EET 306 ELECTRICAL MACHINES. Syafruddin Hasan. syafruddin DC machines

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1 EET 306 ELECTRICAL MACHINES Syafruddin Hasan 1

2 DC Machines The direct current (dc) machine can be used as a motor or as a generator. DC Machine is most often used for a motor. The major advantages of dc machines are the easy speed and torque regulation. However, their application is limited to mills, mines and trains. As examples, trolleys and underground subway cars may use dc motors. In the past, automobiles were equipped with dc dynamos to charge their batteries. 2

3 DC MACHINES Even today the starter is a series dc motor However, the recent development of power electronics has reduced the use of dc motors and generators. The electronically controlled ac drives are gradually replacing the dc motor drives in factories. Nevertheless, a large number of dc motors are still used by industry and several thousand are sold annually. 3

4 DC MACHINES: INTRODUCTION DC machine=stator + rotor (armature) Stator: does not move, the outer frame of the machines is made of ferromagnetic materials. Rotor (Armature): free to move, the inner part of the machine is made of ferromagnetic materials. Field winding: is wound on the stator poles to produce magnetic field (flux) in the air gap. Armature winding: is composed of coils placed in the armature slots. Commutator: is composed of copper bars, insulated from each other. The armature winding is connected to the commutator. Brush: is placed against the commutator surface. Brush is used to connect the armature winding to external circuit through commutator. 4

5 DC MACHINES: INTRODUCTION In DC machines, conversion of energy from electrical to mechanical form or vice versa results from the following two electromagnetic phenomena 1. When a conductor moves in a magnetic field, voltage is induced in the conductor. 2. When a current carrying conductor is placed in magnetic field, the conductor experiences a mechanical forces 5

6 DC MACHINES: CONSTRUCTION 6

7 DC MACHINES: CONSTRUCTION 7

8 DC Machine Construction 8

9 DC Machine Construction (cont..) The stator of the dc motor has poles, which are excited by dc current to produce magnetic fields. In the neutral zone, in the middle between the poles, commutating poles are placed to reduce sparking of the commutator. The commutating poles are supplied by dc current. Compensating windings are mounted on the main poles. These short-circuited windings damp rotor oscillations. The poles are mounted on an iron core that provides a closed magnetic circuit. The motor housing supports the iron core, the brushes and the bearings. 9

10 DC Machine Construction (cont..) The rotor has a ring-shaped laminated iron core with slots. Coils with several turns are placed in the slots. The distance between the two legs of the coil is about 180 electric degrees. The coils are connected in series through the commutator segments. The ends of each coil are connected to a commutator segment. The commutator consists of insulated copper segments mounted on an insulated tube. Two brushes are pressed to the commutator to permit current flow. The brushes are placed in the neutral zone, where the magnetic field is close to zero, to reduce arcing. 10

11 DC Machine Construction (cont..) The rotor has a ring-shaped laminated iron core with slots. The commutator consists of insulated copper segments mounted on an insulated tube. Two brushes are pressed to the commutator to permit current flow. The brushes are placed in the neutral zone, where the magnetic field is close to zero, to reduce arcing. 11

12 DC Machine Construction (cont..) The commutator switches the current from one rotor coil to the adjacent coil, The switching requires the interruption of the coil current. The sudden interruption of an inductive current generates high voltages. The high voltage produces flashover and arcing between the commutator segment and the brush. 12

13 DC Machine Construction (cont..) Rotation I r_dc /2 Shaft Brush I r_dc I r_dc /2 Pole winding N 7 3 S Insulation Rotor Winding I r_dc Copper segment Commutator with the rotor coils connections 13

14 DC Machine Construction (cont..) Details of the commutator of a dc motor. 14

15 DC Machine Construction (cont..) Rotor of a dc machine 15

16 DC Machine Construction (cont..) DC motor stator with poles visible. 16

17 DC Machine Construction (cont..) Cutaway view of a dc motor. 17

18 Armature Windings The armature windings are usually former-wound. This are first wound in the form of flat rectangular coils and are then puller. Various conductors of the coils are insulated each other. The conductors are placed in the armature slots which are lined with tough insulating material. This slot insulation is folded over above the armature conductors placed in the slot and is secured in place by special hard wooden or fibre wedges. Lap and wave Windings There are two types of windings mostly employed: Lap winding Wave winding The difference between the two is merely due to the different arrangement of the end connection at the front or commutator end of armature. 18

19 Types of DC Machines DC machines are usually classified according to the way in which their fields are excited. 1. Separately-excited DC machine 2. Self-excited DC machine (a) Shunt DC machine (b) Series DC machine (c) Compound DC machine (i) Long shunt compound (ii) Short shunt compound 19

20 Generating an AC Voltage The voltage generated in any dc generator inherently alternating and only becomes dc after it has been rectified by the commutator. 20

21 Direct Current Generator 21

22 Different Between AC and DC Generators DC AC AC or DC 22

23 Generated emf or back emf or emf Equation of a DC Generator General form of generated emf, or where Note: A = 2 A = P E = C N Φ (for wave winding) (for lap winding) Φ = flux/pole (Wb) Z = total number of armature conductors = number of slots x number of conductor/slot P = number of poles A = number of parallel paths in armature N = armature rotation (rpm) E = emf induced in any parallel path in armature 23

24 Types of Generators Separately-excited generators are those whose field magnetics are energized from an independent external source of dc current. Equivalent circuit of separately excited dc gen. Separately excited 2-pole generator V = E I a. R a V = terminal (load) voltage E = emf induced I a = armature current I L = load current R a = armature circuit resistance V f = field circuit voltage R f = field circuit resistance I f = field current I a = I L V = I. R f f f 24

25 (b) Self excited generators are those whose field magnets are energized by the current produced by the generators them selves. (1) Shunt Generator. The field windings are connected across or parallel with the armature conductors and the full voltage of the generator across them. DC shunt generator V E I. R a a V = V= I. R = f sh sh Equivalent circuit of dc shunt generator where I sh = shunt field current R sh = shunt field resistance 25

26 (2) Series Generator. In this case, the field windings are joined in series with the armature conductors. As they carry full load current, they consist of relatively few turns of thick wire or strip. Such generators are rarely used. V = E I + a( Ra Rse) where R se = series field resistance I se = series field current 26

27 (3) Compound Generator. It is a combination of a few series and a few shunt windings and can be either short or long-shunt. short-shunt compound dc generator or 27

28 Long-shunt compound dc generator Note : Compound generator also refer as: 1. Cumulatively compounded generator, where both a shunt and a series field are present, and their effects are additive 2. Differentially compounded generator, where both a shunt and a series field are present, and their effects are subtractive. 28

29 An 8-pole dc shunt generator with 778 wave connected armature conductors and running at 500 rpm, supplies a load of 12.5 Ω resistance at a terminal voltage of 250 V. The armature resistance is 0.24 Ω and the field resistance is 250 Ω. Find the armature current, the induced emf and the flux per pole. 29

30 Total Loss in a DC Generator a). Magnetic losses (also known as iron or core losses) Due to the rotation of the iron core of the armature in the magnetic flux of the field poles, there are some losses taking place continuously in the core and are known as iron or core losses. Iron losses consist of (i) Hysteresis loss (ii) Eddy current losses. b) Copper losses (i) Armature copper loss = I a2 R a (not E I a ) (ii) Field copper loss In the case of shunt generator it is practically constant In the case of series generator P sh = I sh2 R sh = V I sh P se = I se2 R se (iii) The loss due to brush contact resistance. This is usually included in the armature copper loss. 30

31 Total Loss in a DC Generator (cont..) c) Mechanical losses These consist of (i) Friction loss at bearing and commutator (ii) Air-fraction or windage loss for rotating armature Note: usually, magnetic and mechanical losses are collectively known as stray losses. Constant or Standing Losses For shunt and compound generator, field Cu loss is constant. Hence, stray losses and shunt Cu losses are constant in their case. These losses are together known as standing or constant losses 31

32 Power-Flow (power stages) Diagram Mechanical Efficiency 32

33 Electrical Efficiency or Overall or Commercial Efficiency or or Note: Useless specified otherwise, commercial efficiency is always to be understood. Condition for Maximum Efficiency Efficiency is maximum when Variable loss = Constant loss 33

34 Problems with Commutation in Real Machine In practice, there are two major effects that disturb the commutation process: 1.Armature Reaction 2. L di/dt voltages If the magnetic field windings of a dc machine are connected to a power supply and the rotor of the machine is turned by an external source of mechanical power, then a voltage will be induced in the conductors of the rotor. This voltage will be rectified into dc output by the action of the machine s commutator Now, connect a load to the terminals of the machine, and a current will flow in its armature windings. This current flow will produce a magnetic field of its own, which will distort the original magnetic field from the machine s poles. This distortion of the flux in a machine as the load is increased is called armature reaction 34

35 Armature reaction causes 2 serious problems in real dc machine. Problem 1 : Neutral-Plane Shift Problem 2 : Flux Weakening Solutions to the Problems with Commutation Three approaches have been developed to partially or completely correct the problems of armature reaction and L di/dt voltages: 1. Brush Shifting 2. Commutation poles or interpoles 3. Compensating windings 35

36 Brush Shifting 36

37 Commutation Poles or Interpoles 37