TYPES OF DC MACHINE

TYPES OF DC MACHINE 

Please go through the DC MACHINE CONSTRUCTION before reading this topic. 

The mmf necessary to establish flux in the magnetic circuit of a dc generator can be obtained by means of 

• i. Permanent magnets
• ii. Field coils excited from some external source and
• iii. Field coils excited by the generator itself.

Generators are generally classified according to these methods of field excitation. On this basis, dc generators are divided into the following two classes:

1. Permanent magnet dc generators
2. Separately excited dc generators
3. Self excited dc generators

In permanent magnet dc machines, permanent magnet is used to establish flux in the magnetic circuit. These generators are not found in industrial applications because of the low power generated from it. Such generators are employed only in small sizes like dynamos in motor cycles. 

The behavior of a dc generator on load depends upon the method of field excitation adopted.

Separately Excited D.C. Generators

A dc generator whose field magnet winding is supplied from an independent external d.c. source (e.g., a battery etc.) is called a separately excited generator. Figure shows the connections of a separately excited generator. The voltage output depends upon the speed of rotation of armature and the field current (Eg = φZNP/60 A). The greater the speed and field current, greater is the generated e.m.f. 

It may be noted that separately excited d.c. generators are rarely used in practice. The d.c. generators are normally of self excited type.

Armature current, Ia = IL

Terminal voltage, V = Eg - IaRa

Electric power developed = EgIa

Power delivered to load = EgIa - I R = I E - I R = VIa 

Self-Excited D.C. Generators

A d.c. generator whose field magnet winding is supplied current from the output of the generator itself is called a self-excited generator. There are three types of self-excited generators depending upon the manner in which the field winding is connected to the armature, namely;

• i. Series generator
• ii. Shunt generator
• iii. Compound generator

Series generator

In a series wound generator, the field winding is connected in series with armature winding so that whole armature current flows through the field winding as well as the load. Figure shows the connections of a series wound generator. Since the field winding carries the whole of load current, it has a few turns of thick wire having low resistance. Series generators are rarely used except for special purposes e.g., as boosters.

Armature current, Ia = Ise = IL = I(say)

Terminal voltage, V = EG - I(Ra + Rse)

Power developed in armature = EgIa

Power delivered to load

Shunt generator

In a shunt generator, the field winding is connected in parallel with the armature winding so that terminal voltage of the generator is applied across it. The shunt field winding has many turns of fine wire having high resistance. Therefore, only a part of armature current flows through shunt field winding and the rest flows through the load. Fig. (1.34) shows the connections of a shunt-wound generator.

Shunt field current, Ish = V/Rsh

Armature current, Ia = IL + Ish

Terminal voltage, V = Eg - IaRa

Power developed in armature = EgIa

Power delivered to load = VIL

Compound generator

In a compound-wound generator, there are two sets of field windings on each pole - one is in series and the other in parallel with the armature. A compound wound generator may be:

• Short Shunt in which only shunt field winding is in parallel with the armature winding.

• Long Shunt in which shunt field winding is in parallel with both series field and armature winding.

Long shunt

Series field current, Ise = Ia = IL + Ish

Shunt field current, Ish = V/Rsh

Terminal voltage, V = Eg - Ia(Ra + Rse)

Power developed in armature = EgIa

Power delivered to load = VIL

Short shunt

Series field current, Ise = IL

Shunt field current,

Terminal voltage, V = Eg - IaRa - IseRse

Power developed in armature = EgIa

Power delivered to load = VIL

In a compound generator the major portion of excitation is usually supplied by the shunt field. The shunt field is slightly weaker and the series field is considerably weaker than those of the corresponding machine in which the entire excitation is produced by a single shunt or a single series winding.

Compound wound generators are of two types, known as cumulative wound and differential wound generators. In cumulative wound generators the series field assists the shund field, whereas in differential wound generators, series field opposes the shunt field.

DC MACHINE CONSTRUCTION

CONSTRUCTION OF DC MACHINE (MOTOR AND GENERATOR)

The dc generators and dc motors have the same general construction. In fact, when the machine is being assembled, the workmen usually do not know whether it is a dc generator or motor. Any dc generator can be run as a dc motor and vice-versa. 

All dc machines have five principal components

• (i) field system
• (ii) armature core  
• (iii) armature winding
• (iv) commutator 
• (v) brushes

The diagram given below represents the various parts of a DC machine 

Field system

The function of the field system is to produce uniform magnetic field within which the armature rotates. 

• It consists of a number of salient poles (of course, even number) bolted to the inside of circular frame (generally called yoke). The yoke is usually made of solid cast steel whereas the pole pieces are composed of stacked laminations. 

• Field coils are mounted on the poles and carry the dc exciting current. The field coils are connected in such a way that adjacent poles have opposite polarity.

• The m.m.f. developed by the field coils produces a magnetic flux that passes through the pole pieces, the air gap, the armature and the frame. Practical d.c. machines have air gaps ranging from 0.5 mm to 1.5 mm. 

• Since armature and field systems are composed of materials that have high permeability, most of the m.m.f. of field coils is required to set up flux in the air gap. By reducing the length of air gap, we can reduce the size of field coils (i.e. number of turns).

 

Armature core

The armature core is keyed to the machine shaft and rotates between the field poles. 

• It consists of slotted soft-iron laminations (about 0.4 to 0.6 mm thick) that are stacked
  
  to form a cylindrical core as shown in Fig. The laminations (See Fig.) are individually coated with a thin insulating film so that they do not come in electrical contact with each other. 

• The purpose of laminating the core is to reduce the eddy current loss. The laminations are slotted to accommodate and provide mechanical security to the armature winding and togive shorter air gap for the flux to cross between the pole face and the armature “teeth”.

Armature winding

The slots of the armature core hold insulated conductors that are connected in a suitable manner. This is known as armature winding. 

• This is the winding in which “working” e.m.f. is induced. The armature conductors are connected in series-parallel; the conductors being connected in series so as to increase the voltage and in parallel paths so as to increase the current. 

• The armature winding of a d.c. machine is a closed-circuit winding; the conductors being connected in a symmetrical manner forming a closed loop or series of closed loops.

Commutator

A commutator is a mechanical rectifier which converts the alternating voltage generated in the armature winding into direct voltage across the brushes. 

• The commutator is made of copper segments insulated from each other by mica sheets and mounted on the shaft of the machine . The armature conductors are soldered to the commutator segments in a suitable manner to give rise to the armature winding. 

• Depending upon the manner in which the armature conductors are connected to the commutator segments, there are two types of armature winding in a d.c. machine viz., 
• (a) lap winding 
• (b) wave winding.

Great care is taken in building the commutator because any eccentricity will cause the brushes to bounce, producing unacceptable sparking. The sparks may bum the brushes and overheat and carbonise the commutator.

Brushes

DC motors are of two types : one is brushed dc motor and the other one is brushless dc motor. Brushless dc motors are mainly used in high speed applications such as multicopters (eg:- quadcopters). 

The purpose of brushes in a dc generator is to ensure electrical connections between the rotating commutator and stationary external load circuit. 

• The brushes are made of carbon and rest on the commutator. The brush pressure is adjusted by means of adjustable springs (See Fig). 

• If the brush pressure is very large, the friction produces heating of the commutator and the brushes. On the other hand, if it is too weak, the imperfect contact with the commutator may produce sparking.

Multipole machines have as many brushes as they have poles. For example, a 4-pole machine has 4 brushes. As we go round the commutator, the successive brushes have positive and negative polarities. Brushes having the same polarity are connected together so that we have two terminals viz., the +ve terminal and the -ve terminal.

CIRCUIT BREAKER

Circuit Breaker, Power System, Substation

Circuit Breaker - Operating Principle and Arcing Phenomenon

How Circuit Breaker Works?

A circuit breaker essentially consists of fixed and moving contacts, called electrodes. Under normal operating conditions, these contacts remain closed and will not open automatically until and unless the system becomes faulty. 

The contacts can be opened manually or by remote control whenever desired. When a fault occurs on any part of the system, the trip coils of the breaker get energised and the moving contacts are pulled apart by some mechanism, thus opening the circuit.

Operating principle of Circuit Breaker

When the contacts of a circuit breaker are separated under fault conditions, an arc is struck between them. The current is thus able to continue until the discharge ceases. The production of arc not only delays the current interruption process but it also generates enormous heat which may cause damage to the system or to the breaker itself. Therefore, the main problem in a circuit breaker is to extinguish the arc within the shortest possible time so that heat generated by it may not reach a dangerous value. 

Arc Phenomenon in Circuit Breaker

When a short-circuit occurs, a heavy current flows through the contacts of the circuit breaker before they are opened by the protective system. At the instant when the contacts begin to separate the contact area decreases rapidly and large fault current causes increased current density and hence rise in temperature. 

The heat produced in the medium between contacts (usually the medium is oil or air) is sufficient to ionise the air or vapourise and ionise the oil. The ionised air or vapour ,acts as conductor and an arc is struck between the contacts. The potential difference between the contacts is quite small and is just sufficient to maintain the arc. The arc provides a low resistance path and consequently the current in the circuit remains uninterrupted so long as the arc persists. 

During the arcing period, the current flowing between the contacts depends upon the arc resistance. The greater the arc resistance, the smaller the current that flows between the contacts. The arc resistance depends upon the following factors:

1. Degree of ionisation - the arc resistance increases with the decrease in the number of ionised particles between the contacts.
2. Length of the arc - the arc resistance increases with the length of the arc i.e. seperation of contacts.
3. Cross section of arc - the arc resistance increase with the decrease in the area of cross section of the arc.

substation earthing system design

substation earthing layout diagram.