# Recent questions and answers in Transformer

Losses which occur in rotating electric machines and do not occur in transformers are?

The efficiencies of transformers compared with that of electric motors of the same power are?

what do not change in transformer?

Impulse testing of transformers is done to determine the ability of: A) bushings to withstand vibrations B) insulation to withstand transient voltages C) windings to withstand voltage fluctuations D) all of the above

Full load copper loss in a transformer is 1600 watts. At half load the copper loss will be: A) 6400 watt B) 1600 watt C) 800 watt D) 400 watt

Which of the following company is associated with manufacture of high capacity transformers in India? A) M/S National Thermal Power Corporation Limited B) M/S Bharat Heavy Electricals Limited C) M/S Heavy Engineering Corporation Limited D) M/S Electricity Authority of India

If the frequency of input voltage of a transformer is increased keeping the magnitude of voltage unchanged, then: A) both hysteresis loss and eddy current loss in the core will increase B) hysteresis loss will increase but eddy current loss will decrease C) hysteresis loss will decrease but eddy current loss will increase D) hysteresis loss will decrease but eddy current loss will remain unchanged

The current transformer that is used to measure a 100 A current by 5 A ammeter is a_____.

A current transformer must at all times have (a) a secondary load (b) short circuited secondary (c) either (a) or (b) (d) none of these

The maximum flux in a transformer is φm when primary voltage is V and the frequency is f. If the primary voltage is 2 V and frequency is f/2, the flux will be (a) 4 φm (b) 0.25 φm (c) φm (d) 0.5 φm

The voltage applied to the H.V. side of a transformer during S.C. test is 2% of the rated voltage. The core loss as the fraction of its rated core loss is (a) 4% (b) 0.4% (c) 0.25% (d) 0.04%

Instruments transformers are used on a.c. circuits for extending the range of (a) ammeters (b) voltmeters (c) wattmeters (d) all of these

The voltage regulation of a transformer, with ohmic drop 2% and reactance drop 5% of the voltage at 0.8 lagging power factor, will be (a) 7% (b) 5.6% (c) 5.3% (d) 4.6%

Three phase step-up transformer installed at power house, just before commencement of transmission line, have:

When two transformer of different kVA rating are connected in parallel they share the load in proportion to their respective kVA rating only when their:

The potential transformers Nominal Ratio is defined as the ratio of:

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.

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.

Two transformers with identical voltage ratings are working in parallel to supply common load. The percentage impedance of one transformer is higher compared to that of other. The load sharing between the two transformer will: (1) Be proportional to their percentage impedance (2) Be independent of their percentage impedance (3) Be inversely proportional of their respective impedance (4) Depend on the resistance to leakage reactance ratio of each transformer

Which of the following are constant losses in Transformers? (1) Winding losses (2) Core losses (3) Both Winding & core losses (4) None of these

The core flux of a practical transformer with a resistive load  A) Is strictly constant with load changes B) Increases linearly with load C) Increases as the square root of the load D) Decrease with increased load

The function of oil in a transformer is to provide  A) Insulation and cooling B) Protection against Lightning C) Protection against short circuit D) Lubrication

A single-phase transformer has a turns ratio of 1 :2 and is connected to a purely resistive load as shown in the figure. The magnetizing current drawn is 1A, and the secondary current is 1A. Ifcore losses and leakage reactances are neglected, the primary current is  A)1.41A B)2A C) 2.24A D)3A

A simple phase transformer has a maximum efficiency of 90% at full load and unity power factor. Efficiency at half load at the same power factor is  (A) 86.7% (B) 88.26% (C) 88.9% (D) 87.8%

A 500 kVA, 3-phase transformer has iron losses of 300 W and full load copper losses of 600 W. The percentage load at which the transformer is expected to have maximum efficiency is  (A) 50.0% (B) 70.7% (C) 141.4% (D) 200.0%

Which three-phase connection can be used in a transformer to introduce a phase difference of 30o between its output and corresponding input line voltages  (A) Star-Star (B) Star-Delta (C) Delta-Delta (D) Delta-Zigzag

Two transformers are to be operated in parallel such that they share load in proportion to their kVA ratings. The rating of the first transformer is 500 kVA ratings. The rating of the first transformer is 500 kVA and its pu leakage impedance is 0.05 pu. If the rating of second transformer is 250 kVA, its pu leakage impedance is  (A) 0.20 (B) 0.10 (C) 0.05 (D) 0.025

A three-phase, three-stack, variable reluctance step motor has 20 poles on each rotor and stator stack. The step angle of this step motor is  (A) 3o (B) 6o (C) 9o (D) 18o

A single-phase, 50 kVA, 250 V/500 V two winding transformer has an efficiency of 95% at full load, unity power factor. If it is re-configured as a 500 V/750 V auto-transformer, its efficiency at its new rated load at unity power factor will be  (A) 95.752% (B) 97.851% (C) 98.276% (D) 99.241%

A single-phase transformer has no-load loss of 64 W, as obtained from an open circuit test. When a short-circuit test is performed on it with 90% of the rated currents flowing in its both LV and HV windings, he measured loss is 81 W. The transformer has maximum efficiency when operated at  (A) 50.0% of the rated current (B) 64.0% of the rated current (C) 80.0% of the rated current (D) 88.8% of the rated current

In transformers, which ofthe following statements is valid?  A) In an open circuit test, copper losses are obtained while in short circuit test, core losses are obtained B) In an open circuit test, current is drawn at high power factor C) In a short circuit test, current is drawn at zero power factor D) In an open circuit test, current is drawn at low power factor

Which losses in a transformer varies significantly with load  a. Hysteresis losses b. Eddy current losses c. Copper losses d. Core losses

For a pulse transformer, the material used for its core and the possible turn ration from primary to secondary are respectively   (A) ferrite : 20 : 1 (B) laminated iron : 1 :1 (C) ferrite : 1 : 1 (D) powered iron : 1: 1

The transformer utilization factor of full wave bridge rectifier is   (A) 0.812 (B) 0.286 (C) 0.693 (D) 0.782

In a power transformer, the fundamental frequency of the hum arising due to magnetostriction is  (a) equal to the line frequency (b) double the line frequency (c) 4 times the line frequency (d) not related to the line frequency

A transformer is used to

If the secondary turns of a transformer are doubled and at the same time primary voltage is reduced to half then the secondary voltage will

In a step-up or step-down transformer, electrical power is transferred from one circuit to another without change in

Transformer oil tester is used to know • Permittivity • Permeability • Dielectric strength • Resistance of the oil

Which of the following is not a basic element of a transformer ?  1. Core 2. Primary winding 3. Secondary winding 4. Mutual flux

A transformer transforms  1. Frequency 2. Voltage 3. Current 4. Voltage and current

The hysteresis and eddy current losses of a single phase transformer working on 200 V, 50 Hz supply are Ph and Pe respectively. The percentage decrease in these when operated on a 160 V, 40 Hz supply are   (a) 32, 36 (b) 20, 36 (c) 25, 50 (d) 40, 80

For a constant load current, transformer maximum efficiency occurs at   (a) any power factor (b) zero power factor leading (c) zero power factor lagging (d) unity power factor

In a transformer, low voltage winding is placed near the core in case of concentric windings so as to  1. Reduce the leakage flux 2. Reduce the insulation requirement 3. Reduce the risk of voltage shock in case of insulation breakdown 4. Reduce the core loss   From these, the correct answer is (a) 1, 2, 3 (b) 2, 3, 4 (c) 1, 2, 3, 4 (d) 2, 3

In a transformer, zero voltage regulation at full load is   (a)Not possible   (b) Possible at unity power factor load   (c) Possible at leading power factor load   (d) Possible at lagging power factor load