Electric Machines PDF

Electric machines  To be uncovered 1. Electric machines 2. Principle of operation 3. Basic laws 4. Generator 5. Motor  Introduction 23.1 Various forms of rotating electrical machines These can be divided into...

Electric machines  To be uncovered 1. Electric machines 2. Principle of operation 3. Basic laws 4. Generator 5. Motor  Introduction 23.1 Various forms of rotating electrical machines These can be divided into: – generators – which convert mechanical energy into electrical energy – motors – which convert electrical energy into mechanical energy Both types operate through the interaction between a magnetic field and a set of windings  Electric Machine Electric machines can be divided into 2 types: AC machines DC machines  Electric Machine All Electric machines have: Stationary members (stator) rotating members (rotor) Air gap which is separating stator and rotor The rotor and stator are coupled magnetically Principle Of Operation Of A d.c. Machine All motors and generators make use of two basic principles: – When two poles of a magnet are brought close together then there will be either a repulsion force or an attraction force – When current flows in a conductor, a magnetic field is created around that conductor and relative motion induces emf. BASIC LAWS  Magnetic Field in a Coil When a current is passed through a coil a magnetic field is generated  Faraday Effect Faraday Effect Basic Concepts Voltage – V – Potential to Move Charge (volts) Current – I – Charge Movement (amperes or amps) Resistance – R – V = IxR (R in =ohms) Power – P = IxV = I2xR (watts) Magnetic Induction and the DC Generator Faraday’s Law e = N dΦ / dt – e = the induced voltage in volts (V) – N = the number of series-connected turns of wire in turns (t) – dΦ/dt = rate of change in flux in Webers/second (Wb/s) e=BLv – B = the flux density in teslas (T) – L = the length of the conductor that is in the magnetic field in meters (m) – v = the relative velocity between the wire and the flux, in meters/second (m/s) Motor Physics Fleming's left-hand rule is used to determine the direction of the force on a current carrying conductor inside a magnetic field. Generator Physics Fleming's right-hand rule is used to determine the direction of the induced current when a conductor cuts through the magnetic field lines perpendicularly. Simple Generator D. C. Generator Principle of Operation Input: Rotational or Mechanical Energy Output: Electrical Energy When a conductor rotates in a magnetic field produced by poles, dynamical ly induced e.m.f is produced in it according to Faraday’s law of electromagnetic induction  Cond… Whenever a conductor is moved within a magnetic field in such a way that the conductor cuts across magnetic lines of flux, voltage is generated in the conductor. The AMOUNT of voltage generated depends on: i. the strength of the magnetic field, ii. the angle at which the conductor cuts the magnetic field, iii. the speed at which the conductor is moved, and iv. the length of the conductor within the magnetic field  THE ELEMENTARY GENERATOR The simplest elementary generator that can be built is an ac generator. Basic generating principles are most easily explained through the use of the elementary ac generator. For this reason, the ac generator will be discussed first. The dc generator will be discussed later. An elementary generator consists of a wire loop mounted on the shaft, so that it can be rotated in a stationary magnetic field. field This will produce an induced emf in the loop. loop Sliding contacts (brushes) connect the loop to an external circuit load in order to pick up or use the induced emf. Elementary Generator AC generator with slip rings and brushes. Elementary Generator (Conclusion) Output voltage of an elementary generator during one revolution  A Simple DC Generator 23.3 The alternating signal from the earlier AC generator could be converted to DC using a rectifier A more efficient approach is to replace the two slip rings with a single split slip ring called a commutator – this is arranged so that connections to the coil are reversed as the voltage from the coil changes polarity – hence the voltage across the brushes is of a single polarity – adding additional coils produces a more constant output DC generator with commutator and brushes.  THE ELEMENTARY DC GENERATO Effects of commutation http://www.sciencejoywagon.com/physicszone/lesson/otherpub/wfendt/generatorengl.htm d.c. Generator DC from Four Armature Loops DC generator output waveform.  Parts of DC Machine 1. Magnetic Frame or Yoke 2. Pole Cores and Pole Shoes 3. Pole coils or Field Coils 4. Armature Core 5. Armature winding or Conductors. 6. Commutator 7. Brushes and Bearings DC Machines - Construction 27 DC Machine Construction Figure 8.3 Details of the commutator of a dc motor. DC Machine Construction Figure 8.4 DC motor stator with poles visible. DC Machine Construction Figure 8.5 Rotor of a dc motor. DC Machine Construction Figure 8.6 Cutaway view of a dc motor.  DC Machines The rotor has a ring- shaped laminated iron core with slots. The commutator consists of insulated copper segments mounted on an insulated tube. Two brushes are pressed to the commutator to permit current flow. The brushes are placed in the neutral zone, where the magnetic field is close to zero, to reduce arcing.  DC Machines The commutator switches the current from one rotor coil to the adjacent coil, The switching requires the interruption of the coil current. The sudden interruption of an inductive current generates high voltages. The high voltage produces flashover and arcing between the commutator segment and the brush. Voltage induced in the armature winding is alternating in nature The armature winding is connected to rotating commutator which rectifies the induced voltage to unidirectional voltage (but pulsating) The brushes which are connected to the armature winding, ride on commutator collect the current from the commutator and deliver it to the external load circuit  Generated EMF Let φ Flux/Pole in Wb Z Total No. of Armature Conductors  No. of Slots No. of Conductors per Slot P  No. of Generator poles A  No. of Parallel Paths in Armature N Armature rotation in rpm E EMF induced in any parallel path in Armature Generated EMF E g EMF generated in any one of the parallel paths i.e E  dφ Average EMF Generated/ Conductor  volt dt Now, Flux cut/conduc tor in 1 revolution , dφ φP Wb No. of Revolution s/sec N/60 60  Time for 1 revolution , dt  sec N Hence, according to Faraday’s Laws of electromagnetic induction, dφ EMF Generated/conductor  volt dt φP φPN   volt 60/N 60 φZN  P  Emf induced per parallel path E g    volt 60  A  Types of D.C. Generators According to the way in which their fields are excited, generators are classified into : i) Separately excited D.C. Generator ii) Self- excited D.C. Generator  Shunt connected  Series connected  Compound connected i) Separately excited D.C. Generators are those whose field magnets are energized from an independent source of D.C. Current If I Brush + L Eg Ra O V A Field Winding Eg D - Brush R Field connected to external source Eg = V + IaRa ii) Self excited D.C. Generators are those whose field magnets are energized from the induced voltage itself (Here residual magnetism should be present and the connection should be such that the flux produced by induced voltage aids the residual flux) a) Shunt wound D.C. Generator Ish I + Ia Ra L Shunt Field Ra O V Eg A Rsh Eg D - Rsh – Shunt Field Resistance Eg = V + IaRa Ra - Armature Resistance b) Series wound D.C. Generator Ia=Ise=I Rse Series Field + L Ra O V Eg Eg A D Rse – Series Field Resistance Eg = V + IaRa + IaRse Ra - Armature Resistance c) Compound wound D.C. Generator I Ish I Series Field Series Field Ish Ia L V Shunt Field L V Ia O O A Eg A D D Shunt Field Eg Long Shunt Short Shunt Shunt winding is parallel Shunt winding is parallel to series combination of only to armature winding armature winding and series field Eg = V + IaRa + IRse Eg = V + IaRa + IaRse Types of DC Generator  DC Motor A motor is an electrical machine which converts electrical energy into mechanical energy. Working principle The principle of working of a DC motor is that "whenever a current carrying conductor is placed in a magnetic field, it experiences a mechanical force". Input is Electrical energy output is Electrical energy  D. C. Motor Applications in rolling mills, traction, overhead cranes Principle of Operation Input: Electrical Energy Output :Mechanical Energy When a current carrying conductor is placed in a magnetic field, it experiences a mechanical force whose direction is given by Fleming’s left hand rule. 11/20/24  Constructionally, D.C. Generator and D.C. motor have no basic difference When the coil is powered, a magnetic Armature is now rotating in the field is generated around the armature. magnetic field and hence there The left side of the armature is pushed will be an induced emf in the away from the left magnet and drawn armature winding. The nature of toward the right, causing rotation. When the induced emf is to appose the the armature becomes horizontally cause producing it and hence aligned, the commutator reverses the opposes the applied voltage direction of current through the coil, which is the cause for rotation reversing the magnetic field. The and hence called as the back process then repeats. emf 11/20/24 Torque Equation of DC Motor Shunt motor Ish I + Ia Ra Shunt Field V Armature Rsh Eb - E b V  I a R a 11/20/24  Series Motor Ia=Ise=I Rs e Series Field + Ra V Eb - E b V  I a R a  I a Rse 11/20/24 Compound wound motor Ish I I Series Field Series Field Ia Ish L V L V Shunt Field O Ia O Eg b A A D D Shunt Field Eg b Long Shunt Short Shunt Shunt winding is parallel Shunt winding is parallel to series combination of only to armature winding armature winding and series field E b V  I a R a  IRse E b V  I a R a  I a Rse  As the back emf is same as induced voltage in the case of generators, the equation of back emf is same as the generated voltage equation of a generator φZN  P  i. Eb    volt e 60  A  Eb  60 A  or N     ZP  E b V  I a Ra N    So we can vary the speed by varying either of these three quantities and hence three types of speed control V – Supply voltage control IaRa – Armature voltage control Φ – flux control 11/20/24 Type of Motor Characteristi Applications Applications of cs DC motor Shunt Motor Approximately Centrifugal pumps constant Machine tools speed, Blowers and fans medium Reciprocating pumps torque Series Motor Variable Traction work speed Electric locomotives High starting Trolley, cars torque Cranes and hoists Conveyors Compound Adjustable Elevators (commulative) varying speed Heavy planers Air compressors Rollilng millls 2 Direction of induced current a Fleming's right-hand rule For a wire cutting through a B-fiel d... motion or force F magnetic field B induced curr ent I This is a rule to help you to remember their directions. (NOT a physics principle!) 2 Direction of induced current b Lenz's law induced I oppos es the motion of magnet (being r epelled) induced I oppos es the motion of magnet (being a ttracted) 2 Direction of induced current b Lenz's law In both cases, magnet moves against a force. Work is done during t he motion & it is trans ferred as electrical en ergy. Induced I always flows to oppose the movement which started it. Parts of Generator Yoke Pole Shoe  Armature Core Consists of slotted soft iron laminations. Purpose of laminations is to reduce eddy current loss.  Laminations are slotted to accommodate and provide mechanical security to armature winding and give shorter air gap flux to cross pole face and the armature teeth.  Armature Winding Slot of armature core hold insulated conductors that are connected in suitable manner, known as armature winding. Here EMF is induced. The armature conductors are connected in series-parallel; the conductor being connected in series so as to increase the voltage and in parallel so on to increase the current. Windings are classified as i) Lap winding and ii) Wave Winding.  Commutator Acts as mechanical rectifier that converts AC induced EMF in the armature winding into DC across the brushes. Made of copper segments insulated from each other by Mica sheets. Mounted on shaft of the machine.  A four-pole DC generator