Quiz: Electrical Engineering
Each question carries 1 mark.
Negative marking: 1/3 mark
Time: 10 Minute
Q1. “M.M.F around any closed path equals the current enclosed by the path”. This is known as
Q2. Electro-dynamometer type of instrument is enclosed in a casing made up of high permeability material to:
(a)To remove electrostatic effect
(b)To provide mechanical strength to the coils present
(c)To protect against external magnetic fields
(d)Provide good damping
Q3. According to IE Rules the maximum load on a power sub circuit should not exceed……… watts.
Q4. A universal motor has high starting torque and a variable speed characteristic because it is……………
(b)Long shunt wound
Q5. Hopkinson’s test for DC motor is conducted with
Q6. In a double squirrel cage motor, outer cage is made of high resistance metals bars primarily for the purpose of increasing its……….
Q7. During regenerative braking the motor operates as:
Q8. The electric supply authority supplies power to the consumers through a low voltage three phase four wire distribution system is called……
(a)Primary transmission system
(b)Primary distribution system
(c)Secondary transmission system
(d)Secondary distribution system
Q9. ………….. wiring is used in residential, commercial and public building to improve appearance.
Q10. When a material becomes a superconductor, its resistivity becomes
(d)About 20% of the normal value
Sol. The integral around a closed path of the component of the magnetic field tangent to the direction of the path equals µ0 times the current intercepted by the area within the path.
i.e. ∫▒〖B ⃗. (dS) ⃗ 〗=µOI
NOTE: In order to apply Ampère’s Law all currents have to be steady (i.e. do not change with time)
Sol. The field produced by the fixed coils of electro dynamometer type instruments is weaker i.e. nearly 0.005 to 0.006 Wb/m*m.
In dc measurements, even the Earth’s magnetic field may affect the readings. Thus, it is necessary to shield and electrodynamometer type instrument from the effect of stray magnetic fields. Air cored electrodynamometer instruments are protected against external magnetic fields by enclosing them in a casing of high permeability alloy. This shunts external magnetic fields around the instrument mechanism and minimizes their effects on the indication.
Sol. The sub-circuits may be divided into the following two groups:
(i) light and fan (5A) sub-circuits: the numbers of such points (i.e. lights, fans, 5A socket outlasts) connected in one sub-circuit should not exceed ten and load connected in one sub-circuit should not exceed 800 watts.
(ii) power (15A) sub-circuits: Single phase 15A sub-circuits (power sub-circuits) are used for connecting heaters, stoves, electric iron, small single-phase motors etc. The load connected in a power sub-circuit normally should not exceed 3,000 watts and the number of outlasts connected on each sub circuit in no case should exceed two.
Sol. A universal motor has high starting torque and a variable speed characteristic because it is series wound.
A universal motor is a series-wound motor — meaning that the field and armature windings are connected in series — and is mechanically commutated with brushes and a commutator. Although its construction is very similar to a series-wound DC motor, a universal motor incorporates several modifications that allow it operate properly on either DC or AC power supply.
Sol. Hopkinson’s test for DC motor is conducted with full load.
Sol. A Double Cage Induction motor is that type of motor in which a double cage or two rotor windings or cages are used.
The outer cage bars have a smaller cross-sectional area than the inner bars and are made of high resistivity materials like brass, aluminium, bronze, etc. the bars of the inner cage are made of low resistance copper. Thus, the resistance of the outer cage is greater than the resistance of the inner cage.
At starting most of the current flows through outer cage despite its large resistance (as total impedance is lower than the inner cage). This will not affect the outer cage because of its low reactance. And because of the large resistance of outer cage starting torque will be large.
Sol. During regenerative braking the motor operates as generator.
Sol. The electric supply authority supplies power to the consumers through a low voltage three phase four wire distribution system is called secondary distribution.
Sol. Conduit wiring is used in residential, commercial and public building to improve appearance. Conduit Wiring is the best system of wiring. There are two methods for laying conduits. In one method conduit is laid on the surface of the wall, ceiling etc. This is called surface wiring. The other method is to lay the conduit recessed in wall or ceiling, and this system is known as concealed wiring.
Advantages of Conduit Wiring:
The safest wiring
Appearance is better
No risk of fire or mechanical wear and tear.
No risk of damage of cable insulation
Safe from humidity, smoke, steam etc.
No risk of shock
Sol. Superconductor has zero resistivity.