hour 1 - tripodet2520.tripod.com/dc_machines.pdfelectrical machines 1 ive (ty) department of...
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slide 1
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Electrical Machines 1
Hour 1
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slide 2
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Electrical Machines• An electromechanical converter which is used
to continuously translate electrical input to mechanical output, or vice versa.
• The process of translation is known as Electromechanical Energy Conversion.
T, ne, i
Electrical system
Mechanical system
MotorGenerator
Electricmachine
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slide 3
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Generator
• The electromechanical energy conversion is from mechanical to electrical
Mechanical Power
Electrical Power
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slide 4
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Motor
• The electromechanical energy conversion is from electrical to mechanical
Electrical Power
Mechanical Power
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slide 5
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Electromagnetic Conversion (P.121 – P.123)
1 When a conductor moves in a magnetic field, voltage is induced in the conductor
e=Blv
2. When a current-carrying conductor is placed in a magnetic field, the conductor experiences a mechanical force.
F=BlI
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slide 6
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Basic Structure of Electrical Machines (P.123 - P.125)
Stator
• This part of the machine does not move and normally is the outer frame of the machine.
Rotor
• This part of the machine is free to move and normally is the inner part of the machine.
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slide 7
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
DC Machines
DC machines are versatile and extensively used in industry. A wide variety of volt-ampere or torque-speed characteristics can be obtained from various connections of the field windings.
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slide 8
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
DC Machine Construction (sect.4.2.1)
• shunt field winding• series field winding
Stator has salient poles that are excited by one or two field windings:
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slide 9
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
DC Machine Construction (cont)
• Armature winding is placed on the rotor
• Voltage induced in armature winding is alternating (Fig.4.12)
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slide 10
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
DC Machine Construction (cont)
• Commutator-brush combination is used as a mechanical rectifier to make the armature terminal voltage unidirectional and also to make the mmf wave due to armature current fixed in space
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slide 11
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Voltage Rectification by Commutators (sect. 4.2.2)
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slide 12
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Multi-turn Machine
In an actual machine a large number of turns are placed in several slots around the periphery of the rotor to reduce the ripple.
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slide 13
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Close-up of Commutator (1/2)
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slide 14
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Close-up of Commutator (2/2)
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slide 15
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Electrical Machines 1
Hour 2
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slide 16
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Armature Windings (Sect.4.2.3)
Terms:• A turn consists of 2 conductors connected to one end
by an end connector.
• A coil is formed by connecting several turns in series
• A winding is formed by connecting several coils in series
• See. Figure 4.15
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slide 17
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Turn, Coil and Winding
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slide 18
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Lap Winding (Fig. 4.17)
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slide 19
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Lap Winding (Fig. 4.17)
• The 2 ends of a coil are connected to adjacent commutator segments.
• The number of brushes is equals to the number of poles.
• The number of parallel paths is equals to the number of poles.
• Each path takes 1/p times the load current (p = no. of poles)
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slide 20
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Lap Winding (Fig. 4.17)
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slide 21
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Wave Winding (Fig. 4.18)
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slide 22
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Wave Winding (Fig. 4.18)
• The 2 ends of a coil are connected to commutatorsegments which separated by twice the pole-pitch.
• Only 2 brushes are necessary, irrespective of the number of poles, but 4 or more may be used.
• The number of parallel paths is always 2.
• Each path takes 1/2 of the load current.
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slide 23
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Wave Winding (Fig. 4.18)
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slide 24
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Example
A 6-pole armature is wound with 498 conductors. The flux and the speed are such that the average emf generated in each conductor is 2V. The current in each conductor is 120A. Find the total current and the generated emf of the armature if the winding is connected (a) wave, (b) lap. Also find the total power generated in each case.
240A, 498V; 720A, 166V; 119.5kW
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slide 25
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Armature Induced Voltage
Emf EquationLet Φ = Total flux per pole in Webers
n = Speed of the armature in rev/sec
p = number of pairs of poles (pole-pair)
Z = Total number of conductors on armature
a = Number of parallel paths through armature
(wave winding, a = 2, lap winding, a = 2p)
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slide 26
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Emf Equation
Total number of poles = 2p
A particular conductor pass 2pn poles/sec
Time taken tp pass one pole = 1/2pn sec
Emf induced per conductor = dΦ/dt
= Φ/1/2pn
= 2pnΦ volts
On the armature, there are Z/a conductors in series
∴ Total induced emf, Ea = 2pnΦZ/a volts
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slide 27
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Armature Voltage (Sect. 4.2.4)
As the armature rotates in the magnetic field produced by the stator poles, voltage is induced in the armature winding:
Ea= KaΦωm (Eq.4.9 P.138)
(where Ka=Np/πa or Ka=Zp/2πa)N=No.of turns, andp= No. of poles Ka
Ea= generated voltage (Generator)
Ea= back emf (Motor)
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slide 28
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Armature Voltage Ex 1
The wave wound armature of a 6-pole DC generator has 30 slots and in each slot there are 8 conductors. The flux per pole is 0.0174Wb. Calculate the value of the emf generated when the speed of the armature is 1200 rev/min.
(Ans: 250.56V)
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slide 29
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Armature Voltage Ex 2
A lap wound DC generator is to have an output voltage of 500V at 26 rev/s. The armature has 28 slots each containing 12 conductors. Calculate the required value of flux per pole.
(Ans: 0.057Wb)
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slide 30
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Magnetization Curve (Sect. 4.2.6)
• Flux-mmf relation (Fig. 4.22)
∝ Field Current
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slide 31
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Magnetization Curve (Fig.4.23)
Also known as:• Saturation Curve• Open-Circuit Characteristic
the magnitude of induced emf is depending on the rotor speed
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slide 32
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Electrical Machines 1
Hour 3
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slide 33
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Armature Reaction (P.147 to P.150)
• With no current flowing in the armature, the flux in the machine is established by the mmf produced by the field current.
• However, if the current flows in the armature circuit it produces its own mmf (hence flux) acting along the q-axis.
• Therefore, the original flux distribution in the machine due to the field current is disturbed.
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slide 34
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Armature Reaction
1. Evenly distributed flux due to Field Current alone
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slide 35
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Armature Reaction
2. Flux Distribution due to Armature Current alone
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slide 36
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Armature Reaction
3. Overall Flux Distribution (M.N.A. Shifted)
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slide 37
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Effects of Armature Reaction
• Distort the air-gap flux pattern of the machine.• In DC generator, the magnetic neutral axis is being
shifted by some angle θ in the direction of rotation.• The distorted flux density weakening one pole tip and
strengthening the other.• Uneven flux density distribution which will result in a
reduction in the total flux (due to magnetic saturation).• Due to the field is distorted, there is an emf between the
commutator segments at the instant when both touch the same brush. This emf generates a brief, high current that causes excessive sparking and arcing as the commutator rotates
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slide 38
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Summary of AR
Leading Pole-tip
TrailingPole-tip
MagneticNeutral
AxisGenerator Weakened Strengthened Shifted
ForwardMotor Strengthened Weakened Shifed
Backward
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slide 39
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Commutation (Sect 4.3.5)
• Ideal commutation curve [Fig. 4.46(c)].
• 2 reasons for non-ideal commutation:
• Coil inductance
• Reactance voltage
• Causes sparking [Fig. 4.46(d)].
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slide 40
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Commutation Curve (Fig.4.46c)
Ideal Actual
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slide 41
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Methods of Reducing Sparking(Remedies for Armature Reaction)
• Use of high-resistance brushes
• Brush Shifting
• Interpoles
• Compensating Winding
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slide 42
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Interpoles (Sect 4.3.5)
• Commutation pole.
• Small poles which are situated between the main poles.
• Its winding carries the armature current in such a direction that its flux opposes the q-axis flux produced by the armature current flowing in the armature winding [see Fig. 4.46(e)].
• The net flux in the interpole region is almost zero.
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slide 43
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Interpoles (Sect 4.3.5)
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slide 44
Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Compensating Winding (P.150 to P.151)
• Winding which is fitted in slots cut on the main pole faces.
• They are arranged that the mmf produced by currents flowing in these windings opposes the armature mmf [see Fig. 4.33(a)].
• Compensating winding is connected in series with the armature winding so that its mmf is proportional to the armature mmf [see
Fig. 4.33(b)].
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Electrical Machines 1IVE (TY)IVE (TY)Department of EngineeringDepartment of Engineering
Compensating Winding (Fig.4.33)