In this thread, I am going to discuss the motorcycle charging system found in motorcycles in general, and with specific focus on the lower capacity motorcycles found here in India and how to modify them. This thread will be somewhat technical in nature and basic understanding of electricity and magnetism is assumed. Those here who have zero knowledge in electronics may refer to websites like Wikipedia.com and understand the concepts discussed here. Ok, let’s get started!

The charging system (and indeed all forms of generators, alternators) works on the basis of a principle called Faraday’s Law of Induction. In the simplest form, it states that a conductor when placed in a changing magnetic field will tend to develop an EMF. (Electromotive force or in simpler word a Voltage is developed.) So, for the charging system to work, we need an electrical coil (which acts as the conductor) and a magnetic field that can be varied with respect to time.

Now, let us discuss the coil. Why do we need a coil when a simple wire could also have acted as a conductor? The answer is that, the more the length of wire that the magnetic field can “affect”, more is the EMF or Voltage generated. In case of vehicles, we need around 12V. Hence, the use of a coil made using turns of wire of certain calculated length. Now, the coil is wound around an iron cylinder. Why? When we use an iron core instead of an air core (or core made of any non magnetic material) the number of turns of wire needed decreases to a lower value. Thus, the system can be made compact.

Now, the magnetic field can be generated using a permanent magnet (natural magnet) or an electromagnet. Electromagnets are formed by applying a voltage across a coil. Another law called Ampere’s Law states that a magnetic field will be generated. The voltage applied to the coil determines the strength of the magnetic field. Natural magnets generate a fixed magnetic field. Now, to vary this field, the arrangement used is described below.


Permanent Magnet Charging System

A practical charging system uses many coils arranged in a radial axis. Refer to figure. ("2" is Rotor, "3" is Stator.) These coils are fixed together to the Stator Plate. The magnet is mounted on the crankshaft and rotates to vary the magnetic field around the coils. It is known as the Rotor. This is how the system works.

Now, let us discuss the various methods to vary the output of the coil. The wires used in the coil affect the output. Thicker wires generate more current as the resistance in the coil decreases. But there is a catch. If we go ahead and use the thickest wire available, the system will not work satisfactorily. Why? A thicker wire will allow a lesser number of turns in the same area as we will simply run out of space. Thus the length of the wire used in the stator would be less than before. This will decrease the EMF generated. The coil will not generate the required 12V. The trick is to use the thickest wire possible while still being able to wound the required number of turns around the iron core. Another factor – the speed of the rotor affects the coil output. The faster the magnetic field varied (faster the rotor rotates) greater is the EMF induced. Hence, output of the coil varies with the speed of rotation of the engine. This is why we need a regulator. The third factor is the magnetic field strength of the rotor. The field strength in case of a natural magnet rotor is constant. We can only replace the rotor with a rotor having a stronger magnet to generate higher output from the coils. But a magnetic field generated by an electromagnet can be varied. This means we can vary the magnetic field to control the output generated.

As we have seen, the EMF varies with engine speed, while all electronic devices including the battery, bulbs etc require a constant voltage of around 12V. So how do we regulate the output voltage? In case of a electromagnet rotor system, we can vary the current applied to the rotor electromagnets as speed varied and keep the coil output to a fixed voltage level. This type of system is used in car alternators and BMW bikes among others. All our Indian bikes as well as most of the international big capacity international bikes use the permanent magnet charging system. The output in this case cannot be directly controlled. What is done is that the coil output is short-circuited to reduce the voltage to the desired level. The regulator consists of 2 parts. A monitoring unit determines if the output voltage from the coils exceed the determined voltage limit. If the limit is found to be exceeded, the second part of the regulator short-circuits the coil output and brings down the voltage level. This process occurs continuously. It is seen that a certain portion of coil output is actually wasted. This system has low efficiency but is very simple and rugged and hence is generally used in low power applications like motorcycles.

The rectifier is often a part of the regulator. This is used to convert the AC output from the coil to DC which is needed by the battery.

Now, after studying the underlying principles and working of the charging system, one may ask, what can be done to increase the coil output? A higher output can support higher wattage bulbs, louder horns etc. Firstly we will have to increase the actual output from the stator coil. (The process is commonly called rewinding) Secondly, we will have to change the regulator as the original one cannot take the increased load.

I have done these mods on my P150 myself and will describe what/how I have done in details in my next few posts. These mods require certain electrical knowledge and are not suitable for newbies.

Any suggestions/clarifications are welcome.