EASA Part 66 : Electrical Question15

Question Number. 37. A transformer has 500 turns on the primary and 100 turns on the secondary. The secondary supplies 20V and is connected to a 10 ohm load. The primary current is. Option A. 0.4 amps. Option B. 2 amps. Option C. 10 amps.

Correct Answer is. 0.4 amps. Explanation. Secondary current = V/R = 20/10 = 2A. Primary current = 2A/5 = 0.4A.

Question Number. 48. A near perfect 4:1 step up transformer has it's primary connected to 120VAC, if the secondary is connected to a load with 1600 ohms resistance, what is the current in the primary?. Option A. 1 amp. Option B. 0.3 amps. Option C. 1.2 amps.

Correct Answer is. 1.2 amps. Explanation. Secondary voltage = 480V, Secondary current = 480/1600 = 0.3A, Primary current = 4 * 0.3 = 1.2A.

Question Number. 41. A transformer to supply a 24V load with a 5:1 turns ratio will have what primary supply?. Option A. 120 V AC. Option B. 48 V AC. Option C. 120 V DC.

Correct Answer is. 120 V AC. Explanation. 5:1 is step down, so the supply voltage must be 5 * 24 = 120 VAC (AC because transformers don't work with DC).

Question Number. 53. In a 4:1 step-up transformer, 120V is applied to the primary, and the load on the secondary is 1600 Ohms. What is the current in the primary?. Option A. 0.075A. Option B. 1.2A. Option C. 3A.

Correct Answer is. 1.2A. Explanation. Voltage in secondary = 4 * 120 = 480V. Current in Secondary = 480/1600 (Ohm's Law) = 3/10A. Current in Primary = 4 * 3/10A = 12/10A = 1.2A (Remember that current is stepped down Primary to Secondary in a step-up transformer).

Question Number. 52. An aircraft transformer is needed to step up a voltage. What sort would be used?. Option A. Air core. Option B. Solid core. Option C. Laminated core.

Correct Answer is. Laminated core. Explanation. Laminated core transformers are most efficient.

Question Number. 33. The dots on the transformer symbol shown are called, and indicate. Option A. phasing dots, point that have the opposite polarity at the same moment in time. Option B. phasing dots, point that have the same polarity at the same moment in time. Option C. quadrature dots, points are 90o out of phase.

Correct Answer is. phasing dots, point that have the same polarity at the same moment in time. Explanation. NIL.

Question Number. 56. An autotransformer is. Option A. primary and secondary coils. Option B. primary coil only. Option C. primary and secondary coils on single bar.

Correct Answer is. primary and secondary coils on single bar. Explanation. NIL. http://www.hyperdictionary.com/dictionary/autotransformer

Question Number. 55. Aircraft autotransformers are of what type?. Option A. Open core. Option B. Twin core. Option C. Laminated core.

Correct Answer is. Laminated core. Explanation. Pg 60 Aircraft Electrical Systems Pallett.

Question Number. 18. Eddy currents in a transformer can be reduced by. Option A. using a single solid piece of metal. Option B. laminations. Option C. using a special non metallic material.

Correct Answer is. laminations. Explanation. NIL.

Question Number. 27. Copper losses in a transformer are caused by. Option A. the resistance in the windings. Option B. wasting of the copper wires due to friction. Option C. circulating currents through the transformer.

Correct Answer is. the resistance in the windings. Explanation. NIL.

Question Number. 12. A step up transformer has. Option A. two windings. Option B. one winding. Option C. three windings.

Correct Answer is. two windings. Explanation. NIL.

Question Number. 26. A transformer with a 5:1 ratio has a 24V output. What is the input?. Option A. 120 V DC. Option B. 4.8 V AC. Option C. 120 V AC.

Correct Answer is. 120 V AC. Explanation. 5:1 is step down. So input must be 5 * 24 = 120 V. Transformers only work on AC.

Question Number. 50. To supply a 24volt output with a transformer with a 5:1 turns ratio, the input required is. Option A. 4.8V. Option B. 24V. Option C. 120V.

Correct Answer is. 120V. Explanation. Assuming this is a step-down transformer (i.e 5:1 means Primary:Secondary).

Question Number. 14. The secondary winding of a transformer has an impedance of 10 ohms and 1500 turns. For the primary winding to have an impedance of 1 kilohm, how many turns must it have?. Option A. 15,000. Option B. 1,500,000. Option C. 150,000

Correct Answer is. 15,000. Explanation. Turns ratio = square root of impedance ratio.

Question Number. 10. The secondary coil of a transformer has 1500 turns and 10 Ω resistance. The primary coil has 1kΩ resistance. How many turns does the primary coil have?. Option A. 15,000. Option B. 1,500,000. Option C. 150,000

Correct Answer is. 15,000. Explanation. Turns ratio = square root of the impedance ratio.

Question Number. 38. A transformer has 1200 turns primary and 4800 turns secondary. What value of load resistance would be required to give a reflected impedance of 1000 ohms?. Option A. 4 kilohm. Option B. 1 kilohm. Option C. 16 kilohm.

Correct Answer is. 16 kilohm. Explanation. Impedance matching formula, Turns Ratio = √(Impedance Ratio).

Question Number. 46. In a 4:1 step up transformer, what would give a reflected impedance of 1Kilohm?. Option A. 1 k ohm. Option B. 16k ohm. Option C. 4k ohm.

Correct Answer is. 16k ohm. Explanation. Turns ratio = √(impedance ratio).

Question Number. 36. A transformer has 500 turns on the primary and 100 turns on the secondary. When connected to a 100 V supply, the output is. Option A. 500 V. Option B. 25 V. Option C. 20 V.

Correct Answer is. 20 V. Explanation. 5:1 step down. 100/5 = 20V.

Question Number. 13. In a transformer core loss is 200 W and copper loss is 220W at no load. What is the core loss at full load?. Option A. 200 W. Option B. 0 W. Option C. 220 W.

Correct Answer is. 200 W. Explanation. Since the flux is (almost) the same when on load as it is off load, the core loss is the same. (However, copper loss will be greater ON load because of the higher current).

Question Number. 9. A 4:1 step down transformer draws 115 V and 1 A. The output power will be. Option A. 460 V at 0.25 A. Option B. 28.5 V at 16 A. Option C. 28.5 V at 4 A.

Correct Answer is. 28.5 V at 4 A. Explanation. NIL.

Question Number. 45. A 3-phase star-delta wound transformer gives. Option A. 30° phase shift. Option B. 90° phase shift. Option C. no phase shift.

Correct Answer is. 30° phase shift. Explanation. A star-delta wound transformer gives a 30 degree phase shift.

Question Number. 42. A transformer has 2000V, 500 turns on the primary and 50 turns on the secondary. What would be the line voltage if the transformer were Delta/Star connected?. Option A. 200V. Option B. 450V. Option C. 346V.

Correct Answer is. 346V. Explanation. It is a 10:1 step down transformer. Phase voltage in secondary = 200V. With star wound secondary, line voltage = 1.73 * 200 = 346V.

Question Number. 3. A transformer has an input of 400V and a ratio of 2:1. If the transformer is delta / star wound, what will the line voltage output be?. Option A. 115V. Option B. 346V. Option C. 200V.

Correct Answer is. 346V. Explanation. Star wound output - line voltage is 1.73 * phase voltage.

Question Number. 47. The input is 115V and is applied across the 300 turn portion of the autotransformer shown. What is the output?. Option A. 115V. Option B. 28V. Option C. 460V.

Correct Answer is. 460V. Explanation. Autotransformer 300:1200 ratio 1:4. Output = 4 * 115V.

Question Number. 31. What is the reflected impedance in the primary of the transformer circuit shown?. Option A. 20 ohms. Option B. 500 ohms. Option C. 50 ohms.

Correct Answer is. 500 ohms. Explanation. Turns ratio = √(turns ratio). 10/1 = √Zin/5. 100 = Zin/5 Zin = 500.

Question Number. 16. Transformer copper loss on full load is 220 Watts. On half load the loss will be. Option A. 440 Watts. Option B. 110 Watts. Option C. 55 Watts.

Correct Answer is. 55 Watts. Explanation. Since copper loss is given by I2R, if the load doubles, the copper loss will quadruple. So it must have been 55 Watts on half load to be 220Watts on full load. (Note: Core loss does not change with load).

Question Number. 28. The copper loss on a single phase transformer is 25 Watts on full load. What will it be on half load?. Option A. 5 Watts. Option B. 6.25 Watts. Option C. 12.5 Watts.

Correct Answer is. 6.25 Watts. Explanation. Copper loss = I2 R . So halve the current (load) and you reduce the copper loss to 1/4.

Question Number. 5. A Transformer has 4500 secondary turns and 750 primary turns. Its turns ratio is. Option A. 1:6. Option B. 1/6. Option C. 6:1.

Correct Answer is. 6:1. Explanation. Turns ratio is Secondary:Primary (using British notation).

Question Number. 4. A Transformer with 115V primary voltage and a ratio of 5:1 is supplying a landing light (load 24V 45 amps) is used, what is the current drawn?. Option A. 205 amps. Option B. 4.5 amps. Option C. 9 amps.

Correct Answer is. 9 amps. Explanation. When the voltage is stepped DOWN, the current has been stepped UP. So with a 45A output current on a 5:1 transformer.

Question Number. 54. A 10:1 step up power transformer has 120 V, and 3A flowing through the primary, and 0.285 A flowing through the secondary what is the efficiency?. Option A. 90%. Option B. 95%. Option C. 85%.

Correct Answer is. 95%. Explanation. Power in primary = 120 * 3 = 360W. Power in secondary = 1200 * 0.285 = 342W. 342/360 = 0.95.

Question Number. 51. What would you use for the core in a generator current transformer?. Option A. Air core. Option B. Laminated core. Option C. Steel Core.

Correct Answer is. Air core. Explanation. A current transformer uses the generator output cable as its primary, so it must have a hollow (air) core.

Question Number. 25. What causes a transformer to be noisy?. Option A. High core losses. Option B. An air gap in the core. Option C. High coil resistance.

Correct Answer is. An air gap in the core. Explanation. An air gap in the magnetic core causes noise.

Question Number. 58. How do you reduce Hysteresis loss of a transformer?. Option A. Wind the coils one over the other and by careful design of the transformer. Option B. By laminating the core. Option C. By using a core material which can be easily demagnetised.

Correct Answer is. By laminating the core. OR By usinga core material which can be easily demagnetised. Explanation. NIL.

Question Number. 1. How many turns does the secondary winding of a 2:1 step-up transformer have?. Option A. Less than primary. Option B. More than primary. Option C. Less turns but with thicker wire.

Correct Answer is. More than primary. Explanation. Clearly using British notation of Secondary:Primary.

Question Number. 40. What voltage can the neutral line carry in a star transformer?. Option A. Zero voltage. Option B. Phase voltage. Option C. Combined line voltages.

Correct Answer is. Zero voltage. Explanation. The neutral line is grounded.

Question Number. 39. An autotransformer windings are. Option A. wired in parallel. Option B. on separate formers. Option C. connected.

Correct Answer is. connected. Explanation. NIL.

Question Number. 7. The primary winding of a 3-phase transformer. Option A. is delta wound. Option B. could be either delta or star wound. Option C. is star wound.

Correct Answer is. could be either delta or star wound. Explanation. NIL.

Question Number. 15. Eddy currents in a transformer core are reduced by. Option A. decreasing the thickness of the laminates. Option B. making the core from a solid block. Option C. increasing the thickness of the laminates.

Correct Answer is. decreasing the thickness of the laminates. Explanation. To reduce eddy currents, the thinner the laminates, the better.

Question Number. 29. A transformer is designed to operate at 60 Hz. It is connected across a 50 Hz supply. The primary windings would. Option A. draw more current. Option B. draw the same current. Option C. draw less current.

Correct Answer is. draw more current. Explanation. The inductive reactance decreases with a decreased frequency - so it will draw MORE current.

Question Number. 11. In a star wound primary transformer, how could you wind the secondary winding?. Option A. star only. Option B. either delta or star. Option C. delta only.

Correct Answer is. either delta or star. Explanation. The opposite type of winding is preferred because this reduces interference between the two. However, the connecting circuits may dictate which is used - either way, any combination is possible.

Question Number. 23. The area of a hysteresis loop is proportional to the. Option A. energy dissipated in putting the material through a complete cycle of magnetization and demagnetization. Option B. relative permeability of the material being magnetized. Option C. energy absorbed in completely demagnetizing the material.

Correct Answer is. energy dissipated in putting the material through a complete cycle of magnetization and demagnetization. Explanation. NIL.

Question Number. 24. In transformer operation, hysteresis losses are. Option A. current overcoming coil resistance. Option B. induced current in the iron core. Option C. energy wasted in reversing core magnetization.

Correct Answer is. energy wasted in reversing core magnetization. Explanation. NIL.

Question Number. 22. The voltage in a transformer secondary coil that contains twice as many loops as the primary coil will be. Option A. less and the amperage greater than in the primary coil. Option B. greater and the amperage greater than in the primary coil. Option C. greater and the amperage less than in the primary coil.

Correct Answer is. greater and the amperage greater than in the primary coil. OR greater and the amperage less than in the primary coil. Explanation. It is a step up transformer. The voltage is therefore stepped up and the current stepped down by the same ratio. It must do this because the power (IV) is the same in the primary and secondary (neglecting efficiency).

Question Number. 44. Eddy current losses in a transformer are. Option A. magnetic flows induced into the core. Option B. current used in magnetising the core. Option C. hysteresis losses.

Correct Answer is. magnetic flows induced into the core. Explanation.

Question Number. 6. The point of coercivity occurs when the. Option A. magnetic flux is zero even though a magnetizing force is being applied. Option B. magnetizing force reaches a peak positive or negative value. Option C. magnetizing force applied causes the material to become magnetically saturated.

Correct Answer is. magnetic flux is zero even though a magnetizing force is being applied. Explanation. NIL.

Question Number. 32. A transformer rated at a frequency of 200 Hz. Option A. may be used at any other frequency. Option B. may only be used at the rated frequency or above. Option C. may only be used at the rated frequency or below.

Correct Answer is. may only be used at the rated frequency or above. Explanation. Since the reactance of the primary coil decreases with decreasing frequency, using below the rated frequency may damage it due to increased current flow.

Question Number. 34. A transformer is built with a material having a. Option A. high coercive force. Option B. wide hysteresis loop. Option C. narrow hysteresis loop.

Correct Answer is. narrow hysteresis loop. Explanation. The area of the hysteresis loop is the power lost in repeatedly reversing the magnetism.

Question Number. 19. What is the ratio of turns between the primary coil winding and the secondary coil winding of a transformer designed to triple its input voltage?. Option A. primary will have one-third as many turns as its secondary. Option B. primary will have three times as many turns as its secondary. Option C. primary will have twice as many turns as its secondary.

Correct Answer is. primary will have one-third as many turns as its secondary. Explanation. NIL.

Question Number. 2. To reduce eddy currents in a transformer you would. Option A. increase the thickness of laminations in the magnetic core. Option B. reduce the number of turns on the primary winding. Option C. reduce the thickness of laminations in the magnetic core.

Correct Answer is. reduce the thickness of laminations in the magnetic core. Explanation. Read up on transformer losses.

Question Number. 8. In a transformer, if the rate of change of current increases, the mutual inductance will. Option A. decrease. Option B. remain the same. Option C. increase.

Correct Answer is. remain the same. Explanation. Transformer ratio (which is dependant upon the mutual inductance) is not a function of frequency.

Question Number. 43. The line voltage in a Wye wound transformer is. Option A. root 3 / phase voltage. Option B. root 3 * phase voltage. Option C. the same as phase voltage.

Correct Answer is. root 3 * phase voltage. Explanation. Line voltage = root 3 * phase voltage in a Wye (star) wound transformer.

Question Number. 20. A transformer should always have its. Option A. primary shorted or off load. Option B. secondary shorted or on load. Option C. secondary open circuit or on load.

Correct Answer is. secondary open circuit or on load. Explanation. If you short out the secondary winding, the primary will burn out.

Question Number. 21. Which of the following would be used to calculate transformer turns ratio?. Option A. secondary turns ÷ primary turns. Option B. primary turns × secondary turns. Option C. primary turns ÷ secondary turns.

Correct Answer is. secondary turns ÷ primary turns. Explanation. If the American version of turns ratio is used it is Primary:Secondary. English notation is Secondary:Primary.

Question Number. 30. A current transformer, when disconnected from a circuit must be. Option A. left open circuit. Option B. short circuited. Option C. have a resistor connected across the terminals.

Correct Answer is. short circuited. Explanation. Current transformers must be short circuited when disconnected.

Question Number. 35. In a circuit which possesses mutual inductance, current is increased to the primary. Option A. the induced EMF in the secondary produces a current in the secondary circuit which opposes the increase in flux due to the increase of current in the primary. Option B. the induced EMF in the secondary produces a current in the secondary circuit which assists the increase in flux due to the increase of current in the primary. Option C. the induced EMF in the secondary produces a current in the secondary circuit whose flux has no effect on the flux in the primary.

Correct Answer is. the induced EMF in the secondary produces a current in the secondary circuit which opposes the increase in flux due to the increase of current in the primary. Explanation. NIL.

Question Number. 17. How can it be determined if a transformer winding has some of its turns shorted together?. Option A. measure the input voltage with an ohmmeter. Option B. the output voltage will be high. Option C. the transformer will get hot in normal operation.

Correct Answer is. the transformer will get hot in normal operation. Explanation. Copper loss (heat) is given by I2R. If some turns are shorted together, the resistance (R) goes down and the current (I) goes up. But since the current is squared, the heat loss will increase on a square law.

Question Number. 49. Flux losses in a transformer can be prevented by. Option A. winding the primary on top of the secondary. Option B. the use of primary shielding. Option C. laminating the core.

Correct Answer is. winding the primary on top of the secondary. Explanation. Flux losses are due to primary flux lines not going through the secondary.

Question Number. 57. The windings on a autotransformer are. Option A. wound on separate legs of core. Option B. wound with no magnetic core. Option C. wound with the primary over the secondary.

Correct Answer is. wound with no magnetic core. OR wound with the primary over the secondary. Explanation. NIL.