UPRVUNL-JE’21 EE: Daily Practices Quiz
UPRVUNL-JE’21 EE: Daily Practices Quiz 18-Oct-2021
Each question carries 1 mark.
Negative marking: 1/4 mark
Total Questions: 06
Time: 06 min.
Q1. Star-delta starter in a 3-phase induction motor is used mainly to
(a) Reduce the starting torque
(b) Reduce the starting current
(c) Reduce the starting speed
(d) Provide constant power
Q2. Consider the following statements with regard to the synchronous machine.
1. Star-delta starters are used to start synchronous motors.
2. Damper windings are used to prevent hunting.
The correct statement(s) is/are:
(a) 1 only
(b) 2 only
(c) Both 1 and 2
(d) Neither 1 nor 2
Q3. In a three-phase induction motor if the rotor input or air gap power is 20,000 W and slip is 3 %. The rotor copper loss per phase is:
(a) 600 W
(b) 100 W
(c) 200 W
(d) 300 W
Q4. A 50 KVA transformer has a core loss of 250 W and total loss of 400 W. Find the copper loss for maximum efficiency:
(a) 400 W
(b) 150 W
(c) 250 W
(d) 500 W
Q5. An alternating voltage υ=200 √2 sin 100 t is connected to a 1 μ F capacitor
through an a.c ammeter. The reading of the ammeter shall be
(a) 40 mA
(b) 80 mA
(c) 100 mA
(d) 20 mA
Q6. A negative sequence relay is commonly used to protect
(a) An alternator
(b) A transformer
(c) A transmission-line
(d) A bus-bar
Sol. In the star-delta starting method of 3-phase induction motor, the starting current is reduced by approx. 1/3 times as compared to the original.
Hence, Star-delta starter in a 3-phase induction motor is used mainly to reduce the starting current.
Sol. Synchronous motors are not self-starting.
Starting with the help of an External Prime Mover
Starting with the help of Damper Windings
Damper windings are used to prevent the rotor from losing synchronism in a synchronous machine and this phenomenon is called Hunting.
Sol. Rotor Cu loss=sP_in=0.03×20,000=600 W
∴Rotor Cu loss per phase=600/3=200 W.
Sol. For maximum efficiency: Copper loss=iron or core loss=250 W
Sol. V_(r.m.s.)=V_m/√2=200 √2 /√2=200V; ω=100 rad/s;
X_C=1/ωC=1/(100×1×10^(-6) )=10^4 Ω
∴ I_(r.m.s.)=V_(r.m.s.)/X_C =200/10^4 =20×10^(-3) A=20 mA
Sol. A negative sequence relay is commonly used to protect an alternator.
A relay which protects the electrical system from negative sequence component is called a negative sequence relay or unbalance phase relay. The negative sequence relay protects the generator(alternator) and motor from the unbalanced load which mainly occurs because of the phase-to-phase faults.