Fixing the Amazon's Alternator/Voltage Regulator

refers to: Volvo Amazon

During the rebuild of my Volvo Amazon, I decided to change the existing dynamo for an alternator, and got a second-hand one from one of the Volvo specialists, with a voltage regulator 'to match'. While I was at it I got a friend of a friend, Jeremy Stevens, to check up on the setup, and he not only rebuilt the alternator, but fixed a problem with the voltage regulator not properly matching it.

The details of what he did are somewhat technical, but for those interested in getting the best out of their Amazon electric setup or just wanting to understand it better are well worth reading. Anyway, here's his report. Many thanks, Jeremy!

Volvo Amazon SEV-Motorola Alternator

The alternator used on the Volvo Amazon is an “odd-ball” made by SEV and of Motorola parentage.  It is designed for an external electro-mechanical regulator and rather than the 9 diode (six main 3 field) used in contemporary Bosch and Lucas machines it uses a series diode mounted on a red heatsink to isolate the field circuit from the battery. This series diode passes the whole machine output and incurs a loss of between 0.7V and 1V depending on the machine loading.

This is all very well for medium output machines such as this one, but becomes a nonsense at higher output ratings because two diodes are required in parallel to handle the current and the cost of this arrangement far outweighs that of the three small auxiliary diodes used by Lucas and Bosch to power the field winding of the machine – and SEV did make just such a machine!

As the car was converted from a dynamo charging system, there was no regulator to suit the alternator and, prior to my involvement, a Bosch Aftermarket regulator (P/N 0192 062 007) had been purchased from a specialist under the “assurance that it was the right one for the job”.  This proved not to be the case as the voltage setting of the regulator was for a conventional alternator (with a trio auxiliary rectifier) and thus was set too low because of the 0.7V inherent drop between the regulator sensing point (D+ terminal) and the main battery connection.

Luckily the regulator was of conventional construction and used discrete electronic components, this meant that it was a relatively easy matter to adjust the voltage from the standard 14.1V setting to 14.85V required by the SEV machine.  This was achieved by adding an 18k resistor across the pre-existing setting resistor (mounted on stand-offs).  With this setting, although the regulator controls to about 14.8V the correct battery voltage of about 14V is maintained.  The Bosch electronic regulator is far superior to the mechanical unit it replaces.

The other interesting feature of this machine is the slip ring arrangement, which is quite unlike any other alternator I have seen.  Rather than a complete ring assembly the slip rings are mounted individually, I was lucky to find a pair of brand new ones at a alternator specialist in Ealing.

The SEV machine used standard semi-shielded bearings but was rebuilt using high-grade sealed bearings for better performance and life.

The output curve was plotted when the rebuilt machine was tested and despite its age, it gave a very creditable performance, far superior to the dynamo that it replaced.  The peak stabilised output of 35A is less important than the low speed performance where the object of the exercise is to balance all electrical loads at idle so that the battery is never discharged in normal running.  The pulley drive ratio is about 2.8:1 which at an idle speed of 750rpm gives an alternator speed of 2100rpm and thus a potential output capability of about 25A, more than enough for this application.

Jeremy Stevens, 16/1/08
 

Tags: alternator bosch motorola rebuild voltage-regulator

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