MASCOT May - June 2005

Ask ASCO

Dear Aunty. Regarding the article 'The Transition From Dynamo To Alternator' in the March-April 2005 MASCOT, and in particular the last paragraph about getting an alternator built into an original dynamo casing; I went to the Classic and Vintage Dynamos stall at Beaulieu to enquire about this, and I was told that it can only be done for cars with negative earth electrical systems. So if, like me, you have a car with a positive earth system, and you want one of these conversions, you will have to convert the electrical system to negative earth as well. Maybe one of our electrical specialists could tell us what would be involved in carrying out this conversion? It can't be as easy as just changing the battery connections around, surely? Peter Watts.

Dear Peter. You are right that converting from Positive to Negative earth isn't just a matter of changing the battery connections I spoke to Barry Paine about your query ands kindly sent the following:

Conversion to Negative Earth

Barry Paine.

Alternators are designed to operate on negative earth systems, so if you decide to fit an alternator to your positively earthed Singer you will need to convert to negative earth. Apart from obvious practical things like reconnecting the battery so that the negative pole is earthed, there is very little you need to do on a car like a Singer Roadster. Most electrical components are not 'polarity conscious' and so can work with either polarity, but you would have to reverse the ammeter connections so that it reads 'Charge' and 'Discharge' correctly, and you should reverse the low tension coil connections, although it would still work the wrong way round, albeit with erosion of the earth tags on the spark plugs.

The car radio is one area where polarity may be critical, but then most of our old Singers weren't fitted with them, and most pre SM1500 cars weren't even provided with heaters ......

You might think you would need to do something to the heater and wiper motors, and perhaps the starter motor, but these should not be affected by a change in polarity. They will not run backwards because their direction of rotation depends on the relative sense of the electromagnetic fields in the armature and field coils. By reversing the polarity of the supply voltage to the motors, both electromagnetic fields will have changed direction, so their relative sense is the same and the direction of rotation remains the same.

The above refers to conventional dc motors as originally fitted to our Singers. There could, however, be specialist items such as modern electric water pumps and fans that could be polarity conscious - an electric pump I have purchased for use on my Roadster is made of plastic. It just needs connecting the correct way round in accordance with the manufacturer's instructions. The pump body, being plastic (or non-conducting) will not be earthed and therefore not connected to one pole of the battery, so there should be no problem.

The conversion would, of course, make the old cut-out or regulator redundant. In order to retain an appearance of originality, some people leave these in situ with the wires connected to them. These would disappear into the loom but not actually be connected anywhere else.