmed by the manufacturer. These tasks are basically giving commands to other devices based on the inputs received. They can be used in engine control systems, medical devices, power tools, appliances, toys, etc. The idea of using microcontrollers is to make systems work automatically without the need of constant human interaction, or at least to minimize it. [10] [11]

There are many types of microcontrollers in the world made by different manufacturers. Each microcontroller has its own advantages and disadvantages depending on the application and the user. Two of the most common types of microcontrollers are the PIC and the Arduino.

PIC stand for Peripheral Interface Controller. Each PIC has its own standard and features such as on-chip program (code) ROM, data RAM ,data EEPROM and number of input and output ports. On example of PICs is the PIC18F4580. It has 40 pins, 32K code ROM, 1536 data RAM, 256 data EEPROM and 36 input and output pins as you can see in figure (13). [12]

Another type of a microcontroller is the Arduino. An Arduino is an open source physical computing platform based on a simple input/output (I/O) board and a development environment that implements the processing language. Arduino can be used to develop standalone interactive objects or can be connected to a computer such as Flash or Max/MSP via software. The board can be assembled by hand or purchased preassembled. The open source IDE (Integrated Development Environment) can be downloaded for free from www.arduino.cc. Figure (14) shows an Arduino microcontroller.

Arduino is different from other platforms on the market because of these features:

1. It can run on Windows, Macintosh, and Linux

2. It is based on the processing programming IDE which is easy to use by artists and designers

3. The program is uploaded by USB cable, not serial port. This feature is useful because many modern computers do not have serial ports.

4. It is an open source hardware and software so the user can make your own, without paying anything to the makers of Arduino.

5. It is cheap. It costs around €20

6. There is an active community of users, so there are plenty of sources to get information from [13]

vi. LCD Screen

It is an electronic visual display that depend on the light modulating properties of liquid crystal. The LCD screen has a huge number of tiny blocks called Pixels, each pixel contain red, blue and green lights these lights are controllable by liquid crystals to switch them on or off in very fast way, due to this behavior moving color picture will create. [14]

A small LCD screen could be used to display information about the output. For example, in this project, the reading of the frequency or the ICEs RPM speed will be needed.

Project Alternatives

This section will list 3 possible alternatives for the system in order to achieve the required objective. The number of possible alternative for the project can reach up to 6 different combinations. It will be noticed that the main difference between them will be the sensor and the microcontroller. This is because the type of ICE and generator will not affect the main objective of the project which is to have a steady 60Hz frequency output. Also, if the type of ICE and generator is added to the possibilities, then there will be over 15 different designs.

Alternative 1:

This alternative uses petrol as the main fuel for the internal combustion engine. The rotational movement will rotate the synchronous generator’s rotor to produce electricity in the armature windings.

For the controlling part, a Variable Reluctance Speed Sensor could be used to measure the rotation speed. This information will then be sent to a PIC18F4580 to calculate and then decide whether to increase or decrease the speed of the engine. Also, the speed can be displayed on an LCD screen.

The advantage of this combination would be that the VR Speed Sensor would withstand the harsh environment and heat from the engine. Also, the PIC18F4580 is a very simple microcontroller to program and apply and it is cheap. On the other side, this sensor would not be able to measure slow speeds and its circuit would be expensive.

Alternative 2:

This alternative uses petrol as the main fuel for the internal combustion engine. The rotational movement will rotate the synchronous generator’s rotor to produce electricity in the armature windings.

For the controlling part, a Hall Effect Speed Sensor could be used to measure the rotation speed. This information will then be sent to an Arduino to calculate and then decide whether to increase or decrease the speed of the engine. Also, the speed can be displayed on an LCD screen.

The advantage of this combination would be that the Hall Effect Speed Sensor would directly provide digital output compatible with the microcontroller. Also, the Arduino is a very simple and user-friendly microcontroller. This would be beneficial for non-computer experts. Also, this sensor would give fairly good results. On the other side, this sensor would not be able to withstand very high temperatures.

Alternative 3:

This alternative uses petrol as the main fuel for the internal combustion engine. The rotational movement will rotate the synchronous generator’s rotor to produce electricity in the armature windings.

For the controlling part, an Eddy Current Speed Sensor could be used to measure the rotation speed. This information will then be sent to an PIC18F4580 to calculate and then decide whether to increase or decrease the speed of the engine. Also, the speed can be displayed on an LCD screen.

The advantage of this combination would be that the Eddy Current Speed Sensor would give very accurate results. Of course, the more accurate the results means the more complex and expensive the circuit. For non-computer experts, this would be a big barrier. Even though the Arduino is used, the circuit would still be too advanced for them.

Cost Estimations* of Each Alternative:

Alternative 1:

Parts Variable Reluctance Speed Sensor PIC18F4580 Synchronous Generator Internal Combustion Engine LCD

Price $230 $7 $53 $189 $14

Total cost $495

Alternative 2:

Parts Hall Effect Speed Sensor Arduino Synchronous Generator Internal Combustion Engine LCD

Price $9 $80 $53 $189 $14

Total cost $345

Alternative 3:

Parts Eddy Current Speed Sensor Arduino Synchronous Generator Internal Combustion Engine LCD

Price $299 $80 $53 $189 $14

Total cost $635

* all cost estimations are based on average prices on Amazon.com

Chosen Alternative

Based on the advantages and disadvantages of each alternative and also based on the total cost estimations, (Alternative 2) seems to be the most efficient and simple approach to attempt this project. Also, this alternative does not exceed any of the constrains in the project charter.

References

[1] A. E. Fitzgerald, Electric Machinery, USA: McGraw-hill, 2003.

[2] K. Daware, “Electricaleasy,” [Online]. Available: http://www.electricaleasy.com/2014/03/salient-pole-rotor-vs-non-salient-pole.html. [Accessed 10 11 2015].

[3] V. B. U.A. Bakshi, Electrical And Electronics Engineering, Technical Publications, 2009.

[4] “Basic Electronics Tutorials,” [Online]. Available: http://www.electronics-tutorials.ws/io/io_1.html. [Accessed 10 11 2015].

[5] [Online]. Available: http://www.mfg.mtu.edu/cyberman/machtool/machtool/sensors/intro.html. [Accessed 10 11 2015].

[6] P. Jain, “EngineersGarage,” [Online]. Available: http://www.engineersgarage.com/articles/speed-sensor-types?page=1. [Accessed 10 11 2015].

[7] W. Arana, “eHow,” Demand Media, Inc., [Online]. Available: http://www.ehow.com/how-does_5019069_tachometers-work.html. [Accessed 10 11 2015].

[8] V. Ganesan, Internal Combustion Engines, New Delhi: Tata McGraw-Hill, 2004.

[9] “Wikipedia,” Wikimedia Foundation, Inc., 05 11 2015. [Online]. Available: https://en.wikipedia.org/wiki/Engine_control_unit. [Accessed 10 11 2015].

[10] Future Electronics, “Microcontrollers,” [Online]. Available: https://www.futureelectronics.com/en/Microcontrollers/microcontrollers.aspx. [Accessed 4 October 2015].

[11] Wikimedia, “Microcontrollers,” 27 September 2015. [Online]. Available: https://en.wikipedia.org/wiki/Microcontroller. [Accessed 4 October 2015].

[12] M. A. Mazidi, R. D. Mckinlay and D. Causey, PIC Microcontrollers and Embedded Systems, Upper Saddle River, New Jersey: Pearson Education Inc., 2008.

[13] “Arduino,” [Online]. Available: https://www.arduino.cc. [Accessed 10 11 2015].

[14] “Wikipedia,” Wikimedia Company Inc., 23 10 2015. [Online]. Available: https://en.wikipedia.org/wiki/Liquid-crystal_display. [Accessed 10 11 2015].

[15] F. Ulabi, Fundamentals of Applied Electromagnetics, Upper Saddle River: Prentice Hall, 2012.

[16] R. Stone, Introduction to Internal Combustion Engines, Hampshire: Macmillan, 1992.

[17] M. J. Nunney, Light and Heavy Vehicle Technology, Oxford: Elsevier, 2007.

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