Below is the phasor diagram of synchronous generator. If the machine is made to run as synchronous motor, what will be the phasor diagram changes then?

In case of synchronous motor, which of the following are true? a. Synchronous motor is not self-starting b. If the rotor poles are excited by field current and the stator terminals are connected to AC supply, motor vibrates. c. Variable frequency supply is used to start synchronous motor. d. By using frequency converter, a synchronous motor can be brought from standstill to its desired speed.

A

Only a and b are true

B

Only b and c are true

C

All are true

D

None of the above

A three-phase cylindrical rotor synchronous generator has a synchronous reactance Xs and a negligible armature resistance. The magnitude of per phase terminal voltage is VA and the magnitude of per phase induced emf is EA. Considering the following two statements, P and Q, P: For any three-phase balanced leading load connected across the terminals of this synchronous generator, VA is always more than EA Q: For any three-phase balanced lagging load connected across the terminals of this synchronous generator, VA is always less than EA Which of the following options is correct?

A

P is false and Q is true

B

P is true and Q is false

C

P is false and Q is false

D

P is true and Q is true

Consider the following statements regarding synchronous machines: 1. When a synchronous motor is over-excited, its back emf is greater than the supply voltage 2. When a synchronous motor is over-excited, its power factor is leading 3. Synchronous motor is used as capacitor where load is so large that construction of a static capacitor is impractical Which of these statements are correct?

A

1 and 2

B

1 and 3

C

2 and 3

D

1, 2 and 3

Two involute gears are designed to mesh for a given centre distance and a given angular velocity ratio (other than 1). During assembly, the centre distance has increased slightly. Then which of the following changes occur? (i) Velocity ratio changes (ii) Pressure angle changes (iii) Pitch circle diameter changes (iv) Working depth changes (v) Base circle radius changes Select the correct answer using the code given below.

A

1, 2 and 3

B

2, 3 and 4

C

2 and 5

D

3 and 5

Transient stability of the generator is dependent on (a) generator reactance; lower reactance decreases peak power and increases initial rotor angle. (b) how heavily the generator is loaded. (c) generator inertia; higher the inertia, slower the rate of the change of angle. (d) generator output during fault Which of the above is incorrect?

A

a

B

b

C

c

D

None of the above

Suppose we want to design a synchronous circuit that processes a string of 0’s and 1’s. Given a string, it produces another string by replacing the first 1 in any subsequence of consecutive 1’s by a 0. Consider the following example. Input sequence: 00100011000011100 Output sequence: 00000001000001100 A Mealy Machine is a state machine where both the next state and the output are functions of the present state and the current input. The above mentioned circuit can be designed as a two-state Mealy machine. The states in the Mealy machine can be represented using Boolean values 0 and 1. We denote the current state, the next state, the next incoming bit, and the output bit of the Mealy machine by the variables s, t, b and y respectively. Assume the initial state of the Mealy machine is 0. What are the Boolean expressions corresponding to t and y in terms of s and b ?

A

t = b

y = sb

B

t = s + b

y = sb

C

t = s + b

y = sb̅

D

t = b

y = sb̅

The phasor diagram of a synchronous machine connected to an infinite bus is shown below. The machine is

A

Generator and operating at lagging pf

B

Generator and operating at leading pf

C

Motor and operating at a leading pf

D

Motor and operating at a lagging pf

A synchronous motor installed at the receiving end substation operates with such an excitation that it takes power at lagging power factor. Now if the applied voltage of the synchronous motor goes down, the power factor of the synchronous motor will

A

remain same

B

go down

C

improve

D

none of the above

An electric motor drives a machine through a speed reducing gear of ratio 9:1. The motor armature, with its shaft and gear wheel, has moment of inertia 0.6 kg-m2. The rotating part of the driven machine has moment of inertia 45 kg-m2. The driven machine has resisting torque of 100 N-m and the efficiency of reduction gear is 95%. Find the power which the motor must develop to drive the machine at a uniform speed of 160 r.p.m.

A

1764 W

B

2764 W

C

3764 W

D

4764 W

The phasor diagram of a synchronous machine connected to an infinite bus is shown in the figure. The machine is acting as

A

generator and operating at leading pf

B

generator and operating at lagging pf

C

motor and operating at leading pf

D

motor and operating at lagging pf

Below is the phasor diagram of synchronous generator. If the machine is made to run as synchronous motor, what will be the phasor diagram changes then?

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