# Electrical Machines-II KEE-503 Btech Aktu Short Question, Quantum Book Pdf

Discover Electrical Machines-II Short Question Notes from the B.Tech. AKTU Quantum Book. Learn about the workings of transformers and AC machines to get useful knowledge for running electrical systems effectively.

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## Unit-I: Synchronous Machine-I (Short Question)

Q1. What are the applications of synchronous machine?

Ans. 1 For generation of 3𝜙 power.

2. For improving power factor.

3. In large low head pumps, reciprocating pumps, compressors, rolling mills, ball mills, crushers and bulb grinders etc.

Q2.  What is the formula of synchronous speed?

Ans.

where,  f = Supply frequency

P = Number of poles

Ns = Synchronous speed.

Q3. What do you understand by excitation emf?

Ans.  It is the emf induced in the armature winding due to field current (If) only.

Q4. What are the advantages of armature winding ?

Ans.

• 1. The harmonic emfs are reduced.
• 2. Distorting harmonics is eliminated.
• 3. It diminishes armature reaction and armature reactance.
• 4. The core is better utilized.

Q5. What are the characteristics of an infinite bus ?

Ans. 1. Terminal voltage is constant.

2. Frequency is constant.

3. Synchronous impedance is very small.

Q6. What are the reasons that alternators are operated in parallel ?

Ans. 1. A group of alternators can provide a greater load than a single alternator.

2. During periods of low load, one or more alternators may be turned off, allowing the remaining alternators to run at or near full load and therefore more efficiently.

3. When one machine is removed, the other machines maintain supply continuity.

Q7. What are the methods of determination of voltage regulation ?

2. Indirect methods:

• i. Synchronous impedance method or EMF method
• ii. Ampere-turn method or MMF method
• iii. Zero power factor method or Potier’s method.

Q8. What do you understand by armature reaction ?

Ans. Armature reaction refers to the influence of armature (stator) flux on the flux produced by the rotor field poles.

Q9. What do you mean by positive and negative voltage regulation of a synchronous alternator ?

Ans. Positive voltage regulation indicates that the synchronous generator has a lagging power factor. Negative voltage regulation indicates that the synchronous generator is working at a high power factor.

Q10. What are the importances of armature reaction in three phase synchronous machine ?

Ans. The armature reaction in a synchronous generator reduces the main field flux with a lagging power factor load while increasing the main field flux with a leading power factor load.

Q11. What are the advantages of having field winding on the rotor and armature winding on the stator in case of synchronous machines ?

Ans. 1. At high voltage, it is easier to insulate stationary armature winding.

2. Stationary armature can be cooled efficiently.

Q12. What do you mean by synchronous reactance ?

Ans. Synchronous reactance (Xs) is the ratio of induced emf and the steady state rms current. It is the sum of the armature reactance (Xal) and the magnetizing reactance (Xar).

Mathematical expression is,

Synchronous reactance, Xs = Xal + Xar.

Q13. Define “distribution factor” and “pitch factor”.

Ans. Coil span factor (or pitch factor): Coil span factor (K) is the ratio of vector sum of emfs to the arithmetical sum of emf induced in coil sides.

where 𝝰 is the angle by which the coil pitch falls short.

Distribution factor (or breadth factor, Kd): It is ratio of vector sum of the emfs in the individual coil to the arithmetical sum if the coils would be one slot only.

Q14. Synchronizing power and synchronous speed of a machine are 628.318 kW/mech degree and 750 rpm. Calculate the synchronizing torque per mechanical degree of rotor displacement at no load.

Ans. Given: Psys 628.318 kW/mech degree, Ns = 750 rpm

To Find: 𝞃sys

## Unit-II: Synchronous Machine-II (Short Question)

Q1. What do you understand by two reaction theory ?

Ans. The idea known as two reaction theory provides a mechanism for analysing the unsettling effects induced by salient pole formation. The proposed theory divides the given armature mmf into two mutually perpendicular components, the direct axis (d-axis) and the quadrature axis (q-axis).

Q2. Discuss the main characteristics of the 3𝜙 synchronous motor.

Ans. 1. Synchronous motor operates only at synchronous speed

2. While not in use, synchronous motors consume very little current from the power supply to compensate for internal losses.

3. If the field excitation is low, the stator inductance is high, causing the motor to draw trailing current and have a low power factor.

4. When the excitation increases, the stator flux is negated by the rotor flux, and the power factor equals one.

Q3. Write the demerits of 3𝜙 synchronous motor.

Ans.

• 1. In general, the cost per kW production is considerable.
• 2. It necessitates DC stimulation, which must be obtained from an external source.
• 3. It cannot be utilized for variable speed projects since the speed cannot be adjusted.
• 4. When overloaded, it may lose synchronism and shut down.

Q4. Write the main applications of the three-phase synchronous motor.

Ans. 1. Used to regulate the voltage at the end of transmission lines.

2. Employed for the loads where constant speed is required.

Q5. What do you understand by locked rotor torque?

Ans. It is the minimal torque created at any angular rotor position when the rotor is locked and rated voltage at rated frequency is delivered to the terminals. The stator windings create this torque.

Q6. What is running torque ?

Ans. It is the torque produced by the motor when it is running. The power rating and speed of the driven machine decide it.

Q7. What do you mean by pull in torque ?

Ans. The pull in torque is the greatest continuous torque at rated voltage and frequency under which a motor will pull a connected load into synchronism when the DC motor stimulation is applied.

Q8. Define pull out torque.

Ans. It is the maximum torque that a synchronous motor can produce at its rated voltage and frequency without losing synchronism.

Q9. What happens when the load on a synchronous motor is increased?

Ans. 1. The torque angle 𝜹 increases.

2. The armature current Ia drawn from the supply increases.

3. The phase angle 𝜙 increases in the lagging direction.

Q10. Draw the V-curve and inverted V-curve of a synchronous motor.

Ans.

Q11. What are the causes of hunting?

Ans.

• 1. Sudden changes of load.
• 2. Faults occurring in the system which the generator supplies.
• 3. Sudden changes in the field current.
• 4. Cyclic variation of the load torque.

Q12. What are the effects of hunting?

Ans.

• 1. It can lead to loss of synchronism.
• 2. It can cause variations of the supply voltage producing undesirable lamp flicker.
• 3. It increases the possibility of resonance.
• 4. Large mechanical stresses may develop in the rotor shaft.
• 5. The machine losses increase and the temperature of the machine rises.

Q13. What are the techniques used in reduction of hunting?

Ans. 1. By using damper windings.

2. By using flywheels.

3. By designing synchronous machines with suitable synchronizing power coefficients.

Q14. Write any two main comparisons of 3𝜙 synchronous motor and induction motor.

Ans.

Q15. What is damper windings ?

Ans. Little slots are constructed in the salient rotor’s pole faces, and copper bars are put into the slots. Copper rings create a closed circuit by short-circuiting the ends of all the bars. The damper winding is the name of this winding.

Q16.  Explain the basic role of damper winding in synchronous machines.

Ans. 1. It aims to immediately bring the rotor to equilibrium and opposes hunting.

2. Because of the damper winding, the synchronous motor can be started like an induction motor.

Q17. What do you understand by term mechanical vibration in a synchronous machine?

Ans. Mechanical vibration is the back and forth motion of a synchronous machine or machine component. It is a system’s reaction to an applied force, either internal or external.

Q18. Why is synchronous motor not self-starting ?

Ans. Due to the rotor’s inertia and the rapid speed of the rotating magnetic field, it is impossible to rotate the rotor beyond its original position. Hence, synchronous motors cannot start themselves, regardless of the rotor’s starting position.

## Unit-III: Three Phase Induction Machine-I (Short Question)

Q1. Write the types of 3𝜙 induction motors.

Ans. 1. Squirrel-cage induction motor.

2. Wound-rotor or slip-ring induction motor.

Q2. What are the advantages of cage rotor and wound rotor ?

Ans. Advantages of the cage rotor:

• 1. Lesser maintenance and cheaper
• 2. Higher power factor
• 3. Reduced risk of sparking because of absence of brushes.

• 1. High starting torque and low starting current
• 2. Speed control is easy due to addition of external resistance.

Q3. Why external resistors are provided in rotor circuit ?

Ans. 1. To increase the starting torque and decrease the starting current.

2. To control the speed of the rotor.

Q4. Draw the equivalent circuit of induction motor.

Ans.

Q5. Draw torque-slip characteristics of 3𝜙 induction motor.

Ans.

Q6. What do you understand by induction generator ?

Ans. An induction generator is a type of electrical generator that produces alternating current and works on the ideas of electromagnetic induction. Induction generators are described as induction machines when they are operated above synchronous speed (Asynchronous generator).

Q7. What are the applications of induction generator?

Ans. 1. In hydro-ohmic power stations having a variable low head water supply.

2. For installation in small power station.

3. For braking purpose in railway work.

Q8. Draw torque-slip characteristics curve of an induction machine.

Ans.

Q9.  Draw the power flow diagram for three-phase induction motor.

Ans.

Q10. Explain the principle of operation of a three-phase induction motor.

Ans. The electromagnetic induction theory underlies its operation. It claims that when the magnetic flux connecting the same coil changes over time, an emf is induced in the coil. It is denoted as

Q11. What is the condition for producing maximum torque in three-phase induction motor?

Ans. The condition for producing maximum torque in three-phase induction motor is that its slip must be equal to the ratio of rotor resistance at standstill to the rotor reactance at standstill. Such a slip is denoted as sm given by R2/X2

Q12. Differentiate between squirrel-cage rotor and wound rotor type induction motors.

Ans.

## Unit-IV: Three Phase Induction Machine-II (Short Question)

Q1. Why starter is necessary for starting an induction motor ?

Ans. A three-phase induction motor requires a starter because, when it first starts, it draws a significant quantity of current that can harm nearby equipment. Hence a starter is required to control beginning current.

Q2. What are the types of starter used in starting of 3𝜙 induction motor ?

Ans. 1. Direct on-line starter

2. Star-delta starter

3. Auto-transformer starter.

Q3. What are the functions of starter?

Ans. 1. To reduce the heavy starting current.

2. To provide overload and under-voltage protection.

Q4. What are the merits of inner and outer cage of double-cage induction motor ?

Ans. Merits of inner cage:

• 1. The leakage reactance is high.
• 2. The resistance is small.

Merits of outer cage:

• 1. It provides high starting torque.
• 2. Its resistance is large enough to limit starting current and improve power pf.

Q5. Draw torque-speed characteristics of deep-bar cage motors.

Ans.

Q6. Sketch torque-speed characteristics of a double-cage induction motor.

Ans.

Q7. State the terms cogging and crawling in three-phase induction motor.

OR

Define “cogging” phenomenon in induction motor.

Ans. Cogging: Cogging or teeth locking refers to the magnetic locking phenomena that occurs between the teeth of the stator and rotor.

Crawling: Crawling of the motor is defined as the tendency of the motor to run at a stable speed as low as one-seventh of the normal speed N and being unable to pick up its regular speed.

Q8. What are the different methods of speed control of induction motors ?

Ans.

• 1. Pole changing
• 2. Stator voltage control
• 3. Supply frequency control
• 4. Rotor resistance control
• 5. Slip energy recovery.

Q9. What are the disadvantages of the rotor resistance control method ?

Ans. 1. Due to additional losses in the resistors connected to the rotor circuit, efficiency is low.

2. Because of larger slides at low speeds, efficiency is decreased.

Q10. Give various applications of three-phase induction motors.

Ans. 1. For loads requiring high starting torque and where a lower starting current is required.

2. Used in conveyors, cranes, pumps, elevators and compressors.

Q11. Why does a 3𝜙 induction motor always run at less than the synchronous speed ?

Ans. If rotor is assumed to run at synchronous speed Ns in the direction of rotating field, then there would be no flux-cutting action, no emf in rotor conductors, no current in rotor bars and therefore no developed torque. Thus, the rotor of 3𝜙 induction motor can never attain synchronous speed.

Q12. Write down the advantages of double-cage rotor induction motor over single squirrel-cage motor.

Ans.

• 1. High starting torque
• 2. Excellent running performance
• 3. Low starting current
• 4. With the proper choice of inner and outer cage parameters, wide range of torque-slip characteristics can be obtained.

## Unit-V: Single Phase Induction Motor (Short Question)

Q1. Why 1𝜙 induction motor is not self starting?

Ans. A pulsing magnetic field is created when the stator winding is linked to a single phase source. Two revolving magnetic fields of similar magnitude but spinning in the opposite directions result from a pulsating magnetic field. The two torques are therefore equal and in opposition. So, at a complete stop, the net torque is zero. A single phase induction motor is not self starting as a result.

Q2. What do you understand by double-revolving field theory?

Ans. A stationary pulsating magnetic field can be split into two rotating magnetic fields with equal magnitudes and angular rotations, according to the double-revolving field theory of single phase induction motors.

Q3. Write the forward field and backward field equation.

Ans.

Q4. What do you mean by forward slip in single phase induction motor ?

Ans.

Therefore, the direction in which the rotor is started initially will be called the forward field.

Q5. What is the relation between forward and backward slip?

Ans.

Q6. Draw torque-speed characteristics of 1𝜙 induction motor.

Ans.

Q7. What are the starting methods of 1𝜙 induction motor ?

OR

What are the types of 1𝜙 induction motor ?

Ans.

• 1. Split-phase motor
• 2. Capacitor-start motor
• 3. Capacitor-start capacitor-run motor
• 4. Permanent split capacitor motor

Q8. Sketch torque-speed characteristics of split-phase motor.

Ans.

Q9. Draw torque-speed characteristic of capacitor-start motor.

Ans.

Q10. What are the applications of capacitor-start motors ?

Ans. 1. In pumps and compressors, and refrigerators and air conditioner compressors.

2. They are also used for conveyors and some machine tools.

Q11. What are the applications of two-value capacitor motor or capacitor-start capacitor-run motors ?

Ans. 1. Used for loads of higher inertia requiring frequent starts where the maximum pull out torque and efficiency required are higher.

2. They are used in pumping equipment, refrigeration, air compressors etc.

Q12. Write the applications of permanent-split capacitor motors.

Ans. 1. Used for fans and blowers in heaters and air conditioners.

2. To drive refrigerator compressors.

3. They are also used to drive office machinery.

Q13. What do you understand by shaded-pole motor  ?

Ans. A single phase induction motor that contains an auxiliary short circuited winding or winding that is located differently magnetically from the primary winding is known as a shaded pole motor.

Q14. What are the applications of universal motor?

Ans.

• 1. Potable drills
• 2. Hair dryers
• 3. Grinders
• 4. Table fans.

Q15. What are the applications of stepper motor ?

Ans. The paper feed motor in typewriters and printers, the pen in X-Y plotters, the recording heads in computer hard drives, and the placement of the work table and tools in numerically controlled machine equipment all employ stepper motors.

Q16. Calculate the stepping angle for a three-phase, 24-pole permanent magnet stepper motor.

Ans.

Q17. Explain brushless DC motor.

Ans. A polyphase synchronous motor with a permanent magnet rotor is a brushless DC motor. Without its electronic controller, this motor is unable to function. A solid state inverter, an AC motor, and a rotor position sensor are all included in a brushless DC motor, which is a motor drive system.