Electrical world

Elelectrical energy and mechanical energy are transformed into each other by rotating machines

Rotating electrical machines -- generators and motors -- are devices that transform mechanical power into electrical power, and vice-versa. Electrical power from a central power station can be transmitted and subdivided very efficiently and conveniently. The operation of electrical machines is explained by four general principles, that will be briefly presented below. These principles are not difficult to understand, and illuminate most of the reasons for the stages in the historical development of electrical power, and especially of electric railways. This page discusses motors in general, but the specific application to electric locomotives is emphasized. Electricity is the medium that carries power from the prime mover, whether at a central power station or on the locomotive, to its point of application at the rail, and allows it to be controlled conveniently.

Power is rate of doing work. One horsepower means lifting 550 pounds by one foot in one second. Mechanical power is force times speed. One watt is a current of one ampere (A) flowing in a potential difference or voltage of one volt (V). Electrical power is current (in amperes, A) times voltage (in volts, V). 746W is equivalent to 1 hp. A medium-sized electric locomotive might have a rating of 2000kW, or 2680 hp. At 85% efficiency, and a voltage of 15kV, 157A is drawn from the overhead contact wire. Torque is the rotational equivalent of force, often useful in speaking of motors. It is force times perpendicular distance, and power is torque times rotational speed in radians per second.

The first principle is that an electrical current causes a magnetic field which surrounds it like a whirlpool, and that this field, which is not material but rather a region of influence on other currents and magnets, is guided and greatly strengthened (by more than a thousand times) by passing through iron. When the current reverses in direction, so does the magnetic field. Currents deep in the earth cause its magnetic field, and the energy to drive them comes from either the rotation of the earth or the flow of heat within the earth. The field acts on the compass needle, which is a magnet. This principle can be called "electromagnet action."

The second is that an electrical current in a magnetic field (produced by some other currents) experiences a force perpendicular to both the direction of the current and the direction of the magnetic field, and reverses if either of these reverses in direction. The force is proportional to the current and to the strength of the magnetic field. This principle can be called "motor action."

The third is that an electrical conductor, such as a copper wire, moving in a magnetic field has an electrical current induced in it. This is expressed by the creation of an electromotive force or voltage, which causes current to flow just as the voltage of a battery does. The effect is maximum when the wire, the motion, and the magnetic field are all mutually perpendicular. This principle can be called "generator action."

And the fourth principle is that a changing magnetic field causes a voltage in any circuit through which it passes. The change can be caused by changing the current producing the magnetic field, or by moving the sources of the magnetic field. This principle can be called "transformer action."

A rotating electrical machine consists of a field and an armature that rotate with respect to each other. The armature is the part of the machine in which the energy conversion takes place. The field provides the magnetic field to aid this process. In DC machines, the field is stationary (the stator) and the armature rotates within it (the rotor), because the rotation is necessary to switch the armature connections by means of the commutator, but it is only the relative motion that counts. In an alternator, the armature is stationary and the field rotates. The field consists of an iron core to carry the magnetic field, and a winding to excite the magnetic field by the current passing through it (first principle). The magnetic field is a passive but essential component in the operation of the machine. Like the field, the armature also consists of iron to complete the magnetic circuit, and is separated by a short air gap from the iron of the field. It is important that the air gap be as small as possible and remain uniform as the armature rotates.

The armature also has windings. In a generator, these conductors are moved in the magnetic field producing a voltage (generator action). If a circuit is completed and current flows in these windings, a force is produced resisting the rotation of the armature (motor action) so that the driving machinery experiences a mechanical resistance and does work, which is being transformed into electrical energy. In a motor, these conductors are supplied with an electrical current, so that a force acts on them in the magnetic field (second principle), and this force can do external work. When the armature rotates while exerting the force, work is done, but a voltage is also produced opposing the applied voltage, resisting the flow of current in the armature (third principle), implying a change of electrical work into mechanical work.

This opposing voltage generated when the armature of a motor turns is called counter-electromotive force. It might seem that it resists current flow through the motor, and of course it does, but it is really the essential factor in turning electrical into mechanical energy. Only the current that is driven into a counter-emf appears as mechanical work at the motor shaft; all else is wasted, the energy going into heat instead of mechanical work. Early inventors of electric motors did not realize this, and tried simply to get as much current into the motor as possible, which only burned the motor up without producing any mechanical effect.

Current is supplied to the armature through sliding contacts formed by graphite blocks (called brushes because originally brushes of phosphor bronze wire were used instead) pressing against copper rings. It is usually necessary to change the connections of the armature windings as they rotate with respect to the magnetic field, and this can conveniently be done by making the copper rings in segments. The result is the rotary switch called the commutator. These days, semiconductor switches can be used for this purpose in small motors, eliminating the commutator, but the principle is the same. The commutator and brushes are the only parts of a machine that normally require maintenance, except for the bearings and other mechanical elements. If it is not necessary to switch the current, as in AC machines, the moving contacts are called slip rings.

Contemplate now the complete chain of energy flow from the prime mover, a steam or internal combustion engine, to the point where the mechanical power is finally applied. The transformation at each end must take place with a smooth mutual reaction, based on the second and third principles. This was not properly understood until the 1880's, so that practical electric transmission of power was delayed until that time. Power that is not delivered to the load is lost as heat in electrical resistance, which is equivalent to mechanical friction. Heat is produced in the generator, transmission lines, and motor, and limits the amount of power that can be handled.

Electrical motors were invented early, in the 1830's, as soon as the magnetic effects of electrical currents and the magnetic properties of iron became known. The motors of Christie and Pixii are typical of these, which used the repulsion and attraction between electromagnetic poles switched by a commutator. Small motors of this kind are still made for classroom demonstration. Attempts in the 1840's to make these motors more powerful and larger failed completely, because the magnetic forces do not scale proportionally to distance, and the significance of counter-emf was not known. The motors of Davenport and of Long in the United States are examples of these unsuccessful attempts to scale up classroom demonstrations to practical size.

More success was encountered in making generators, usually by moving permanent magnets (thereby creating a moving magnetic field) with respect to coils of wire wound around iron cores, to generate alternating currents for supplying arc lights and direct currents for electrolysis tanks (transformer action). These generators all ran quite hot because of their lack of efficiency, but supplied the greater currents required for these applications more cheaply than chemical batteries. This industry evolved into the electrical power industries of later years.

Siemens and Gramme solved the problem of efficiency in the late 1870's by introducing magnetic circuits that did not change as the armature rotated, so that the electrical reactions were smooth and constant. Siemens' first machine (a generator) of 1866 is shown at the right, and a Gramme dynamo, which could also serve as a motor if the brushes were repositioned, is shown at the left. These machines had smooth armatures with conductors on their surfaces. It was still thought that the conductors actually had to be immersed in the magnetic field to produce forces. Soon it was discovered that if the conductors were put into slots in the armature surface, the same result was obtained. This was far superior mechanically, and also made a smaller air gap possible.

The first long-distance transmission of electrical power took place in 1886 over the 8 km between Kriegstetten and Solothurn in Switzerland. Two Gramme machines in series were used as generators, and two similar machines in series at the other end were used as motors. The line voltage was 2000V, and the wire 6mm in diameter (1/4"). The shaft-to-shaft efficiency was 75%, and the installation remained in service until 1908.

Edison's famous Z-type dynamos (as direct-current generators are often called) appeared in 1879 to supply his carbon-filament incandescent lamps. These had long fields on the mistaken assumption that this gave a more powerful magnet (like a longer lever), showing how little magnetic circuits were understood at the time. This arrangement allowed a great deal of magnetic leakage between the long arms, and made the flux distribution in the armature nonuniform. Hopkinson, an engineer with Edison's British company, rationalized the field geometry, making a very good generator of the modern type a few years later. The field was symmetrical with respect to the armature, and short. A closely related type, the Manchester dynamo, is shown below. Compare its compact and short magnetic circuit with that of the Edison Z. Note the brush holder and the brass commutator on the armature. This is a two-pole machine, because the field has one N pole and one S pole.

One thing that may worry you if you examine an electrical machine closely also worried early designers. They put the wires on the surface of the armature where they would actually be in the magnetic field and experience motor or generator action, in the way we have explained it here by our principles. However, wires are now always placed in slots cut in the armature iron, allowing the air gap to be made smaller and the magnetic circuit much more efficient. The overall result is the same as if the wires were actually in the magnetic field, but the mechanism is slightly different. Now the armature current in the motor magnetizes the armature iron, and the interaction of this magnet with the field poles provides the force. In a generator, the field magnetizes the armature iron, and this field moves past the conductors as the armature rotates, with an effect like a transformer. Siemens, I believe, was the one who first saw this and the great improvement it could make in electrical machines.

The ways that windings of wire are arranged in modern machines are shown at the right. The windings are either around the pole pieces, or placed in slots on the surface. The part that rotates is called the rotor, and the part that remains at rest is called the stator. Both are of a magnetic core iron alloy, and are laminated if they are subject to alternating magnetic fields, to reduce eddy-current losses. DC machines typically have a salient-pole field on the stator, with the field windings on the pole pieces, and a non-salient pole winding on the armature, forming the rotor. The magnetic field of the stator is constant, while the field in the armature alternates. Therefore, the armature is laminated. The actions of salient and non-salient pole windings are equivalent. A non-salient pole winding can be arranged to give any desired spatial distribution of magnetic field. The typical salient-pole winding of a DC machine provides field-free regions between the poles that aids commutation, since switching can be done while the armature conductors are in this region and not generating any emf. In both salient and non-salient pole machines, the windings are firmly held mechanically.

The windings of motors and generators can be connected in one of two basic fashions. If the field windings and the armature windings are in series, they are called series-connected. In this case, the field windings are of heavy-gauge wire to carry the main motor current. 

The field becomes stronger as the armature current increases, leading to a very great force at low speeds. If the field and armature are in parallel, they are called shunt-connected. The field winding consists of rather fine wire. If the voltage applied to the motor is constant, then the field strength is also constant. If a generator is rotated at constant speed, then the output voltage is independent of the load. There are intermediate cases where the field has both series and shunt windings, and such machines are called compound.

Most direct-current power-station generators are mainly shunt-connected, and most traction motors mainly series-connected, as you might expect from the requirements of the two services: constant voltage in the first case, high starting torque in the second. Rotating machines can be made for voltages up to about 2000V, the restrictions being insulation and flashover at the commutator.

It is not easy to change DC voltages. One way to do this was to use a dynamotor, which had a normal field winding, but dual armature windings and two commutators. One winding was supplied at the input voltage and drove the dynamotor by motor action. The other winding supplied the output voltage. This can really be considered a kind of AC transformer. The input commutator creates AC from DC, and the output commutator changes the new AC voltage to DC. In World War II, when radios required a plate supply of, say 300 V, dynamotors were used to obtain this voltage from 6 V battery supply.

The speed of a direct-current motor is determined by both the field strength and the load. If there is no load, the speed is such that the voltage produced in accordance with the third principle exactly balances the applied voltage, and the armature current is zero. As the load is increased, the speed decreases to allow current to be drawn so the necessary electrical power can be converted. When the motor stalls, it is exerting its maximum force. Therefore, the speed of a shunt motor, or one in which the field is produced by a permanent magnet, is determined by the applied voltage, and can be adjusted finely.

If voltage is applied to a series motor without a load, the motor speeds up. As it does so, the field current decreases so the motor must speed up some more to generate the same back voltage. This keeps up until the motor flies apart. The loss of load on a series motor is a serious thing, and must be guarded against. When a loaded series motor is rotating with the maximum voltage applied to it, the current just produces the required amount of force with the existing field strength. If the field is weakened by reducing the current in it (by putting a resistance in series with it, for example) the motor must speed up to compensate. This is one method of speed control for direct-current motors.

A large direct-current motor must not be started by applying the full voltage across it while it is at rest, especially a series motor. The heavy current and field will create a great jolt that may damage the motor and its mechanical connections. A starting resistance is used to limit the initial current to only the amount necessary to put the motor into rotation. As it speeds up, the starting resistance can be removed in steps. In normal operation, the starting resistance should be removed, since it represents a significant loss of power. For further speed control when more than one motor is used, as on a streetcar or locomotive, the motors can be connected in series to start, and in parallel to run . In each case, the field can be weakened to give a higher speed. With four motors, series, series-parallel, and parallel connections, with field weakening, gives six speed levels that can be designed for service requirements. This could give, for example, speeds of 10, 15, 20, 30, 40, and 60 mph with starting resistance switched out.

A direct-current motor can be reversed by reversing the direction of the current in either the field or the armature.