Continuing with the subject of Automotive alternators, this post talks about the in-built safety and temperature compensation
In-built safety in Alternator design against short circuit
The internal circuit of the alternator and the selection of the diodes in the rectifier are designed to prevent damage against two normally met with conditions in the vehicle. One is the reverse polarity protection. This is to take care of the possibility of the alternator terminals are connected inadvertently to a reverse polarity and the circuit becomes a near short circuit through the internal bridge rectifier which becomes forward biased. Such a situation could arise during some servicing work or when jump starting of the engine is attempted from an external battery through some loose temporary cables. Normally a fuse is provided inside the alternator for the purpose. Any fuse can also become a nuisance in terms of long term durability and may tend to heat up locally over a period of use. Nowadays as a trade off of, the risk of reverse polarity is considered very minimal the fuse is being deleted in alternator designs. The other safety protection which is usually provided is against the high voltages which result due to the vehicle system phenomenon called “load dump”. Basically the alternator gets its energy input from the engine through the belt drive and converts it into electrical energy at its output terminals. Depending on the electrical load there is a corresponding energy input at the alternator shaft. This energy develops the current output to the required level. The alternator has an internal inductance which stores energy of the classical I2L form where the I is the current being delivered and L is the inductance of the winding. Any inductance resists change in current and when the load is suddenly switched off from the output the stored energy in the inductance seeks to dissipate the energy through the paths available and depending on the impedance the energy meets with, a peak surge voltage is generated which reduces exponentially within a short time which has a relation to the collective resistance which dissipates the energy. Exact quantification of the voltage peak and its decay time , in relation to the vehicle system electrical elements’ parameters and in different vehicle electrical system environments, is not possible because every vehicle system is unique and load switching can be random at various levels. To arrive at a most probable peak voltage under usually encountered vehicle environments, experiments have been conducted on representative vehicle structures and based on the test results specific guidelines have been introduced in industry applicable standards like SAE, ISO and DIN. These standards are normally used as the average basis for designing units with a capacity to withstand such short time voltage surges without damage to the electronic devices; such precautions in design ensures functional reliability under load dump situations met with on vehicles. The alternator itself uses within itself a number of electronic devices like diodes and transistors and to absorb the voltage spikes and limit the level of spikes, the bridge rectifiers are formed using zener diodes instead of normal diodes and these zener diodes provide the path for the spike energy to dissipate; this limits the spike levels and ensures overall electrical system reliability under load dump conditions. .
Built-in Temperature compensation for maintaining the set voltage output over the expected ambient temperature range
The regulated output voltage of the alternator is set normally to generate around 14 volts for vehicles using a nominal 12 volts system. This is to ensure that the alternator will be able to charge from a sufficiently higher voltage level even as the battery voltage keeps increasing with progressively higher levels of SOC. Apart from the alternator output voltage the battery’s capacity to absorb charge also depends on the rate of chemical reaction in the electrolyte. The rate of chemical reaction is dependent on the ambient temperature. The experience of the vehicle power system designers has been that the charging efficacy of the system can be improved by marginally modifying the set point of the voltage regulator to be more at colder temperatures and lower at higher temperatures; the aim is to compensate for the changes in the rates of reaction in the battery electrolyte by having an automatic voltage setting mechanism in relation to the ambient temperature. The alternators now have regulators with a temperature compensation circuit to dynamically change the setting of the regulator with respect to the ambient temperature. A typical graph showing the compensation is given in Fig 3.7 .
Fig 3.7 Typical graph of temperature compensation for voltage regulator setting
The automatic in-built compensation ensures that when the ambient temperature is cold the alternator voltage is enhanced to compensate for the reduced chemical reaction time of the electrolyte by driving the charging at a higher voltage. Similarly at higher ambient temperatures the output voltage is reduced to compensate for the increased chemical reaction time of the electrolyte so as to avoid overcharging and gassing in the battery.
The next post will be on data / signal interfaces between the alternator and the Engine management system
Automobile Electrical System – Components, Control Units, Systems & Integration 