What is D.C. Generator

 

D.C. Generator

Construction and working principle of DC Generator

DC Machines are mainly divided in two types Motor and Generator. Here we see only about Generator. Motor will be covered in another blog.

Working principle of DC generator :

All the generators work on a principle of dynamically induced E.M.F. This principle nothing but the Faraday’s law of electromagnetism induction. It states that, “When electric supply given to any conductor it produces magnet fluxes around it these fluxes link with a conductor or a coil. Then voltage is produced in that conductor or coil.’ But this happened only when there exists a relative motion between a conductor and the flux.”

The relative motion can be achieved by rotating conductor with respect to flux or by rotating flux with respect to a conductor. So, a voltage gets generated in a conductor, as long as there exists a relative motion between conductor and the flux. Such an induced EMF which is due to the physical movement of coil or conductor with respect to flux or movement of flux with respect to coil or conductor is called dynamically induced E.M.F.

 

Key Point: So, a generating action requires following basic components to exist,

i)                The conductor or a coil

ii)               The relative motion between conductor and flux.

 In a particular generator, the conductors are rotated to cut the magnetic flux, keeping flux stationary. To have a large voltage as the output, the number of conductors is connected together in a specific manner, to form a winding. This winding is called armature winding of a D.Sc. machine.

The part on which this winding is kept is called armature of a D.Sc. machine. To have the rotation of conductors, the conductors placed on the armature are rotated with the help of some external device. Such an external device is called a prime mover. The commonly used prime movers are diesel engines, steam engines, steam turbines, water turbines etc.

The necessary magnetic flux is produced by current carrying winding which is called field winding. The direction of the induced EMF can be obtained by using Fleming’s right hand rule.

This rule says that if you stretch thumb, index finger and middle finger of your right hand perpendicular to each other, then thumbs indicates the direction of motion of the conductor, index finger indicates the direction of magnetic field i.e. N - pole to S - pole, and middle finger indicates the direction of flow of current through the conductor.

 

 Fig. Flemings Right Hand Rule 

            
                                                         Fig. A Single Loop Generator          

Let’s understand this concept in details. A Single Loop Generator In the figure above, a single loop of conductor of rectangular shape is placed between two opposite poles of magnet. Let's us consider; the rectangular loop of conductor is ABCD which rotates inside the magnetic field about its own axis ab. When the loop rotates from its vertical position to its horizontal position, it cuts the flux lines of the field. As during this movement two sides, i.e. AB and CD of the loop cut the flux lines there will be an emf induced in these both of the sides (AB and BC) of the loop. Single Loop Generator As the loop is closed there will be a current circulating through the loop. The direction of the current can be determined by Fleming’s right hand Rule. This rule says that if you stretch thumb, index finger and middle finger of your right hand perpendicular to each other, then thumbs indicates the direction of motion of the conductor, index finger indicates the direction of magnetic field i.e. N - pole to S - pole, and middle finger indicates the direction of flow of current through the conductor. Now if we apply this right hand rule, we will see at this horizontal position of the loop, current will flow from point A to B and on the other side of the loop current will flow from point C to D. Figure: Single Loop Generator Now if we allow the loop to move further, it will come again to its vertical position, but now upper side of the loop will be CD and lower side will be AB (just opposite of the previous vertical position). At this position the tangential motion of the sides of the loop is parallel to the flux lines of the field. Hence there will be no question of flux cutting and consequently there will be no current in the loop. If the loop rotates further, it comes to again in horizontal position. But now, said AB side of the loop comes in front of N pole and CD comes in front of S pole, i.e. just opposite to the previous horizontal position as shown in the figure beside.

 

Figure: Single Loop DC Generator

 Here the tangential motion of the side of the loop is perpendicular to the flux lines, hence rate of flux cutting is maximum here and according to Fleming’s right hand Rule, at this position current flows from B to A and on other side from D to C. Now if the loop is continued to rotate about its axis, every time the side AB comes in front of S pole, the current flows from A to B and when it comes in front of N pole, the current flows from B to A. Similarly, every time the side CD comes in front of S pole the current flows from C to D and when it comes in front of N pole the current flows from D to C. If we observe this phenomena in different way, it can be concluded, that each side of the loop comes in front of N pole, the current will flow through that side in same direction i.e. downward to the reference plane and similarly each side of the loop comes in front of S pole, current through it flows in same direction i.e. upwards from reference plane. From this, we will come to the topic of principle of DC generator. Now the loop is opened and connected it with a split ring as shown in the figure below. Split ring is made out of a conducting cylinder which cuts into two halves or segments insulated from each other. The external load terminals are connected with two carbon brushes which are rest on these split slip ring segments. It is seen that in the first half of the revolution current flows always along ABLMCD i.e. brush no 1 in contact with segment a. In the next half revolution, in the figure the direction of the induced current in the coil is reversed. But at the same time the position of the segments a and b are also reversed which results that brush no 1 comes in touch with the segment b. Hence, the current in the load resistance again flows from L to M. The wave from of the current through the load circuit is as shown in the figure. This current is unidirectional. Fig: Output waveform of generator This is basic working principle of DC generator, explained by single loop generator model. The position of the brushes of DC generator is so arranged that the changeover of the segments a and b from one brush to other takes place when the plane of rotating coil is at right angle to the plane of the lines of force. It is so become in that position, the induced emf in the coil is zero.

Construction of a DC Machine:

A DC generator can be used as a DC motor without any constructional changes and vice versa is also possible. Thus, a DC generator or a DC motor can be broadly termed as a DC machine. These basic constructional details are also valid for the construction of a DC motor. Hence, let's call this point as construction of a DC machine instead of just 'construction of a DC generator. The above figure shows constructional details of a simple 4-pole DC machine. A DC machine consists of two basic parts; stator and rotor.

Basic constructional parts of a DC machine are described below.

1. Yoke: The outer frame of a dc machine is called as yoke. It is made up of cast iron or steel. It not only provides mechanical strength to the whole assembly but also carries the magnetic flux produced by the field winding.

2. Poles and pole shoes: Poles are joined to the yoke with the help of bolts or welding. They carry field winding and pole shoes are fastened to them. Pole shoes serve two purposes; (I) they support field coils and (ii) spread out the flux in air gap uniformly.

3. Field winding: They are usually made of copper. Field coils are former wound and placed on each pole and are connected in series. They are wound in such a way that, when energized, they form alternate North and South poles.

4. Armature core: Armature core is the rotor of a dc machine. It is cylindrical in shape with slots to carry armature winding. The armature is built up of thin laminated circular steel disks for reducing eddy current losses. It may be provided with air ducts for the axial air flow for cooling purposes. Armature is keyed to the shaft.

5. Armature winding: It is usually a former wound copper coil which rests in armature slots. The armature conductors are insulated from each other and also from the armature core. Armature winding can be wound by one of the two methods; lap winding or wave winding. Double layer lap or wave windings are generally used. A double layer winding means that each armature slot will carry two different coils.  There are two types of Armature Winding/coil of DC machine.

1. Lap winding

2. Wave winding

6. Commutator and brushes: Physical connection to the armature winding is made through a commutator-brush arrangement. The function of a commutator, in a dc generator, is to collect the current generated in armature conductors. Whereas, in case of a dc motor, commutator helps in providing current to the armature conductors. A commutator consists of a set of copper segments which are insulated from each other. The number of segments is equal to the number of armature coils. Each segment is connected to an armature coil and the commutator is keyed to the shaft. Brushes are usually made from carbon or graphite. They rest on commutator segments and slide on the segments when the commutator rotates keeping the physical contact to collect or supply the current.

fig. Commutator

Types of DC Generators

The DC generator converts the Mechanical power into electrical power. The magnetic flux in a DC machine is produced by the field coils carrying current. The circulating current in the field windings produces a magnetic flux, and the phenomenon is known as Excitation.

DC Generator is classified according to the methods of their field excitation. By excitation, the DC Generators are classified as Separately excited DC Generators and Self-excited DC Generators.

1.     Separately Excited DC Generator.

2.     Self-Excited DC Generator.

i)      DC Series Generator

ii)    DC Shunt Generator

iii)   DC Compound Generator

a)     DC Long Shunt Generator

b)     DC Short Shunt Generator

There are also Permanent magnet type DC generators. The field poll of the DC generator is stationary, and the armature conductor rotates. The voltage generated in the armature conductor is of alternating nature, and this voltage is converted into the direct voltage at the brushes with the help of the commutator. The detailed description of the various types of generators is explained below.

Permanent Magnet type DC Generator In this type of DC generator, there is no field winding is placed around the poles. The field produced by the poles of these machines remains constant. Although these machines are very compact but are used only in small sizes like dynamos in motorcycles, etc. The main disadvantage of these machines is that the flux produced by the magnets deteriorates with the passage of time which changes the characteristics of the machine.

 Separately Excited DC Generator A DC generator whose field winding or coil is energized by a separate or external DC source is called a separately excited DC Generator. The flux produced by the poles depends upon the field current with the unsaturated region of magnetic material of the poles. i.e. flux is directly proportional to the field current. But in the saturated region, the flux remains constant. The figure of self-excited DC Generator is shown below. Separately Excited DC Generator

Here, I_A = I_L where IA is the armature current and I_L is the line current. Terminal voltage is given as I_f the contact brush drop is known, then the equation (1) is written as the power developed is given by the equation shown below. Power output is given by the equation (4) shown above.

Self-Excited DC Generator

Self-excited DC Generator is a device, in which the current to the field winding is supplied by the generator itself. In self-excited DC generator, the field coils mat be connected in parallel with the armature in the series, or it may be connected partly in series and partly in parallel with the armature windings.

The self-excited DC Generator is further classified as

1.     Shunt Generator

2.     Series Generator

3.     Compound Generator  

1.     Shunt Generator

In a shunt generator, the field winding is connected across the armature winding forming a parallel or shunt circuit. Therefore, full terminal voltage is applied across it. A very small field current I_s, flows through it because this winding has many turns of fine wire having very high resistance R_ash of the order of 100 ohms. The connection diagram of shunt wound generator is shown below. Shunt Wound DC Generator Shunt field current is given as Where R_ash is the shunt field winding resistance. The current field I_s is practically constant at all loads. Therefore, the DC shunt machine is considered to be a constant flux machine. Armature current is given as Terminal voltage is given by the equation shown below. If the brush contact drop is included, then extra 2Volt added in equation.

2.     Series Generator

A series-wound generator the field coils are connected in series with the armature winding. The series field winding carries the armature current. The series field winding consists of a few turns of wire of thick wire of larger cross-sectional area and having low resistance usually of the order of less than 1 ohm because the armature current has a very large value. Its convectional diagram is shown below. Series Wound DC Generator Series field current is given as Rise is known as the series field winding resistance. Terminal voltage is given as If the brush contact drop is included, the terminal voltage equation is written as the flux developed by the series field winding is directly proportional to the current flowing through it. But it is only true before magnetic saturation after the saturation flux becomes constant even if the current flowing through it is increased.

3.     Compound Wound Generator

In a Compound Wound Generator, there are two sets of the field winding on each pole. One of them is connected in series having few turns of thick wire, and the other is connected in parallel having many turns of fine wire with the armature windings. In other words, the generator which has both shunt and series fields is called the compound wound generators. If the magnetic flux produced by the series winding assists the flux produced by the shunt winding, then the machine is said to be cumulative compounded. If the series field flux opposes the shunt field flux, then the machine is called the differentially compounded. It is connected in two ways. One is a long shunt compound generator, and another is a short shunt compound generator. If the shunt field is connected in parallel with the armature alone then the machine is called the short compound generator. In long shunt compound generator, the shunt field is connected in series with the armature.

The two types of generators are discussed below in details.

a.     Long Shunt Compound Generator

 In a long shunt wound generator, the shunt field winding is parallel with both armature and series field winding. The connection diagram of long shunt wound generator is shown below. Long Shunt Compound Wound Generator Shunt field current is given as Series field current is given as Terminal voltage is given as If the brush contact drop is included, the terminal voltage equation is written as Short Shunt Compound Wound Generator In a Short Shunt Compound Wound Generator, the shunt field winding is connected in parallel with the armature winding only.

b.     Short Shunt Compound Wound Generator

Series field current is given as Shunt field current is given as Terminal voltage is given as If the brush contact drop is included, the terminal voltage equation is written as in this type of DC generator, the field is produced by the shunt as well as series winding. The shunt field is stronger than the series field. If the magnetic flux produced by the series winding assists the flux produced by the shunt field winding, the generator is said to be Cumulatively Compound Wound generator. If the series field flux opposes the shunt field flux, the generator is said to be Differentially Compounded.

Voltage build up in self excited Generator or Dc Shunt Generator`

A self-excited DC generator supplies its own field excitation A self-excited generator shown in figure is known as a shunt generator because its field winding is connected in parallel with the armature. Thus, the armature voltage supplies the field current. This generator will build up a desired terminal voltage. Assume that the generator in figure has no load connected to it and armature is driven at a certain speed by a prime mover. we shall study the condition under which the voltage build up takes place. Due to this residual flux, a small voltage will be generated. It is given by This voltage is of the order of 1V or 2V. It causes a current If to flow in the field winding in the generator. The field current is given by This field current produces a magneto motive force in the field winding, which increases the flux. This increase in flux increases the generated voltage Ea. The increased armature voltage Ea. increases the terminal voltage V. with the increase in V, the field current If increases further. This in turn increases Φ and consequently Ea. increases further. The process of the voltage build-up continues. Figure shows the voltage build-up of a dc shunt generator.

Compensating Winding and Interpoles in DC Generator

In DC compound machine setup by armature current opposes magnetic field flux, this is known as armature reaction.

The armature reaction has two effects

(I) Demagnetizing effect and

(ii) Cross magnetizing effect.

 Demagnetizing effect weakens the main field flux which in turn decreases the induced a.m. (as E ∝ Ø)). To overcome this effect a few extra turns/poles are added in series to main field winding. This creates a series field which serves two purposes, (I) It helps to neutralize the demagnetizing effect of armature reaction. (ii) If wound for cumulative compounded machine the electrical performance will be improved. Compensating winding: All armature conductors placed under the main pole’s region produces a.m. which is at right angle (90°) to the main field e.m.f. This e.m.f causes distortion in main field flux. This is known as cross magnetizing effect. To minimize the cross-magnetizing effect compensating winding is used. This compensating winding produces an m.m.f which opposes the m.m.f produced by armature conductors. This objective is achieved by connecting compensating winding in series with armature winding. In absence of compensating winding, cross magnetizing effect causes sparking at the commutators and short circuiting the whole armature winding. Let, Zc = Number of compensating conductors/pole Za = Number of active armature conductors/pole Ia = Armature current. ZcIa= Za (Ia/A) Where, Ia/A=Armature current/conductor Zc= Za/A Compensating Winding Disadvantages This winding neutralizes the cross magnetizing effect due to armature conductors only but not due to interpolar region. This winding is used in large machine in which load is fluctuating.

Interpoles

Cross magnetizing effect in interpolar region is by interpoles (also known as compoles (or) commutating poles). These interpoles are small in size and placed in between the main poles of yoke. Like compensating winding, interpoles are also connected in series with armature winding such that the m.m.f produced by them opposes the m.m.f produced by armature conductor in interpolar region. In generators, the interpole polarity is same as that of main pole ahead such that they induce an e.m.f which is known as commutating or reversing e.m.f. This commutating e.m.f minimizes the reactance e.m.f and hence sparks or arcs are eliminated. Fig: Intepoles Compensating winding and Interpoles are used for same purpose but the difference between them is, interpoles produce e.m.f for neutralizing reactance e.m.f whereas compensating winding produces an m.m.f which opposes the m.m.f produced by conductors.

COMMUTATION:

The process of reversal of current in the short-circuited armature coil is called ‘Commutation’. This process of reversal takes place when coil is passing through the interpolar axis (q-axis), the coil is short circuited through commutator segments. Commutation takes place simultaneously for ‘P’ coils in a lap wound machine and two coil sets of P/2 coils each in a wave-wound machine. The process of commutation of coil ‘B’ is shown below. In figure ‘1.29’ coil ‘B’ carries current from left to right and is about to be short circuited in figure ‘1.30’ brush has moved by 1/3 rd. of its width and the brush current supplied by the coil are as shown. In figure ‘1.31’ coil ‘B’ carries no current as the brush is at the middle of the short circuit period and the brush current in supplied by coil C and coil A. In figure ‘1.32’ the coil B which was carrying current from left to right carries current from right to left. In fig ‘1.33’ spark is shown which is due to the reactance voltage. As the coil is embedded in the armature slots, which has high permeability, the coil possesses appreciable amount of self-inductance. The current is changed from +I to –I. So due to self-inductance and variation in the current from +I to –I, a voltage is induced in the coil which is given by L dI/dt.

Methods of improving commutation:

Methods of Improving Commutation: There are two practical ways of improving commutation i.e. of making current reversal in the short-circuited coil as sparkless as possible.

These methods are known as

(i) Resistance Commutation and

(ii) Emf Commutation (which is done with the help of either brush lead or interpoles, usually the later).

Resistance Commutation: This method of improving commutation consists of replacing low-resistance Cu brushes by comparatively high-resistance carbon brushes. When current I from coil C reach the commutator segment b, it has two parallel paths open to it. The first part is straight from bar ‘b’ to the brush and the other parallel path is via the short-circuited coil B to bar ‘a’ and then to the brush. If the Cu brushes are used, then there is no inducement for the current to follow the second longer path, it would preferably follow the first path. But when carbon brushes having high resistance are used, then current I coming from C will prefer to pass through the second path. The additional advantages of carbon brushes are that (i) they are to some degree self-lubricating and polish the commutator and (ii) should sparking occur, they would damage the commutator less than when Cu brushes are used. But some of their minor disadvantages are:

(i) Due to their high contact resistance (which is beneficial to sparkless commutation) a loss of approximately 2 volts is caused. Hence, they are not much suitable for small machines where this voltage forms an appreciable percentage loss.

(ii) Owing to this large loss, the commutator has to be made somewhat larger than with Cu brushes in order to dissipate heat efficiently without greater rise of temperature.

(iii) because of their lower current density (about 7-8 A/cm2 as compared to 25-30 A/cm2 for Cu brushes) they need larger brush holders.

EMF Commutation: In this method, arrangement is made to neutralize the reactance voltage by producing a reversing emf in the short-circuited coil under commutation. This reversing emf, as the name shows, is an emf in opposition to the reactance voltage and if its value is made equal to the latter, it will completely wipe it off, thereby producing quick reversal of current in the short-circuited coil which will result in sparkless commutation. The reversing emf may be produced in two ways:

(i) either by giving the brushes a forward lead sufficient enough to bring the short-circuited coil under the influence of next pole of opposite polarity or

(ii) by using Interpoles. The first method was used in the early machines but has now been abandoned due to many other difficulties it brings along with.

Interpoles of Compoles:

These are small poles fixed to the yoke and spaced in between the main poles. They are wound with comparatively few heavy gauges Cu wire turns and are connected in series with the armature so that they carry full armature current. Their polarity, in the case of a generator, is the same as that of the main pole ahead in the direction of rotation. The function of interpoles is two-fold: (i) As their polarity is the same as that of the main pole ahead, they induce an emf in the coil (under commutation) which helps the reversal of current. The emf induced by the compoles is known as commutating or reversing emf. The commutating emf neutralizes the reactance emf thereby making commutation sparkless. With interpoles, sparkless commutation can be obtained up to 20 to 30% overload with fixed brush position. In fact, interpoles raise sparking limit of a machine to almost the same value as heating limit. Hence, for a given output, an interpole machine can be made smaller and, therefore, cheaper than a non-interpolar machine. As interpoles carry armature current, their commutating emf is proportional to the armature current. This ensures automatic neutralization of reactance voltage which is also due to armature current.

(ii) Another function of the interpoles is to neutralize the cross-magnetising effect of armature reaction. Hence, brushes are not to be shifted from the original position.