Quiz: Electrical Engineering
Exam: SSC JE
Each question carries 1 mark.
Negative marking: 1/4 mark
Time: 10 Minute
Q1. A DC motor starter is
(a) Fixed resistance
(b) Variable resistance
(c) Variable inductance
(d) Variable capacitor
Q2. Which type of winding is generally preferred for generating large currents on DC generators?
(a) Progressive wave winding
(b) Retrogressive wave winding
(c) Lap winding
(d) Current depends on design
Q3. The range of efficiency for shaded pole motors is
(a) 95% to 99%
(b) 80% to 90%
(c) 50% to 75%
(d) 5% to 35%
Q4. Demand factor is
(a) (maximum demand)/(connected load)
(b) (maximum demand)/(average load)
(c) (connected load)/(maximum demand)
(d) (average load)/(maximum demand)
Q5. High tension cables are generally used up to
(a) 11 KV
(b) 33 KV
(c) 66 KV
(d) 132 KV
Q6. What does the bedding on the cable consists of?
(a) Jute strands
(b) Hessian tape
(c) Paper tape compounded with a fibrous material
(d) Any of these
Q7. A 6 pole Lap wound dc generator has 300 conductors. Emf induced per conductor is 5 volts. This generator will generate emf of:
(a) 60 V
(b) 250 V
(c) 300 V
(d) 1800 V
Q8. …………. are integrating instruments?
(d) Ampere-hour and Watt-hour meters
Q9. Resistances can be measured with the help of a …………
(d) ohmmeter and resistance bridge
Q10. ………….. instruments indicate the instantaneous value of the electrical quantity being measured at the time at which it is being measured?
Sol. Sol. At the time of starting of motor it is at rest and no back e.m.f. is generated. On application of full voltage, armature winding draws a heavy current due to small armature resistance. This high armature current may damage the armature windings, commutator and brushes. To prevent high armature current during the starting of motors, variable resistance is connected in series with the armature winding. The starting resistance is reduced as the motor speeds up. The resistance is cut off fully when the motor attains full speed. This arrangement is known as starter. For very small D.C. motor (e.g. 6v, 12v, motor) starter is not required and it can be started directly.
Sol. LAP winding: large current & low voltage applications
WAVE winding: high voltage & low current applications
Sol. The range of efficiency for shaded pole motors is 5% to35%.
Shaded pole motors are used where low torque is acceptable (such as fans) and are usually less than ¼ HP. Due to their very low efficiency, shaded pole motors should only be used in applications where the motor is very small or operates for very short period of time (e.g. shower fan motor).
Sol. Demand factor is the ratio of the sum of the maximum demand of a system (or part of a system) to the total connected load on the system (or part of the system) under consideration. Demand factor is always less than one.
Sol. The classification of cables on the basis of voltage is more common, according to which they can be divided into the following categories:
1. Low-tension cables — up to 1000 V or 1 KV
2. High-tension cables — the operating voltage of high-tension cables is up to 11000 V or 11 KV
3. Super-tension cables — the operating voltage of super tension cable is from 22 kV to 33 kV
4. Extra high-tension cables — from 33 kV to 66 kV
5. Extra super voltage cables — beyond 132 kV
Over the metallic sheath is applied a layer of bedding which consists of a fibrous material like jute or hessian tape. The purpose of bedding is to protect the metallic sheath against corrosion and from mechanical injury due to armouring.
Sol. Emf induced per conductor=5 V
For lap wound, number of parallel paths(A)=P=6
No. of conductor per parallel path=300/6=50
∴total emf generated by generator=50×5=250 V
Sol. Integrating instruments: These instruments record the consumption of the total quantity of electricity, energy etc., during a particular period of time. That is, these instruments totalize events over a specified period of time. No indication of the rate or variation or the amount at a particular instant are available from them. Some widely used integrating instruments are: Ampere-hour meter: kilowatt-hour (kWh) meter.
Sol. ohmmeter and resistance bridge
Sol. Indicating instruments: Indicating instruments indicate, generally the quantity to be measured by means of a pointer which moves on a scale. Examples are ammeter, voltmeter, wattmeter etc.