Fundamentals of Electrical Machines PDF

Fundamentals of electrical machines : Fleming’s left hand and right hand rule, mutual inductance and mutual coupling phenomena in transformer, transformer – working, concept of turns ratio and applications, transformer on DC, instrument transformers, auto- transformer, dc machines- w...

Fundamentals of electrical machines : Fleming’s left hand and right hand rule, mutual inductance and mutual coupling phenomena in transformer, transformer – working, concept of turns ratio and applications, transformer on DC, instrument transformers, auto- transformer, dc machines- working principles, classification, starting, speed control and applications of dc motors, working principle of single and three phase induction motors, applications of ac motors The relation between the direction of induced emf and the direction of motion of the conductor is? a)Parallel b)Equal c)Not related d)Perpendicular According to Fleming’s right hand rule, the thumb points towards? a)Current b)E.M.F. c)Motion of the conductor d)Magnetic flux  TRANSFORMER Principle of Operation: Mutual inductance and mutual coupling phenomena in transformer Construction Working Concept of Turns Ratio Applications Transformer on DC Autotransformer Instrument transformers  Necessity of a Transformer Usually, electrical power is generated at 11Kv. For economical reasons AC power is transmitted at very high voltages say 220 kV or 440 kV over long distances. Therefore a step-up transformer is applied at the generating stations. Now for safety reasons the voltage is stepped down to different levels by step down transformer at various substations to feed the power to the different locations and thus the utilisation of power is done at A Transformer is a static electrical machine which transfers AC electrical power from one circuit to the other circuit at the constant frequency, but the voltage level can be altered that means voltage can be increased or decreased according to the requirement. In Brief, A transformer is a static electrical device that transfers electrical energy between two or more circuits through electromagnetic induction. Function of transformer is to a)Convert AC to DC b)Convert DC to AC c)Step down or up the DC voltages and currents d)Step down or up the AC voltages and currents TRANSFORMER SYMBOLS Principle of operation It is based on principle of MUTUAL INDUCTION. According to in which an coil e.m.f. c aurre is induced when neighbouri in nt ng the changes. coil  PRINCIPLE OF TRANSFORMER It works on the principle of Electromagnetic induction. The current flowing in the primary winding of the transformers creates a magnetic field, magnetic flux flows to the secondary side of the transformers, which induces EMF in the winding and current flows when the circuit is closed. A varying current in one coil of the transformer produces a varying magnetic field, which in turn induces a varying electromotive force (emf) or "voltage" in a second coil. If secondary number of turns are higher then, transformer is called a)Step-down b)Step-up c)One-one d)Autotransformer If primary number of turns are higher then, transformer is called a)Step-down b)Step-up c)One-one d)Autotransformer PARTS OF THE TRANSFORMER BASIC PARTS OF A TRANSFORMER  These are the basic components of a transformer.  Laminated core  Windings  Insulating materials  Transformer oil  Tap changer  Oil Conservator  Breather  Cooling tubes CORE CORE  The core acts as support to the winding in the transformer. It also provides a low reluctance path to the flow of magnetic flux.  It is made of laminated soft iron core in order to reduce eddy current loss and Hysteresis loss.  The composition of a transformer core depends on such as factors voltage, current, and frequency.  The diameter of the transformer core is directly proportional to copper loss and is inversely proportional to iron loss.  When the diameter of the core is increased, the vice versa occurs. Transformer core is designed to reduce a)Hysteresis loss b)Eddy current loss c)Hysteresis loss and Eddy current loss d)Cannot be determined WINDING WINDING  Two sets of winding are made over the transformer core and are insulated from each other. Winding consists of several turns of copper conductors bundled together, and connected connected in series.  Winding can be classified in two different ways: Based on the input and output supply Based on the voltage range  WINDING  Within the input/output supply classification, winding are further categorized:  Primary winding - These are the winding to which the input voltage is applied.  Secondary winding - These are the winding to which the output voltage is applied  WINDING  High voltage winding - It is made of copper conductor. The number of turns made shall be the multiple of the number of turns in the low voltage winding. The conductor used will be thinner than that of the low voltage winding.  Low voltage winding - It consists of fewer number of turns than the high voltage winding. It is made of thick copper conductors. This is because the current in the low Transformers windings are generally made of a)Steel b)Iron c)Copper d)Steel iron alloy INSULATING MATERIALS  Insulating paper and cardboard are used in transformers to isolate primary and secondary winding from each other and from the transformer core.  Transformer oil is another insulating material.  Transformer oil performs two important functions: in addition to insulating function, it can also cool the core and coil assembly.  The transformer's core and winding must be completely immersed in the oil.  CONSERVATO R  The conservator conserves the transformer oil. It is an airtight, metallic, cylindrical drum that is fitted above the transformer.  The conservator is connected to the main tank inside the transformer, which is completely filled with transformer oil through a pipeline. BREATHER BREATHE R  The breather controls the moisture level in the transformer. Moisture can arise when temperature variations cause expansion and contraction of the insulating oil, which then causes the pressure to change inside the conservator.  If the insulating oil encounters moisture, it can affect the paper insulation or may even lead to internal faults. Therefore, it is necessary that the air entering the tank is moisture-free.  The transformer's breather is a cylindrical container that is filled with silica gel. TAP CHANGER  TAP CHANGER  The output voltage of transformers vary according to its input voltage and the load.  During loaded conditions, the voltage on the output terminal decreases, whereas during off-load conditions the output voltage increases. In order to balance the voltage variations, tap changers are used.  Tap changers can be either on-load tap changers or off-load tap changers. In an on-load tap changer, the tapping can be changed without isolating the transformer from the supply.  Automatic tap changers are also available. COOLING TUBES  COOLING TUBES  Cooling tubes are used to cool the transformer oil.  The transformer oil is circulated through the cooling tubes.  The circulation of the oil may either be natural or forced. In natural circulation, when the temperature of the oil rises the hot oil naturally rises to the top and the cold oil sinks downward.  Thus the oil naturally circulates through the tubes. In forced circulation, an external pump is used to circulate the oil. As the transformer is basically a linear device, a ratio now exists between the number of turns of the primary coil divided by the number of turns of the secondary coil. This ratio, called the ratio of transformation, more commonly known as a transformers “turns ratio”, ( TR ). This turns ratio value dictates the operation of the transformer and the corresponding voltage available on the secondary winding. TRANSFORMER ON DC SUPPLY What will happen if the Primary of a Transformer is Connected to D.C. Supply????  Transformer doesn't work on a DC supply According to the principle of Transformer operation It doesn't work on a DC supply since the rate of change of fl ux is zero  Applications of Transformers 1. For step up and step down of voltage. 2. For isolation between two circuits. 3. For impedance matching. 4.For measurement of current and voltage( current transformer and potential transformer are used to measure high currents and high voltages respectively) 5. In transmission and distribution network. 6. Also used in rectifier circuits. 7. It is also used in voltage regulators, power stabilizers.  Applications and uses of According to the necessity, transformers are classified into: Transformers Power Transformers: These kinds of transformers are used for high voltage power transfer applications (more than 33 KV). They are usually bigger in size and can occupy larger space. Distribution Transformers: These type of transformers are used to distribute the generated power to distant locations. It is used for distributing electricity at low voltage that is less than 33 KV in industry or 220-440 V for household purposes. Measurement Transformers: This kind of uses of transformer helps in measuring voltage, current, and power, etc.  Auto- An Auto-transformer is an electrical Transformer transformer with only one winding. An autotransformer (or auto transformer) is a type of electrical transformer with only one winding. The “auto” prefix refers to the single coil acting alone (Greek for “self”) – not to any automatic mechanism. An auto transformer is similar to a two winding transformer but varies in the way the primary and secondary winding of the transformer are interrelated. 3. Total windings present in a autotransformer are a)1 b)2 c)3 d)4  Instrument Transformers Applications of Instrument Transformers: For measurement of high ac current, it is usual to use low range ac ammeter with suitable shunt. For measurement of high ac voltage, low range ac voltmeters are used with high resistances connected in series. For measurement of very high ac current and voltage, we cannot use these methods. Instead, we use specially constructed HV instrument transformers to insulate the high voltage circuit from the measuring circuit in order to protect the measuring instruments from burning.  Current What is current Transformer (CT)?: Transformers A current transformer is a transformer, which produces in its secondary winding low current, which is proportional to the high current flowing in its primary winding. The secondary current is usually much smaller in magnitude than the primary current. The design of CT depends on which type of instrument is connected to its secondary winding. Measuring instrument OR Protective instrument. -Measuring instrument CT is expected to give accurate results up to a maximum of 125% of its normal full-load rated current. -Protective instrument CT is expected to be accurate for up to 20 times of its normal full-load rated current (about 2000% of its full-load rated current!!..??). Based on the type of equipment for which the Ct is used for, its saturation point will vary. At the same time it is expected to be linear in the entire working range.  Potential Transformers What is a Potential Transformer (PT) or (VT)?: A PT or sometimes called VT is a step- down transformer having many primary turns but few secondary turns. In a step-down transformer the voltage decreases and the current increases, thus voltage can be easily measured by using a low-range voltmeter instrument. The voltage is stepped-down in a known ratio called the voltage ratio.  Introduction The Dc machines are of two types namely DC generators and DC motors. A DC generators converts mechanical energy into electrical energy whereas a DC motor converts the electrical energy into mechanical energy. In order to understand the operating principle of a DC motor, it is necessary to understand how does a current carrying conductor experience a force, when kept in a magnetic field.  Force on current carrying conductor:  If a straight conductor is placed in the magnetic field produced by a permanent magnet, the current flowing through a conductor in anti clockwise direction.  Due to the presence of two magnetic fields simultaneously, an interaction between them will take place as shown in fig.(1). Fig.1(a): Interaction of the fields Fig.1(b):Resultant field  As shown in fig.(1), the flux lines produced by the magnet and the conductor are in opposite direction to each other at left side and hence cancel each other. Therefore the no of flux lines at left side will reduced.  At the right side, the individual fields are in the same direction, hence will add or strengthen each other. Therefore the no. of flux lines at right side will increase.  Magnitude of Force:  The magnitude of the force experienced by the current carrying conductor placed in the magnetic field is given by, F = BIl Newton Where B = Flux density produced by Magnet I = current flowing through conductor l = Length of the conductor  Direction of force:  The direction of rotation of a motor depends on the direction of force exerted on the the armature winding and the direction of force experienced by a current carrying conductor is given by Fleming’s left hand rule.  Statement of Fleming’s left hand rule: It states that if the first three fingers of the left hand are held mutually at right angles to each other and if index finger indicates the direction of the magnetic field, and if middle finger indicates the direction of current flowing through the conductor, then thumb indicates the direction of force exerted on the conductor. This is shown in fig (2). Fig.(2):Fleming’s left hand rule thumb  DC Motor Principle of operation:  When current carrying conductor is placed in a magnetic field, it experienced a force.  In case of DC motor, the magnetic field us developed by the field current i.e. current flowing in field winding and armature winding plays the role of current carrying conductor  So armature winding experienced a force and start rotating.  Principle of Operation ARMATURE winding are defined as the winding which a voltage is induced. FIELD windings are defined as the windings that produce the main flux in the machines. The magnetic field of the field winding is approximately sinusoidal, thus AC voltage is induced in the armature winding as the rotor turns under the magnetic field of stator. The COMMUTATOR and BRUSH combination converts the AC generated voltages to DC. 2 4 Direction of rotation of motor is determined by a)Faraday’s law b)Lenz’s law c)Coulomb’s law d)Fleming’s left-hand rule WORKING OF DC MOTOR 2 6 Current in DC Motor 2 7 Magnetic Field in DC Motor 2 8 Force in DC Motor 2 9 Torque developed by a DC motor depends upon a)magnetic field b)active length of the conductor c)current flow through the conductors d)Current, active length, no. of conductors, magnetic field all  DC Machines Construction A DC motor is constructed with: A Stator A Rotor A Yoke Poles Field windings Armature windings Commutator Brushes CONSTRUCTION OF DC MACHINES DC motor stator Rotor of a dc motor  DC Machines Constructi DC machines, like on other. electromechani cal energy conversion devices have two sets of – electrical field windings windings - on stator – amarture windings - on the rotor. 3 3  DC Machines Construction A DC motor is constructed with: A Stator A Rotor A Yoke Poles Field windings Armature windings Commutator Brushes 1)Yoke: The yoke make by cast steel for large machines and cast iron for a small machine. It uses to protect the internal parts of the DC machine and gives mechanical support to the poles. The yoke provides a return path for magnetic flux. In the yoke, the laminations are not required, but the modern machines uses the laminations in yoke. 2)Poles and Pole shoe: The pole core use to provide housing to the field winding. When field winding excites, it behaves like a magnet. The pole shoes provide mechanical support to the field winding and due to a large area, it reduces the magnetic reluctance. The pole and pole shoe make by cast steel. Pole is not necessary to laminate. The pole shoe is always laminated because it is close to the armature. 3)Armature: Armature core provides housing to the armature winding. It completes low reluctance path for magnetic flux. The armature slots are skew at some angle to reduce the mechanical vibration. Armature core is made with silicon steel. It is laminated to reduce the eddy current losses. In a DC machine, open slots are use to reduce leakage flux, inductance, and leakage reluctance.  4) Armature winding and Field winding: There are two types of armature windings; Lap winding and Wave winding. Lap winding is known as complete winding because, after completion of winding, all slots does fill with armature winding. Wave winding is known as incomplete winding because, after completion of winding, all slots does not fill with armature winding. Some slots remain empty. These slots do fill with dummy coils. The dummy coils only use in wave winding to fill empty slots and give mechanical balance. It is not used in lap winding. In lap winding, due to unbalance flux and unbalance voltage, the circulating current is more. It causes more copper loss and heat. The circulating current can minimise by using the equalizer ring. In wave winding, circulating current does not exist. The armature of DC motor is laminated to a)To reduce mass b)To reduce hysteresis loss c)To reduce eddy current loss d)To reduce inductance 5) Commutato r: case of a generator, the commutator uses to convert AC voltage into In DC voltage. The commutator uses as a rectifier. In the case of the motor, the commutator use to produce unidirectional torque. To reduce wear and tear, the commutator make by hard drawn copper. The number of armature slots is equal to the number of commutator segments. 6)Brush: Brushes use to carry the current or give the current to the armature conductors through the commutator. The brushes make by copper or carbon materials for small machines. Electro-graphite brushes use for large machines. Carbon-graphite brushes use for large current low voltage machines. 7)Shaft: The shaft use to transfer mechanical power. In case of DC motor, mechanical power is transfer from DC machine to load. In the case of a DC generator, mechanical power is transfer from Prime mover to the DC generator. What are the materials used for brushes in dc machines? a)Iron b)Carbon c)Aluminum d)Steel  Starting of DC motors A starter is a device to start and accelerate a motor. A controller is a device to start the motor, control and reverse the speed of the DC motor and stop the motor. While starting the DC motor, it draws the heavy current which damages the motor. The starter reduces the heavy current and protects the system from damage.  Need of Starters for DC Motors: The dc motor has no back emf. At the starting of the motor, the armature current is controlled by the resistance of the circuit. The resistance of the armature is low, and when the full voltage is applied at the standstill condition of the motor, the armature current becomes very high which damage the parts of the motor. Since at the time of starting the DC Motor, the starting current is very large. At the time of starting of all DC Motors, except for very small motors, an extra resistance must be connected in series with the armature. This extra resistance is added so that a safe value of the motor is maintained and to limit the starting current until the motor has attained its stable speed.  Speed Control of DC motors According to the speed equation of a dc motor N ∞ Eb/φ ∞ V- Ia Ra/ φ Thus speed can be controlled by: Flux control method: By Changing the flux by controlling the current through the field winding. Armature control method: By Changing the armature resistance which in turn changes the voltage applied across the armature  Flux Control Method Advantages: It provides relatively smooth and easy control Speed control above rated speed is possible As the field winding resistance is high the field current is small. Power loss in the external resistance is small. Hence this method is economical Disadvantages: Flux can be increased only upto its rated value High speed affects the commutation, motor operation becomes unstable  Armature Voltage Control Method The speed is directly proportional to the voltage applied across the armature. Voltage across armature can be controlled by adding a variable resistance in series with the armature Potential Divider Control If the speed control from zero to the rated speed is required , by rheostatic method then the voltage across the armature can be varied by connecting rheostat in a potential divider arrangement.  Advantages of DC motors  Efficient and reliable.  Quick response since high ratio of torque to rotor inertia.  Controlled speed and reversible.  More sturdy and Compact. 4 4  Disadvantages of DC motors  Brush wear: Since they need brushes to connect the rotor winding. Brush wear occurs, and it increases dramatically in low‐ pressure environment. So they cannot be used in artificial hearts. If used on aircraft, the brushes would need replacement after one hour of operation.  Sparks from the brushes may cause explosion if the environment contains explosive materials.  RF noise from the brushes may interfere with nearby t.v. sets, or electronic devices.  Starting of DC motors A starter is a device to start and accelerate a motor. A controller is a device to start the motor, control and reverse the speed of the DC motor and stop the motor. While starting the DC motor, it draws the heavy current which damages the motor. The starter reduces the heavy current and protects the system from damage.  Need of Starters for DC Motors: The dc motor has no back emf. At the starting of the motor, the armature current is controlled by the resistance of the circuit. The resistance of the armature is low, and when the full voltage is applied at the standstill condition of the motor, the armature current becomes very high which damage the parts of the motor. Since at the time of starting the DC Motor, the starting current is very large. At the time of starting of all DC Motors, except for very small motors, an extra resistance must be connected in series with the armature. This extra resistance is added so that a safe value of the motor is maintained and to limit the starting current until the motor has attained its stable speed. Why starters are required in a DC motor? a)Back emf of these motors is zero initially b)These motors are not self-starting c)These motors have high starting torque d)To restrict armature current as there is no back emf at starting The speed of a DC motor can be varied by changing a)Field current b)Applied voltage c)Resistance in series with armature d)Field current, applied voltage or resistance in series with armature any method will work  Speed Control of DC motors According to the speed equation of a dc motor N ∞ Eb/φ ∞ V- Ia Ra/ φ Thus speed can be controlled by: Flux control method: By Changing the flux by controlling the current through the field winding. Armature control method: By Changing the armature resistance which in turn changes the voltage applied across the armature  Flux Control Method Advantages: It provides relatively smooth and easy control Speed control above rated speed is possible As the field winding resistance is high the field current is small. Power loss in the external resistance is small. Hence this method is economical Disadvantages: Flux can be increased only upto its rated value High speed affects the commutation, motor operation becomes unstable  Armature Voltage Control Method The speed is directly proportional to the voltage applied across the armature. Voltage across armature can be controlled by adding a variable resistance in series with the armature Potential Divider Control If the speed control from zero to the rated speed is required , by rheostatic method then the voltage across the armature can be varied by connecting rheostat in a potential divider arrangement.  Advantages of DC motors  Efficient and reliable.  Quick response since high ratio of torque to rotor inertia.  Controlled speed and reversible.  More sturdy and Compact. 9  Disadvantages of DC motors  Brush wear: Since they need brushes to connect the rotor winding. Brush wear occurs, and it increases dramatically in low‐ pressure environment. So they cannot be used in artificial hearts. If used on aircraft, the brushes would need replacement after one hour of operation.  Sparks from the brushes may cause explosion if the environment contains explosive materials.  RF noise from the brushes may interfere with nearby t.v. sets, or electronic devices.  Applications of DC Motor i. Various machine tools such as lathe machines, drilling machines, milling machines etc. ii. Printing machines iii. Paper machines iv. Centrifugal and reciprocating pumps v. Blowers and fans etc. i. Electric trains ii. Diesel-electric locomotives iii. Cranes iv. Hoists v. Trolley cars and trolley buses vi. Rapid transit systems vii. Conveyers etc. i. Elevators ii. Rolling mills iii. Planers iv. Punches v. Shears Induction Motor  INDUCTION MOTORS An induction motor (also known as an asynchronous motor) is a commonly used AC electric motor. In an induction motor, the electric current in the rotor needed to produce torque is obtained via electromagnetic induction from the rotating magnetic field of the stator winding. The rotor of an induction motor can be a squirrel cage rotor or wound type rotor. Construction of Single Phase Induction Motor 1 6  HIGH STARTING TORQUE LOW STARTING TORQUE I SLIP RING I SQUIRREL CAGE ROTOR ROTOR I I  Operating principle of single phase induction motor Link: https://w ww.youtube.com /watch? v=awrUxv7B-a8 10/8/20 1 20 8  7.6 Single Phase Induction Motor The single-phase induction motor operation can be described by two methods: – Double revolving field theory; and – Cross-field theory. Double revolving theory is perhaps the easier of the two explanations to 10/8/20 20 understand 1 9 Learn the double revolving theory only  Single Phase Induction Motor Double revolving field theory A single-phase ac current supplies the main winding that produces a pulsating magnetic field. Mathematically, the pulsating field could be divided into two fields, which are rotating in opposite directions. 10/8/20 20 The interaction between 2 0 the fields and the current induced in the rotor bars generates Single Phase Induction Main winding Motor The interaction flux between the -t +t fields Main and the winding current induced in the rotor bars generates opposing torque. Under these Starting conditions, winding with only the main field energized the Single-phase motor main motor will winding generates two not start However, if rotating fields, which oppose an and counter-balance one external another. 2 torque moves the 1 motor in any direction, the motor will  Applications of single phase Induction The single phase motors are simple in construction, cheap in motor cost, reliable and easy to repair and maintain. Due to all these advantages, the single phase motor finds its application in vacuum cleaners, fans, washing machines, centrifugal pumps, blowers, washing machines, etc. These are used in low power applications and widely used in domestic applications as well as industrial. And some of those are mentioned below Pumps Compressors Small fans Mixers Toys High speed vacuum cleaners Electric shavers Drilling machines Induction Motors  Introducti on Three-phase induction motors are the most common and frequently encountered machines in industry – simple design, rugged, low-price, easy maintenance – wide range of power ratings: fractional horsepower to 10 MW – run essentially as constant speed from no- load to full load – Its speed depends on the frequency of the power source not easy to have variable speed control requires a variable-frequency power- electronic drive for optimal speed control  Which is fixed part of Motor A. Stator B. Rotor C. Both D. None  Constructi onmotor has two main An induction parts – a stationary stator consisting of a steel frame that supports a hollow, cylindrical core core, constructed from stacked laminations (why?), having a number of evenly spaced slots, providing the space for the stator winding Stator of IM  Constructi on – a revolving rotor composed of punched laminations, stacked to create a series of rotor slots, providing space for the rotor winding one of two types of rotor windings conventional 3-phase windings made of insulated wire (wound-rotor) » similar to the winding on the stator aluminum bus bars shorted together at the ends by two aluminum rings, forming a squirrel-cage shaped circuit (squirrel-cage)  Constructi on Squirrel cage rotor Wound rotor Notice the slip rings Workin g  The stator of a 3-phase induction motor produces ……… magnetic filed. a) steady b) rotating c) alternating d) none of the above  Rotating Magnetic windings,Field Balanced three phase i.e. mechanically displaced 120 degrees form each other, fed by balanced three phase source A rotating magnetic field with spee constant magnitude is produced, d rotating with a 120 f nsync  e rpm P Where fe is the supply frequency and P is the no. of poles and nsync is called the synchronous speed in rpm (revolutions per minute)  Synchronous P speed 50 Hz 60 Hz 2 3000 3600 4 1500 1800 6 1000 1200 8 750 900 10 600 720 12 500 600 Rotating Magnetic Field Rotating Magnetic Field  Principle of operation This rotating magnetic field cuts the rotor windings and produces an induced voltage in the rotor windings Due to the fact that the rotor windings are short circuited, for both squirrel cage and wound-rotor, and induced current flows in the rotor windings The rotor current produces another magnetic field A torque is produced as kBRa result Bs of the interaction of those ind two magnetic Where fields torque and B and B are  is the induced ind R S the magnetic flux densities of the rotor and the stator respectively The operation of an induction motor is based on a)Lenz’s law b)Ampere’s law c)mutual induction d)self induction  Induction motor speed At what speed will the IM run? –Can the IM run at the synchronous speed, why? – If rotor runs at the synchronous speed, which is the same speed of the rotating magnetic field, then the rotor will appear stationary to the rotating magnetic field and the rotating magnetic field will not cut the rotor. So, no induced current will flow in the rotor and no rotor magnetic flux will be produced so no torque is generated and the rotor speed will fall below the synchronous speed – When the speed falls, the rotating magnetic field will cut the rotor windings and a torque is produced The rotor of a three phase induction motor can never attain synchronous speed. a)True b)False  Induction motor speed So, the IM will always run at a speed lower than the synchronous speed The difference between the motor speed and the synchronous speed is called the Slip nslip  nsync  nm Where nslip= slip speed nsync= speed of the magnetic field nm = mechanical shaft speed of the motor  The Slip nsync  nm s Where s is the slip nsync Notice that : if the rotor runs at synchronous speed s=0 if the rotor is stationary s=1 Slip may be expressed as a percentage by multiplying the above eq. by 100, notice that the slip is a ratio and doesn’t have units Induction Motors and Transformers Both IM and transformer works on the principle of induced voltage – Transformer: voltage applied to the primary windings produce an induced voltage in the secondary windings – Induction motor: voltage applied to the stator windings produce an induced voltage in the rotor windings – The difference is that, in the case of the induction motor, the secondary windings can move – Due to the rotation of the rotor (the secondary winding of the IM), the induced voltage in it does not have the same frequency of the stator (the primary) voltage An induction motor can be said analogous to a)transformer b)synchronous motor c)universal motor d)stepper motor  Frequen cyof the voltage The frequency induced in the rotor P  n is given by fr  120 Where fr = the rotor frequency (Hz) nP = slip speed = number of stator (rpm) f rpolesP  (ns  nm )  120 P  sns  sf e  120 If a 4-pole induction motor has a synchronous speed of 1500 r.p.m., then, supply frequency is …….. a)50 Hz b)25 Hz c)60 Hz d)none of the above  Frequen cythe frequency of What would be the rotor’s induced voltage at any speed nm? fr  s When the rotorf eis blocked (s=1) , the frequency of the induced voltage is equal to the supply frequency On the other hand, if the rotor runs at synchronous speed (s = 0), the frequency will be zero  Torqu While the input e to the induction motor is electrical power, its output is mechanical power and for that we should know some terms and quantities related to mechanical power Any mechanical load applied to the motor shaft will introduce a Torque on the motor shaft. This torque is related to the motor output power and the rotor speed  load  Pout N.m and  2 nm rad /  m  m s 60  Horse power Another unit used to measure mechanical power is the horse power It is used to refer to the mechanical output power of the motor Since we, as an electrical engineers, deal with watts as a unit to hp  746 watts measure electrical power, there is a relation between horse power and watts