Definition Application of electrical machines Electromagnetism: review Analogies between electric and magnetic circuits Faraday s Law Electromagnetic Force Motor action Generator action Types and parts of machines 1
1. Definition Electric machines are energy conversion devices that convert electrical energy to mechanical energy and vice versa through the medium of magnetic field. An electric machine is called a generator when it is used to convert mechanical energy to electrical energy. On the contrary, when an electric machine is operated to convert electrical energy to mechanical energy, it is called a motor. 2. Application of Machines Electric motors are used to operate washing machines, elevators, cranes etc while electric generators are used to generate electricity for power generation and alternator for charging car battery. 2
Applications of Machines 3
Applications of Machines 4
Applications of Machines 5
Electromagnetism Magnetic fields are the fundamental mechanism by which energy is conserved from one form to another in motors, generators and transformers. A magnetic field is a condition resulting from electric charges in motion. For convenience in visualization and analysis, magnetic fields are represented on diagrams by close loops. Theses loops, called magnetic flux lines 6
Magnetic Circuit Magnetic circuit showing an arrangement of ferromagnetic materials (core) that forms the path and guide the magnetic flux. The flux always takes the shortest path 7
Megnetomotive force It is the driving force that causes the magnetic filed to appear in a magnetic circuit. Magnetic Field Intensity It is the mmf per unit length of the magnetic circuit and it may vary from point to point throughout the circuit. Flux Density It s a measure of the concentration of lines of flux in a particular section of magnetic circuit 8
Reluctance It s a measure of the opposition the magnetic circuit offers to the flux. The reluctance of a magnetic circuit is related to its length, cross sectional area, and permeability. 9
Relative permeability and magnetization curves Relative permeability is the ratio of the permeability of a material to the permeability of free space. It is very useful for comparing the magnetizability of different magnetic materials whose relative permeabilities are known. 10
The ratio µ=b/h is called magnetic permeability and has different values for different magnetic materials. 11
Analogies between Electric and Magnetic Circuits The relationship between mmf, flux, and reluctance in a magnetic circuit is an analog of the relationship bwteen emf, current and resistance respectively, in an electric circuit. 12
Magnetic Circuit and its Electrical Analog 13
Magnetic Circuit and its Electrical Analog A ferromagnetic core is shown in figure below. The depth of the core is 5 cm. The other dimensions of the core are as shown in the figure. Find the value of the current that will produce a flux of 0.005 Wb. With this current, what is the flux density at the top of the core? What is the flux density at the right side of the core? Assume that the relative permeability of the core is 1000. 14
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Magnetic Hysteresis and hysteresis losses Hysteresis means lagging behind. In magnetic hysteresis, flux density B lags behind the field intensity H Hysteresis loop shows the characteristics of magnetic material. It is obtained by plotting values of flux density B for periodically reversing field intensity 17
Magnetic Field play an important role in electric machines. Magnetic fields are produced when current flows in coils of wire. Magnetic fields have 2 funtions: - produce torque for motor operation - generate voltage for generator operation 18
If a conductor is moved by an external force, D in a magnetic field, an electromagnetic force is produced between the two ends of the conductor. This is the basis for the operation of a generator. According to Faraday s Law, a magnetic field will induce a voltage, e in a coil. This forms the basis for the operation of a generator. The voltage, e is given by, e dnφ dt NdΦ dt N Number of turns of each coil Φ Magnetic flux (Weber) 19
Force F is produced and it acts on the conductor. Force is given by, F = BI (Newtons) Current flows in a conductor and produces a force and a mechanical output i.e. conductor motion. This serves as the basis for motor operation. 20
Michael Faraday showed that passing a current through a conductor freely suspended in a fixed magnetic field creates a force which causes the conductor to move through the field. The force created by the current, now known as the Lorentz force, acts between the current conductor and the magnetic field, or the magnet creating the field.the magnitude of the force acting on the conductor is given by: F = BLI where F is the force on the conductor, L is the length of the conductor and I is the current flowing through the conductor 21
Faraday also showed that the converse is true - moving a conductor through a magnetic field, or moving the magnetic field relative to the conductor, causes a current to flow in the conductor. The magnitude of the EMF generated in this way is given by: E = BLv where E is the generator EMF (or back EMF in a motor) and v is the velocity of the conductor through the field 22
In practice, both the motor and the generator effects take place at the same time. Passing the current through a conductor in the magnetic field causes the conductor to move through the field but once the conductor starts moving it becomes a generator creating a current through the conductor in the opposite direction to the applied current. Thus the motion of the conductor creates a "back EMF " which opposes the applied EMF. Conversely moving the conductor through the field causes a current to flow through the conductor which in turn creates a force on the conductor opposing the applied force. 23
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1. 3 types of rotating machines: - Synchronous AC Machines - Asynchronous Machines or Induction Machines - DC Machines 26
- Shaft is fasten to the rotor so that both rotate at the same angular speed - Connection of the machine to load is through the shaft - 3 main parts for both rotor and stator: - core - winding - insulation 27