EE 123 HANDOUTS : ELECTRICAL MACHINES

Generators – converts mechanical energy to electrical energy

Motor – converts electrical energy to mechanical energy

Transformers – converts electrical energy of one form to electrical energy of another form.

NOTES ON DC Machines

PARTS OF A DC MACHINE: a. Two Main Parts:

1. Stationary part designed mainly for producing magnetic flux 2. Rotating part called armature, where mechanical energy is converted into

electrical energy (generator) or electrical energy converted into mechanical energy (motor).

b. Various parts 1. Frame 4. Field poles 7. Commutator poles 2. Armature 5. Brush Gear 8. Armature windings 3. Commutator 6. Armature shaft bearings

Two types of Armature Windings according to the degree of closure

1. Open Coil Winding 2. Closed Coil Winding

Two Types of Closed Armature Winding

1. Ring Winding 2. Drum Winding

Two Types of Drum Winding 1. Lap winding – suitable for comparatively low voltage but high current generators.

The finish of each coil is connected to the start of the next coil so that winding or commutator pitch is unity.

2. Wave winding – suitable for comparatively low current but high voltage generators. The finish of coil is connected to the start of another coil electrical degrees away from the first coil.

Types of DC Generators:

1. According to method of Excitation: a. Separately excited generators – generators whose field magnets are

energized from an independent external source of DC current. b. Self excited – generators whose field magnets are energized by the current

produced by the generators themselves. 2. According to how the field windings are connected: (Refer to Figure)

a. Shunt wound generators b. Series woung generators c. Compound wound generators

1. Short Shunt 2. Long SHUNT

Losses in a DC generator: 1. Copper Losses

a. Armature Copper Loss Ia2Ra Ia - armature current

b. Shunt Copper Loss Ish2Rsh or VtRsh Ra – armature resistance

c. Series Copper Loss Ise2Rse Ish – Shunt field current

d. Loss due to brush contact resistance Rsh – shunt field resistance

Vt - terminal voltage 2. Iron Loss (Core Loss) Rse – Series Field Resistance

a. Hysteresis loss Ph = Kh (Bmax)1.6N watts Ise - series field current b. Eddy Current Loss Pe = Ke(BmaxNt)2 watts Pe – Eddy Current loss c. Mechanical Loss – Friction and Windage Ph – Hyteresis Loss

Kh , Ke – proportionality constant 3. Mechanical Loss – Friction & Windage Bmax – maximum flux density

N – speed of armature rotation t – thickness of armature core lamination

Requirements for the parallel operation of DC generator: 1. the same external characteristics or behaviors when loaded 2. terminal voltage of each machine must be numerically equal 3. terminal polarity must be the same

Advantages of operating in parallel over single operation

1. no generator will be overloaded 2. good maintenance procedure 3. continuity of service

Alternator – also called as synchronous generators. An alternating current generator. Requirements for parallel operation of alternator

1. operating frequency must be equal 2. line to line voltage must be equal 3. phase sequence must be the same

TRANSFORMERS: Characteristics:

a. According to the core used 1. Core type transformer 2. Shell type transformer

b. According to method of cooling the windings

1. Self-cooled transformer 2. Oil-self cooled transformer 3. Forced-oil cooled transformer 4. Force-air cooled transformer

c. According to purpose of applications 1. Distribution type transformer 2. Instrument type transformer 3. Power transformer 4. Welding transformer 5. Rectifier transformer 6. Regulating transformer 7. Lighting transformer

d. According to voltage transformation 1. Step-up transformer (low to high) 2. Step-down transformer (high to low)

Parallel operation of transformers Requirements:

1. voltage ratio must be the same 2. transformers must be properly connected as to polarity 3. the ratio of the equivalent resistance to reactance of all transformers should be the

same Autotransformer – a transformer with only one winding common to both primary and secondary windings. Instruments transformers – used in conjunction with an ammeter or a voltmeter to measure relatively large values of current or voltage.

a. Current transformer (CT) – the primary terminals is connected in series to line in which the current flowing through it is to be measured while an ammeter of suitable range is connected across the secondary terminals.

b. Potential transformer (PT) – the primary terminals is connected across the high voltage

line in which the voltage across it is to be measured while a voltmeter of suitable range is connected across the secondary terminals.

Standard kVA ratings of SINGLE PHASE TRANSFORMERS

1, 1 ½, 2, 3, 5, 7 ½,

15, 20, 25, 30, 37 ½,

50, 75, 100, 150, 167,

200, 250, 333 and 500

Standard kVA ratings of THREE PHASE TRANSFORMERS

3, 6, 9,

15, 20, 25, 30, 37 ½,

45, 50, 60, 75, 100,

112 ½, 150, 200, 225, 300,

400, 500, 750, 1000,

1500 and 2000