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Unit -5 AC Machines 2 marks What is the principle of induction motor? Answer: Principle of induction motor: An induction motor is such an electromechanical device which converts electrical energy into a mechanical energy. Principle of induction motor is based on mutual induction. When a three phase supply is given to the stator conductors a rotating magnetic field is developed in the stator which is of constant value. When the rotor is placed in this magnetic field it experiences force by which the rotor starts rotating in the direction of magnetic field. 2. Define Slip and Slip speed in an induction motor? Answer: Slip in Induction motor is defined as the ratio of relative speed between magnetic field and rotor to the synchronous speed and is given as follows Where, (rpm) (rpm) And %slip= Slip speed: Slip speed is probably defined as the relative speed between rotating magnetic field and rotor speed. 3. What is synchronous speed and write its expression. Answer: The speed of magnetic field produced in a three magnetic field is termed as synchronous speed. The synchronous speed (Ns) of the stator field is directly related to the supply frequency and the number of (magnetic) poles in the stator and is expressed in revolutions per minute [rpm]: Where, f= supply frequency P=number of stator poles 4. What is the working principle of alternator? Answer: The working principle of alternator depends upon Faraday's law of electromagnetic induction which says the current is induced in the conductor inside a magnetic field when there is a relative motion between that conductor and the magnetic field. 5. Define Pitch factor and Distribution factor? Answer: Pitch factor: Pole pitch in an Alternator is defined as the distance from centre of one pole to the centre of the next pole. Coil span in an Alternator is defined as the distance between two conductors which are connected end to end. When the coil span is less than the pole pitch, the coil is said to be short pitch coil and the factor is termed as Pitch factor. Distribution factor: If the windings in the Alternator are confined only to particular slots leaving all the remaining slots empty, the winding is termed as concentrated winding. If the windings are distributed uniformly in all the given slots the winding is said to be distributed winding and the factor is termed as Distribution factor. 3 Marks What are the losses occurring in an induction motor? Answer: The power losses in an Induction motor are as follows Core or Iron losses (Pi):- These losses occur in the stator and rotor parts of the induction motor. These are further classified as Stator core loss and Rotor core loss. These losses are of hysteresis and eddy current losses and occur in the stator and rotor core due to the alternating flux When three phase supply is given to the stator winding of an Induction motor, three phase currents flow in the windings. As a result of this current a constant rotating magnetic field is produced in the air gap. Due to the reversal of magnetization of the core material, the loss occurred is termed as Hysteresis loss. Hysteresis losses = kh f Bm(1.6) During the operation of the Induction motor small currents flow in the core material which is termed as eddy currents. The loss in the induction motor because of these currents is termed as Eddy current loss and it is given Eddy current losses = ke f2Bm 2t2 ii) Copper losses:The loss in an Induction motor because of the ohmic resistance of both the stator and rotor windings are termed as Copper losses. Total Cu losses, Pc = I12R1 + I22R2 Where I12R1 = Stator copper loss I22R2 = Rotor copper loss I1 = Stator current I2 = Rotor current R1 = stator resistance R2 = rotor resistance These losses can be minimized by designing the winding with low resistance. iii) Mechanical losses: An induction motor involves moving parts in conversion electrical energy into mechanical energy so it has bearings and brushes attached to the shaft. Therefore friction and windage losses take place which are termed as mechanical losses. These losses can be minimized by using grease and lubricants in the moving parts. 2. Explain the torque slip characteristics in an induction motor? Answer: The torque slip characteristics of an Induction motor is a graph showing the variation of torque with respect to slip. At low slips the value of SX2 << R2 so we get the torque as T= 0.159 * sE22/R2. From this it is clear that torque is directly proportional to slip and graph is a straight line passing through centre. At high slips the value SX2 >> R2 so we get the torque as T= 0.159 * E22R2/sX22. From this it is clear that torque is inversely proportional to slip and the graph is rectangular hyperbola. 3. Mention any three differences between a transformer and induction motor? Answer: TRANSFORMER INDUCTION MOTOR The secondary winding .is stationary. The secondary winding (rotor) is rotating in nature. short Rotor winding is always short circuited. Secondary winding is not circuited. No-load current is about 1% of full load. No-load current is approximately 30 to 50% current. Frequency of primary and secondary Frequency of stator current (f) and rotor currents are same. current (sf) are not the same. 4. Compare salient pole and cylindrical pole rotors? Answer: Salient pole rotor Cylindrical rotor This is also termed as projected poleType rotor where all the poles are projected Out from the surface of the rotor. These rotors have large diameters and smaller axial lengths. The field winding is provided on the pole shoe. This is also termed as non-salient pole type rotor or non-projected pole type rotor. These rotors have small diameters and large axial lengths. The unslotted portions of the cylinder acts as poles. As mechanical strength of salient pole rotors is Cylindrical rotor is mechanically strong and less. It is preferred for low speed applications. preferred for high speed applications. 5. What is coil and coil span in an alternator? Answer: The Alternator consists of windings in the form of single layer and multi-layer in nature. These windings are wound in such a way that maximum Emf is induced in the conductors. When one end of the conductor is connected to the other end of the conductor, it is termed as one turn. These turns in particular number is termed as coil. Coil span in an Alternator is defined as the distance between one side of the coil to the other side of the coil. 10 marks Explain the construction of a 3 phase Alternator? Answer: ALTERNATOR: An alternator is an a.c generator that converts mechanical energy to electrical energy in the form of alternating current by using Faradays law of electromagnetic induction. It says that when there change in flux in conductor an Emf is induced at the ends of the conductor. CONSTRUCTION: An alternator consists of 1. Stator 2. Rotor STATOR: Stator is a stationary part in an Alternator. It consists of core (i.e. silicon steel laminations) and slots to hold the armature winding and it i s laminated to reduce the eddy current losses. The choice of material of the core is steel to reduce the hysteresis losses. The entire core is fabricated in a frame made of cast iron. The three phase windings are placed in these slots and serves as the armature windings of the Alternator. The armature windings are always connected in star and the neutral is connected to ground. These armature conductors are cut by the magnetic field produced by the rotor poles and an Emf is induced at the terminals of the windings of the three phase Alternator. This induced Emf is given by E= Kc Kd 4.44 ϕfTph, where Tph is number of turns per phase. ROTOR: The rotor is rotating part of the Alternator. It is like a flywheel having alternate north and south poles attached to its outer rim. It carries a field winding which is supplied with dc current through two slip rings by a separate dc source. This dc source is generally a small dc generator mounted on the shaft of the Alternator. There are two types of rotors: 1. Salient pole rotor 2. Cylindrical pole rotor. 1. SAILENT POLE ROTOR: A salient pole is a magnetic pole that is projected out of the rotor surface. So, it is termed as projected pole type rotor. Salient type rotor has non-uniform air-gap and two or four poles. These poles are made up of thick lamination. . This type of rotor consists of large number of projected poles (called salient poles), bolted on a magnetic wheel. The poles and pole shoe are also laminated to minimize the eddy current losses. Alternators featuring this type of rotor are large in diameters and short in axial length. As a mechanical strength of salient pole rotor is less, it is preferred for low speed applications. 2. CYLINDRICAL POLE ROTOR: This is also termed as non-salient type rotor. It consists of smooth solid steel cylinder having number of slots to accommodate the field winding. The unslotted portion of cylinder acts as poles. The central polar areas surrounded by the field winding placed in slots. The field coils are so arranged around these polar areas that flux density is maximum on the polar central line and gradually falls away on either side. These Rotors have small diameter and large axial length. The main advantage of cylindrical rotor is, it is mechanically strong and preferred for high speed applications. It is used for steam driven turbo Alternator which run at very high speeds of 3600 rpm. Derive the EMF equation of an Alternator? Answer: An alternator is an a.c generator that converts mechanical energy to electrical energy in the form of alternating current by using Faradays law of electromagnetic induction. It says that when there change in flux in conductor an Emf is induced at the ends of the conductor. The Induced EMF in the case of 3 phase alternator is given by Faraday’s Law of Electromagnetic Induction. i.e, E = Induced Emf Φ = Flux per pole, in Webbers P = Number of poles Ns = Synchronous speed in r.p.m. f = Frequency of induced e.m.f. in Hz Z = Total number of conductors dΦ =change in flux or total flux produced by poles dΦ = Φ*P and dt = change in time Consider a single conductor placed in a slot. The average value of e.m.f. induced in a conductor Eavg = dΦ/dt Total flux in one revolution = Φ * P Time taken for one revolution = 60/Ns seconds. Eavg per conductor = ΦP / (60/Ns) = Φ (PNs/60) We know that, Ns= (120f/P) Eavg = (ΦP / 60) * (120f/P) Eavg = 2Φf. This is induced EMF in a single turn of a coil We know that in an Alternator, Number of conductors (Zph) = 2* Number of turns (Tph) EMF per turn = 2 x (e.m.f. per conductor= 2 x (2 f Φ) Eavg = 4 f Φ volts Average Induced EMF in each phase is given by Eavg = Tph Average Eph = Tph x (Average e.m.f. per turn) x4fΦ The induced Emf is an alternating quantity. The form factor is 1.11 for a sinusoidal waveform. Form factor = (R.M.S.) values/Average values = 1.11 Erms = 1.11 x Average value EMF = 1.11 * 4 f Φ Tph EMF = 4.44 Φ f Tph volts volts EMF =Kc Kd 4.44 f Φ Tph Kc = Pitch factor 3. volts and Kd = distribution factor Explain working principle of 3-phase Induction motor? Answer: An electrical motor is such an electromechanical device which converts electrical energy into a mechanical energy. In case of three phase AC operation, most widely used motor is 3-phase induction motor as this type of motor does not require any starting mechanism. For better understanding the principle of three phase induction motor, the basic constructional features of this motor must be known to us. This Motor consists of two major parts: Stator: Stator of three phase induction motor is made up of numbers of slots to construct a 3 phase winding circuit which is connected to 3 phase AC source. The three phase winding are arranged in such a manner in the slots that they produce a rotating magnetic field after 3-phase AC supply is given to them. Rotor: Rotor of three phase induction motor consists of cylindrical laminated core with parallel slots that can carry conductors. Conductors are heavy copper or aluminum bars which fits in each slots & they are short circuited by the end rings. The slots are not exactly made parallel to the axis of the shaft but are slotted a little skewed. This arrangement reduces magnetic humming noise & can avoid stalling of motor. Working of Three Phase Induction Motor: The stator of the motor consists of overlapping winding offset by an electrical angle of 120°. When the primary winding or the stator is connected to a 3 phase AC source, it establishes a rotating magnetic field which rotates at the synchronous speed. Secrets behind the rotation: According to Faraday’s law an Emf induced in any circuit is due to the rate of change of magnetic flux linkage through the circuit. As the rotor winding in an induction motor are either closed through an external resistance or directly shorted by end ring, and cut the stator rotating magnetic field, an Emf is induced in the rotor copper bar and due to this Emf a current flows through the rotor conductors. Here the relative speed between the rotating flux and static rotor conductor is the cause of current generation. Hence as per Lenz's law the rotor will rotate in the same direction to reduce the cause i.e. the relative velocity. Thus from the working principle of three phase induction motor it may observed that the rotor speed should not reach the synchronous speed produced by the stator. If the speeds equals, there would be no such relative speed, so no Emf induced in the rotor, & no current would be flowing, and therefore no torque would be generated. Consequently the rotor cannot reach the synchronous speed. The difference between the stator (synchronous speed) and rotor speeds is called the slip. The rotation of the magnetic field in an induction motor has the advantage that no electrical connections need to be made to the rotor. 4. Explain the concept of rotating magnetic field in an induction motor? Answer: Rotating magnetic field can be defined as field or flux having constant amplitude but whose axis rotates in a plane at a certain speed .e.g. Permanent magnet rotating in a space produces a rotating magnetic field .similarly if an arrangement made to rotate the poles, with constant excitation supplied, the resulting magnetic field is rotating magnetic field .so a field produces in the air gap of a rotating field type alternator is of rotating type. PRODUCTION OF ROTATING MAGNETIC FIELD Consider a three phase winding displaced in a space by 120°, supplied by a three phase A.C. supply. The three phase currents are also displaced from each other by 120° and the flux produced by each phase currents is also sinusoidal in nature and all three flux are separated from each other by 120°if the phase sequence of the windings is1-2-3, then the mathematical equation for the instantaneous values of the fluxes ∅1, ∅2, ∅3 can be given as, …………………………………………………………………. (1) ………………………………………… (2) ………………………………………… (3) As windings are identical and supply is balanced the amplitude of each flux is same i.e. ∅m. the wave form of three fluxes are shown in the figure.(4) while the assumed positive direction of these fluxes in space are shown in the figure .(5) assumed positive direction means whenever the instantaneous value of the flux is positive,in vector diagram it must be represented along its assumed positive direction.aand if flux has negative instantaneous value then must be represented in opposite direction to the assumed positive direction,in the vector diagram. [1] Let ∅1, ∅2, and ∅3 be the instantaneous values of the fluxes. The resultant flux ∅T, at any instant is given by phasor combination of ∅1, ∅2, and ∅3 at that instant .let us find out ∅T at four different instant 1, 2, 3, and 4 respectively at θ= 0°, 60°, 120°, and 180°. Case 1: θ=0°, substituting in equation (1), (2), (3) we get, =0 And ……………………………..vector sum Show positive values in assumed positive directions and negative in opposite directions to assumed positive directions. Hence vector diagram looks like as shown BD is perpendicular drawn from B on ‘∅T’ Since OD=DA=∅T/2 Since in So OBD, BOD=30° =OD/OB = (∅T/2) 0.866∅m ∅T = 2*0.866∅m * ∅T = 1.5∅m So magnitude of resultant flux is 1.5 times the maximum value of an individual flux. Case 2: θ=60° Substituting in equations 1), (2), (3) we get, = So is positive and is negative so vector diagram looks like as shown It can be seen from the figure that ∅T = 1.5∅m So magnitude of the resultant is same as before but it is rotated in space by 60° in space in clockwise direction, from its previous position. Case 3: θ=120° Substituting in equations 1), (2), (3) we get, = 0 So is positive and is zero and is negative. so vector diagram looks like as shown ∅T = 1.5∅m So magnitude of the resultant is 1.5∅m, same as before but it is rotated in space by 60° in space in clockwise direction, from its previous position. Case 4: θ=180° Substituting in equations 1), (2), (3) we get, =0 So is zero and is positive and is negative. So vector diagram looks like as shown From figure it can be prove easily that, ∅T = 1.5∅m So magnitude of the resultant flux is once again 1.5∅m but it is further rotated by 60° in clockwise direction from its position for θ=120°. The resultant of the three alternating fluxes, separated from each other by 120°, has constant amplitude of 1.5∅m where ∅m is maximum amplitude of an individual flux due to any phase. The resultant always keeps on rotating with a certain speed in space. This is nothing but satisfying the definition of a rotating magnetic field. Hence we can conclude that the 3- phase stationary winding when connected to a three phase A.C. supply produces a rotating magnetic field. 5. What are the losses in induction motor and explain them briefly? Answer: There are two types of losses occur in three phase induction motor. These losses are Constant or fixed losses Variable losses. Constant or Fixed Losses: Constant losses are those losses which are considered to remain constant over normal working range of induction motor. The fixed losses can be easily obtained by performing no-load test on the three phase induction motor. These losses are further classified as Iron or core losses, Mechanical losses and Brush friction losses. Iron or Core Losses: Iron or core losses are further divided into hysteresis and eddy current losses. Eddy current losses are minimized by using lamination on core. Since by laminating the core, area decreases and hence resistance increases, which results in decrease in eddy currents. Hysteresis losses are minimized by using high grade silicon steel. The core losses depend upon frequency of the supply voltage. The frequency of stator is always supply frequency, f and the frequency of rotor is slip times the supply frequency, (sf) which is always less than the stator frequency. For stator frequency of 50 Hz, rotor frequency is about 1.5 Hz because under normal running condition slip is of the order of 3 %. Hence the rotor core loss is very small as compared to stator core loss and is usually neglected in running conditions. Mechanical and Brush Friction Losses: Mechanical losses occur at the bearing and brush friction loss occurs in wound rotor induction motor. These losses are zero at start and with increase in speed these losses increases. In three phase induction motor the speed usually remains constant. Hence these losses almost remain constant. b. Variable Losses: These losses occur due to the current flowing in stator and rotor windings. As the load changes, the current flowing in rotor and stator winding also changes and hence these losses also changes. Therefore these losses are called variable losses. The copper losses are obtained by performing blocked rotor test on three phase induction motor. 6. Derive the expression for torque in an Induction motor? Answer: The torque produced by three phase induction motor depends upon the following three factors: Firstly the magnitude of rotor current, secondly the flux which interact with the rotor of three phase induction motor and is responsible for producing Emf in the rotor part of induction motor, lastly the power factor of rotor of the three phase induction motor. Combining all these factors together we get the equation of torque as Where, T is the torque produced by induction motor, φ is flux responsible for producing induced Emf, I2 is rotor current, Cosθ2 is the power factor of rotor circuit. The flux φ produced by the stator is proportional to stator Emf E1. i.e φ ∝ E1 We know that transformation ratio K is defined as the ratio of secondary voltage (rotor voltage) to that of primary voltage (stator voltage). Rotor current I2 is defined as the ratio of rotor induced Emf under running condition , sE2 to total impedance, Z2 of rotor side, And the total impedance Z2 on rotor side is given by , Putting this value in above equation we get, s= slip of Induction motor We know that power factor is defined as ratio of resistance to that of impedance. The power factor of the rotor circuit is Putting the value of flux φ, rotor current I2, power factor cosθ2 in the equation of torque we get, Combining similar term we get, Removing proportionality constant we get, Where ns is synchronous speed in r. p. s, ns = Ns / 60. So, finally the equation of torque becomes, 7. Explain the slip torque characteristics of a 3-phase Induction motor? Answer: The torque slip curve for an induction motor gives us the information about the variation of torque with the slip. The slip is defined as the ratio of difference of synchronous speed and actual rotor speed to the synchronous speed of the machine. The variation of slip can be obtained with the variation of speed that is when speed varies the slip will also vary and the torque corresponding to that speed will also vary. The curve can be described in three modes of operationdescribed in three modes of operation Motoring Mode: In this mode of operation, supply is given to the stator sides and the motor always rotates below the synchronous speed. The induction motor torque varies from zero to full load torque as the slip varies. The slip varies from zero to one. It is zero at no load and one at standstill. From the curve it is seen that the torque is directly proportional to the slip. That is, more is the slip, more will be the torque produced and vice-versa. Generating Mode: In this mode of operation Induction motor runs above synchronous speed and it should be driven by a prime mover. The stator winding is connected to a three phase supply in which it supplies electrical energy. Actually, in this case, the torque and slip both are negative so the motor receives mechanical energy and delivers electrical energy. Induction motor is not much used as generator because it requires reactive power for its operation. Braking Mode: In the Braking mode, the two leads or the polarity of the supply voltage is changed so that the motor starts to rotate in the reverse direction and as a result the motor stops. This method of braking is known as plugging. The kinetic energy stored in the revolving load is dissipated as heat. Also, motor is still receiving power from the stator which is also dissipated as heat. So as a result of which motor develops enormous heat energy. For this stator is disconnected from the supply before motor enters the braking mode.