AY-2021-2022�Branch : Electrical Engineering�Semester : 4th�Subject : Energy Conversion – I�Chapter : 01�Topic : DC Generator�Faculty : Dr. Mrutyunjay Das

Mayurbhanj School Of Engineering, Baripada

Principle of generation

Action of commutator

According to Fleming’s right hand rule, the direction of induced current changes whenever the direction of motion of conductor changes.

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Action of commutator

Magnitude of induced emf at different positions of the coil:

Action of commutator

Induced emf with more number of Armature coils:

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Constructional features

Cross sectional view of dc machine:

Constructional features

Cut view of dc machine:

Constructional features

Yoke:

The external structure of the DC generator is Yoke, and it is made with cast iron otherwise steel. It gives the protection against external mechanical stress and gives the path for the magnetic-flux to flow through it.

Poles:

These are mainly used to hold the field windings. These are bolted to the yoke. When current flows through the field windings the pole gets magnetized. To reduce the eddy current loss, poles are laminated.

Pole Shoe:

The pole shoe is mainly utilized for spreading the magnetic flux uniformly through out the air gap as well as to avoid the field coil from falling.

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Constructional features

Commutator:

It is located on the shaft of the machine. Copper strips are placed on its surface separated by mica sheets. It collects the induced current from armature windings and sent to the load. Also it makes the alternating induced current in to unidirectional.

Brushes:

The electrical connections can be ensured between the commutator as well as the exterior load circuit with the help of brushes. It is made up of carbon material in order to ensure less damage to the commutator due to friction.

Armature windings

Armature along with commutator:

Armature windings

• Turn: A turn consists of two conductors connected to one end by an end connector.
• Coil: A coil is formed by connecting several turns in the series.
• Winding: A winding is formed by connecting several coils in series.

Armature windings

Pole Pitch:

The pole pitch is defined as peripheral distance between center of two adjacent poles in DC machine. This distance is measured in term of armature slots or armature conductor come between two adjacent pole centers. It is equal to the total number of armature slots divided by the number of poles in the machine.

Coil span / Coil pitch:

Whatever may be the number of conductors per side of the coil, each coil side is placed inside one armature slot only. Coil span is defined as the peripheral distance between two sides of a coil, measured in term of the number of armature slots between them.

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Armature windings

If the coil span is equal to the pole pitch, then the armature winding is said to be full – pitched.

If the coil span is less than the pole pitch, then the winding is referred as fractional pitched.

Advantages of Short Pitch Coil or Chording:

It shortens the ends of the winding and, therefore, there is a saving in the conductor’s material.

It Reduces the effects of distorting harmonics and thus the waveform of the generated voltage is improved and making it a sine wave.

Armature windings

Back Pitch (Yb):

Back pitch is the distance measured between top and bottom coil sides of a coil measured around the back of the armature, away from the commutator.

Front Pitch (Yf)

The number of armature conductors or elements spanned by a coil on the front is called front pitch.

Alternatively, we define the front-pitch as the distance between the second conductor of the next coil which connects the front, i.e., commutator end of the armature.

Resultant Pitch (Y)

It is the distance between the beginning of one coil and the beginning of the next coil to which it is connected.

Commutator Pitch

Commutator pitch is defined as the distance between two commutator segments which two ends of same armature coil are connected. We measure commutator pitch in term of commutator bars or segment.

Armature windings

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Armature windings

Lap winding:

Lap Winding is a form of two-layer winding for electric machines in which each coil is connected in series with the one adjacent to it. lap winding is used for high current and low voltage machines.

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Armature winding

Armature winding

Wave winding:

In wave winding the end of one coil is connected to the starting of another coil of the same polarity as that of the first coil.

Armature winding

Armature winding

Armature winding

Armature winding

Armature windings

Equalizer rings:

The induced emfs in different paths tend to differ slightly due to the non-uniformities in the magnetic circuit. This will be more with the increase in the number of poles in the machine. If this is left uncorrected, circulating currents appear in these closed parallel paths. This circulating current wastes power, produces heat and over loads the brushes under loaded conditions. One method commonly adopted in d.c. machines to reduce this problem is to provide equalizer connections. These connections identify similar potential points of the different parallel paths and connect them together to equalize the potentials. Any difference in the potential generates a local circulating current and the voltages get equalized. Also, the circulating current does not flow through the brushes loading them.

Armature winding

Lap winding:

E. M. F. Equation

E. M. F. Equation

Armature reaction

Armature reaction

Armature reaction

Armature reaction

Cross-magnetising & De-magnetising AT/pole

Cross-magnetising & De-magnetising AT/pole

The armature conductors other than de-magnetizing conductors carry current in such a direction as to send the flux pointing vertically downwards i.e. at right angles to the main field flux. Hence these conductors are called cross magnetizing armature conductors which will cause distortion in main field flux.

Cross-magnetising & De-magnetising AT/pole

Cross-magnetising & De-magnetising AT/pole

Commutation

The process by which the current in the short circuited coil is reversed while it crosses the MNA is called commutation.

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Commutation

The period during which the coil remain short circuited is known as commutation period.

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Commutation

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The computation is called Ideal commutation when the reversal of current is completed by the end of the commutation period.

If the current reversal is not completed during the commutation period, sparking occurs at the contact of brushes and overheating occurs damaging the surface of the commutator. This is known as under commutation or delayed commutation.

Commutation

Commutation

Resistance commutation:

• The reversal of current in a coil takes place while the coil is short-circuited by the brush. Therefore, there are two parallel paths for the current as long as the short circuit exists.
• If the contact resistance between the brush and the commutator is made large, then current would divide in the inverse ratio of contact resistances.
• This is achieved by using carbon brushes (instead of Cu brushes) which have high contact resistance.
• The main cause of sparking during commutation is the production of reactance voltage and carbon brushes cannot prevent it.

Limitations:

1. Power loss and Voltage drop up to 2 volts due their high resistance.

2. Need of large brush holders due low current density of carbon.

3. Due to the higher losses it increases the size of commutator for effective heat dissipation.

Commutation

EMF Commutation:

• In this method, an arrangement is made to neutralize the reactance voltage by producing a reversing voltage in the coil undergoing commutation.
• The reversing voltage acts in opposition to the reactance voltage and neutralizes it to some extent.
• If the reversing voltage is equal to the reactance voltage, the effect of the latter is completely eliminate and we get sparkless commutation.

We can produce reversing emf in two ways:

1. Brush Shift:

Commutation can be improved by shifting the

brushes in forward direction for the DC

generator and in backward direction for the

DC motor for producing the sufficient

reversing emf for eliminating the reactance

voltage.

Commutation

2. By using Interpoles:

These are small poles fixed to the yoke

and spaced mid-way between the main

poles. They are wound with comparatively

few turns and connected in series

with the armature so that they carry

armature current. Their polarity is the

same as the next main pole ahead in the direction of rotation for a generator. Hence they induce an e.m.f. in the coil (undergoing commutation) which opposes reactance voltage. This leads to sparkless commutation. The e.m.f. induced by compoles is known as commutating or reversing e.m.f. Since the interpoles carry the armature current and the reactance voltage is also proportional to armature current, the neutralization of reactance voltage is automatic.

�Commutation�

Compensating winding:

The effect of armature reaction varies with the load current. Therefore, each time the load current varies, the neutral plane shifts. This means the brush position must be changed each time the load current varies.

Compensating winding consists of conductors embedded in the pole face that run parallel to the shaft and carry an armature current in a direction opposite to the direction of current in the armature conductors under that pole arc. This also reduces armature circuit’s inductor and improves system response.

Methods of excitation

DC generators are classified based on their method of excitation.

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 generators

Self excited generators

Self excited generators

Self excited generators

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.

• 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.

Self excited generators

Self excited generators

Build up of EMF

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Build up of EMF

Conditions to build up voltage in dc shunt generator:

• poles should contain residual flux
• field winding and armature winding should be correctly connected so that initial mmf will act on residual flux
• resistance of the field winding should be less than Critical field resistance
• speed of the generator greater than critical speed

Critical field resistance:

The critical field resistance is

defined as the maximum field

resistance (for a given speed)

with which the shunt generator

would be able to excite.

Critical speed

Characteristics of DC Generators

Characteristics Of Separately Excited DC Generator

Characteristics of series generators

Load characteristics of shunt generators

Load characteristics of compound generators

• If series winding amp-turns are adjusted so that, increase in load current causes increase in terminal voltage then the generator is called to be over compounded.
• If series winding amp-turns are adjusted so that, the terminal voltage remains constant even the load current is increased, then the generator is called to be flat compounded.
• If the series winding has lesser number of turns than that would be required to be flat compounded, then the generator is called to be under compounded.

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