Electrical advice needed:
Some of the known suppliers of cooling fans sell separate units for positive earth cars. Other fan manufacturers have informed me to just switch the wire connections on their negative earth fans. Is this correct and wouldn’t the fan direction be effected? I want to retain the positive earth on the car.
Thanks,
Alan
Alan,
I know nothing about fans sold for XKs but I offer the following general comment. DC fan motors are of two main types: series-wound and permanent magnet.
Series-wound or “universal” motors have field coils and armature coils wired in series. The direction of rotation is determined by magnetic attraction between the two coils, “north” pole of one to “south” pole of the other. Reversing the polarity of the applied voltage reverses the fields of both coils simultaneously–north becomes south, south becomes north. Thus the rotation direction stays the same. You can even apply AC (changing polarity 100 or 120 times per second, and the motor will happily run in its designed direction. To change that direction, one must take the motor apart and reverse the connections between field and armature coils (the latter supplied by brushes of course). So in practice, two motors are often sourced by vendors. You can select either direction, and it will be the same regardless of pos or neg ground.
Permanent magnet motors use permanent magnets in lieu of field coils. Therefore, the field’s magnetic polarity is physically determined by magnet orientation. The magnetic polarity, and therefore direction of rotation, of the armature is determined by which of two power wires (which in this case supply only the armature) is grounded and which is pos. So only one fan model is needed to choose either direction.
Permanent magnet motors are the newer design. They provide more power for a given size due to reduced bulk and power consumption afforded by the permanent magnet field. So I would recommend them.
Trivia: XJ6 dual heater fan motors switched from field coil type (two motor part numbers since each fan ran in a different direction) to permanent magnet type (same motor both sides; just reverse polarity) in 1972. So not all that new. IMHO.
Hi Alan…best to take advice from a manufacturer…it will depend on which fan you use…the plastic body fans typically have two wires that require a pos and neg…as the fan is plastic it is insulated from the car body…so it wont matter if your car is pos or neg earth…Steve
For quite a few years I had a Bosch fan I pulled from a Mercedes on my XK120, no problem with positive ground. I didn’t look but I imagine the Mercedes was negative ground.
I removed it because I found I never needed it.
But a couple of years ago I bought a new starter motor from Rock Auto for my negative ground modern S-Type, and it turned backwards new out of the box straight from China. Rock exchanged it and the new one was fine.