Emec 1 question
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This document contains 20 multiple choice questions about electrical machines, including transformers, DC motors, and generators. The questions cover topics such as: how transformers work through inductive linkage between windings; why DC series motors must start under load; typical generation voltages; output waveforms from transformers driven by different input waveforms; characteristics of DC motor types like the armature magnetic field orientation; transformer voltage regulation points; effects of load on DC generator speed; phase angle differences between transformer windings; calculation of DC motor EMF; factors affecting DC motor torque; the purpose of commutators; effects of armature reaction and interpoles; load sharing between transformers; no load current phase relationship in transformers; and characteristics of hysteresis motors.
Emec 1 question
- 1. Electrical Machines Q.1 The two windings of a transformer are: (A) conductively linked. (B) inductively linked. (C) not linked at all. (D) electrically linked. Ans : B Q.2 The d.c. series motor should always be started with load because (A) at no load, it will rotate at dangerously high speed. (B) it will fail to start. (C) it will not develop high starting torque. (D) all are true. Ans: A Q.3 The generation voltage is usually (A) between 11 kV and 33 kV. (B) between 132 kV and 400 kV. (C) between 400 kV and 700 KV. (D) None of the above. Ans: A Q.4 If a transformer primary is energised from a square wave voltage source, its output voltage will be (A) A square wave. (B) A sine wave. (C) A triangular wave. (D) A pulse wave. Ans: A Q.5 In a d.c. machine, the armature mmf is (A) stationary w.r.t. armature. (B) rotating w.r.t. field. (C) stationary w.r.t. field. (D) rotating w.r.t. brushes. Ans: C Q.6 In a transformer the voltage regulation will be zero when it operates at (A) unity p.f. (B) leading p.f. (C) lagging p.f. (D) zero p.f. leading. Ans: B Q.7 The primary winding of a 220/6 V, 50 Hz transformer is energised from 110 V, 60 Hz supply. The secondary output voltage will be (A) 3.6 V. (B) 2.5 V. (C) 3.0 V. (D) 6.0 V. Ans: C Q.8 The emf induced in the primary of a transformer (A) is in phase with the flux. (B) lags behind the flux by 90 degree. (C) leads the flux by 90 degree. (D) is in phase opposition to that of flux. Ans: C Q.9 The most suitable servomotor for low power applications is (A) a dc series motor. (B) a dc shunt motor. (C) an ac two-phase induction motor. (D) an ac series motor. Ans: B
- 2. Q.10 A dc shunt generator has a speed of 800 rpm when delivering 20 A to the load at the terminal voltage of 220V. If the same machine is run as a motor it takes a line current of 20A from 220V supply. The speed of the machine as a motor will be (A) 800 rpm. (B) more than 800 rpm. (C) less than 800 rpm. (D) both higher or lower than 800 rpm. Ans: C Q.11 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. (A) delta side lags by -30. (B) star side lags by -30. (C) delta side leads by 30. (D) star side leads by -30. Ans: C Q.12 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? (A) 100 Volts. (B) 0.1 Volts. (C) 1 Volts. (D) 10 Volts. Ans: D Q.13The torque, in a _____________is proportional to the square of the armature current (A) DC shunt motor (B) stepper motor (C) 2-phase servomotor (D) DC series motor Ans: D Q. 14 A commutator in a d.c. generator (A) Reduces power loss in armature. (B) Reduces power loss in field circuit. (C) Converts the induced a.c armature voltage into direct voltage. (D) Is not necessary. Ans: C Q.15 A d.c. shunt motor runs at no load speed of 1140 r.p.m. At full load, armature reaction weakens the main flux by 5% whereas the armature circuit voltage drops by 10%. The motor full load speed in r.p.m. is (A) 1080 (B) 1203 (C) 1000 (D) 1200 Ans: A Q.16 The introduction of interpoles in between the main pole improves the performance of d.c. machines, because (A) The interpole produces additional flux to augment the developed torque. (B) The flux waveform is improved with reduction in harmonics. (C) The inequality of air flux on the top and bottom halves of armature is removed. (D) A counter e.m.f. is induced in the coil undergoing commutation. Ans: D
- 3. Q.17 In DC generators, armature reaction is produced actually by (A) Its field current. (B) Armature conductors. (C) Field pole winding. (D) Load current in armature. Ans: D Q.18 Two transformers operating in parallel will share the load depending upon their (A) Rating. (B) Leakage reactance. (C) Efficiency. (D) Per-unit impedance. Ans: A Q.19 No load current in a transformer: (A) lags the applied voltage by 90 (B) lags the applied voltage by somewhat less than 90 (C) leads the applied voltage by 90 (D) leads the applied voltage by somewhat less than 90 Ans: B Q.20 A hysteresis motor (A) is not a self-starting motor. (B) is a constant speed motor. (C) needs dc excitation. (D) can not be run in reverse speed. Ans: B