Published on July 23, 2014
1 READING MATERIAL FOR B.E. STUDENTS OF RGPV AFFILIATED ENGINEERING COLLEGES SUBJECT BASIC ELECTRICAL AND ELECTRONICS ENGG Professor MD Dutt Addl General Manager (Retd) BHARAT HEAVY ELECTRICALS LIMITED Professor(Retd) in EX Department Bansal Institute of Science and Technology KOKTA ANANAD NAGAR BHOPAL Presently Head of The Department ( EX) Shri Ram College Of Technology Thuakheda BHOPAL Sub Code BE 104 Subject Basic Electrical & Electronics UNIT III Rotating Electrical Machines
2 RGPV Syllabus BE 104 BASIC ELECTRICAL & ELECTRONICS ENGINEERING UNIT III ROTATING ELECTRICAL MACHINES Construction details of DC machines, induction machine and synchronous machine. Working principle of 3 ph induction motor, EMF equation of 3 ph induction motor. Concept of slip in 3 ph induction motor. Explanation of torque slip characteristic of 3 ph induction motor . Classification of series DC Motor and DC Generator. INDEX S No Topic Page 1 Construction details of DC machines 3,4,5,6 2 Construction details of 3ph induction motor 6,7,8,9 3 Construction details of Synchronous machines 10,11 4 Working principle of 3 ph induction motor reaction 12,13,14 5 EMF equation of 3 ph induction motor 14,15 6 Concept of slip in 3 ph induction motor 15,16 7 Starting Torque of 3ph induction motor; 16,17 8 Explanation of torque slip characteristic of 3 ph induction motor 18,19 9 Series excited DC motor and Generators 20,21,22,23 10 References 24
3 Construction Details of DC Machines There are four main parts of a DC machine 1)Field Magnet 2)Armature 3) Commutator 4)Brush and Brush gear Field System:- The purpose of field system is to create a uniform magnetic field within which armature rotates. It consists of following four parts. a) Yoke or frame b) Pole cores c) Pole Shoes d) Magnetizing coils Cylindrical yoke or frame is used which acts as frame and carries the magnetic flux produced by the poles. Poles are used to carry coils of insulated wires carrying the exciting current. The pole shoes acts as support to the coils and spread out flux over the armature periphery more uniformly. The magnetizing coils is to provide number of ampere turns of excitation required to give the proper flux through the armature to induce the desired potential difference. ARMATURE:- It is the rotating part of the DC machine and is built up in a cylindrical drum. The purpose of armature is to rotate the conductors in the uniform magnetic field , It consists of coils insulated wires wound around a iron
4 core and is so arranged that the electric current are induced in these wires when armature is rotated in the magnetic field. The armature core is made from high permeability silicon sheet steel stampings. A small airgap exists so that armature can rotate freely without rubbing or touching poles. Armatures are LAP or WAVE wound. COMMUTATOR:- The commutator is a form of switch (rotating) placed between the armature and the external circuit so arranged that the input is fed (incase of motor) and the output is taken out (in case of generator) through commutator by brushes and brushgear. Two important functions it is doing in case of DC machine. 1) It connects the rotating armature conductors to the external circuit through brushes. 2) It converts the alternating alternating current induced in the armature conductors into unidirectional current to the external load circuit in generating action, where as it converts the alternating torque into unidirectional torque in motor action. The commutator is of cylindrical shape and is made up of wedge shaped hard drawn copper segments of mica. These segments are insulated from each other by thin sheet of mica . The segments are held together by two
5 Vee rings that fits into the groove cut into the segments. Each armature coil is connected to the commutator segments through riser. BRUSHES :- The brushes usually made from carbon are pressed upon the commutator and from the connecting link between armature winding and external circuit. They are made from carbon because is conducting material and at the same time in powdered form provides lubrication effect on the commutator surface. The brushes are held in brush holders and brushgear on commutator. END HOUSINGS:- End housings are attached to the ends of the main frame and supports bearings. The commutator side supports brushgear assembly. Where as NCE side supports only bearing. BEARINGS :- The ball and roller bearings are fitted in the end housings. The function of bearing is to reduce the friction between the rotating part and stationary part.
6 SHAFT :- The shaft is made of steel having maximum breaking strength. The shaft is used to transfer mechanical power from or to the machine. The rotating parts like armature core, cooling fan etc are keyed to the shaft CONSTRUCTION OF INDUCTION MOTOR:- A three phase induction motor consists mainly two parts namely stator and rotor. Stator It is the stationary part of the motor. It consists of following parts i) Outer frame ii) Stator core iii) Stator winding Outer Frame:- It is the outer body of the motor. Its function is to support stator core and protects the inner parts of the machine. For small machines it is casted and for large machines it is fabricated. It also supports the motor to be placed on the foundation through feet’s at the bottom. Stator Core:- The stator core is to carry the alternating field which is produced hysteresis and eddy losses.. The core is made from high grade silicon sheet steel. The stampings are assembled under hydraulic pressure
7 and are keyed to the frame. Each lamination is insulated with a thin layer of varnish. The thickness of lamination varies from 0.3mm to 0.5mm .Slots are punched on the inner periphery of the stampings Stator Winding:- The stator core carries three phase winding which is supplied from three phase supply. Six terminals of winding ( two each per phase) are connected to terminal box of the motor. The stator winding are wound for definite number of poles depends upon the required speed. Ns = 120f/P The three phase winding can be connected in star or delta externally. Through starter. ROTOR:- It is the rotating part of the motor. There are two type of rotors for induction motor. 1) Squirrel Cage Rotor:- Most of the induction motors are of this type of rotor because of simple and rugged construction of rotor. In cage construction, copper bars or aluminum bars are placed, the rotor bars are almost placed parallel to the shaft. The rotor conductors are short circuited with short circuiting rings made up similar material of rotor bar. This resembles as squirrel cage. The slots in the rotor stampings are semi closed or closed type. The use of semi closed or closed slots is for reducing the magnetizing current.
8 1) WOUND ROTOR :- The rotor is wound with an insulating winding similar to that of stator except that the number of slots are smaller and fewer turns per phase of heavier conductors are used. A large number of turns increases secondary voltage and reduces the current that flows through the sliprings. Based on secondary voltage the insulation of rotor winding is decided. The voltage of rotor and current influences the value of rotor resistance to be put across the slip rings. The rotor is wound for the same number of poles as that of stator. The finish terminals are connected in star and are connected to the three phosphor Bronze sliprings, which is mounted on the shaft. The rotor current is carried to the external resistance through brushes mounted on the three sliprings. Since sliprings are used that is why this type of motor is called as SLIPRING MOTORS.
10 CONSTRUCTION OF SYNCHRONOUS MOTOR:- The synchronous motor essentially consists of two parts mainly the armature ( stator) and field magnet system ( rotor). STATOR : - The armature is an iron ring formed of laminations of special magnetic material ( silicon sheet steel) . It is having slots on the inner periphery to accommodate armature conductors and is known as stator. The whole structure is held in a cast iron or fabricated frame. The field rotates in between the stator, flux of rotating field cuts the stator core continuously and causes eddy current losses in the core. The laminations are insulated from each other by thin layer of varnish. ROTOR:- Similar to DC field system the rotor field system of synchronous machine is excited by DC 125 250V DC supply from exciter which is mounted on the same shaft. Rotors are of two type 1) Salient pole type rotor 2) Smooth cylindrical rotor The rotor of this type is used entirely for low speed alternators. These type of machines are called projected pole type machines. The poles are made from lamination punched from silicon sheet steel and joined together by pole rivets. The each lamination is insulated by this layer varnish. The damper
11 windings are provided at the pole shoes for avoiding hunting. The pole faces are so shaped that airgap is minimum at centre and increases from the pole centre for the sinusoidal flux so that the induced EMF is sinusoidal. The end of the field windings are connected through sliprings to a DC source. They have following special features:- i) Salient pole field structure has large diameter and short shaft lengths ii) The pole shoes cover about ⅔ of pole pitch iii) These are employed in HYDRO turbine or diesel engines, where RPM is low ( 100rpm to 325 rpm) SMOOTH CYLINDRICAL ROTOR The rotor of this type is used in very high speed alternators. ( Steam Turbine) To reduce the peripherals velocity the diameter of this type of rotor is small and the axial length is increased. Such rotor normally have two or four poles. It consists of steel forgings with radial slots in which field copper ,usually strips are placed. The coils are held by steel or bronze wedges and coil ends are fastened by metal strips. This type of rotor have uniform air gap. For getting sinusoidal EMF slots are shapes machined in the rotor forging. i) Less windage loss ii) Very high operating speed ( 3000rpm) iii) Robust construction and noiseless operation.
12 WORKING PRINCIPAL OF 3 PH INDUCTION MOTOR:- In a induction motor there is no electrical connection to the rotor, but the currents are induced in the rotor circuit and therefore , the same condition exists as in the case of DC motor. The rotor conductors carry current in the stator magnetic field and thereby have force exerted upon them tending to move them at right angle to the field. When the stator winding of 3ph induction motor is connected to a 3ph supply, a rotating field is established which rotates at synchronous speed. The direction of rotation of the magnetic field will depend upon the phase sequence of the stator current. The direction of magnetic field can be reversed by reversing the phase sequence of 3ph supply. This can be done by interchanging any two leads of 3ph supply. The number of poles of the revolving field will be same as the number of poles for each phase of stator winding is wound. The speed at which the field is rotating is called synchronous speed. Ns = 120f P Here P is number of Poles, and f is supply frequency As magnetic field sweeps across the rotor conductors, and EMF is induced in these conductors, ( as similar to the transformers). Since the rotor circuit is either shorted or closed through some external resistance, the induced EMF due to revolving field causes current to flow in rotor conductors. A section of induction motor stator and rotor, with the magnetic field assumed to be rotating in a clockwise direction and with rotor stationary, as at starting. The relative motion of the rotor with respect to the stator field is anticlockwise. Now by the effect of combined field and LHS rule . the rotor
13 conductor experiences a force tending to move the rotor conductor to the right, one half cycle late, the stator field direction will be reversed, the rotor current will also be reversed, so the force on the rotor is still same. Likewise rotor conductors under stator poles will have a force exerted upon them, all tending to turn the rotor in the clockwise direction. If the developed torque is great enough to overcome the resisting torque of the load, the rotor will accelerate in the clockwise direction or in the same direction of the stator field. When the rotor is stationary and about to start, the frequency of the induced EMF in the rotor is equal to the supply fed to the stator. Because of relative motion is at synchronous speed . As the rotor picks up the speed, the relative motion between rotor and the synchronously rotating magnetic field becomes less and the frequency induced in the rotor decreases. The magnitude of rotor induced EMF, induced rotor current so that the torque developed depends upon the relative motion. In case of relative motion is zero the rotor runs at synchronous speed. There will be no induced EMF, no current, no torque, Thus we can say that the induction motor cannot run at synchronous speed. Induction motor at no load will have speed very near to the synchronous speed, therefore EMF in the rotor will also be very small. The small EMF will produce little rotor current producing a torque just sufficient to overcome the losses due to friction and maintain the rotor in motion. As
14 mechanical load is applied on the motor shaft, it must slowdown, as the rotor slows down the relative speed between the magnetic field and rotor is increased , This results in greater rotor current and greater torque. Thus as the load increases the motor slows down until the relative motion between rotor and rotating magnetic field is just sufficient to result in development of the torque necessary for particular load. EMF EQUATION OF 3 PH INDUCTION MOTOR Cross sectional view of a three phase induction motor is shown in figure. The stator is supplied from a 3ph supply. The rotor is wound 3ph and for the same number of poles as that of stator. The rotor is short circuited, Neglecting stator resistance and leakage reactance being negligibly small we get Terminal Voltage V Per phase = Stator induced EMF per Phase E V = E = 4.44 Kw1N1 Ø r f Kw1 = stator winding factor N1 = Number of series turn per phase Ø r = Resultant air gap flux per pole f = frequency of supply in Hz The resultant airgap flux Ø r per pole is constant and is related to the supply voltage V in view of assumption made. The MMF Fr with associated
15 flux density Br , which causes for generating Ø r, rotates at synchronous speed because it is associated with the 3 ph balanced supply to the stator. Due to the relative speed between Br and the rotor, an EMF is induced in the rotor winding which causes a current to flow in rotor conductors. The torque is developed due to interaction between Br and rotor currents torque so developed tends to turn the rotor in the direction of Br, so as to reduce the relative speed. Thus the motor is self starting and rotor attends steady speed N ( where N<Ns) depending upon the load coupled to it. In case rotor runs at Ns, There would be no induced EMF and no rotor currents in conductors, no field, hence no torque because relative speed between Br and rotor is zero. CONCEPT OF SLIP IN 3PH INDUCTION MOTOR The speed of polyphase induction motor is always be less than the synchronous speed, as load increase the speed of rotor decreases. The difference between the speed of the stator field known as synchronous speed Ns and actual speed of the rotor N is known as SLIP and is defined by s .Though the SLIP can be expressed in rpm or in radians per second, but usually it is expressed in fraction or percentage of synchronous speed. s = Syn Speed - Rotor Speed Syn Speed = Ns – N Ns % slip = Ns – N X 100 Ns At normal load slip of induction motor is between 2 to 5 percent. At no-load the slip is very small 0.5%. As the load is applied, the natural effect of the load torque ids to cause the rotor to slow down. As it does so, increases and with it increase the current and torque increase until the driving torque of the machine balances with retarding torque of the load. FREQUENCY OF ROTOR
16 Rotor emf frequency f' = Relative speed 120/P Ns – N = sNs = s 120f/P f' = Ns –N 120/P We know and /replacing Ns –N with s120f/P We get f' = s 120 f P = s f P 120 EXPLANATION OF TORQUE /SLIP CHARACTERISTICS OF INDUCTION MOTOR. Rotor Torque :- The torque is a induction motor is produced due to the interaction of rotor and stator fields. The in induction motor is:- i) Proportional to rotor current I2 ii) Stator flux ϕ iii) Proportional to the rotor power factor Cos ϕ₂ Torque α I2 ϕ Cos ϕ₂ E2 α ϕ T = K I2 E2Cos ϕ₂ Under running condition I2 = sE2 √ R₂² + s² X₂² Cos ϕ 2 = R2 √ R₂² + s² X₂²
17 Putting the value of I2 andCos ϕ 2 We get T = K sE2 X E2 X R2 √ R₂² + s² X₂² √ R₂² + s² X₂² T = K s E2² R2 R₂² + s² X₂² This is the torque under running condition STARTING TORQUE:- At the start the rotor is stationary and s=1 Tst = K E2² R2 R₂² + X₂² Since the supply voltage is constant the flux also will be constant and E2 will be constant Let K1 = K E2² Tst = K1 R2√ R₂² + X₂² We know that R₂² + X₂² = Z² So Tst == K1 R2 Z²
18 CONDITION FOR MAXIMUM TORQUE T = K s E2² R2 √ R₂² + s² X₂² This will be maximum when s R2 or R2 R₂² + s² X₂² R₂²/s + s X₂² is maximum R2/√s - √sX₂ = 0 Or R2/ X₂ = s Putting the value of s we get Tmax = ( K E2² R2 ) R2/ X₂ R₂² + R₂² So Tmax = (K E2² R2 ²) 2 R2 ² X₂ Tmax = K E2² X₂ From this equation it is obvious that i) Maximum torque is independent of rotor resistance ii) The slip at which the maximum torque occurs is generated rotor resistance and rotor reactance becomes equal, this can be easily done in the case of slip ring motors. iii) Maximum torque various inversely with standstill rotor reactance. It is kept minimum by placing the rotor bars near the rotor periphery. iv) Maximum torque varies directly square of supply voltage.
19 TORQUE SPEED, TORQUE SLIP CURVES 1) When the speed is synchronous i.e slip is zero , the torque is also zero so that the torque slip curve starts from 0 2) When the speed is very near to the synchronous speed Ns i.e slip is very small , the value of sx2 is very small and is negligible in comparison with rotor resistance, therefore T is proportional to slip s, and is straight line 3) As the slip increases i.e the speed drops with increase in load, torque increases reaches maximum when s = R2/X2 , The maximum torque is known as pullout torque or breakdown torque and the slip is known as breakdown slip Sb . 4) When further in increase in slip the speed drops due to increase in load beyond the point of maximum torque the torque begins to decrease, The reason is that motor slows down and eventually stops.
20 CLASSIFICATION OF SELF EXCITED DC MOTORS AND GENERATORS A DC motor or generator whose field winding is supplied itself is called self excited DC machine In a self excited DC machine the field coils are connected in parallel with armature winding . Self excited DC motor are classified as follows:- i) Shunt excited DC motor ii) Series excited DC motor iii) Compound DC motor Shunt excited DC motor :- In a shunt excited DC motor the field coil winding is connected parallel to the armature winding Ish = V/ Rsh Ia = Il – Ish Eb = V – Ia Ra -2Vb SERIES EXCITED DC MOTOR In case of series excited DC motor the line current passes through series field winding and also armature current. Ia = Il = Ise Eb = V – Ia(Ra +Rse) -2 Vb
21 COMPOUND DC MOTOR:- There are two type of compound DC motors i) Cumulative compound motor ii) Differential compound motor In the compound wound DC motor the field is produced by the shunt field as well as series field. Generally shunt field is stronger than series field. When the series field assists the shunt field it is called cumulative compound wound DC motor. When series field opposes the shunt field the motor is called differentially wound DC motor. Cumulative compound dc motor Differential compound DC motor SELF EXCITED DC GENERATORS There are following type of self excited generators. a) Shunt excited DC generator b) Series excited DC generator c) Compound DC generator
22 Series Wound DC Generator :- In series wound Dc generator full line current or armature current flows from the series winding. The series field current Ia = Il = Ise V = Eg – Ia(Ra +Rse) -2 Vb Power developed Eg Ia power output = V Ia Shunt wound DC generator The field winding is connected in parallel to the armature winding Ish = V/ Rsh Ia = Il + Ish V = Eg – Ia Ra -2Vb COMPOUND WOUND DC GENERATOR A compound generator may be called short shunt or long shunt Short shunt compound
23 Ise = Il Ish = V + IlRse = Eg -IaRa Rsh Rse Ish + Il = Ia V = Eg – IaRa – Il Rse -2Vb Power developed =Eg Ia Power output = V Il LONG SHUNT Ish = V/Rsh Ise = Ia= Il + Ish V = Eg – IaRa – IseRse -2Vb = Eg - Ia(Ra +Rse) -2Vb Power Developed EgIa Power output V Il LONG SHUNT
24 References 1. Basic electrical and Electronics Engineering By Pankaj Swarnakar and Shiv Shankar Mishra Tech India publication 2. Electrical & Electronics Engineering By RK Chaturvedi and SK Sahdev Dhanpatrai Publication. 3. Electrical & Electronics Engineering By JB Gupta KATSON Books 4. Basic Electrical Engineering by Vincent Deltoro 5. Basic Electrical Engineering by De an Sen TMH Publication 6.
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