Tuesday, August 31, 2021

HOW D.C GENERATORS AND MOTORS WORKS

 D.C GENERATORS AND MOTORS

We must now see how the principles of electromagnetism can be applied to practical generators and motors. For the present, we shall confine our attention to generators in which the current supplied to the external circuit flows always in the same direction, and to motors suitable for operation on such current. Machines of this kind are called direct current (d.c) generators and motors. Sometimes the term continuous current is used instead of direct current, and sometimes generators are called dynamos.

PRACTICAL GENERATORS

The output of a single wire loop rotating in a magnetic field is very small. It is also far from uniform, for although the two-section commutator ensures that the current in the external circuit flows in only one direction, it does not alter the fact that during each half revolution the current grows gradually from zero to a maximum, and then dies away again.

In an attempt to produce a steadier current, we might arrange two wire loops at right angles, so that instead of two conductors cutting the field we had four. One pair of conductors would then produce their maximum e.m.f while the other pair were producing their minimum, and if we could arrange for the two pairs to be connected in series, we should have a current which, while still not uniform, was much steadier than before.



In practice, this principle is carried farther, and a large number of conductors are arranged on the periphery of a drum and rotated together. The commutator has four sections instead of two. The brushes are represented by solid blocks. Suppose that the rotation is clockwise, and that the direction of the field is again from left to right. Then all the conductors on the left-hand side will carry current flowing down into the paper, and all those on the right-hand side current flowing up out of the paper. The action can be better understood if we imagine each pair of conductors to be replaced by an electric cell. Since the conductors are sources of e.m.f., this is a permissible assumption.

We have supposed so far that the conductors are surrounded by air. In order to obtain a magnetic path of low reluctance, it is desirable that as much as possible of the space not occupied by conductors should be filled with iron. The conductors are therefore placed in slots on a cylindrical iron core, the whole being termed an armature. Except in the smaller sizes, internal channels are provided for ventilation.

Rectangular coils of several turns may be used instead of single loops, all the turns of a coil being accommodated in the same pair of slots. As the number of slots and the number of commutator sections is often large, the connexions of a complete armature may appear very complicated, but the principle is similar to that of the simple example we have described.

 


Commutators are usually built up from copper sections(segments) clamped together on the shaft and insulated from each other and from the shaft by mica. Brushes are made from special grades of carbon and are allowed to bed themselves down onto the curved surface of the commutator. The brushes slide in brush-holders and are kept in contact with the commutator by means of springs.

EDDY CURRENTS

As the iron armature is rotated with the conductors, it too cuts the magnetic field, and if proper precautions were not taken, considerable currents would be induced in it. These eddy currents would circulate in the iron, heating it up and wasting energy.

The armature is therefore laminated or made up of thin sheets or stampings. The stampings are of the general shape, and a stack of them being clamped together and mounted on the armature shaft. Each stamping has a thin coating of insulation on one side(sometimes the natural coating of oxide is sufficient) so that eddy currents are confined to individual sheets and the building up of an appreciable e.m.f. along the length of the armature is prevented.

Eddy currents can be further reduced by using iron of high ohmic resistance.

FIELD MAGNET

The magnet which produces the field in which the conductors rotate is called the field magnet. It is nearly always an electromagnet, and in order to effect a further improvement in the magnetic the circuit it is fitted with iron pole-pieces, shaped so as to come as close to the armature as possible

Although the minimum number of poles on the field magnet is naturally two, one north and one south, it is usually desirable to have more than two, so that each conductor passes north and south poles alternately several times during one revolution. The number of poles is an important factor in the design of the generator. A four-pole field magnet with the armature in position is shown in the diagram above.

ARMATURE REACTION

We have assumed so far that the only magnetic field present is that of the field magnet. Actually, the armature has a field of its own while current is flowing in its winding, and this field is at right angles to that of the field magnet.

The result is to distort the original field so that it appears to have moved around slightly in the direction in which the armature rotates.

The effect of the armature upon the field is known as the armature reaction. As the correct position of the brushes depends upon the direction of the field, it is necessary to move them around too; if this were not done, serious sparking would occur. The amount by which they are moved is called the angle of lead.

For some reason, this is not always enough to ensure sparkles commutation, particularly when the current drawn by the external circuit varies from time to time. It is therefore a common practice to provide the field magnet with interpoles. These are small auxiliary poles placed between the main poles and having windings arranged in series with those of the armature. As their strength depends upon the armature current, they are able to provide the necessary compensation.

FIELD CONNECTIONS

The windings of the field magnet are sometimes fed from a separate source of power when they are said to be separately excited. the normal practice, however, is for the machine to supply its own field current. It is able to do this because the residual magnetism of the iron allows a small e.m.f to be generated without any current in the field-magnet winding. The current which this e.m.f causes to flow assists the residual magnetism, and so the flux and the e.m.f build-up to their full values. Generators of this kind are said to be self-excited.

In the shunt-wound generator, the voltage across the field winding is the same as that at the terminals. If the extra current is taken by the external circuit, the voltage drop in the armature is increased and the terminal voltage decreases. The consequent drop in the field current causes a further fall in the terminal voltage. The effect is particularly marked at heavy loads.

In the series-wound generator, if the extra current is taken by the external circuit, extra current flows through the field winding. Owing to the rapid rise in flux for small increases in current, the terminal voltage, within certain limits, rises as the current taken increases. This renders the series-wound generator unsuitable for most purposes, but it is sometimes used as an auxiliary generator to compensate for the voltage drop in a long cable. It is then called a booster.

The voltage of a generator can be regulated by hand if means are provided for altering the strength of the field. Thus in the shunt-wound generator, the field strength may be controlled by a variable resistance in series with the field-magnet winding.

It is often required that the terminal voltage of a generator should remain constant on various loads without regulation by hand. Since the terminal voltage of a shunt-wound machine decreases as the load increases. While that of a series-wound machine increases, a combination of the two can be made to give a nearly constant voltage. The field magnet then has two windings, one in series with the armature and one in parallel, and the generator is said to be a compound wound.

MOTORS

The simple wire loop connected to a two-section commutator would hardly be suitable for running as a motor, owing to the supply being short-circuited when both brushes made contact with the same half of the commutator. This difficulty does not arise when the number of commutator segments is increased, and most commercial d.c generators can be made to run as motors.

The effect of armature reaction in the motor is opposite to that in a generator, and in order to allow for it, the brushes are given an of lag instead of the angle of lead; that is, they are moved from the mid position in a direction contrary to that of rotation. As in the case of the generator, however, interpoles are often employed to ensure sparkles commutation.

When the armature of a motor rotates, the conductors cut the field, and an e.m.f is produced in them just as it is in the case of a generator. This back e.m.f opposes the applied e.m.f, the difference between them being the voltage which the current through the armature resistance. If we multiply the back e.m.f by the armature current we obtain (negligible losses)the electrical equivalent in watts of the mechanical power developed by the machine.



Motors, like generators, may be either series, shunt, or compound wound. The speed of a shunt-wound motor does not vary much with changes in load, but that of a series-wound motor falls as the load increases. A series-wound motor is more suitable for starting under heavy loads but is liable to “race “if the load is suddenly removed. Compound-wound motors have characteristics of the other two.

Note that the direction of rotation of a motor is not reserved by reversing the supply current, because this would change the polarity of both the field and the armature. To reverse the rotation it is necessary to reverse the connections of either the field or the armature, but not both.

The speed of a motor can be varied by varying the strength of the field, the speed increasing as the field is weakened. The field strength can be controlled by means of a variable resistance connected in series with the field-magnet winding of a shunt-wound machine or in parallel with that of a series-wound machine. Alternatively, the speed of either type of machine can be controlled by resistance in series with the armature, but this leads to heavy losses.

 STARTERS FOR SHUNT/SERIES WOUND MOTOR

When a motor is running, the back e.m.f is nearly as great as the applied e.m.f., but at the moment of starting there is no rotation, and therefore no back e.m.f. It follows that if the normal voltage was suddenly applied, a greatly excessive current would flow through the armature.

Motors of any size are therefore provided with a starter. This is a tapped resistance connected to a series of contact studs so arranged that when a switch arm is moved over the studs from an “off “ position the resistance is gradually cut out of the armature circuit. The circuit is closed while the resistance is all in a circuit, and the arm is then brought slowly over the studs as the machine starts up.

In order to ensure that the starter is used on every occasion, it is necessary to prevent the switch arm from being left in its final position when the current is switched off. In the starters commonly used for shunt-wound motors, the arm is held in its final position by a small electromagnet is known as a low-voltage release. The winding of this magnet is included in the field-magnet circuit, and when the supply is switched off (or if for any other reason the field-magnet circuit is broken) it no longer hold the switch arm, which is then returned to its “ off “position by a spring.

Very often, a second small electromagnet, known as an overload release, is connected in series with the machine. Should the current increase unduly, the overload release operates contacts which short-circuit the low-voltage release, and again the arm returns to its “ off “ position.

LOSSES IN GENERATORS AND MOTORS

The object of a generator is to convert mechanical into electrical energy, and that of a motor is to convert electrical into mechanical energy. Owing to losses in the machine, however, not all the energy put in is available in the changed form. The following are the chief sources of loss.

COPPER LOSSES: These are caused by the passage of current through the resistance of the armature and field-magnet windings. The lost energy appears as heat in the windings.

IRON LOSSES: These are caused by eddy currents and hysteresis. In both cases, the lost energy appears as heat in the iron parts of the machine.

MECHANICAL LOSSES: These are caused by air resistance and bearing friction. In this case, too, the lost energy is ultimately converted into heat. The smaller these losses, the greater is the efficiency of the machine. The statement that a machine has an efficiency of 90% at full load means that, at the load for which it is designed, nine-tenths of the energy put into it is available for external use in the changed form.

Example: The combined copper, iron, and mechanical losses in a generator total 1200 watts. If it supplies 20 amperes at 240 volts, what is its efficiency?

Solution:

Output = 20 amps at 240 volts = 4800 watts

Input = 4800 + 1200 = 6000 watts.

Efficiency = (4800/6000*100) % = 80%

Large machines are more efficient than small ones, and the efficiency of a large generator may approach 95 %.

 

 

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