21. If dΦ is the luminous flux incident normally on an area dA, then Illuminance is
given as ?
Illuminance E = dΦ/dA
illuminance is the total luminous flux incident on a surface, per unit area.
प्रति इकाई एरिया पर पड़ने वाले ल्युमिनस फ्लक्स को प्रदीपन कहते है।
इसका मात्रक लक्स या कैंडल - मीटर होता है।
*इसका बड़ा मात्रक फोट होता है।
1 phot = 10000 Lux .
22. starting method CANNOT be used for starting a 3-phase squirrel cage induction
motor ?
Rotor-resistance method used for Starting Of Slip-Ring Motors
Slip-ring motors are started with full line voltage, as external resistance can be easily added in the rotor circuit with the help of slip-rings. A star connected rheostat is connected in series with the rotor via slip-rings
23. Hunting occurs in a/an _ motor ?
The Hunting process occurs in a synchronous motor as well as in synchronous generators if an abrupt change in load occurs.
The steady state or stable operation of a synchronous motor is a condition of equilibrium.
Causes of Hunting in Synchronous Motor
1-Sudden change in load.
2-Sudden change in field current.
3-A load containing harmonic torque.
4-Fault in supply system.
Effects of Hunting in Synchronous Motor
1-It may lead to loss of synchronism. 2-Produces mechanical stresses in the rotor shaft. 3-Increases machine losses and cause temperature rise. 4-Cause greater surges in current and power flow. 5-It increases possibility of resonance.
Reduction of Hunting in Synchronous Motor
Two techniques should be used to reduce hunting. These are –
Use of Damper Winding: It consists of low electrical resistance copper / aluminum brush embedded in slots of pole faces in salient pole machine.
Damper winding damps out hunting by producing torque opposite to slip of rotor.
The magnitude of damping torque is proportional to the slip speed.
Use of Flywheels: The prime mover is provided with a large and heavy flywheel. This increases the inertia of prime mover and helps in maintaining the rotor speed constant.
Designing synchronous machine with suitable synchronizing power coefficients.
24. Let T be the net torque developed by the rotor runs at ω rad/s. What is the
mechanical power developed ?
Mechanical power developed by a system is given by Pmech = ωT
Where, ω = speed of rotor
T = total torque
25. In a single-phase capacitor start-and-run motor, the minimum number of
capacitors to be used in it is ?
One There are two capacitors in this method one is used at the time of the starting and is known as starting capacitor.
The other one is used for continuous running of the motor and is known as RUN capacitor.
but we can use only one capacitor for starting and as well as runing time.
26. Split-phase, capacitor and shaded-pole single phase motors classified as ?
The single phase induction motors are made self starting by providing an additional flux by some additional means. Now depending upon these additional means the single phase induction motors are classified as:
1-Split phase induction motor.
2-Capacitor start inductor motor.
3-Capacitor start capacitor run induction motor (two value capacitor method).
4-Permanent split capacitor (PSC) motor.
5-Shaded pole induction motor.
27. Which of the following plants experiences chain reactions ?
Nuclear chain reaction
A chain reaction refers to a process in which neutrons released in fission produce an additional fission in at least one further nucleus. This nucleus in turn produces neutrons, and the process repeats. The process may be controlled (nuclear power) or uncontrolled (nuclear weapons).
28. When only current source is active in the circuit, find the current through the
10 Ω resistor ?
when only current source working then voltage source as act short circuit because The internal resistance is zero of ideal voltage source and thus it behaves as a short circuit.
so apply current division rule between 5Ω and 10Ω
current in 10Ω resistance = 2*5/(5+10) = 10/15 =0.66 A
29. 4F2D is a/an __ number ?
hexadecimal number 4F2D
F→1111
2→0010
D→1101
Hence, (4F2D)16 = (0100 1111 0010 1101)2
30. Which of the following is the correct relation regarding coupled coils ?
Generally, the amount of inductive coupling that exists between the two coils is expressed as a fractional number between 0 and 1 instead of a percentage (%) value, where 0 indicates zero or no inductive coupling, and 1 indicating full or maximum inductive coupling.
In other words, if k = 1 the two coils are perfectly coupled, if k > 0.5 the two coils are said to be tightly coupled and if 0.5 >k the two coils are said to be loosely coupled. Then the equation above which assumes a perfect coupling can be modified to take into account this coefficient of coupling, k and is given as:
1 2 3 4 5 6 7 8 9 10
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