Electrical drives and control unit 2
- 1. ELECTRICAL DRIVES AND CONTROL 32044 M – SCHEME 2019-2020 𝐈𝐈 𝐧𝐝 Year – B section DEPARTMENT OF MECHANICAL ENGINEERING Prepared by, Mr.B.Don Dougles.,M.E., Lecturer., D.M.E
- 2. UNIT – II AC CIRCUITS AND AC MACHINES Questions and topics to be focussed on, ➢ Fundamentals of AC. ➢ Three phase star and delta connection-Relationship between line voltage, phase voltage, line current and phase current in star and delta connection. ➢ Single phase transformer construction and operation. ➢ EMF equation of transformer and losses in transformer. ➢ Alternator-Construction and operation. ➢ Operation of single phase capacitor start induction motor. ➢ Three phase squirrel cage induction motor and speed control. ➢ AC starters-DOL starter and Star delta starter. Slide – 01EDC – Unit II
- 3. FUNDAMENTALS OF AC • FREQUENCY: The number of cycles completed in one second is called the frequency (f). The unit of frequency is Hertz (Hz). 1 Hertz = 1 cycles per second. • TIME PERIOD: The time required to complete one cycle is called the periodic time or simply the period (T). The unit of time period is second(s). Slide – 02EDC – Unit II
- 4. • AMPLITUTE: The maximum positive or negative value of an alternating quantity is called amplitude. • PEAK VALUE (OR) MAXIMUM VALUE: This is the maximum value of the alternating quantity attained by it in a cycle. Maximum value is called the peak value, crest value, or the amplitude. Thus Im denotes maximum current and Vm denotes the maximum voltage. • AVERAGE OR MEAN VALUE: The average or mean of an alternating quantity over a given interval is the sum of all instantaneous values divided by the number of values taken over that interval. Average value = Area under the curve Length of the base of the curve Slide – 03EDC – Unit II
- 5. • ROOT MEAN SQUARE VALUE: RMS defined as the amount of AC power that produces same heating effect as equivalent DC power. The RMS value is the square root of the mean (average) value of the squared function of the instantaneous values. The symbols used for defining an RMS value are VRMS or IRMS. IRMS = Io 2 or VRMS = Vo 2 • POWER: Power is defined as the rate of doing work, or the rate of change of energy. The unit of power is Watts. P = Energy Time Watts • POWER FACTOR: The ratio of active power (P) in watts to the apparent power (S) in volt amperes in an ac circuit is defined as the power factor of the circuit. Power factor = Active power (VI cos∅) Volt amperes (VI) = PF = cos∅ Slide – 04EDC – Unit II
- 6. THREE PHASE STAR AND DELTA CONNECTION SYSTEM • In a three phase system, there are three windings or phases. Each phase has two terminals i.e., start and finish. The three windings are interconnected in two methods. They are • Star or Wye connection (Y) • Delta or Mesh connection (∆) STAR CONNECTION SYSTEM • In this method similar ends of the three phases are joined together to form a common junction N. The junction N is called the star point or neutral point. Slide – 05EDC – Unit II
- 7. Relationship between line, phase voltages and line, phase currents: • Let VRY, VYB, VBR are the line voltages and the voltage between neutral point N and any of the lines are phase voltages of star connection respectively. • The line voltage equation is expressed as, 𝐕𝐋= 𝟑 𝐕𝐩𝐡 • Thus, for star connection line voltage = 𝟑 X phase voltage. N • The line current equation is, 𝐈 𝐋= 𝐈 𝐩𝐡 • Line current = Phase current. Slide – 06EDC – Unit II
- 8. DELTA CONNECTION SYSTEM • In this method of interconnection, the dissimilar ends of three phase windings are joined together i.e., finishing end of one phase is connected to the starting end of the other phase. Relationship between line, phase voltages and line, phase currents • In this system, there is no Neutral connection. Therefore the phase voltage relation is, 𝐕𝐋= 𝐕𝐩𝐡. • Line voltage = Phase voltage. • The line current relationship with phase current would be, 𝐈 𝐋= 𝟑 𝐈 𝐩𝐡 • Line current = 𝟑 Phase current. Slide – 07EDC – Unit II
- 9. DIFFERENCE BETWEEN STAR CONNECTION AND DELTA CONNECTION SYSTEM STAR CONNECTION SYSTEM • This is also called as 3 phase, 4 wire system. • There is a Neutral point (N). • Line current = Phase current, IL= Iph. • Line voltage = 3 X Phase voltage VL= 3 Vph DELTA CONNECTION SYSTEM • This is also called as 3 phase, 3 wire system. • There is no Neutral point. • Line voltage = Phase voltage, VL= Vph. • Line current= 3 X Phase current IL= 3 Iph Slide – 08EDC – Unit II
- 10. SINGLE PHASE TRANSFORMER • A transformer is a static device which transfers electric power from one circuit to another electric circuit without change of same frequency. Principle of Operation • The main principle of operation of a transformer is mutual inductance between two coils which is linked by common magnetic flux. • According to Faraday’s law of electromagnetic induction, when the current carrying conductor cuts the magnetic flux lines an emf is induced in the conductor. Slide – 09EDC – Unit II
- 11. Construction • The important parts of transformer are, core and windings. Transformer Core: The two most common and basic designs of transformer construction are the Closed-core Transformer and the Shell-core Transformer. • In the “closed-core” type (core form) transformer, the primary and secondary windings are wound outside and surround the core ring. • In the “shell type” (shell form) transformer, the primary and secondary windings pass inside the steel magnetic circuit (core) which forms a shell around the windings. • In both types of transformer core design, the magnetic flux linking the primary and secondary windings travels entirely within the core with no loss of magnetic flux through air. Slide – 10EDC – Unit II
- 12. Transformer windings: Transformer windings form another important part of a transformer construction, because they are the main current-carrying conductors wound around the laminated sections of the core. • In a single-phase two winding transformer, two windings would be present The one which is connected to the voltage source and creates the magnetic flux called the primary winding, and the second winding called the secondary winding in which a voltage is induced as a result of mutual induction. • If the secondary output voltage is less than that of the primary input voltage the transformer is known as a “Step-down Transformer”. If the secondary output voltage is greater then the primary input voltage it is called a “Step-up Transformer”. Slide – 11EDC – Unit II
- 13. Working of transformer • The transformer works on the principle of mutual induction. According to Faraday’s law of electromagnetic induction, when a conductor cuts the magnetic flux lines, an emf is induced in the conductor. • The working schematic has been shown above. The primary winding of the transformer is connected to the external alternating current supply. Slide – 12EDC – Unit II
- 14. • The primary winding creates the magnetic flux and leakage flux in the primary side of transformer core. • This magnetic flux is created in the entire core part of the transformer and thus the flux travels along the secondary part of the core. • On the basis of Faraday’s law of electromagnetic induction, the magnetic flux of the core cut by the secondary winding which is wound on the transformer core. • Due to such action a voltage is induced in the secondary winding of the transformer core. This phenomenon is called mutual induction. • The induced voltage is transferred to the load. Slide – 13EDC – Unit II
- 15. EMF EQUATION OF TRANSFORMER • Transformer works on Faraday’s law, when the primary winding is supplied by an external power supply magnetic flux is induced in the transformer core. This magnetic lines are cut by secondary winding and thus produces an emf in secondary side of the transformer. • The EMF developed in the transformer is expressed below. • Let, N1 = Number of turns in primary winding N2 = Number of turns in secondary winding Φm = Maximum flux in the core (in Wb) = (Bm x A) f = frequency of the AC supply (in Hz) Slide – 14EDC – Unit II
- 16. • From Faraday’s law of electromagnetic induction the average e.m.f. induced in each turns is proportional to the average rate of change of flux. Average e.m.f. per turn = Average rate of change of flux • Average e.m.f per turn = d∅ dt (1) • Now, d∅ dt = change in flux/Time required. Consider 1/4th cycle of the flux as shown above. i.e., from 0 to Φm Slide – 15EDC – Unit II
- 17. • Rate of change of flux is maximum, d∅ = Φ 𝑚 • Rate of change of time for one cycle, dt = 1/4f • On substituting (2) and (3) in (1), we obtain, (1) = > Average emf per turn = Φm 1/4f • Average emf per turn = 4f Φ 𝑚volts. • For sinusoidal quantity, Form factor = 1.11 • R.M.S value = 1.11 X Average value. • Therefore, R.M.S value induced emf per turn = 1.11X4f Φ 𝑚 = 4.44f Φ 𝑚 volts. (2) (3) Slide – 16EDC – Unit II (4)
- 18. • There are N1 number of primary turns hence the R.M.S. value of induced e.m.f. of primary denoted as E1 is, E1 = N1 × 4.44f Φm volts. • While as there are N2 number of secondary turns hence the R.M.S. value of induced e.m.f. of secondary denoted as E2 is, E2 = N2 × 4.44f Φm volts. • The above equations (3) (4) (5) are called as EMF equation of single phase transformer. Losses in Transformer: In Transformer losses occur due to loop currents and heating effects. • The losses occur in core of the transformer are called as Core or iron losses. • Core loss is classified as Hysteresis loss and Eddy current loss. • The losses occur in transformer windings are called Copper loss. (4) (5) Slide – 17EDC – Unit II
- 19. ALTERNATOR CONSTRUCTION AND OPERATION ➢ An alternator is commonly called as AC generator, an electrical machine which converts mechanical energy into direct current electricity. Principle: ➢ According to Faraday’s laws of electromagnetic induction, whenever a conductor is placed in a varying magnetic field an emf (electromotive force) is induced in the conductor. Construction: Slide – 18EDC – Unit II Rotor
- 20. • Basic constructional parts of an alternator machine are described below. 1. Stator: The stator consists of cast iron frame, which supports armature core. It has the slots on its inner periphery for housing the armature conductors. The armature core is made of laminations of special magnetic iron or steel alloy. The core is laminated to reduce eddy current loss. The laminations are stamped out in complete rings for small machines or in segments for large machines. 2. Rotor: Two types of rotor are salient pole type and smooth cylindrical type. • Salient pole types are projecting in nature. It is used in low and medium speed alternators. . It has a large number of projecting poles, having their cores bolted or dove tailed on to a heavy magnetic wheel of cast iron. Slide – 19EDC – Unit II
- 21. • Smooth cylindrical type used for turbo driven alternators. The rotor consists of a smooth solid forged steel cylinder, having number of slots along the outer periphery. The field coils are placed in the slots. • To avoid excess peripheral velocity at high speeds, these rotors have smaller diameter (about 1 meter). Hence turbo alternators are characterized by small diameter and larger axial length. Working of Alternator: • In an alternator, field coils produce an electromagnetic field and the armature conductors are rotated into the field. Thus, an electromagnetically induced emf is generated in the armature conductors. It’s given by Flemming’s right hand rule. Slide – 20EDC – Unit II
- 22. • According to Fleming’s right hand rule, the direction of induced current changes whenever the direction of motion of the conductor changes. • Let’s consider an armature rotating clockwise and a conductor at the left is moving upward. When the armature completes a half rotation, the direction of motion of that particular conductor will be reversed to downward. • Hence, the direction of current in every armature conductor will be alternating. • If you look at the above figure, you will know how the direction of the induced current is alternating in an armature conductor. Slide – 21EDC – Unit II
- 23. SINGLE PAHSE CAPACITOR START INDUCTION MOTOR • A Capacitor Start Motors are a single phase Induction Motor that employs a capacitor in the auxiliary winding circuit to produce a greater phase difference between the current in the main and the auxiliary windings. • The name capacitor starts itself shows that the motor uses a capacitor for the purpose of the starting. Slide – 22EDC – Unit II
- 24. • Capacitor start induction motor has two stator windings namely main winding and starting winding. Main winding is highly inductive and starting winding is highly resistive. • The capacitor start induction motor has a capacitor connected in series with the starting winding. A centrifugal switch is connected with this capacitor in series. • Main winding is connected in parallel to the rotor of the motor and the single phase input supply. The main winding and starting winding are in series to each other along with a capacitor. Working: When the stator windings are energized, main winding carries Im and starting winding carries Ist. • The phasor diagram shows that, the Im current in the main winding is lagging the starting winding current Ist. Thus the single phase supply current is split into two phases. Slide – 23EDC – Unit II
- 25. • The two windings are displaced apart by 90 degrees electrical, and their MMF’s are equal in magnitude but 90 degrees apart in time phase. • As the motor approaches its 75% of rated speed, the auxiliary winding and the starting capacitor is disconnected automatically by the centrifugal switch provided on the shaft of the motor. Characteristics of the Motor: Slide – 24EDC – Unit II
- 26. • The graphical representation of characteristics of capacitor start motor is drawn above. This characteristics is drawn between synchronous speed and torque. • In the beginning, the torque is above 300% in main winding and starting winding. When the motor attains 75% or 80% of its synchronous speed, the centrifugal switch opens and disconnects the stator windings. Applications of Single phase capacitor start induction motor: • It is used in fans, blowers, centrifugal pumps, washing machine, wet grinder, drilling machine and compressors. Slide – 25EDC – Unit II
- 27. THREE PHASE SQUIRREL CAGE INDUCTION MOTOR • 3 phase induction motor is widely used in industrial drives. The construction of it is discussed below. The three phase induction motor consists essentially of two main parts: • Stator • Rotor • Stator: Stator frame is in the form of cylinder. It is made of cast iron and used for supporting the stator core. Terminal box is fixed in the outer surface. • . At both ends of the stator frame, provision is made to fit the end bells. Eye bolt is fitted at the top of frame. Eye bolts are used for lifting the motor. • Stator core is made up of number of silicon steel stampings. The core is laminated to reduce eddy current loss. Slide – 26EDC – Unit II
- 28. • The three phase winding fed from a three phase supply is placed in the slots. The winding is wounded for a defined number of poles. • Squirrel cage rotor: The rotor consists of a cylindrical laminated core. It consists of laminated silicon steel punching, bolted together and mounted on a shaft. The cylindrical core has the slots on the outside surface. • Rotor conductors made of heavy bars of copper, aluminium or alloys are placed in the rotor slots. Slide – 27EDC – Unit II
- 29. • It should be noted that the rotor bars are permanently short-circuited on themselves, hence it is not possible to add any external resistance in series with the rotor circuit for starting purposes. • Skewing is done to avoid magnetic locking of stator and rotor and to reduce magnetic hum. • Working: When a 3 phase supply is given to the stator winding it sets up a rotating magnetic field in space. This rotating magnetic field has a speed which is known as the synchronous speed. Slide – 28EDC – Unit II
- 30. • This rotating magnetic field induces the voltage in rotor bars and hence short-circuit currents start flowing in the rotor bars. These rotor currents generate their self-magnetic field which will interact with the field of the stator. • In this case, the cause which produce the rotor current is the relative velocity between rotating flux of the stator and stationary rotor conductor. Hence to reduce the relative speed, the rotor starts running in the same direction as that of the flux and tries to catch up with the rotating flux. Applications: • They are preferred for driving fans, blowers, grinders, lathe machines, drilling machines, water pumps, printing machines etc. Slide – 29EDC – Unit II
- 31. Speed control of three phase induction motors: Control from Stator side: • By changing the applied voltage: This method is cheapest and the easiest. But it is rarely used because large change in voltage is required for small change in speed. • By changing the applied frequency: Synchronous speed of the rotating magnetic field of an induction motor is given by, Ns= 120f p where, f=frequency of the supply and P = number of stator poles. Hence, the synchronous speed of the motor changes with change in supply frequency. • Constant V/F control of induction motor: This is the most popular method for controlling the speed of an induction motor. By keeping V/F constant, the developed torque remains approximately constant. Slide – 30EDC – Unit II
- 32. Speed control from rotor side: • Rotor resistance control: This method is only applicable to slip ring motors, as addition of external resistance in the rotor of squirrel cage motors is not possible. The speed of the motor is reduced by adding external resistance • Cascade operation: In this method of speed control, two motors are used. Motor A is called the main motor and motor B is called the auxiliary motor. Both are mounted on a same shaft so that both run at same speed. By this cascading two motors the speed of motor shall be controlled. Slide – 31EDC – Unit II
- 33. STARTERS IN AC MOTORS Need of a starter: • At starting time, rotor is initially at rest positon. Hence the e.m.f induced in the rotor is maximum value and it circulates very high current through the rotor consequently the stator draws a very high current from the supply. • Due to such heavy inrush current, there is a possibility of damage of the motor winding. Hence in order to reduce the starting current of induction motors, starters are used. • The different type of starter used are listed below. • D.O.L starter • Star-delta starter • Auto Transformer starter • Rotor Resistance starter Slide – 32EDC – Unit II
- 34. Direct Online Starter (DOL Starter) • This type of starter is used for motor of less than 5 H. P rating. This type of starter connects stator directly to the supply line without any voltage reduction. Hence this starter is known as Direct Online Starter. Power Circuit: The three phase 440 V, 50 Hz supply is connected to motor through fuse, thermal overload relay and contactor. The contactor has 3 power contacts to control the supply input to the motor. Control Circuit: The ON push button has NO contact (normally open). The OFF push button has contact NC(normally closed). • The thermal overload relay has NC contact. The NC contacts Off push button, thermal overload relay and NO contact of ON push button are connected in series with the contactor coil. The sealing contact of contactor is connected in parallel with ON push button. Slide – 33EDC – Unit II
- 35. Operation: For starting the motor, ON push button is pushed for fraction of second. Hence contactor coil gets energized and attracts the contactor. • As the power contacts (M1, M2, M3) are closed, stator directly gets 3 phase supply. The sealing contact (M4) holds the coil in ON state even if the ON push button is de-pressed. • For stopping the motor, the OFF push button is pressed. Now the NC contact is opened and the coil circuit gets opened due to which coil gets de-energized and motor gets switched OFF from the supply. Slide – 34EDC – Unit II
- 36. • Overload condition: Current drawn by the motor increases. Hence an excessive heat is produced is produced in the motor, which increases temperature beyond limit. • The NC contact of thermal relay gets opened due to high temperature, due to which coil gets de- energized and motor gets switched OFF from the supply. Thus, the motor is protected from overload conditions. Advantages: • Simple construction. • Easy to maintain. • Low cost Disadvantages: • Used only in small HP motor. • Starting current is high. Slide – 35EDC – Unit II
- 37. Star Delta Starter • This type of starter is used for starting the induction motors which are running with delta connection normally. The connection diagram of Star – delta starter is shown below: Working: The switch connects the stator winding in Star at start and then delta for normal running. • During starting, the switch connects the motor in star. Hence per phase voltage gets reduced by the factor 1/√3. • Due to this reduced voltage, the starting current is reduced to 1/√3 times that of current taken with direct starting. Slide – 36EDC – Unit II
- 38. • During running, when motor reaches 75 % of the rated speed, the switch is thrown on other side. Now the winding gets connected in delta, across the supply. So it gets normal rated voltage. • The operation of the switch can be automatic by using relay which ensures that motor will not start with the switch in Run position. Advantages • Cheap • Maintenance free operation Disadvantages • It is suitable only for normal delta connected motors. Applications • Used in machine tools, pumps, motor – generator sets Slide – 37EDC – Unit II