Quiz: Electrical Engineering

Exam: UPSSSC- JE

Topic: Miscellaneous

Each question carries 1 mark.

Negative marking: 1/4 mark

Time: 10 Minute

Q1. The voltage regulation of a short transmission line will be maximum when the

value of tan ϕ_Rfor lagging power factor is

(a) unity

(b) zero

(c) R/X_L

(d) X_L/R

L1Difficulty 3

QTags Power generation, transmission and distribution

QCreator Devbrat Kumar

Q2. In a.c. system, the skin effect

(a) reduces effective area of conductor

(b) increases resistance of conductor

(c) causes greater power loss

(d) all of the above

L1Difficulty 3

QTags Power generation, transmission and distribution

QCreator Devbrat Kumar

Q3. To measure a. c. as well as d. c. power, we use

(a) induction wattmeter

(b) dynamometer wattmeter

(c) sometimes induction and sometimes dynamometer wattmeter

(d) none of the above

L1Difficulty 3

QTags Measurement and measuring instrument

QCreator Devbrat Kumar

Q4. A 230 V, 50 Hz single-phase energy meter has a load current of 10 A at p.f. of 0.8 lagging. The energy consumed by the load in 2 minutes is

(a) 1.2 kWh

(b) 0.06 kWh

(c) 2.4 kWh

(d) 4.2 kWh

L1Difficulty 3

QTags Measurement and measuring instrument

QCreator Devbrat Kumar

Q5. 1 kWh = …………. kcal

(a) 610

(b) 746

(c) 520

(d) 860

L1Difficulty 3

QTags Electrical

QCreator Devbrat Kumar

Q6. A 100mA meter has accuracy of +2%. Its accuracy while reading 50mA will be:

(a) +1%

(b) +2%

(c) +4%

(d) +20%

L1Difficulty 3

QTags Measurement and measuring instrument

QCreator Devbrat Kumar

Q7. Field control of a DC shunt motor gives–

(a) Constant kW drive

(b) Constant torque drive

(c) Constant speed drive

(d) Variable load speed drive

L1Difficulty 3

QTags DC Motor

QCreator Devbrat Kumar

Q8. The ‘cogging’ of an induction motor can be avoided by:

(a) proper ventilation

(b) using DOL starter

(c) auto-transformer starter

(d) having number of rotor slots more or less than the number of stator slots (not equal)

L1Difficulty 3

QTags 3-Phase induction motor

QCreator Devbrat Kumar

Q9. If an induction motor with certain ratio of rotor to stator slots, runs at 1/7 of the normal speed, the phenomenon will be termed as:

(a) humming

(b) hinting

(c) crawling

(d) cogging

L1Difficulty 3

QTags 3-Phase induction motor

QCreator Devbrat Kumar

Q10. One lux is:

(a) 1 lumen/m²

(b) 4π lumen/mt²

(c) 1 lumen mt

(d) 1 candela

L1Difficulty 3

QTags Utilization of electrical energy

QCreator Devbrat Kumar

SOLUTIONS

S1. Ans.(d)

Sol. For lagging power factor, we have,

Voltage regulation = V_S-V_R=IR cos〖ϕ_R+I X_L sin〖ϕ_R 〗 〗

For maximum regulation, d/(dϕ_R ) (IR cos ϕ_R+I X_L sin〖ϕ_R 〗)=0

Or –IR sin ϕ_R+I X_L cos〖ϕ_R=0〗

∴ tan ϕ_R=X_L/R

S2. Ans.(d)

S3. Ans.(b)

Sol. Induction wattmeters have low accuracy and high-power consumption. Therefore, they are inferior to dynamometer wattmeter. For this reason, dynamometer wattmeters are almost universally used for the measurement of a.c. as well as d.c. power.

S4. Ans.(b)

Sol. Energy consumed by the load in 2 minutes

=(V I cosϕ)/1000×t=(230 × 10 × 0.8)/1000×2/60 = 0.06 k Wh

S5. Ans.(d)

Sol. 1 kWh = 1000 watts × 3600 seconds = 36 × 10^5 joules

=(36×10^5)/(4.18 ) cal = 860 × 10³ cal = 860 kcal

S6. Ans.(c)

Sol. Error = 100 mA ×(±2)/100= ±2mA

% error = (±2mA)/50mA×100= ±4%

S7. Ans.(a)

Sol. There are 3 types of speed control for dc drives: –

(i) Armature voltage control

(ii) Field flux control

(iii) Armature resistance control

We know▭(N∝1/ϕ)

Armature voltage control technique gives high efficiency but below base speed.

So, above base speed, Field flux control is used, where by weakening the field, speed above base speed can be achieved because these two are inversely proportional to each other.

S8. Ans.(d)

Sol. “Cogging” and “Crawling” are associated with squirrel cage induction motors.

Cogging comes into picture when motor refuse to start at all cogging is caused by-

Low supply voltage

Stator slots = rotor slots (i.e. magnetic locking)

Harmonic frequency coincides with the slot frequency.

S9. Ans.(c)

Sol. Crawling is associated with squirrel cage induction motor due to which motor has a tendency to run at very low speed compared to its synchronous speed. The fundamental runs at synchronous speed and harmonics 3^rd,5^th,7^th may rotate in backward or forward direction at Ns/3,Ns/5,Ns/7 speed respectively.

Hence harmonics torque developed along with fundamental.

5^th Harmonics = 120°×5= 600°= -120°, so will run in backward direction with speed (Ns/S) and 7th Harmonics = 120×6 = 720°= +120°, so will run in forward direction with speed Ns/7.

S10. Ans.(a)