UNIT 1 fundamentals of AC rotating machine

Contents

1.1 Introduction

1.3 Parts of AC machines

1.4 Synchronous speed

1.5 AC winding

1.6 Winding factor (Coil pitch and distributed winding)

1.7 Distribution factor

1.8 EMF equation of alternator or generator

1.9 Numericals

1.10 Harmonic

1.11 Slot harmonics

1.12 Harmonics minimization

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1.1 Introduction

In the rotating electrical machine an electromechanical energy conversion take place. In all rotating electrical machine a changes in flux is closely associated with the mechanical motion to cause electromechanical energy conversion. When mechanical input energy is converted to an electrical energy the machine is called generator. The electrical energy is produce in the form of generated  voltage or emf. If this emf is constant the machine is called DC generator . While if it is alternating the machine is called motor. If input electrical energy is in the form of AC voltage then it is called AC motor. Thus various electrical rotating machine are classified as AC and DC machine.

Whenever there is relative motion between conductor and flux the emf is induced in a conductor. This is the principle of generator while whenever current carrying conductor is placed in magnetic field then it experiences a mechanical force which is the principle of motor.

Every rotating machine must posses the following parts.

1. Stationary member called stator

2. Rotating member called rotor

3. Shaft

4. Slip ring or brush assembly

5. Bearing

These are the mechanical part which machine are posses. In addition to this machine has

1. An arrangement of winding which is used as primary source of flux. When current is passed through it. This is called field winding or exciting winding.

2. An arrangement of conductor to form a winding in which the emf is induced this is called armature winding.

3. The current flowing through filed winding used to produced main flux called magnetising current, exciting current or filed current.

4. The current flowing through armature winding varies as load on machine varies it is called load current or armature current.

5. The current in field winding is always DC there is an appropriate air gap between a stator and rotor of machine.

In most of machine armature winding is place on stator while field winding is placed on rotor from practical convince point of view.

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In practices most of alternator have rotating magnetic field type construction with stationary armature due to following reason.

1. The generation level of AC voltage may be in the range of 11kV which gets induced in armature it is possible to provide large spaces to accommodate any conductor and insulation for stationary armature. The ventilation arrangements for high voltage side can be improve.

2. It is high voltage armature stationary it is possible to avoid interaction of mechanical stress.

3. It is very easy to collect high current at high voltages from stationary member.

4. Overall construction is simplified with rotating field and hence high output can be obtained.

5. Cooling arrangement become very easily.

6. Since a field coils need very low voltage it can be easily be supplied with the help of slip ring and brush assembly. At low voltage it is possible to avoid sparking at slip ring.

7. Since the low voltage level is on field side insulation cost is reduce. Field has low inertia and it can also rotated at high speed.

8. Only two Slip rings are required to excited the field by AC supply.

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