* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

# Download File

Electrical ballast wikipedia, lookup

Spark-gap transmitter wikipedia, lookup

Resistive opto-isolator wikipedia, lookup

Power inverter wikipedia, lookup

Stray voltage wikipedia, lookup

Current source wikipedia, lookup

Utility frequency wikipedia, lookup

History of electric power transmission wikipedia, lookup

Opto-isolator wikipedia, lookup

Pulse-width modulation wikipedia, lookup

Power engineering wikipedia, lookup

Commutator (electric) wikipedia, lookup

Power electronics wikipedia, lookup

Brushless DC electric motor wikipedia, lookup

Switched-mode power supply wikipedia, lookup

Resonant inductive coupling wikipedia, lookup

Electrification wikipedia, lookup

Mains electricity wikipedia, lookup

Buck converter wikipedia, lookup

Transformer types wikipedia, lookup

Distribution management system wikipedia, lookup

Voltage optimisation wikipedia, lookup

Electric motor wikipedia, lookup

Alternating current wikipedia, lookup

Electric machine wikipedia, lookup

Three-phase electric power wikipedia, lookup

Brushed DC electric motor wikipedia, lookup

Variable-frequency drive wikipedia, lookup

P= VIL= 240 x 100= 24Kw EE6352 ELECTRICAL ENGINEERING AND INSTRUMENTATION 6. What are the conditions to be fulfilled for the self-excitation of a dc shunt generator? (i) There must be some residual magnetism. (ii) Shunt field coils should be properly connected to the armature terminals. (iii) Shunt field resistance should be less than the critical resistance. Important Short Answer Questions with answers Class/Sem : II/III Staff Name: Mrs.,S.Panchali A.P/EEE 1. A dc motor operates at from a 240 v supply. The armature resistance is 0.2 ohm. Determine the back emf if armature current is 50 A. Eb= V-IaRa; V=240V; Ra= 0.2; Ia= 50 Eb= 240- (50x0.2)= 240-10= 230 V 1. Define armature reaction. If load current starts to flow in armature conductor it will produce some flux. This flux may distort the main flux. Due to this drop will occur in the armature side. E 7. The emf per turn of single phase 6.6 kv/440 v , 50 hz transformer is approximately 12 v. calculate the number of turns in HV and LV windings and calculate the net cross sectional area of the core for the maximum flux density of 1.5 T. Emf/Turn = 12 V; V1= 6.6 Kv; V2= 440 V; F = 50 Hz; Bm = 1.5 T E1 = N1 * Emf/Turn N1= 6.6 x 103 / 12 = 550 turns N2 = 440/12 = 37 turns A = E1/( 4.44 F N1 Bm) = 0.036 m2 2. Define Back EMF. It is the counter EMF produced by the motor which is in opposite direction to the supply voltage due to the rotation of armature. Eb = ϕZN/60 (P/A) 3. Draw the speed torque characteristics of DC series motor and write the Applications of shunt motor. 8. Define the voltage transformation ratio of a transformer. V1= E1; V2=E2 V2/V1 = E2/E1 9. Define voltage regulation of a transformer. The regulation of a transformer is defined as reduction in magnitude of the terminal voltage due to load, with respect to the no load terminal voltage % Regulation = (( V2 on no load)- (V2 when loaded))/( V2 on no load) x 100 10. Why the transformer rating is in KVA? Copper loss depend on current and iron loss depend on voltage. Hence the total loss in a transformer depend on Volt-Ampere(VA) only and not on the power factor. That is why the rating is in KVA 11. State the condition for maximum efficiency of a transformer? Iron Loss= Copper Loss (or) Constant Loss = Variable Loss 5. A separately excited generator has induced emf of 250 V and a full load terminal voltage of 240 V. if the value of Ra = 0.01 Ω, find the full load current and output of the generator. Neglect armature reaction and brush drop. V= 240V; Eg= 250V; Ra= 0.01 Eg=V+IaRa Ia=(Eg-V)/Ra = 100A = IL 12. Draw the equivalent circuit of a transformer referred to primary? 3 phase induction motor is producing rottaing MMF in the airgap which produces some rotation where as 1 phase induction motor is producing alternating flux in the air gap which will not produce any rotation. That’s why the 1 phase motor is not a sekf starting motor . 13. A 3 phase 2 pole motor is to have synchronous speed of 6000 rev/min. calculate the frequency of supply voltage. F= N P/120 = 6000 x 2 /120 = 100 Hz 14. A stator winding supplied from a 3 phase 60 hz system is required to Produce the magnetic flux rotating at 900 rev/min. determine the number of poles. P= 120 F/N = 120 X 60 / 900 = 8 15. Define slip of an induction motor. And what is the relation between speed and frequency. S =( Ns- Nr) / Ns Nr =( 1-S) Ns Fr = S F 16. Draw the torque-speed characteristics of a 3 phase induction motor. 19. Distinguish between accuracy and precision. Accuracy Precision 1 Accuracy refers to degree of Precision refers to degree of agreement closeness of the measured value to among group of readings the true value 2 Accuracy gives the maximum error Precision of a measuring system gives that is maximum departure of the its capability to reproduce a certain final result from its true value reading with a given accuracy 20. List the different types of possible errors in measurements. (i) gross error (ii) systematic error (iv) environmental error (v) observation error (vi) random error (vii) limiting error 21. What is the difference between analog and digital instruments? Aspects Analog Digital 1 Information form As the position of As a number pointer against a calibrated scale or dial 2 Human error Exists Does not exists 3 Accuracy + 0.25% +0.005% 4 Construction Simple in Construction is construction complex 22. What is the need for measurements? The need of measurement is to know the unknown magnitude. 17. Why synchronous motor is called so. The synchronous motor rotate in synchronous speed (i.e Ns =120 F /P) or in constant speed that’s why this motor is called as synchronous motor. 18. Why the 3 phase induction motor is a self-starting motor and why not in 1 phase? 23. List the main functional elements used in most of the measurement systems. (i) primary sensing unit (ii) variable conversion element (iii) data presentation element 24. How the transducers are classified. Transducers are classified (i) on the basis of transduction form used (ii) as primary and secondary transducers (iii) (iv) (v) as active and passive transducers as analog and digital transducers as transducers and inverse transducers. 25. What are the advantages of digital instruments over analog instruments? (i) high accuracy (ii) high precision (iii) better resolution (iv) no parallax and observational error (v) very high input resistance et… 26. How are the digital voltmeter classified. (i) integrating (a) V/F converter type (b) potentiometric type (c) dual slope integrating type (ii) Non integrating type (a) potentiometer type (i) servo type (ii) null balance type (iii) successive approximation type (b) Ramp type (i) linear type (ii) staircase type 27. Define resolution and sensitivity of DVM. Resolution R= 1/ 10n ; n- number of full digits in a digital display Sensitivity S = (Fs)min x R ; Fs min – Full scale value of minimum range 28. What is sampling oscilloscope and list a few applications of CRO. Sampling oscilloscope: Above the range of 50-300 MHz, sampling techniques have to be employed to obtain suitable display and CRO employing such sampling methods are called sampling oscilloscopes. Applications of CRO It is capable of measuring (i) voltage (ii) current (iii) time (iv) frequency (v) rise & fall time. 29. What is AC bridges and what are the 2 conditions must be satisfied to make an bridge balance. These type of bridges are used to measure the impedances consisting of inductances and capacitances. Here the excitation voltage is AC. Example (i) maxwel bridge (ii) hay bridge (iii) schering bridge (iv) Anderson bridge (v) Owen bridge (vi) Wien bridge Conditions (i) (ii) Z1Zx = Z2 Z3 Θ1 + θx = θ2 + θ3 30. What is kelvin and kelvin double bridge? Wheatstone bridge is not suitable for measurement of very low resistance. Kelvin bridge is modification of W Bridge and is used to measure vales of resistance below 1 ohm. In kelvin double bridge it include second set of ratio arms. This circuit is mainly used for measuring very low resistance from 1 to 0.00001 ohm. 31. Why cannot induction motor run at synchronous speed? Induction motor always run at a speed of less than synchronous speed. If the rotor catch the MMF then ther will not be any relative velocity in turn the EMF produced is also zero and due to this there is no torque to produce further rotation 32. What do you meant by step down transformer? If number of turns in secondary winding is less than the number of turns in primary winding ( N2 < N1) then the transformer is said to be a step down transformer. 33. Draw the equivalent circuit of induction motor with respect to primary? 34. Mention some applications of synchronous motor? (i) (ii) For constant speed applications. Power factor improvement 35. What are the types of induction motor? (i) 3 phase induction motor. (a) squirrel cage (b) slip ring or wound rotor (ii) 1 phase induction motor. 36. Why should an alternator always runs at synchronous speed? Alternator is meant for producing three phase voltage at constant frequency. As the frequency of generated emf changes then it can’t feed to the transmission line. So to produce the emf at constant frequency it must rotate at synchronous speed. 37. Write down the emf equation of alternator? RMS value of EMF/Phase = 4.44 KpKd F ϕ T volts Where KP – pitch factor (or) coil span factor = cos (α/2) Kd – Distribution factor = (sin mβ/2) / (m sin β/2) F – frequency of induced EMF in Hz. Φ – flux/ pole in Webbers T – number of coils (or) turns / phase 38. Write down the EMF equation of transformer. RMS value of EMF = 4.44 F ϕ N (or) = 4.44 F N Bm A Where F- frequency; ϕ – flux ; N- number of turns; Bm – maximum flux density and A- Area of cross section of the core. There are two types of induction motor (i) squirrelcage induction motor (ii) wound rotor or slip ring induction motor. 42. Explain the working principle of induction motor. Three phase supply produces stator current in the winding which in turn produces rotating magnetic field. Rotating magnetic field produces emf if the mmf cuts the stationary conductor. Due to this rotor current will produce and this current produces the rotor field. The interaction of stator and rotor fields develops torque. 43. Write the torque equation of an 3 phase induction motor under running and Starting condition. Trunning = (KsE22R2)/ (R22 + sX22) Tstarting = (KE22R2)/ (R22 + X22) 45. What are the losses in a 3 phase induction motor? (i) Core loss (or) Iron Loss ( Constant loss) (ii) Copper loss (i) Mechanical loss ( friction loss + Windage loss) (Constant loss) 46. Explain the no load current in a 1 phase transformer. The current which flows in the primary when secondary is in open.(I 0) I0 = is comprises two components Iw and Iµ. Iw = I 0 cos ϕ0 and Iµ = I0 sin ϕ0 ; I0 = √𝐼𝑊 2 + 𝐼𝜇 2 47. Relate the slip power ( PCU), Air gap power ( P2) and Power developed in the rotor( Pm). 1: (1-S): S = P2 : Pm : Pcu 39. Define voltage regulation of alternator. The percentage regulation is defined as the ratio of change in terminal voltage from full load to no load rated terminal voltage. % Reg = (E0 – V ) / V *100 where E0 – No load terminal voltage and V- full load rated terminal voltage. 40. Why the core is laminated? To minimize the eddy current loss. 44. Draw the power flow diagram of a 3 phase induction motor. 41. Write the classifications of an 3 phase induction motor. Dc motor is a self regulating machine, back EMF which causes a changes in the load or armature current to produce the required torque or torque developed by the motor to satisfy the load requirements internally. 52. What are the different methods of excitation of generator? Self and separate excitation In self excitation it is classified as series, shunt and compound excitation. 53. What is a dc compound generator? A generator has both series and shunt field windings is called compound generator. It is classified as flat compound, over compound and under compound. 54. What are the functions of interpoles? To compensate the drop in main field flux and to provide better commutation. 55. What are the advantages of digital voltmeter? 48. Define shaft torque. Motor develop a torque called armature torque or gross torque(Tm) which is not maintained in the shaft. Due to some frictions the torque is lost(Tf). The remaining torque is available in the shaft(Tsh) Tsh= Tm-Tf 49 . Why series motor cannot be started without any load. Speed of a dc motor is expressed as N= Eb/ϕ, initially the value of flux is zero (i.e) field current or armature current is zero. Therefore the speed becomes infinite. To avoid this series motor should be started with some load. 50 Mention the applications of series and compound motor. Series Motor applications: cranes, hoist load, traction etc.. Compound motor applications: driving a heavy machine tools, shear, punching machines. 51. What is the significance of back emf? (i) (ii) (ii) (iii) (iv) high accurate reading can be taken. better resolution. High input impedance. Reading speed is very high. Digital output can be directly recorded. 56. List out the general characteristics of DVM. (i) input range. (ii) Absolute accuracy. (v) Resolution. (vi) Stability (vii) Input resistance (viii) Input capacitance (ix) Calibration. 57. What is Q meter. It is an instrument designed to measure some of the electrical properties of coils and capacitors. The basic principle is series resonance. Q = XL/R 58. What are the various sources of errors in Q meter? (i) due to distributed capacitance. (ii) (iii) (iv) due to residual inductance Due to shunt resistor. Due to conductance of Q voltmeter. 59. List the applications of Q meter. (i) to measure the Q of a coil. (ii) to measure the inductance and capacitance. (iii) To measure the distributed capacitance of a coil. (iv) To measure q and power factor of a dielectric material. (v) To measure the mutual inductance of coupled circuits. Etc…