Quiz: Electrical Engineering

Quiz: Electrical Engineering

Exam: UPSSSC JE

Topic: Machines

 

Each question carries 1 mark.

Negative marking: 1/4 mark

Time: 10 Minute

 

Q1. In a synchronous alternator, which of the following coils will have emf closer to sine wave form?

(a) Concentrated winding in full pitch coils

(b) Concentrated winding in short pitch coils

(c) Distributed winding in full pitch coils

(d) Distributed winding in short pitch coils

 

L1Difficulty 3

QTags Alternators

QCreator Devbrat Kumar

 

Q2. The armature reaction effect in a synchronous machine depends on ……

(a) Load current

(b) Power factor of the load

(c) Speed of the machine

(d) Both load current and power factor of the load

 

L1Difficulty 3

QTags Alternators

QCreator Devbrat Kumar

 

Q3. What is the frequency of voltage generated by an alternator having 6-poles and rotating at 900 rpm?

(a) 50 rad/s

(b) 50 Hz

(c) 45 Hz

(d) 45 rad/s

 

L1Difficulty 3

QTags Alternators

QCreator Devbrat Kumar

 

Q4. A synchronous generator is feeding a zero power factor (lagging) load at rated current. The armature reaction is….

(a) Magnetizing

(b) Demagnetizing

(c) Cross-magnetizing

(d) Ineffective

 

L1Difficulty 3

QTags Alternators

QCreator Devbrat Kumar

 

Q5. If the excitation and terminal voltage of synchronous motor are kept constant and the load is increased then…………

(a) armature current decreases and power factor becomes power leading

(b) armature current increase and power factor becomes more leading

(c) armature current increases and power factor become more lagging

(d) None of these

 

L1Difficulty 3

QTags Synchronous motor

QCreator Devbrat Kumar

 

Q6. In a synchronous motor, the stator frame is made of………

(a) Stainless steel

(b) CRGOS

(c) Cast iron or welded steel plates

(d) Laminated silicon steel

 

L1Difficulty 3

QTags Synchronous motor

QCreator Devbrat Kumar

 

Q7. In a synchronous machine, in case the axis of field flux is in line with the armature flux then.

(a) the machine is working as synchronous motor

(b) the machine is working as synchronous generator

(c) the machine is said to floating

(d) the machine will Vibrate violently

 

L1Difficulty 3

QTags Synchronous motor

QCreator Devbrat Kumar

 

Q8. For 100% excitation in synchronous motor, the relationship between back emf E and applied voltage V is-

(a)E>V
(b) E<V
(c) E=V
(d) E V

 

L1Difficulty 3

QTags Synchronous motor

QCreator Devbrat Kumar

 

Q9. A synchronous motor is supplying its rated load. Its excitation is increased

(a) the power factor becomes more leading

(b) the power factor becomes more lagging

(c) the power factor remains constant

(d) None of these

 

L1Difficulty 3

QTags Synchronous motor

QCreator Devbrat Kumar

 

Q10. Which one of the following methods gives voltage regulation higher than the actual value in an alternator?

(a) ZPE method

(b) mmf method

(c) emf method

(d) ASA method

 

L1Difficulty 3

QTags Alternators

QCreator Devbrat Kumar

 

 

solutions

 

S1. Ans.(d)

Sol. Due to short-pitching the coil, distortion due to harmonics will be less and waveform will be sinusoidal. In alternator, distributed winding is used.

If coil pitch =  pole pitch, then it is said to be full pitch, otherwise short pitch.

 

S2. Ans.(d)

Sol. The effect of armature (stator) flux on the flux produced by the field poles is called “Armature Reaction”.

In synchronous machine, the armature reaction effects depends on load current and power factor both.

Suppose when pure inductive load is connected to the alternator, the armature flux will entirely demagnetize the main flux. Therefore, net flux in the airgaps reduces with load.

 

S3. Ans.(d)

Sol.N_s=120f/P
⇒ f = (N_s P)/120=(900×6)/120

= 45 HZ

 

S4. Ans.(b)

Sol. Armature reaction of alternator at zero pf logging will be purely demagnetizing because at zero pf lagging  and  will be opposite to each other.

Zero pf lagging → purely demagnetizing

 

S5. Ans.(b)

Sol. in synchronous motor (when excitation kept constant) load is increases –

  1. Pf will be more leading
  2. Armature current increased
  • Torque angle increases

 

S6. Ans.(c)

Sol. in synchronous motor: –

  • Stator frame → made up of last iron/welded steel plates
  • Core → made up of silicon steel

 

S7. Ans.(c)

Sol. when the axis of field flux is in line with the armature flux then the condition is called “FLOATING” in case of synchronous motor.

In case of floating

  1. = V ie armature current = 0
  2. Losses = zero
  • = 0

 

S8. Ans.(c)

Sol. 100% excitation means full or normal excitation:

Excitation Relation b/w &V
Normal Eb=V
Under Eb<V
Over Eb>V

 

S9. Ans.(a)

Sol.

So, when excitation is increased from normal then Pf will be more leading.

 

S10. Ans.(c)

Sol. # EMF method or pessimistic method because gives VR higher than actual.

# MMF method or optimistic method because VR lower than actual.

 

Leave a comment

Your email address will not be published. Required fields are marked *