MASCOT July - August 2004

How Does It Work? No 10 - The Charging System - Part 1 - The Generator, or Dynamo, and Cut-out.

from the Tool Chest

Car batteries need to be kept fully charged, and for this they need a direct current (dc) source of electricity of slightly higher voltage than that of the battery. It is the function of the Generator to provide this voltage, or Electro Motive Force (EMF). We saw in HDIW No 8 that when an electrical conductor (wire) moves within a magnetic field, an emf is induced in the conductor - see Fig 1 - and it is this basic principle that is harnessed in the generator. The direction of the current flow can be determined using-Fleming's Right Hand Rule - see Fig 2

The conductor is formed into a loop which is wound onto a rotating former known as the armature, and the current is collected by means of a simple brush system - see Fig 3. As the armature rotates, the EMF varies from zero when the coil is vertical, to a maximum when in line with the magnetic field. Also, using Fleming's RH Rule, you can see that the direction of current flow changes during the second half of the rotation, the result being that an alternating current (ac) is generated. This is in fact an Alternator, and is fine for modern cars with electronic rectifiers for changing the ac to dc, but our dynamos were made long before the technology was available to make alternators viable.

So our generators use a commutator, in which the brush ring is split, each piece insulated from the other by a mica strip, so that the brushes collect the emf only in one direction, resulting in the required dc voltage, although it fluctuates as the conductor rotates in and out of the magnetic field - see Fig 4. Adding more coils reduces this fluctuation and increase the output. The commutator brush ring is divided into segments, each made of copper insulated and insulated from each other by mica strips - see Fig 5. The brushes are made of carbon and are held in contact with the commutator by light springs.

The output from generators using permanent magnets is very difficult to control, so electromagnets are used. Coils are wound around the magnet pole pieces - these are called the Field Coils - and they are connected across the generator output as shown in Fig 6. A small amount of residual magnetism in the pole pieces is sufficient to form the initial magnetic field, then, as the armature is rotated, the induced voltage is fed to the field coils, creating the electromagnet. As the speed of rotation increases so the output increases, and this can reach a very high value, therefore some form of control is required before it is suitable for charging our battery.

We will look at the various methods of voltage control/regulation in Part 2, but if a stationary generator were to be connected directly across a battery there would be a current flow back through the generator, which effectively becomes an electric motor, and the battery would be discharged. A method of disconnecting the generator from the battery is therefore required for when the engine is not running, or when the voltage generated is less than the battery voltage. This is the job of the cut-out. The cut-out has an iron core with two windings - see Fig 7. One, the `shunt' winding, consisting of many coils of fine wire, is connected across the generator. The other - the series winding has fewer coils of heavier wire connected in series with the battery through a set of contact points that are held open by a spring. The contacts are set to close when the generator output is a little higher than the nominal battery voltage. When the generator output is less than the battery voltage, the magnetic field created by the shunt winding is not strong enough to overcome the spring and close the contacts, but as the generator speed/output is increased, a point is reached when the contacts close. This allows current to flow through the series winding, which assists the action of the shunt winding, and the generator output can now pass through to charge the battery. When the generator speed decreases until its output drops to a voltage less than that of the battery, a discharge current will flow through the generator windings. The passage of this `reverse flow' of current through the cut-out series winding produces a magnetic field of opposing polarity to that created by the shunt winding. This weakens the magnetic field. enabling the spring to open the contacts. Thus disconnecting the battery from the generator.

The cut-out also controls the ignition warning lamp, which is connected across the cut-out contacts. As the lamp is only required when the ignition is switched on, it is connected between the D terminal of the cut-out and the coil side of the ignition switch When the contacts are open with the ignition switched on, the lamp will light, as one side is connected to the battery and the other earthed through the generator. When the output voltage rises and the contacts close, the lamp is short-circuited and goes out and you know that the generator is charging the battery.