explain the working of dc generator with a neat diagram|dc motor generator

working of dc generator with a neat diagram

dc motor generator

 BASIC STRUCTURE of electrical MACHINES:

A rotating electrical machine has 2 main elements, stator coil and rotor, separated by the air gap.

→ the stator coil of the machine doesn't move and ordinarily is that the outer frame of the machine.

→ the rotor is absolve to move and ordinarily is that the inner a part of the machine. each stator coil and rotor square measure manufactured from magnetic force materials. Slots square measure cut on the inner outer boundary of the stator coil and therefore the outer outer boundary of the rotor. Conductors square measure placed within the slots of the stator coil or rotor. they're interconnected to make winding. The winding during which voltage is iatrogenic is termed the coil winding.

          The winding through that a current is passed to provide the most flux is termed the sphere winding.

          Permanent magnets square measure employed in some machines to supply the most flux of the machine.

There square measure 2 varieties of D.C. machines, the D.C. generator and therefore the D.C. motor. The D.C. generator converts energy into voltage. The D.C. motor converts voltage into energy. The D.C. generator is predicated on the principle that once a conductor is revolved in an exceedingly D.C. magnetic flux, a voltage are going to be generated within the conductor.

working of dc generator:

A D.C. generator consists of 3 main elements.

1. Magnetic-field system

2. Armature

3. electrical switch and brush gear

Magnetic-field System of dc motor generator:

The magnetic flux system is that the stationary (fixed) a part of the machine

It produces the most magnetic flux. The outer frame or yoke could be a hollow of solid steel or rolled steel. an excellent range of pole cores square measure locked to the yoke.

The yoke serves the subsequent 2 purposes:

(a) It supports the pole cores and acts as protective cowl to the machine

(b) It forms a region of the magnetic circuit.

Since the poles project inwards they're referred to as salient poles. Each pole.com incorporates a pole shoe having a wiggly surface.

The pole shoe serves 2 purposes:

(1) It supports the sphere coils.

(2) It will increase the cross-sectional space of the magnetic circuit and reduces its reluctance.

 The pole cores square measure manufactured from sheet steel laminations that square measure insulated from one another and riveted along. The poles square measure laminated to scale back eddy-current loss.

Each pole core has one or additional field coils (windings) placed over it to provide a magnetic flux. the sphere coils (or exciting coils) square measure connected asynchronous with each other such once the present flows through the coils, alternate north and south poles square measure created within the direction of rotation.

Armature of dc motor generator:

The rotating a part of the D.C. machine is termed the coil. The coil consists of a shaft upon that a laminated cylinder, referred to as coil core, is mounted. The mature core has grooves or slots on its outer surface. The laminations square measure sulated from one another and tightly clamped along. In tiny machines the minations square measure keyed on to the shaft. In massive machines they're mounted on epider. the aim of exploitation laminations is to scale back eddy-current loss.

The insulated conductors square measure place within the slots of the coil core. The conductors square measure impacted and bands of steel wire square measure fixed around the core to

Event them flying below centrifugal forces. The conductors square measure appropriately connected. This connected arrangement of conductors is termed coil winding. 2 varieties of windings square measure used-wave and lap.

Commutator and Brush gear:

Alternating voltage is created in an exceedingly coil rotating in an exceedingly magnetic flux. to get DC within the external circuit a electrical switch is required. The electrical switch, that rotates with the coil, is formed from variety of wedge-shaped hard-drawn copper bars or segments insulated from one another and from the shaft. The segments kind a hoop round the shaft of the coil. every electrical switch phase is connected to the ends of the coil coils .

Current is collected from the coil winding by suggests that of 2 or additional carbon brushes mounted on the electrical switch. every brush is supported in an exceedingly metal box referred to as a brush box or brush holder. The pressure exerted by the brushes on the electrical switch may be adjusted and is maintained at a continuing worth by suggests that of Springs, Current created within the coil winding is passed on to the electrical switch and so to the external circuit by suggests that of brushes.

MAGNETIC CIRCUIT OF A dc motor generator:

The magnetic circuit of a four-pole D.C. generator is shown in image. The broken lines indicate the most flux ways. Flux created by the sphere winding of a generator is established within the pole cores, air gap, coil core and gap is that the area between the coil surface and therefore the pole face. This as tiny as doable.

 

TYPES OF D.C. MACHINE :

The magnetic flux in an exceedingly D.C. machine is created by field coils carrying current. the assembly of magnetic flux within the machine by current current within the coil is termed excitation. There square measure 2 ways of excitation, particularly separate excitation and self excitation. In separate excitation the sphere coils square measure energized by a separate d.c. me. In self-excitation the present flowing through the sphere winding is equipped by the machine itself.

Direct current machines square measure named in step with the affiliation of the sphere winding with the coil.

The principal varieties of D.C. machine are:

(1)Separately excited D.C. dc motor generator .

(2)Shunt wound or shunt machine.

(3)Series wound or series machine.

(4)Compound wound or compound machine.

The four varieties of machines given higher than can be either generators or motors.

ARMATURE REACTION IN DC GENERATORS:

Armature reaction is that the impact of magnetic flux created by coil current upon the distribution of flux underneath the most poles.

A 2-pole D.C. generator. once there's no load connected to the generator, this within the coil conductors is zero. underneath these conditions there exists in it solely the mmf of the most poles that manufacture the most the most.This flux is distributed symmetrically with regard to the polar axis, that is, the centre line of the north and also the south poles. The direction of is shown by associate degree arrow. The magnetic neutral axis or plane (MNA) could be a plane perpendicular to the axis of the flux.

The MNA coincides with the geometrical neutral axis or plane (GNA). Brushes square measure continually placed on MNA. Hence, MNA is additionally referred to as the axis of commutation.

Armature conductors carrying current with no current in field coils. The direction of current within the coil conductors is also determined by Fleming's right-hand rule. this flows within the same direction altogether the conductors lying underneath one pole. The direction of flux created by coil conductors is also determined by cork-screw rule. The conductors on the left-hand facet of the coil carry current within the direction into the paper. The flux created by current in these coil conductors in shown in image.

These conductors mix their mmfs to supply a flux through the coil within the downward direction. Similarly, the conductors on the right-hand facet of the coil carry current within the direction out of the paper. These conductors conjointly mix their mmfs to supply a flux through the coil within the downward direction. Thus, the conductors on either side of the coil mix their mmfs in such a way on send a flux through the coil within the downward direction. This flux φ, is pictured by associate degree arrow as shown in image.

 

The coil flux created is analogous to it created within the equivalent iron-cored magnet with its axis on the comb axis.

Shows the condition once the sphere current and coil current square measure acting at the same time. this happens once the generator is on load. currently there square measure 2 fluxes within the machine, one created by the most field poles of the generator and also the different by this within the coil conductors. These 2 fluxes currently mix to make a resultant flux are as shown in image.

 

It is seen that the sphere flux getting into the coil isn't solely shifted however a distorted. The distortion produces state of affairs of the flux (increase within the flux density) within the higher pole basketball shot the N-pole and within the lower pole basketball shot the S-pole. Similarly, there's a discount of flux (decreased flux density) within the lower tip of the N-pole and within the higher pole tip of the S-pole. The direction of the resultant

Has shifted within the direction of rotation of the generator. Since the MNA is usually perpendicular to the axis of the resultant flux, the MNA is additionally shifted.

Because of the nonlinear behavior and saturation of the core, the rise in one pole tip is a smaller amount than the decrease in flux within the different pole tip sequent, the most pole flux is weakened. Since E =kNφ, the reduction within the within the decreases the terminal voltage of a generator with enlarged load. Effects of coil Reaction

The effects of coil reaction square measure summarized below:

1. Magnetic concentration is enlarged over one 1/2 the pole and weakened over the opposite [*fr1]. however the full flux created by every pole is slightly reduced and, therefore, the terminal voltage is slightly reduced. The impact of total flux reduction by coil reaction is

Known as demagnetizing impact.

 2. The flux wave is distorted and there's shift within the position of the magnetic neutral axis (MNA) within the direction of rotation for the generator and against the direction of rotation for the motor.

3. coil reaction establishes a flux within the neutral zone (or commutating zone). coil reaction flux within the neutral zone can induce conductor voltage that aggravates the commutation drawback.

COMMUTATION of dc motor generator :

The currents iatrogenic within the coil conductors of a d.c. generator square measure alternating in nature. The commutation method involves the amendment from a generated electricity to associate degree outwardly applied DC. These iatrogenic currents flow in one direction once the coil conductors square measure underneath North Pole. {they square measure|they're} in other way once they are underneath South Pole. As conductors pass out of the influence of North Pole and enter the South Pole, this in them is reversed. The reversal of current takes place on the MNA or brush axis. Whenever a brush spans 2 electric switch segments, the winding part connected to those segments is brief circuited. By commutation we have a tendency to mean the amendment that crop up in a very winding part throughout the amount of short by a brush. These changes square measure shown in Fig. 6.14. For simplicity, contemplate a straightforward ring winding.

In position, this I flowing towards the comb from the left-hand facet pass around the coil in a very dextral direction.

In position, this coil carries constant current within the same direction, however the coil is to short circuited by the comb.

In position, the comb makes contact with thereby short circuiting coil one. this continues to be I from the left-ban from the right-hand facet. it's seen that these 2 currents will while not passing through coil one.

The picture Shows that bar b has simply left the comb and also the short of a has terminated. it's currently necessary for this I reaching the comb from the Hand facet within the anticlockwise direction. From the on top of discussion it's seen that in the amount of tangency of coil coil by a brush the present in this coil should be reversed and conjointly referred to to its full worth within the reversed direction. The time of tangency is named the amount of commutation.

The fig shows however the present within the short-circuited coil varies throughout the temporary interval of the tangency. Curve B shows that the present changes from + I - Linearly within the commutation amount. Such a commutation is named ideal amputation or straight-line commutation.

 

If the present through coil one has not reached its full worth within the position shown in Fig. 6.14(d), then since coil two is carrying full current, the distinction between the currents, through components two and one must jump from switch bar b to the comb within the kind of a spark. Thus, the explanation for sparking at the switch is that the failure of the present within the short-circuited components to succeed in the complete worth within the reversed direction by the tip of tangency.

This is called underneath commutation or delayed commutation. The curve of current against the clock in such a case is shown is Fig. 6.15 by curve A. In ideal commutation (curve B) the present of the commutating coils changes linearly from + I to -I within the commutation amount. Curve C represents over commutation or accelerated commutation once the present reaches its final worth with a zero rate of amendment at the tip of the commutation amount. sometimes this may lead to a satisfactory commutation.

In actual follow, the present within the short-circuited coil when commutation amount doesn't reach its full worth. this can be because of the very fact that the short-circuited coil offers coefficient additionally to resistance. the speed of amendment of current is therefore nice that the coefficient of the coil sets up a back electrical phenomenon that opposes the reversal. Since the present within the coil must amendment from + I to - I, the whole amendment fifteen twenty one. If t, is that the time of tangency and L is that the inductance of the coil = coefficient of the short-circuited coil + mutual inductances of the neighboring coils), then the typical worth of the self iatrogenic voltage is

This is known as the electrical phenomenon voltage.

The large voltage showing between switch segments to is connected causes sparking at the brushes of the machine switch is way harmful and it'll harm each commutare brushes. Its impact is additive which can result in a brief circuit with associate arc around the switch from brush to brush. Circuit of the machine

METHODS OF UP COMMUTATION of dc motor generator:

There area unit 3 ways of getting sparkles commutation:

> Resistance commutation

>Voltage commutation

>Compensating windings

Resistance Commutation

This methodology of up commutation consists of exploitation carbon heat This makes the contact resistance between switch segments and he high. This high contact resistance has the tendency to force the present within the shoe circuited coil to alter consistent with the commutation needs, name, reverse so build up within the reversed direction.

Voltage Commutation

In this methodology, arrangement is formed to induce a voltage within the coil undergoing commutation, which can neutralize the electrical phenomenon voltage. This injected voltage is con to the electrical phenomenon voltage. If the worth of the injected voltage is formed capable the electrical phenomenon voltage, fast reversal of current within the short-circuited coil can come about and there'll be sparkless commutation.

Two ways could also be wont to manufacture the injected voltage con to the electrical phenomenon voltage :

1. Brush shift.

2. Commutating poles or interpoles.

Brush Shift:

The impact of coil reaction is to shift the magnetic neutral axis the direction of rotation for the generator and against the direction of roll the motor. coil reaction establishes a flux on the neutral zone. Voltage is elicited within the commutating coil since it's cutting the flux

Commutating Poles or Interlopes:

Interpoles ar slim poles connected to the stator coil yoke, and set up midway between the most poles. Interpoles also are known as commutati compoles. The interpole windings ar connected serial with the coil the interpoles should turn out fluxes that ar directly proportional to the present. The coil and interpole mmfs is affected concurrent same coil current. Consequently, the coil flux within the reverse zone that tends to point out tends to shift the magnetic neutral axis, is neutralised by associate applicable onent of interpole flux. The neutral plane is, therefore, mounted in position despite the load.

          The interpoles should induce a voltage within the conductors undergoing switch that's opposite to the voltage caused by the neutral-plane shift and electrical phenomenon sve. For a generator, the neutral plane shifts within the direction of rotation. Thus, onductors undergoing commutation have a similar polarity of the voltage as a pole they solely left. To oppose this voltage, the interpoles should have the alternative that is that the flux of the most pole ahead per the direction of rotation. For a motor, the neutral plane shifts opposite to the direction of rotation, and he conductors undergoing commutation have a similar flux because the main pole then approaching. For opposing this voltage, the interpoles should have a similar polarity because the previous main pole. therefore we've got the subsequent rules for the polarity of the interpoles

1. for a generator, the polarity of the interpole should be a similar as that of succeeding main pole any ahead within the direction of rotation.

2. For a motor, the polarity of associate interpole is opposite thereto of succeeding main pole within the direction of rotation.

It is to be noted that the interpoles serve solely to produce enough flux to assure smart commutation. they are doing not overcome the distortion of the flux ensuing from cross-magnetizing mmf of the coil.

The use of interpoles is extremely common to almost all dc machines of quite one power unit.

During severe overloads or quickly dynamic  hundreds the voltage between adjacent switch segments could become terribly high. this might ionize the air round the switch to the extent that it becomes sufficiently conductive . associate arc is established from brush to brush. This development is thought as arc. his arc is sufficiently hot to soften the switch segments. It ought to be destroyed quickly. so as to forestall arc compensating windings ar used.

Compensating Windings:

Compensating windings are the most effective means for elimina problems of armature reaction and flashover by balancing the armatu Compensating windings are placed in slots provided in pole faces parali Totor (armature) conductors. These windings are connected in

series armature windings OF dc motor generator:

The direction of currents in the compensating wind be opposite to that in the armature winding just below the pole face compensating winding produces an mmf that is equal and opposite to the mmf. In effect the compensating winding demagnetizes or neutralizes the flux produced by the armature conductors lying just under the pole faces. The per pole is then undisturbed by the armature flux regardless of the load condition

The major drawback with the compensating windings is that they are costly. Their use can only be justified in the following special cases :

1. in large machines subject to heavy overloads or plugging.

2. in small motors subject to sudden reversal and high acceleration of the dc motor generator.

 images for dc motor generator bellow: