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22nd August 1918, Page 15
22nd August 1918
Page 15
Page 16
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The Third Article,

[The upkeep of the commercial electric vehicle is not a mailer that presents any great difficulty, but ignorance of the mere fundamentals is largey apt to prevent a man from attempting to gam the knowledge that would enable him to approach the task of driving, tending and maintaining a %chicle run under electric power. To assist in that dissemination of knowledge that should serve to develop driving talent for electric vehicles, we are publishing a very short series of non-technical articles from the pen of a competent electrical engineer.—Eo.j THE SERIES-WOUND electric motor is always used for traction work because of its peculiar properties. It has the largest torque or starting effort at the lowest speed. Fig. 14 is what is called the mechanical characteristic curve of the series motor. It will be noticed that, as, the speed falls, the torque increases and vice versa; also that, at high speeds, the torque decreases rather rapidly, and increases rather rapidly again as the speed falls.

Why the Series-'wound. Motor is Used.

The largest torque is required from the electric traction motor during the period of starting, and immediately after, and the greatest demand that is made upon the battery furnishing current for the motor is during the same periods, the adceleration period making the largest demand.

In the series-wound motor the field magnet coils consist of a comparatively few number of turns of wire of about double the thickness of that on the armature coils. Each coil is wound on a former and is carefully insulated, and subjected to the processes de-, scribed in connection with armature coils before lieifld. fixed in its place, A completed coil is shown in Fig. 15, whilst a Reid magnet, with its core, pole piece, field coil, and a portion ,.of the containing cylinder are shown in Fig. 16. The four field magnets are arranged in pairs, as shown in Fig. 17. They have alternately north and south polarity—thus N.kN.S. The coils of a pair of field magnets, one N. and one S., are connected together, their inner ends being joined by a junction piece, as shown in Fig. 18, the end of one coil and the opposite end of the other coil being carried to screw terminals to which the leads from the controller are brought. When the serieswound motor is running, the current from the battery flows through the field magnet coils, then through the armature coils, and then back to the battery, or vice versa. This arrangement is termed connecting the

field coils and the armature coils in series. The series-wound motor is distinguished from the shuntwound motor that is so much used in factories by the fact that tht current flows continuously through the ;mils of field magnets and armature, whereas, with the shunt-wound motor, the current divides at the brushes between the armature coils and the field magnet coils, the latter consisting of a large number of turns of fine wire, so that the shunt, coils only take a small portion of the current. Shunt-wound motors are not suitable for traction work, because the torque at low speed does not increase in the way that the torque of the series-wound motor does.

The power delivered by the axle of any given motor

depends directly upon the strength of the current flowing through the armature coils and that flowin through the field magnet coils. With the series-wouriti motor the two increase together, and both are at their highest at the time when most power is wanted, during the starting and acceleration period. With the shunt-wound motor, the strength of the current in the field coils is less during the starting period than at any other. The series-wound motor also lends itself to the variations in the quantity of power required' under running conditions very much more readily than does the shunt-wound motor iii particular, it Tends itself to variations of power with yariations of

speed. . . . _ . _

The Operation of the Electric Motor.

• It may, perhaps,,he as well to give a short description of the waT in which an electric motor works. Its armature furnishes .power because a 8exies of attractions and repulsions are set up between the currents flowing in its coils and the magnetism of the pole pieces of the field magnets. This is not the usual technical description given in the text books, but it is practically correct and will probably be mpre easily understood. The stronger the magnetic field in which the armature revolves and the stronger the currents flowing in the armature coils, the greater is the power generated. Also, with any given motor, the greater the electric pressure delivered at the terminals of the motor the greater is the current flowing in both armature and field magnet coils.

But there is a very peculiar feature about the running of the armature of an electric motor. When it is running, a back electric pressure is produced in its coils which tends to choke back the initial pressure-that of the battery in this case—delivered at its ter

animals. The result is that the current flowing through both armature and field coils depends on the difference between the pressure delivered by the battery through the controller and the back-pressure induced in the armature. The back-pressure varies with the currents flowing in the armature and field coils and also with the speed at which the armature is revolving. Hence, during the starting period, the back-pressure is at first nil and gradually increases as the speed rises, and this means that, during the starting peridd,

a powerful current from the battery flows through both armature and field coils of the series wound motor, giving the necessary heavy starting torque. As the speed increases, the back-pressure reduces the current strength more and more, thus reducing the torque to the figure required to drive the car and also reducing the drain on the battery.

As will be described later, special arrangements are made by the aid of the controller to reduce the strength of the current flowing during the starting period. This self-regulating property of the electric motor is also of great assistance when running, in meeting the various conditions of roads, of traffic, etc. When going up-hill, for instance, the motor irnmedi,, ately slows down, taking the necessary additional current from the battery to meet the extra work to be done, and at once furnishes the necessary additional torque. Immediately the level or an easier gradient is reached, the motor increases its speed, thereby increasing its back-pressure and cutting off the additional current that it now no longer requires.

Replacing a Field Coil.

It ought very rarely to be necessary to replace field coil, be taken to prevent oil, moisture and dirt, horn getting into; the ;machine. Occasionally, however, something. ha.ppens, a small piece of metal may get in and -damages the insulation, or one of the wire bands of the armature may break and a portion of it may damage the insulation of a field coil. In such a case, it will be necessary to remove the armature as described in a previous article and then to remove either the field magnet core of the coil that is faulty, or the pole piece. The field magnet cores are sometimes east as part of the containing cylinder, the pole pieces being bolted to their inner ends. In those eases, the pole pieces are removed, the field coils slipped off, and the spare ones put in their place. Care must be taken, when removing the field coil, o see that it is disconnected from the other coil and its terminal screw, and, when replacing the new coil, care must also be taken to connect the saineenj of the new coil to the-same end of the other coil to which the old one was connected, and the same end of the new coil to the terminal sdrew.

As mentioned above; usually the inner ends of the two coils on the N. and,S. field magnets are connected together, and the outer ends to the terminal screws. If the connections should be reversed, or if, by acci

B36 dent, the inner end of one coil be .connected to the other, the pair of electro magnets will furnish no magnetism. • In some cases, the pole pieces and the cores of the field magnets are practically one. They are built up in a special -manner, to avoid eddy currents in them, and they are arranged to be moved together. In those cases, they are held by bolts passing inwards from the outside of the containing cylinder, and the removal of these--bolts allows the core and the pole piece to be slipped gently through the field magnet coil, So that the latter can then be disconnected and removed. In fitting the new -coil, it is slipped over the iron core, which is then put in its place and the holding bolts are screwed home. It is wise to look at the hplding bolts occasionally, whether they are on. the outside of the containing cylinder, or areMerely holding the pole pieces on to the pores. It is of great importance that neither the pole pieces nor the cores should move out of their position by the very smallest fraction of an inch.

Care in Connection with Terminal Screws.

It has -been mentioned in the previous article that the insulating collars of the arms holding the brush shoes should be carefully looked after to see that there. is no deposit of oil, -copper or carbon dust on them. The same precautions should be taken with the insulation of the terminal screws. If that should be destroyed, or if it is bridged over by moisture or dust, connection is made between the terminal screw whose insulation is damaged and the containing cylinder. If a second such connection is made, say, at the other terminal of a pair of field magnet coils, that pair of coils is bridged over by the -containing cylinder, and the current that should have passed through them and created the magnetic field in which the armature revolves passes through the containing cylinder instead, withthe result that the armature becomes unduly heated and the motor furnishes very little power.

If the insulation of the brush spindles be destroyed, the armature coils are bihriged over, and very heavy currents pass through the field coils, heating them unduly, destroying their insulation and obligkig them to be replaced, whilst,• again, the motor furnishes no power. All the materials that are used for insulating


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