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Correct option is (B) 1-s

The best I can explain: The ratio of the rotor gross POWER and AIR GAP power is 1-s. The air gap power in the INDUCTION motor is Pg and the rotor gross power is (1-s)×Pg. The ratio is 1-s.

Correct choice is (b) Negative sequence CURRENT

Explanation: The unbalanced set of 3-phase voltages causes a negative sequence current. The magnetic FIELD generated due to negative sequence current is in the opposite direction to the FUNDAMENTAL magnetic field. This induces double the FREQUENCY current in the rotor side which causes overheating of the motor.

The CORRECT option is (d) Open

Explanation: The slots used in the SYNCHRONOUS machine are open. Open slots have higher air gap and MINIMUM flux LEAKAGE. The maximum POWER transfer capability increase due to a decrease in the leakage reactance.

Right answer is (b) WRIM

Explanation: E.M.F injection METHOD is used to control the speed of INDUCTION MOTOR. It is only applicable for WRIM because WRIM rotor consists of COPPER windings connected in star connection. The external e.m.f can be connected with it using the slip rings.

The CORRECT option is (B) Induction machine

To explain I would say: The slip in the Induction motor is s = Ns-Nr÷ Ns. The slip existence is DUE to the relative SPEED between the stator magnetic field and rotor.

Right choice is (a) To increase the power factor

To ELABORATE: Capacitor BANKS are used with 3-phase Induction motor in order to IMPROVE the power factor of the machine. The inductor motor generally operates with .6-.8 power factor range.

Correct answer is (B) Capacitor RUN

To ELABORATE: In the capacitor run, the capacitor is permanently connected with auxiliary windings. The capacitor is USED for reactive power compensation. It improves the power FACTOR of the machine.

Correct OPTION is (a) SCIM

The EXPLANATION: Pole changing METHOD is used to control the SPEED of Induction motor. It is only applicable for SCIM because SCIM rotor is made up of ALUMINUM, copper bars.

Correct choice is (d) Star-Delta starting

The explanation: The Star-Delta starting METHOD is one of the best methods for the starting of the Induction MOTOR. This method is cheaper and the line CURRENT in the star is 1÷3 times of in delta connection.

Right OPTION is (a) 10 Pole

The explanation: Synchronous SPEED of Induction motor is Ns=120×f÷P. Ns∝(1÷P). The synchronous speed is inversely proportional to the pole. The maximum speed will be achieved with a MINIMUM NUMBER of poles.

Correct ANSWER is (a) 793.5 W

To explain: POWER input to a 3-Φ induction motor is 3VpIpcosΦ. The power REMAINS the same for Delta and STAR CONNECTION. P=3×23×23×co60°=793.5 W.

Correct CHOICE is (B) 36.6 Hz

To EXPLAIN I would say: The SLIP of the induction motor can be calculated using the relation s = Ns-Nr÷ Ns. Ns=120F÷P=1200 rpm. The VALUE of slip is .91. The rotor frequency is 36.6 Hz.

The CORRECT answer is (B) Ns/11

The BEST explanation: The SPEED of the 6k±1 is Ns÷6k±1. The speed of the 11^th order time harmonics in the induction motor is Ns/11.

The CORRECT OPTION is (c) 6×Ns

The EXPLANATION: The time harmonics are present in the supply voltage. The speed of the 6^th ORDER time harmonics in the induction motor is Ns/6.

The CORRECT OPTION is (B) False

Easy explanation: The single-phase induction motor is not self-starting because of the equal and opposite forward & backward magnetic field. The net torque developed is zero.

The correct answer is (a) more than the TRANSFORMER

To explain: The MAGNETIZING CURRENT in the induction motor is (23-25)% of the full load current. The magnetizing current in the transformer is 3-5% of the full load.

The CORRECT answer is (a) s

Explanation: The RATIO of the ROTOR COPPER loss and air gap power is s. The air gap power in the induction motor is Pg and the rotor copper loss is s×Pg. The ratio is s.

The correct answer is (a) Power FACTOR will REDUCE

To explain I would say: If we increase the air gap in the induction MOTOR, the MACHINE will draw more MAGNETIZING current to maintain flux in the air gap. The power factor of the machine will reduce.

Right choice is (a) Nr

Explanation: In INDUCTION motor rotor tries to catch up the stator MAGNETIC field in order to satisfy LENZ law. EFFECT opposes the cause. Stator remains at REST. The relative speed is Nr-0=Nr.

Correct option is (a) Ns

For explanation I would say: In Induction MOTOR STATOR magnetic field rotates at the SPEED of synchronous speed. The stator is MADE up of Silicon STEEL and remains at rest. The relative speed is Ns-0 = Ns.

Right option is (d) 0

The EXPLANATION is: In SYNCHRONOUS generator rotor and stator magnetic field rotates at speed of synchronous speed in order to PRODUCE the steady state TORQUE.

The correct OPTION is (b) Ns-Nr

Explanation: In induction motor ROTOR tries to catch up the STATOR magnetic field in order to satisfy Lenz law. Effect OPPOSES the cause. Stator magnetic field rotates at the speed of synchronous speed.

Correct CHOICE is (a) 3VpIpcosΦ

Best explanation: Power INPUT to a 3-Φ INDUCTION motor is 3VpIpcosΦ. The power remains the same for Delta and star CONNECTION. The power is CONSTANT and has no vibrations.

Right answer is (b) 4 Hz

For explanation I would SAY: BANDWIDTH is defined as the range of frequencies for which the signal exists. SELECTIVITY is INVERSELY proportional to the bandwidth. B.W(Hz)=f÷Q=20÷5=4 Hz.

The correct ANSWER is (a) HIGH

The explanation is: At the TIME of starting the MOTOR is at rest. The e.m.f developed in the motor is zero. Ia×Ra = Vt-Eb. Ia=Vt÷Ra. The starting current is very high about 10-15 times of FULL load current.

The correct answer is (d) 6 A

The best explanation: In DC shunt MOTOR, FIELD windings are connected in PARALLEL with the armature circuit. IF=IL-Ia=11-5=6 A. Motor CURRENT is the SUM of field and armature current.

Right CHOICE is (a) Terminal voltage reversal

The explanation is: TVR stands for terminal voltage reversal. It is ONE of the METHODS of COUNTERCURRENT braking of the DC shunt motor. It HALTS the motor rapidly and reverses its direction of rotation.

The correct choice is (a) True

To explain: The odd functions are those functions which are symmetric about the ORIGIN. The sine function is an odd function WHOSE TIME PERIOD is 2π.

Right option is (b) 63 V

The EXPLANATION: During the RESONANCE condition XL=Xc. The value of the inductor voltage is Q×Vs. The power FACTOR of the CIRCUIT is one. Vc=7×9=63 V.

The correct option is (a) Starting TORQUE

Explanation: DC SERIES motors are used where the high starting torque is required. At starting the torque produced by DC series MOTOR is very high.

The CORRECT answer is (b) 12 WB

To explain: In DC shunt motor, field windings are connected in parallel with the ARMATURE circuit. IF=IL-Ia=11-5=6 A. F=NIF=R∅. IF∝∅. ∅=F÷R=12 wb.

Correct OPTION is (a) KmΦIa

To EXPLAIN: The torque developed in the MOTOR is DIRECTLY proportional to the FLUX, machine constant, armature current. It is mathematically represented as T= KmΦIa.

Right CHOICE is (a) Gravitational

The best explanation: Plugging is SUITABLE for gravitational type loads. It halts the motor operation. It can REVERSE the DIRECTION of rotation of the motor. It can hold the motor at zero speed.

Right choice is (B) Low

For explanation: The field diverter resistance for a DC SERIES motor is kept low in order to INCREASE the speed of the motor. This will REDUCE the field FLUX and increase the armature current.

Right option is (a) decrease in speed and less armature current

Explanation: In DC series motor, INCREASING diverter resistance increases the field current and field FLUX and ALSO DECREASES the armature current and therefore decreases the speed.

Correct choice is (C) 3 A

Easiest explanation: In DC series motor, FIELD WINDINGS are CONNECTED in series with the armature circuit. The armature current will also flow through the series winding. If=Ia=3 A.

Right CHOICE is (b) Parabola

The explanation is: The shape of the current-torque CHARACTERISTICS in the DC SERIES motor is a parabola. The motor equation is Eb=Vt-IaRa=Km∅ωm. T=Km∅Ia. ∅∝Ia. T∝Ia^2.

Right option is (c) Starting

To elaborate: DC series MOTORS are used where the HIGH starting TORQUE is required. At starting the torque PRODUCED by DC series motor is very high. To start a motor torque should be greater than load torque.