Quiz: Electrical Engineering

Exam: DSSSB-JE

Topic: Miscellaneous

Each question carries 1 mark.

Negative marking: 1/4 mark

Time: 10 Minute

Q1. In an actual transformer the iron loss remains practically constant from no-load to full load because:

(a) Primary voltage remains constant

(b) Core flux remains practically constant

(c) Value of transformation ratio remains constant

(d) Permeability of transformer core remains constant

Q2. Which of the following losses in transformer is zero even at full load?

(a) Core loss

(b) Eddy current loss

(c) Hysteresis loss

(d) Friction loss

Q3. The residual magnetism of a self-excited dc generator is lost. To build up its emf again

(a) The armature connection must be reversed

(b) The field winding connection must be reversed

(c) Field winding must be excited by low voltage dc supply

(d) The field winding must be replaced

Q4. If the full load copper loss and iron loss of a transformer are 200 W and 100 W respectively. Their corresponding value at half load will be respectively:

(a) 200 W and 100 W

(b) 100 W and 100 W

(c) 200 W and 25 W

(d) 50 W and 100 W

Q5. In a circuit, the current lags behind the voltage by a phase difference of π/2, the circuit will contain which of the following:

(a) Only R

(b) Only L

(c) Only C

(d) R and C

Q6. In a series L-C-R circuit, resistance R = 20 Ω and the impedance Z = 20Ω. The phase difference between the current and the voltage is

(a) 0⁰

(b) 30⁰

(c) 45⁰

(d) 60⁰

Q7. For a 3-phase induction motor, if T is the starting torque when a DOL starter is used, then what is the starting torque when a star-delta starter is used?

(a) T/2

(b) T/3

(c) T/4

(d) T/√3

Q8. Reactance relay is used for protection in:

(a) Long transmission lines

(b) Short transmission lines

(c) Medium transmission lines

(d) Both long transmission lines and short transmission lines

Q9. The earthing electrode should be situated at a place at least ………. meters away from the building whose installation system is being earthed?

(a) 4

(b) 2.5

(c) 1.5

(d) 0.5

Q10. If A is the number of parallel paths and P is the number of poles, then the number of parallel paths in lap winding and in wave winding is

(a) A = P, A = 2

(b) A = 2P, A = P

(c) A = 2, A = P

(d)A = P, A = 2P

**SOLUTIONS**

S1. Ans.(b)

Sol. The flux set up in the core of the transformer remains constant from no-load to full load. Hence, iron loss is independent of the load and is known as constant losses.

S2. Ans.(d)

Sol. Because transformer is a static device.

S3. Ans.(c)

Sol. Residual magnetism is very important to start a generator. If its absent, generator will not be started. so, to build up its emf we need to excite the field winding by low voltage dc supply or in practical by a battery.

S4. Ans.(d)

Sol. In a transformer, copper loss depends on load.

At half load: copper loss(P_cu )=(1/2)^2×P_cu (full-load)=1/4×200=50 W

Iron losses are constant irrespective of load.

So, iron loss at half load=iron loss at full-load=100 W

S5. Ans.(b)

Sol. In an inductor, current lags behind the input voltage by a phase difference of π/2.

Current and voltage are in same phase in resistor whereas current leads the voltage by π/2 in a capacitor.

So, the circuit must contain an inductor only.

S6. Ans.(a)

Sol. For series resonant circuit: Z=R(i.e.X_L=X_C)

So, circuit is basically resistive in nature. Hence, phase difference between V and I is 0⁰.

S7. Ans.(b)

Sol. T_((star-delta))=1/3×T_DOL

S8. Ans.(b)

Sol. Relay used for different configuration of transmission lines……

Short transmission line: reactance relay

Medium transmission line: impedance relay

Long transmission line: mho relay

S9. Ans.(c)

Sol. The earthing electrode should be situated at a place at least 1.5 meters away from the building whose installation system is being earthed

The earthing electrode should always be placed in a vertical position inside the earth or pit so that it may be in contact with all the different earth layers.

S10. Ans.(a)

Sol. number of parallel paths for LAP winding: A = P

Number of parallel paths for wave winding: A = 2