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DMRC Paper 
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Re: DMRC Paper
I am providing you the sample question paper of Delhi Metro Rail Corporation (DMRC) Exam DMRC Exam question paper Technical 1. The ability of a material to remain magnetized after removal of the magnetizing force is known as Permeability reluctance hysteresis retentivity AnsD 2. When a solenoid is activated, the force that moves the plunger is an electromagnetic field a permanent magnetic field varying voltage a steady current AnsA 3. Which of the following capacitors is polarized mica ceramic plasticfilm electrolytic AnsD 4. In a series resonant bandpass filter, a lower value of Q results in a higher resonant frequency a smaller bandwidth a higher impedance a larger bandwidth AnsD 5. A steadystate condition is reached when the output voltage reaches the average value of the input voltage the output voltage reaches the input voltage the output voltage reaches approximately 63% of the input voltage the output voltage reaches the effective value of the input voltage AnsA 6. An RC differentiator acts as a lowpass filter highpass filter bandpass filter bandstop filter AnsB 7. A balanced threephase, 50 Hz voltage is applied to a 3 phase, 4 pole, induction motor. When the motor is delivering rated output, the slip is found to be 0.05. The speed of the rotor m.m.f. relative to the rotor structure is 1500 r.p.m. 1425 r.p.m. 25 r.p.m. 75 r.p.m. AnsD Explanation NS = 120f /P = 120 x 50 /4 =1500rpm N = NS ( 1s) = 1500 (10.05) = 1425 \relative speed = 1500 1425 = 75 rpm 8. A ceiling fan uses splitphase motor. capacitor start and capacitor run motor universal motor. capacitor start motor. AnsD 9. The drive motor used in a mixergrinder is a dc motor. induction motor. synchronous motor. universal motor. AnsD 10. A 15 stepup transformer has 120V across the primary and 600 ohms resistance across the secondary. Assuming 100% efficiency, the primary current equals 0.2 Amp. 5 Amps 10 Amps. 20 Amps. AnsA Explanation I1= V1 /R1 = 120/600 = 0.2 (h = 100%, losses are zero \V1 = VR = I1R1) 11. A 50 Hz, 3phase induction motor has a full load speed of 1440 r.p.m. The number of poles of the motor are 4. 6. 12 8. AnsA Explanation N= Ns (1S) = NS NS x S 1440 = Ns (1S) Ns = 1440 / (1S) Ns = (120 f/ p) = 120 x 50/p = 6000 p Ns will be closer to N i.e 1440 When P=2 ; Ns = 3000 rpm , not close to N When P=4 ; Ns = 1500 rpm , it is closer to N Therefore P =4 for N=1440 12. In a 3phase synchronous motor the speed of stator MMF is always more than that of rotor MMF. the speed of stator MMF is always less than that of rotor MMF. the speed of stator MMF is synchronous speed while that of rotor MMF is zero rotor and stator MMF are stationary with respect to each other. AnsD Explanation Because, Motor is magnetically locked into position with stator, the rotor poles are engaged with stator poles and both run synchronously in same direction Therefore, rotor & stator mmf are stationary w.r.t each other. 13. In a three phase transformer, if the primary side is connected in star and secondary side is connected in delta, what is the angle difference between phase voltage in the two cases. delta side lags by 30. star side lags by 30. delta side leads by 30. star side leads by 30. AnsC Explanation This is vector group and has +30 displacement. Therefore, delta side leads by +30. 14. Slip of the induction machine is 0.02 and the stator supply frequency is 50 Hz. What will be the frequency of the rotor induced emf? 10 Hz. 50 Hz. 1 Hz. 2500 Hz. AnsC Explanation Given  s = 0.02 ; f = 50 Hz Therefore, frequency of rotor induced emf = s f = 0.02 50 = 1.0 Hz 15. A 4 pole lap wound dc shunt motor rotates at the speed of 1500 rpm, has a flux of 0.4 mWb and the total number of conductors are 1000. What is the value of emf? 100 Volts. 0.1 Volts. 1 Volts. 10 Volts. AnsD Explanation Given N = 1500 rpm, F = 0.4 mWb, Z = 1000, P = 4, & A= 4 Therefore, Eb = NFPZ / 60 A = 1500 0.4 4 x 1000 103 / 60 4 = 60/6 = 10 volts 16. A 3 stack stepper motor with 12 numbers of rotor teeth has a step angle of ____________. 120 8? 24? 10? AnsD Explanation Given m = 3, Nr = 12 Step angle = 360 / m x Nr = 360 /3 x 12 = 10? 17.Oilfilled cable has a working stress of __________ kV/mm 10 12 13 15 AnsD Explanation This is defined by dielectric strength of mineral oil i.e. 15 kV/mm. 18. The rotor frequency for a 3 phase 1000 RPM 6 pole induction motor with a slip of 0.04 is________Hz 8 4 6 2 AnsD Explanation Given N=1000 rpm ; P= 6; s= 0.04; and f = N_P/ 120 = 1000_6/120 = 50 Hz Rotor frequency fr=s_f = 0.04_50 = 2.0 Hz 19. The synchronous speed for a 3 phase 6pole induction motor is 1200 rpm. If the number of poles is now reduced to 4 with the frequency remaining constant, the rotor speed with a slip of 5% will be _________. 1690 rpm 1750 rpm 1500 rpm 1710 rpm AnsD Explanation Given  Ns1 =1200 , P1= 6, P2 = 4, s = 0.05, Frequency f = Ns_P/120 = 120_6/120 = 60 Hz rotor frequency f/ = s.f = 0.05 _60 = 3.0 Hz Now, Ns2 = 120 _60 /4 = 1800 and Ns N = 120 f / P2 Therefore, N=Ns 120 f / P2 = 1800120_0.05_60/4 = 180090 = 1710 20. The efficiency of a transformer is mainly dependent on core losses. copper losses stray losses. dielectric losses. AnsA 21. A synchronous motor is operating on noload at unity power factor. If the field current is increased, power factor will become Leading & current will decrease Lagging & current will increase. Lagging & current will decrease Leading & current will increase. AnsA Explanation Initially synchronous motor is operating at no load and unity power factor. When field current increases, the excitation will increase. Therefore, p.f will be leading and current will be I Cos??< I 22. If the phase angle of the voltage coil of a directional relay is o 50 , the maximum torque angle of the relay is 130 100 50 25 AnsC Explanation Torque _ Power Power _ Voltage Therefore, It has same angle as V ha 23. A 220/440 V, 50 Hz, 5 KVA, single phase transformer operates on 220V, 40Hz supply with secondary winding open circuited. Then Both eddy current and hysteresis losses decreases. Both eddy current and hysteresis losses increases. Eddy current loss remains the same but hysteresis loss increases. Eddy current loss increases but hysteresis loss remains the same. AnsA Explanation Wh = khfBm 1.6 and We = kef2Bm 2.k Therefore, hysteresis and eddy current losses will be decreased when frequency decreases. 24. Electric ovens using heating elements of _______________ can produce temperature upto 3000C. Nickel Graphite Chromium Iron AnsC Explanation Chromium has high melting point. 25 Electric resistance seam welding uses __________ electrodes. Pointed Disc. Flat Domed AnsB Explanation Disc type electrodes are used for electric resistance seam welding. 11. The line trap unit employed in carrier current relaying offers high impedance to 50 Hz power frequency signal offers high impedance to carrier frequency signal offers low impedance to carrier frequency signal Both (A) & (C) AnsB Explanation The line trap unit employed in carrier current relaying offers high impedance to carrier frequency signal. Because carrier frequency range is 35 km 500 kHz XL = 2_ f l Where f increases XL will also increases 26 The temperature of resistance furnaces can be controlled by changing the applied voltage number of heating elements circuit configuration All of the above AnsD Explanation Temperature of resistance furnaces can be controlled by changing either applied voltage or by number of heating elements or by circuit configuration. 27 A single phase Hysteresis motor can run at synchronous speed only can run at sub synchronous speed only can run at synchronous and super synchronous speed can run at synchronous and sub synchronous speed AnsA Explanation The rotor revolves synchronously because the rotor poles magnetically lock up with the revolving stator poles of opposite polarity 28. The armature of a dc machine is laminated to reduce Eddy current loss Hysteresis loss copper losses friction and windage losses AnsA Explanation Thinner the laminations, greater is the resistance offered to the induced e.m.f., smaller the current and hence lesser the I2R loss in the core. 29. For LV applications (below 1 kV), ______________ cables are used. Paper insulated. Plastic. Single core cables. Oil filled. AnsC 
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