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D.C. Motor Starter

April 11, 2019
D.C. Motor Starter

If a d.c. motor whose coil is stationary is switched on to its provide voltage, it's doubtless that the fuses protective the motor can give way.

This is as a result of the coil resistance is tiny, ofttimes being but one ohm. Thus, extra resistance should be additional to the coil circuit at the moment of closing the switch to start out the motor.

 because the speed of the motor will increase, the coil conductors are cutting flux and a generated voltage, acting con to the applied voltage, is made, that limits the flow of coil current. therefore the worth of the extra coil resistance will then be reduced.

When at traditional running speed, the generated e.m.f. is specified no extra resistance is needed within the coil circuit. to realize this varied resistance within the coil circuit on beginning, a d.c. motor starter is employed, as shown in Figure 21.28.

The crank is touched slowly during a clockwise  direction to begin the motor. For a shunt-wound motor, the field winding is connected to stud one or to L via a slippy contact on the crank, to allow most field current,

Hence most flux, thus most torsion on beginning, since T/8Ia. an analogous arrangement while not the field affiliation is employed for series motors.


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D.C. Machine losses

April 11, 2019
D.C. Machine losses
As explicit  in Section twenty one.1, a generator may be a machine for changing energy into voltage and a motor may be a machine for changing voltage into energy. once such conversions crop up, sure losses occur that ar dissipated within the sort of heat.

The principal losses of machines are:

(i) Copper loss,

 Copper loss, because of I^2R heat losses within the coil and field windings.

ii) Iron (or core) loss

 Iron (or core) loss, because of physical phenomenon and eddy-current losses within the coil. This loss may be reduced by constructing the coil of element steel laminations having a high impedance and low physical phenomenon loss. At constant speed, the iron loss is assumed constant.

(iii) Friction and windage losses,

Friction and windage losses, because of bearing and brush contact friction and losses because of air resistance against moving elements (called windage). At constant speed, these losses ar assumed to be constant.

(iv) Brush contacts

 Brush contact loss between the brushes and electric switch. This loss is roughly proportional to the load current.


The total losses of a machine may be quite significant and operational efficiencies of between eightieth and ninetieth ar common.
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D.c. generators

April 11, 2019
D.c. generators

D.c. generators ar classified per the strategy of their field excitation. These groupings are:

(i) Separately-excited generators

 Separately-excited generators, wherever the field winding is connected to a supply of provide aside from the coil of its own machine.

 (ii) Self-excited generators

 Self-excited generators, wherever the field winding receives its provide from the coil of its own machine, and that ar sub-divided into (a) shunt, (b) series, and (c) compound wound generators.

 Types of d.c. generator and their characteristics

(a) Separately-excited generator



A typical separately-excited generator circuit is shown in Figure twenty one.5. once a load is connected across the coil terminals, a load current Hawkeye State can flow. The terminal voltage V can fall from its open-circuit e.m.f. E thanks to a V drop caused by current flowing through the coil resistance, shown as Ra, i.e.,

                                   
Characteristics

 The 2 principal generator characteristics ar the generated voltage/field current characteristics, referred to as the open-circuit characteristic and therefore the terminal voltage/load current characteristic, referred to as the load characteristic.
     A typical separately-excited generator open-circuit characteristic is shown in Figure twenty one.6(a) and a typical load characteristic is shown in Figure twenty one.6(b).



 A separately-excited generator is employed solely in special cases, like once a large variation in terminal p.d. is needed, or once actual management of the field current is important. Its disadvantage lies in requiring a separate supply of DC.





(b) Shunt-wound generator

 during a shunt wound generator the field winding is connected in parallel with the coil as shown in Figure twenty one.7. The field winding includes a comparatively high resistance and so the present carried is simply a fraction of the coil current.



Characteristics

The generated e.m.f., E, is proportional to 8ω, thus at constant speed, since ω D2n, E/8. conjointly the flux eight is proportion to field current If till magnetic saturation of the iron circuit of the generator happens. thus the electrical circuit characteristic is as shown in Figure twenty one.9(a). 

As the load current on a generator having constant field current and running at constant speed will increase, the worth of coil current will increase, thus the coil V drop, IA Ra will increase. The generated voltage E is larger than the terminal voltage V and therefore the voltage equation for the coil circuit is VDEIaRa. Since E is constant, V decreases with increasing load. The load characteristic is as shown in Figure twenty one.9(b). In apply, the autumn in voltage is regarding 100 percent between no-load and full-load for several d.c. shunt-wound generators. 

The shunt-wound generator is that the sort most employed in apply, however the load current should be restricted to worth|a worth|a price} that's well below the most value. This then avoid successive variation of the terminal voltage. Typical applications square measure with battery charging and motor automotive generators.

(c) Series-wound generator

In the series-wound generator the field winding is connected nonparallel with the coil as shown in Figure 21.10.


Characteristic

The load characteristic is that the terminal voltage/current characteristic. The generated e.m.f. E, is proportional to 8ω and at constant speed ω (D 2n) could be a constant. so E is proportional to eight. For values of current below magnetic saturation of the yoke, poles, air gaps and coil core, the flux eight is proportional to this, hence E/I. For values of current on top of those needed for magnetic saturation, the generated e.m.f. is more or less constant. The values of field resistance and coil resistance during a series wound machine ar tiny, therefore the terminal voltage V is incredibly nearly capable E. A typical load characteristic for a series generator is shown in Figure twenty one.11. 

In a series-wound generator, the field winding is nonparallel with the coil and it's unimaginable to possess a worth of field current once the terminals ar open circuited, so it's unimaginable to get Associate in Nursing electrical circuit characteristic. 

Series-wound generators ar seldom utilized in observe, however is used as a ‘booster’ on d.c. transmission lines.

(d) Compound-wound generator

In the compound-wound generator 2 ways of association area unit used, each having a combination of shunt and series windings, designed to mix the benefits of every. Figure 21.12(a) shows what's termed a long shunt compound generator, and 21.12(b) shows a short-shunt Figure 21.1 compound generator. The latter is that the most usually used sort of d.c. generator.

Characteristics

In cumulative-compound machines the magnetic flux made by the series and shunt fields are additive. enclosed during this cluster are over-compounded, level-compounded and under-compounded machines—the degree of combination obtained betting on the amount of turns of wire on the series winding.

A large variety of series winding turns leads to associate degree over-compounded characteristic, as shown in Figure 21.14, during which the full-load terminal voltage exceeds the no-load voltage. A level-compound machine offers a full-load terminal voltage that is up to the no-load voltage, as shown in Figure 21.14.

 associate degree under-compounded machine offers a full-load terminal voltage that is a smaller amount than the no-load voltage, as shown in Figure 21.14. but even this latter characteristic may be a very little higher than that for a shunt generator alone.

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D.C. Machines

April 10, 2019
 D.C. Machines

Introduction

When the input to Associate in Nursing electrical machine is voltage, (seen as applying a voltage to the electrical terminals of the machine), and therefore the output is energy, (seen as a rotating shaft), the machine is termed an electrical motor. therefore an electrical motor converts voltage into energy.

 once the input to Associate in Nursing electrical machine is energy, (seen as, say, a diesel engine, coupled to the machine by a shaft), and therefore the output is voltage, (seen as a voltage showing at the electrical terminals of the machine), the machine is termed a generator. Thus, a generator converts energy to voltage.

 The action of a commutator

In an electrical motor, conductors rotate in a very uniform magnetic field. A single-loop conductor mounted between permanent magnets is shown in Figure twenty one.1. A voltage is applied at points A and B in Figure twenty one.1(a).


Figure. 21.1

A force, F, acts on the loop because of the interaction of the magnetic field of the permanent magnets and also the magnetic field created by the present flowing within the loop. This force is proportional to the flux density, B, the present flowing, I, and also the effective length of the conductor, l, i.e. FDBIl. 

The force is formed of 2 elements, one acting vertically downwardly because of the present flowing from C to D and also the different acting vertically upwards because of the present flowing from E to F (from Fleming’s paw rule). If the loop is unengaged to rotate, then once it's turned through 180°, the conductors area unit as shown in Figure twenty one.1(b). For rotation to continue within the same direction, it's necessary for the present flow to be as shown in Figure twenty one.1(b), i.e. from D to C and from F to E. This apparent reversal within the direction of current flow is achieved by a method referred to as commutation. With relation to Figure twenty one.2(a), 

When an immediate voltage is applied at A and B, then because the single-loop conductor rotates, current flow can continuously be removed from the electric switch for the a {part of} the conductor adjacent to the N-pole and towards the electric switch for the part of the conductor adjacent to the S-pole. therefore the forces act to present continuous rotation in AN anti-clockwise direction. The arrangement shown in Figure twenty one.2(a) is named a ‘two-segment’ electric switch and also the voltage is applied to the rotating segments by stationary brushes, (usually carbon blocks), that slide on the electric switch material, (usually copper), once rotation takes place. 

In follow, there area unit several conductors on the rotating a part of a d.c. machine and these area unit hooked up to several electric switch segments. A schematic diagram of a multi-segment electric switch is shown in Figure twenty one.2(b).

 D.c. machine construction

The basic components of any d.c. machine area unit shown in Figure twenty one.3, and comprise: 

(a) a stationary half known as the mechanical device having, 

(i) a steel ring known as the yoke, to that area unit connected
(ii) the magnetic poles, around that area unit the 
(iii) field windings, i.e. several turns of a conductor wound around the pole core; current passing through this conductor creates Associate in Nursing magnet, (rather than the permanent magnets shown in Figures twenty one.1 and 21.2), 

(b) a rotating half known as the coil mounted in bearings housed within the mechanical device and having,Figure 21.3 

(iv) a laminated cylinder of iron or steel known as the core, on that teeth area unit move house the 
(v) coil winding, i.e. one or multi-loop conductor system and 
(vi) the electric switch, (see Section twenty one.2).

Armature windings is divided into 2 teams, looking on however the wires area unit joined to the electric switch. These area unit known as wave windings and lap windings.

(a) In wave windings there area unit 2 methods in parallel no matter the amount of poles, every path provision 0.5 the entire current output. Wave wound generators turn out high voltage, low current outputs. 

(b) In lap windings there area unit as several methods in parallel because the machine has poles. the entire current output divides equally between them. Lap wound generators turn out high current, low voltage output.

 Shunt, series and compound windings

When the field winding of a d.c. machine is connected in parallel with the coil, as shown in Figure 21.4(a), the machine is claimed to be shunt wound. If the field winding is connected asynchronous with the coil, as shown in Figure 21.4(b), then the machine is claimed to be series wound. A compound wound machine contains a combination of series and shunt windings. 



Depending on whether or not the electrical machine is series wound, shunt wound or compound wound, it behaves otherwise once a load is applied. The behaviour of a d.c. machine below varied conditions is shown by suggests that of graphs, known as characteristic curves or simply characteristics. The characteristics shown within the following sections ar theoretical, since they neglect the consequences of coil reaction. 



Armature reaction is that the impact that the magnetic field created by the coil current has on the magnetic field created by the field system. in an exceedingly generator, coil reaction ends up in a reduced output voltage, and in an exceedingly motor, coil reaction ends up in exaggerated speed.

 some way of overcoming the impact of coil reaction is to fit compensating windings, settled in slots within the pole face.

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Current transformers

April 10, 2019
Current transformers
For measurement currents in far more than concerning one hundred A a current electrical device is often used. With a d.c. moving-coil meter the present needed to convey full scale deflection is extremely small—typically many milliamperes. once larger currents square measure to be measured a shunt electrical device is value-added to the circuit .








 However, even with shunt resistors value-added it's uphill to live terribly giant currents. When a.c. is being measured a shunt can't be used since the proportion of the present that flows within the meter can rely upon its electrical phenomenon, that varies with frequency.


In current transformers the first sometimes consists of 1 or 2 turns while the secondary will have many hundred turns. A typical arrangement is shown in Figure twenty.19.

 If, as an example, the first has a pair of turns and therefore the secondary two hundred turns, then if the first current is five hundred A,

Current transformers isolate the meter from the most circuit and permit the utilization of a typical vary of ammeters giving complete deflections of one A, 2 A or 5 A.

For very large currents the transformer core can be mounted around the conductor or bus-bar. Thus the primary then has just one turn. It is very important to short-circuit the secondary winding before removing the ammeter. 

This is because if current is flowing in the primary, dangerously high voltages could be induced in the secondary should it be open circuited. Current transformer circuit diagram symbolsare shown in Figure 20.20.

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Auto transformers

April 10, 2019
Auto transformers

 Auto transformers;

 An automotive vehicle electrical device could be a electrical device that has a part of its winding common to the first and secondary circuits. Figure 20.14(a) shows the circuit for a double-wound electrical device and Figure 20.14(b) that for AN automotive vehicle electrical device.



The latter shows that the secondary is really a part of the first, the present within the secondary being (I2 I1). Since the present is a smaller amount during this section, the cross-sectional space of the winding are often reduced, that reduces the number of fabric necessary.



Figure 20.15 shows the circuit diagram symbol for an auto transformer.

 Saving of copper in an auto transformer

 For an equivalent output and voltage quantitative relation, the automobile electrical device needs less copper than a normal double-wound electrical device. this is often explained below.
 The volume, and thence weight, of copper needed in a very winding is proportional to the quantity of turns and to the cross-sectional space of the wire. successively this is often proportional to this to be carried, i.e., volume of copper is proportional to Ni.

                                   i.e., a saving of 80%

 Advantages of auto transformers

 The advantages of automotive vehicle transformers over double-wound transformers include:

 1 a saving in value since less copper is required (see above) 

 2 less volume, therefore less weight 

 3 the next efficiency, ensuing from lower I^2R losses 

 4 a unceasingly variable output voltage is accomplishable if a slippery  contact is employed 
5 a smaller share voltage regulation.

 Disadvantages of auto transformers

 The primary and secondary windings don't seem to be electrically separate, therefore if Associate in Nursing open-circuit happens within the secondary coil the complete primary voltage seems across the secondary.

 Uses of automobile transformers
 automobile transformers area unit used for reducing the voltage once beginning induction motors  and for interconnecting systems that area unit operational at close to an equivalent voltage.
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Transformer losses and efficiency

April 10, 2019
 Transformer losses and efficiency
Transformer losses

There area unit generally 2 sources of losses in transformers on load, these being copper losses and iron losses.

(a) Copper losses

(a) Copper losses area unit variable and end in a heating of the conductors, because of the actual fact that they possess resistance. If R1 and R2 area unit the first and coil resistances then the overall copper loss is I^2 1R1 +I^2 2R2 

(b) Iron losses area unit constant for a given worth of frequency and flux density and area unit of 2 types—hysteresis loss and eddy current loss.

(i) Physical phenomenon loss/Hysteresis  loss

(i) physical phenomenon loss is that the heating of the core as a results of the interior molecular structure reversals that occur because the magnetic flux alternates. 

The loss is proportional to {the area unita|the world|the realm} of the physical phenomenon loop and therefore low loss nickel iron alloys are used for the core since their physical phenomenon loops have little areas.

(ii) Eddy current loss

(ii) Eddy current loss is that the heating of the core because of e.m.f.’s being iatrogenic not solely within the electrical device windings however additionally within the core. These iatrogenic e.m.f.’s created current currents, referred to as eddy currents. thanks to the low resistance of the core, eddy currents may be quite extensive and may cause an outsized power loss and excessive heating of the core.

 Eddy current losses may be reduced by increasing the ohmic resistance of the core material or, additional sometimes, by laminating the core (i.e., cacophonic it into layers or leaves) once terribly skinny layers of insulant may be inserted between every combine of laminations.

 This will increase the resistance of the eddy current path, and reduces the worth of the eddy current.

Transformer efficiency, 


and is typically expressed as a share. it's not uncommon for power transformers to possess efficiencies of between ninety fifth and ninety eight.


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