41669728-DC-machines

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UNIT-1 DC MACHINES:

UNIT-1 DC MACHINES CONSTRUCTION

PARTS OF DC MACHINE :

PARTS OF DC MACHINE Major parts of the machine are, Magnetic frame or Yoke Poles, interpoles, windings, pole shoes Armature Commutator Brushes, Bearings and shaft.

CROSS SECTIONAL VIEW OF DC MACHINE:

CROSS SECTIONAL VIEW OF DC MACHINE

MAGNETIC FRAME:

MAGNETIC FRAME It serves two purposes protecting cover to the whole machine and mechanical support to the poles Allow the flux to pass through Materials used For small machine – cast iron For large machine – cast steel

FIELD POLES:

FIELD POLES

FIELD POLES:

FIELD POLES To minimize the eddy current losses, the poles are laminated

INTERPOLES:

INTERPOLES The interpoles are located at the geometric neutral points midway between the main poles and provide reversing magnetic field of proper strength and polarity. The interpole must have sufficient strength to overcome the armature reaction and provide a reversing field, therefore, it is connected in series with the armature winding.

ARMATURE:

ARMATURE

ARMATURE:

ARMATURE

LOSSES OCCUR IN ARMATURE:

LOSSES OCCUR IN ARMATURE Hysteresis losses To reduce losses low hysteresis steel with few percent of silicon is used in armature. Eddy current losses To minimize the losses, the armature core is laminated.(stampings : 0.4mm – 0.5mm thick)

BRUSHES AND COMMUTATORS:

BRUSHES AND COMMUTATORS

COMMUTATOR:

COMMUTATOR The commutator converts the alternating emf into unidirectional or direct emf. The armature coil leads are soldered to each commutator segment by a riser.

BRUSHES:

BRUSHES Made up of carbon or graphite, collects the current from the commutator and convey it to the external load resistance.

BEARINGS:

BEARINGS

END BELLS:

END BELLS

END BELLS:

END BELLS

CUTAWAY VIEW:

CUTAWAY VIEW

PRINCIPLE OF OPERATION:

PRINCIPLE OF OPERATION

PRINCIPLE OF OPERATION:

PRINCIPLE OF OPERATION According to faraday’s law e = -N d Φ /dt Initially, e = 0 After time t, e = -d Φ /dt (N = 1) Φ = B lb cos θ e = Em sin ω t (Em = B lb ω )

UNIDIRECTIONAL OUTPUT:

UNIDIRECTIONAL OUTPUT

INDUCED EMF EQUATION:

INDUCED EMF EQUATION Emf induced in the conductor = rate of change of flux cut. For 1 complete revolution Φ = P Φ webers time = 60 / N e = d Φ /dt = P Φ / (60/ N). e = P Φ N/ 60 volts Z/A conductors in series in each parallel path, then the emf induced is Eg = Φ ZN/ 60 *(P/A)

INDUCED EMF EQUATION:

Induced emf Eg = Φ ZN/ 60 *(P/A) For lap winging, A=P, Eg = Φ ZN/ 60 For wave winding, A=2, Eg = Φ ZN/ 60 *(P/2) INDUCED EMF EQUATION

Field Excitations of DC generators :

Field Excitations of DC generators Type of DC Motors • Separately Excited generator • Self Excited generator -- Permanent magnet generator -- Shunt excited generator -- Series excited generator -- Compound excited generator

Separately excited DC generator:

Separately excited DC generator Field winding is excited by separate DC supply

Separately excited DC generator:

Separately excited DC generator Field winding is excited by separate DC supply Armature current Ia = load current I L Terminal voltage V = Eg – Ia Ra – Vbrush Vbrush is drop across brush, low value, so neglected. Generated emf Eg = V + Ia Ra + Vbrush

Self excited DC generator:

Self excited DC generator Generator field winding is supplied from the armature of the generator itself. Excitation occurs due to presence of residual flux in the poles. Process: armature cut the residual flux, small emf will induced, this produces small field current in the field winding. Then flux per pole increases. Then by cumulative process, generator produces its rated voltage.

Series DC generator:

Series DC generator Field winding is connected in series with the armature. Ia = Ise = IL Eg = V + Ia Ra + Ia Rse + Vbrush V = Eg - Ia Ra - Ia Rse – Vbrush Vbrush – neglected (low value)

Shunt DC generator:

Shunt DC generator Field winding is connected across the armature. Eg = V + Ia Ra V = Eg – Ia Ra Ish = V /Rsh Ia = IL + Ish

Compound DC generator:

Compound DC generator It consists of both shunt and series field winding. One winding in series (Rse) and other winding is in parallel (Rsh) with the armature. Two types long shunt compound generator short shunt compound generator

Long shunt compound generator:

Long shunt compound generator Shunt field winding is connected across both series field and armature winding I se = Ia = I L + Ish Ish = V / Rsh Eg = V + Ia (Ra + Rse) + Vbrush V = Eg - Ia (Ra + Rse) – Vbrush Vbrush – neglected (low value)

Short shunt compound generator:

Short shunt compound generator Shunt field winding is connected in parallel with the armature and this combination is series to series field winding I se = I L Ia = I se + Ish Ish = V / Rsh Eg = V + Ia Ra + I se Rse + Vbrush V = Eg - Ia Ra - I se Rse – Vbrush Vbrush – neglected (low value)

Short shunt compound generator:

Voltage across shunt field winding = Ish Rsh Ish Rsh = Eg – Ia Ra – Vbrush Substitute Eg value in the above equation Ish Rsh = V + Ise Rse Shunt field current Ish = (V + Ise Rse)/ Rsh Power developed in armature = Eg Ia Power delivered to load = V I L (power formula is same for all types of DC generator ) Short shunt compound generator

Characteristic of self excited DC generator:

Characteristic of self excited DC generator Types of characteristics open circuit characteristics (occ) or magnetisation characteristics (Eg vs I f ) Internal characteristics or total characteristics (E vs Ia) External characteristics or voltage regulated characteristics (V vs I L )

DC series generator:

DC series generator Ia = Ise =I L Curve 1 : OC characteristics Curve 2 : Internal characteristics (drop due to armature reaction) Curve 3 : External characteristics (drop due to armature resistance & series field resistance) Increase in IL, decreases the terminal voltage V V = E – Ia (Ra + Rse)

DC shunt generator:

OCC Due to residual magnetism, field current is not zero initially. Due to this voltage, field current increases and emf also increases. DC shunt generator

DC shunt generator:

Critical resistance (Rc) A tangent line is drawn linear to occ from origin. Slope Rc = OM / ON; (Eg / If) Conditions to build excitation Presence of residual magnetism Shunt field coil should be properly connected to armature terminals Shunt field resistance is less than Rc DC shunt generator

DC shunt generator:

DC shunt generator Ia = Ish + IL Curve 1 : Ideal DC characteristics (IL , V = const ) (Eg = V) Curve 2 : Internal characteristics (drop due to armature reaction) Curve 3 : External characteristics (drop due to armature resistance) Increase in IL, decreases the terminal voltage V V = E – Ia Ra

Compound generator:

Compound generator It consists of series field and shunt field winding. Curve 1 : Flat compound (Eg = V) Flux drop in shunt field is compensated by flux rise in series field Curve 2 : over compound (V > Eg) Series field excitation is more than shunt field Curve 3 : under compound (V < Eg) Series field excitation is less than shunt field

DC MOTOR:

DC MOTOR

Principle of operation:

Principle of operation Construction of DC machines are same. Principle: “whenever a current carrying conductor is placed in a magnetic field, it experiences a force tending to move it”

Principle of operation:

Magnitude of force experienced by the conductor in a motor F = B I l Newtons where, B = field density Wb/ m2 I = current in amperes l = length of the conductor in metres. Principle of operation

Principle of operation:

Direction of motion is given by Fleming’s left hand rule Thumb – direction of motion of conductor Fore finger – direction of field Middle finger – direction of current Three fingers are mutually perpendicular to each other Principle of operation

Back EMF:

Back EMF Due to generator action take place, emf induced i.e even when the machine is working as a motor, voltages are induced. Back emf cause rotation which in turn opposes the supply voltage. (Lenz’s law) Back emf = Φ ZN / 60 (P / A) volts Eb α K Φ N Voltage of the dc motor V = Eb + Ia Ra

Torque equation:

Torque equation Magnitude of torque developed by each conductor T = B I l r Nm Total torque developed by the armature ( Z conductors) Ta = B I l r Z Nm I = Ia / a ; B = Φ / A ; A = 2 Π rl / P Ta = Z Φ Ia P / 2 Π a Nm Ta = K Φ Ia ; K = ZP / 2 Π a - constant

Characteristic of self excited DC motor:

Three characteristics Speed – armature current characteristics Torque – armature current characteristics (electrical characteristics) Speed – torque characteristics (mechanical characteristics) Characteristic of self excited DC motor

Shunt motor characteristics:

Shunt motor characteristics N Vs Ia N = k (V – Ia Ra) / Φ Ish and Φ are nearly constant Speed nearly constant except Ia Ra drop. T Vs Ia T α Ia ( Φ = constant) T ↑ ; Ia ↑ N Vs T Ia ↑ ; N ↓ ; T ↑

Series motor characteristics:

Series motor characteristics N Vs Ia N = k (V – Ia Ra) / Φ ; I L = Ia = Ise Φ α Ia Ia ↑ ; N ↓ (series motor is started with some load) T Vs Ia T α Ia Φ ; T α Ia 2 T ↑ ; Ia ↑ (increases parabolic) N Vs T Ia ↑ ; N ↓ ; T ↑

Compound motor:

It is the combination of series and shunt characteristics. Cumulative – series and shunt field windings are assist each other Differential - series and shunt field windings are opposing each other Compound motor

Starters :

Starters Necessity of starters : Amount of line current can be controlled at the time of starting Types of starters: Two point starter Three point starter Four point starter

Speed control of DC shunt motor:

Speed control of DC shunt motor Three methods of speed control By varying the resistance in the armature circuit (Rheostatic control) By varying the flux (flux control) By varying the applied voltage (voltage control) (Also ward leonard method)

Testing of DC machine:

Testing of DC machine Brake test direct method to find efficiency Swinburne’s test predetermine the efficiency

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