SESSION:17

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1)       What is the classification of PMSM?

Based on construction:

·         Surface mounted PMSM

·         Interior mounted PMSM

Based on the nature of voltage induced

·         Sinusoidally excited

·         Trapezoidally excited

2)       Give some applications of PMSM.

In Robotics and Machine Tools.

3)       Define torque angle.

The angle between excitation emf and supply voltage. This angle is called torque angle or load or power angle.

4)       What are the different classifications of synchronous motor? 

a)            Wound field synchronous motor

      Cylindrical rotor construction
      Salient pole rotor construction

b)             Permanent-Magnet Synchronous Motors

      Surface mounted (Projecting Type and Inset Type)
      Interior Buried Type

c)            Synchronous Reluctance Motors

d)            Hysteresis motors

5)       Why a Synchronous Motor without damper winding is used, in the drive system employing a Cycloconverter?

A Synchronous Motor without the damper winding is used, because the damper winding reduces the inductance of the machine, and therefore is ability to filter out harmonics in the output voltage of Cycloconverter. Since the drive operates in self-controlled mode, the damper winding is not needed for its converter.

6)       What are the conditions to be satisfied for proper synchronising of alternators?

a)      The terminal voltage of the incoming alternator must be the same as bus-bar voltage.

b)      The speed of the incoming alternator must be such that its frequency equals to the bus-bar frequency

c)      The phase sequence of the incoming alternator must be the same as that of the other alternators or busbars.

7)       What are the advantages of Parallel operation of alternators?

a)      It ensures continuity of power supply to consumers in case of breakdown of an alternator in an generation station.

b)      This is economical and improves the efficiency of the generating station.

c)      When the demand of power increases, new alternators can be installed to operate in parallel.

d)      It is not possible to built single large size alternator to meet the requirement.

8)       What are types of rotors used in alternators?

There are two types of rotors used in alternators namely Salient pole rotor and Smooth cylindrical type rotor.

9)       What are the advantages of providing damper winding?

The damper winding is useful in preventing the hunting ( momentary speed fluctuations) in generators.

The damper winding also used to maintain balanced 3 phase voltage under unbalanced load conditions.

10)    What are the various methods to determine the voltage regulation of the large alternators?

In case of small machines, the regulation can be found by direct loading.
For large alternators, to find the voltage regulation indirect methods are used. They are as follows:

a)     Synchronous Impedance Method.

b)     The Ampere-turn method.

c)     Zero power factor or Pointer Method.

11)    What is the basic principle of alternators?

Alternators are nothing but a AC generators. They operate on the fundamental principle of electromagnetic induction as dc generators.
i.e., when the rotor rotates, the emf is induced in the stator.

12)    Which type of rotor is suitable for low speed alternators? Salient pole type or cylindrical type rotor?

Salient pole type alternators are suitable for low and medium speed alternators. It has large number of projecting poles. It has large diameters and short axial lengths.

13)    What is meant by turbo alternators?

High speed alternators are called as Turbo alternators.
As it runs at very high speed, salient pole rotors are not used. Smooth cylindrical type rotor is suitable for turbo alternators.

14)    How does the Induction motor work? (OR) Why does the Rotor rotate?

·         When the 3 phase stator windings are fed by 3 phase supply, a magnetic flux of constant magnitude which is rotating at synchronous speed is set up. 

·         The flux passes through the air-gap and sweeps past the rotor surface thus it cuts the rotor conductors. 

·         Due to the relative speed between the rotating flux and the stationary rotor conductors, an emf is induced in the stationary rotor conductors as per the Faraday's laws of electromagnetic induction.

·         The frequency of the induced emf is the same as the supply frequency.

·         Its magnitude is proportional to the relative velocity between the flux and the conductors.

·         Its direction will be as per Fleming's right hand rule.

·         Since the rotor conductors form a closed circuit the rotor current is produced.

·         This current's direction will oppose the very cause producing it ( as per Len's law)

·         Here the cause is the relative velocity between the rotating flux of the stator and the stationary rotor conductors.

·         Hence, in order to reduce the relative speed, the rotor begins to rotate in the same direction as that of the rotating magnetic flux and tries to catch up with the rotating flux. Thus the rotor of induction motor starts to rotate.

15)    What is the advantage of skewed stator slots in the rotor of Induction motors?

·         In the induction motor design, the rotor slots are purposely made with a slight skew arrangement. It will not be parallel to the shaft. 

·         This is for the purpose of reducing magnetic locking or reducing magnetic attraction between stator and rotor teeth. 

·         In addition to that this arrangement will help to reduce the magnetic hum and noise.