Automotive alternators – In-built safety and temperature compensation

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 .

Sketch 3

  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


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